10Base2 - Mbps baseband Ethernet specification using 50-ohm thin coaxial cable. 10Base2, which is part of the IEEE 802.3 specification, has a distance limit of 185 meters per segment.

10Base5 - 10-Mbps baseband Ethernet specification using standard (thick) 50-ohm baseband coaxial cable. 10Base5, which is part of the IEEE 802.3 baseband physical layer specification, has a distance limit of 500 meters per segment.

10BaseT - 10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Category 3, 4, or 5): one pair for transmitting data and the other for receiving data. 10BaseT, which is part of the IEEE 802.3 specification, has a distance limit of approximately 100 meters per segment. 10BaseT indicates the use of a signaling speed of 10 megabits per second using a baseband signaling scheme transmitted over a twisted pair networking media employing a star or extended star topology.

10Broad36 - 10-Mbps broadband Ethernet specification using broadband coaxial cable. 10Broad36, which is part of the IEEE 802.3 specification, has a distance limit of 3600 meters per segment.

10BaseF - 10-Mbps baseband Ethernet specification that refers to the 10BaseFB, 10BaseFL, and 10BaseFP standards for Ethernet over fiber-optic cabling.

Access Method - 1.) Generally, the way in which network devices access the network medium. 2.) Software within an SNA processor that controls the flow of information through a network. Network architectures provide a regulatory mechanism that allows devices on a network to share the networking media equitably and in some cases cost effectively as well. In computing, this regulatory mechanism is referred to as the access method. An access method is simply the way in which devices that are attached to a network access the networking media to transmit signals. More formally stated, an access method is a set of predefined rules or protocols that determine how a device gains access or entry to the networking media. The two most widely used access methods are token passing and CSMA/CD.

Active Hub - Multiported device that amplifies LAN transmission signals.

Adapter - Network interface card. Board that provides network communication capabilities to and from a computer system. (see NIC Card)

Address Mask - Bit combination used to describe which portion of an address refers to the network or subnet and which part refers to the host. Sometimes referred to simply as mask.

ARP - Address Resolution Protocol - Internet protocol used to map an IP address to a MAC address (provides a method for finding MAC addresses based on IP addresses). to allow communication on a multiaccess medium such as Ethernet. To determine a destination MAC address for a datagram, a table called the ARP cache is checked. If the address is not in the table, ARP sends a broadcast which will be received by every station on the network, looking for the destination station. The term local ARP is used to describe resolving an address when the requesting host and the destination host share the same medium or wire. Prior to issuing the ARP, the subnet mask was consulted. The mask determined that the nodes were on the same subnet. Defined in RFC 826. Each computer on a network maintains its own ARP table. Whenever a network device wants to send data on the network it uses information provided by its ARP table to accomplish this. Once the source has determined the IP address for the destination, the source's Internet protocol will look into its ARP table in order to locate the MAC address for the destination. If the Internet protocol locates a mapping of destination IP address to destination MAC address in its table, it will bind the IP address with the MAC address and use them to encapsulate the data. The data packet is then sent out over the networking media to be picked up by the destination. If the source knows the destination IP address, but is unable to locate a MAC address for it in its own ARP table. If the destination is to retain the data and pass it along to the higher layers of the OSI model, the source must use both a destination MAC address and a destination IP address. Therefore, the device initiates a process called an ARP request that is designed to help it discover what the destination MAC address is. First, the device builds an ARP request packet and sends it to all devices on the network. To ensure that the ARP request packet will be seen by all devices on the network, the source uses a broadcast MAC address. The broadcast address that is used in a MAC addressing scheme results when all places are set to F. Thus, a MAC broadcast address would have the form FF-FF-FF-FF-FF-FF. Generally speaking, a device on one network cannot send an ARP request to a device on another network because ARP requests are sent in broadcast mode, which are not forwarded to other networks by routers. (see Proxy ARP)

Algorithm - Well-defined rule or process for arriving at a solution to a problem. In networking, algorithms are commonly used to determine the best route for traffic from a particular source to a particular destination.

AC - Alternating Current - Electrical current that reverses its direction regularly and continually. It is the form of electrical power found in residential and commercial buildings.

ARP Reply - Once the device that originated the ARP request receives the ARP reply, it extracts the MAC address from the MAC header and updates its ARP table. Now that it has all of the information it needs, the device can properly address its data with both a destination MAC address and a destination IP address. It uses this new information to encapsulate the data before it sends it out over the network. This time when the data arrives at the destination, a match is made at the data link layer. The data link layer strips off the MAC header and transfers the data to the next highest layer, the network layer. The network layer examines the data and finds that its IP address matches the destination IP address carried in the IP header of the data. The network layer strips off the IP header and transfers the data to the next highest layer in the OSI model, the transport layer. The process will be repeated until the rest of the packet reaches the application where the data will beread.

ARP Table - ARP tables kept by routers different from ARP tables kept by other devices on a network because, generally speaking, network devices map IP addresses and MAC addresses they see on a regular and repeated basis. In short, this means that a typical device contains mapping information pertaining only to devices on its own network. It knows very little about devices beyond its local area network. Because routers build tables that describe all networks connected to them, ARP tables kept by routers can contain IP addresses and MAC addresses of devices located on more than one network. In addition to mapping IP addresses to MAC addresses, router ARP tables also map ports, because a router may have more than one IP address, and the router's table maps each IP address to the appropriate MAC address. In this way, the router uses network IP addresses to make path selections so it can forward data from one network to another. (also see Address Resolution Protocol)

ARCnet - Attached Resource Computer Network - ARCnet is no longer used. Increasingly, Ethernet network architecture is becoming the dominant one used. A 2.5-Mbps token-bus LAN developed in the late 1970s and early 1980s by Datapoint Corporation.

Attenuation - Loss of communication signal energy.

Authority Zone - Associated with DNS, an authority zone is a section of the domain-name tree for which one name server is the authority.

Backbone - The part of a network that acts as the primary path for traffic that is most often sourced from, and destined for, other networks.

Backbone Cabling - Cabling that provides interconnections between wiring closets, wiring closets and the POP, and between buildings that are part of the same LAN. Backbone cabling consists of the backbone cabling runs, intermediate and main cross-connects, mechanical terminations and patch cords used for backbone-to-backbone cross-connection. Included in this are the vertical networking media between wiring closets on different floors, networking media between the MDF and the POP, and networking media used between buildings in a multi-building campus. EIA/TIA-568 specifies four types of networking media that can be used for backbone cabling. These include100 ohm unshielded twisted pair, 150 ohm shielded twisted pair, 62.5/125 micron optical fiber, and single-mode optical fiber. Although EIA/TIA-568 recognizes 50 ohm coaxial cable, generally, it is not recommended for new installations, and it is anticipated that it will be removed the next time the standard is revised. Most installations today use the 62.5/125 � fiber-optic cable as a matter of course for backbone cabling. To use single-mode fiber-optic cable for the backbone cabling to connect the horizontal cross-connect to the main cross-connect as described above, then the maximum distance for the backbone cabling run is 3000 meters or 9840 feet. However, if the backbone cabling is used to connect the horizontal cross-connect to an intermediate cross-connect and the intermediate cross-connect to the main cross-connect, then the maximum distance of 3000 meters must be split between the two sections of backbone cabling. When this occurs, the maximum distance for the backbone cabling run between the HCC and the ICC is 500 meters or 1640 feet. The maximum distance for the backbone cabling run between the ICC and the MCC is 2500 meters or 8200 feet. (see IDF)

Backoff - The retransmission delay enforced when a collision occurs. (see Collision)

Baseband - Characteristic of a network technology where only one carrier frequency is used. Ethernet is an example of a baseband network. Also called narrowband.

Best-Effort Delivery - Describes a network system that does not use a sophisticated acknowledgment system to guarantee reliable delivery of information. (see Ethernet)

BISDN - Broadband ISDN - ITU-T communication standards designed to handle high-bandwidth applications such as video. BISDN currently uses ATM technology over SONET-based transmission circuits to provide data rates from 155 to 622 Mbps and beyond. Contrast with N-ISDN. See also BRI, ISDN, and PRI.

Bit - Binary digit used in the binary numbering system. Can be 0 or 1.

BRI - Basic Rate Interface - ISDN interface composed of two B channels and one D channel for circuit-switched communication of voice, video, and data. Compare with PRI. See also BISDN, ISDN, and N-ISDN.

Bridge - Device that connects and passes packets between two network segments that use the same communications protocol. Bridges operate at the data link layer (layer 2) of the OSI reference model. In general, a bridge will filter, forward, or flood an incoming frame based on the MAC address of that frame. Bridges only look at the MAC address, and they build tables of all MACs located on a network (and others it connects to) and maps them. When traffic between network segments becomes too heavy, the bridge can become a bottleneck. There are two types of bridges: Transparent and Source Route (for Token Ring).

Broadcast - Data packet that will be sent to all nodes on a network. Broadcasts are identified by a broadcast address.

Broadcast Address - Special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all ones. Usually designated by all of the host octets having the value of 255.

Broadcast Storm - Undesirable network event in which many broadcasts are sent simultaneously across all network segments. A broadcast storm uses substantial network bandwidth and, typically, causes network time-outs.

Brownout - See Sag

Bus - Common physical signal path composed of wires or other media across which signals can be sent from one part of a computer to another. Sometimes called highway.

Bus Topology - Linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Typical of 10Base2 and 10Base5 Ethernet. A bus topology is one in which all devices on the LAN are attached to a linear networking media. This linear networking media is often referred to as the trunk line, bus, or highway. Every device such as workstations or servers is independently attached to the common bus wire through some kind of connection. The bus wire must end in a terminating resistance, or terminator, which absorbs electrical signals so they don't bounce, or reflect, back and forth on the bus. The local area network architecture that best represents bus topology is Ethernet. When a source transmits data over the networking media in a bus topology, the signal travels in both directions from the source. These signals are made available to all devices on the local area network. As you have learned in previous lessons, each device checks the data as it passes by. If the destination MAC address and the destination IP address carried by the data does not match that of a device, the device ignores the data. However, if the destination MAC address and the destination IP address carried by the data does match that of a device, the device copies the data and passes it up to the data link and network layers of the OSI model. When a signal reaches the end of the bus, it is absorbed by the terminator. This prevents signals from bouncing back and being received again by workstations attached to the bus. In order to ensure that only one workstation transmits at a time, collision detection is used. If more than one node attempts to transmit at the same time, a collision will occur. On a bus topology when a network device detects that a collision has occurred, a backoff is issued by the NIC card. Because it is based on an algorithm, the length of this enforced retransmission delay is different for every device on the network, thus minimizing the likelihood of another collision. Typical bus topology has a simple wiring layout that uses short lengths of networking media. This makes it obvious that the cost of implementing this type of topology is usually low when compared with that of other topologies. However, the low cost implementation of this topology is offset by the its high management costs. In fact, the biggest disadvantage it has is that fault diagnosis and isolation of networking problems can be difficult simply because a bus topology has few points of concentration. Because the networking media does not pass through the nodes attached to it, if one device goes down on the network, it doesn't affect other devices on the network. While this can be considered an advantage of bus topology, it, too is offset by the fact that the single cable used in this type of topology can act as a single point of failure. In other words, if the networking media used for the bus breaks, then none of the devices located along it will have the ability to transmit signals. (see Collision, Collision Domain, and Backoff) Compare with ring topology, star topology, and tree topology.

Byte - Term used to refer to a series of consecutive binary digits that are operated upon as a unit (for example, an 8-bit byte).

Cancellation - When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. The direction of these magnetic lines of force is determined by the direction in which current flows along the wire. If two wires are part of the same electrical circuit, electrons flow from the negative voltage source to the destination along one wire and from the destination to the positive voltage source along the other wire. When two wires in such an electrical circuit are placed in close proximity, their magnetic fields will be the exact opposite of each other. Thus, the two magnetic fields will cancel each other out. Moreover, they will cancel out any outside magnetic fields as well. Twisting the wires can enhance this cancellation effect. By using cancellation in combination with twisting of wires, cable designers can provide an effective method of providing self-shielding for wire pairs within the network media. (see Signal Degradation)

CSMA/CD - Carrier Sense Multiple Access Collision Detect - Media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time. CSMA/CD performs three functions. The first function is transmitting and receiving data packets. The second is decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model. The third function of CSMA/CD is detecting errors within data packets or on the network when a device wants to send data it must first check to see whether the networking media is busy. It looks to see if there are any signals on the networking media. After is has listened for a while, if the networking media has not been busy for a certain amount of time, a device will begin to transmit its data. While it transmits its data in the form of signals, the device also listens. It does this to ensure that no other station is transmitting data to the networking media at the same time. After it completes transmitting its data, the device will return to a listening mode. CSMA/CD access is used by Ethernet and IEEE 802.3.

Catchment Area - Zone that falls within area that can be served by an internetworking device such as a hub.

Category 1 Cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 1 cabling is used for telephone communications and is not suitable for transmitting data.

Category 2 Cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 2 cabling is capable of transmitting data at speeds up to 4 Mbps.

Category 3 Cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 3 cabling is used in 10BaseT networks and can transmit data at speeds up to 10 Mbps.

Category 4 Cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 4 cabling is used in Token Ring networks and can transmit data at speeds up to 16 Mbps.

Category 5 Cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 5 cabling is used for running CDDI and can transmit data at speeds up to 100 Mbps. Uses RJ-45 connectors. Most frequently recommended and implemented in installations today.

CCITT - Consultative Committee for International Telegraph and Telephone - International organization responsible for the development of communications standards. Now called the ITU-T.

CDP - Cisco Discovery Protocol - Provides a single proprietary command that enables network administrators to access a summary of what the configurations look like on other directly connected routers. CDP runs over a data link layer connecting lower physical media and upper-network-layer protocols. Because CDP operates at this level, CDP devices that support different network-layer protocols can learn about each other. (Remember that the data link address is the same as the MAC address.) When a Cisco device running Cisco IOS Release 10.3 or later boots up, CDP starts up automatically. CDP can then automatically discover neighboring Cisco devices running CDP. Discovered devices extend beyond those having TCP/IP. CDP discovers directly connected Cisco devices regardless of which Layer 3 and 4 protocol suite they run. The command show cdp neighbors works at the data link layer to display the following information: neighbor device ID, local port type and number, decremental holdtime value in seconds, Neighbor’s device capability code, Hardware platform of neighbor, Neighbor’s remote port type and number. (see SHOW CDP Interface)

Cheapernet - Industry term used to refer to the IEEE 802.3 10Base2 standard or the cable specified in that standard. Compare with Thinnet. (see Coaxial)

Coaxial - Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. Two types of coaxial cable are currently used in LANs: 50-ohm cable, which is used for digital signaling, and 75-ohm cable, which is used for analog signal and high-speed digital signaling. Coaxial cable consists of two conducting elements. One of these is located in the center of the cable and is copper, which is surrounded by a layer of flexible insulation. Over this insulating material is a shield composed of woven copper braid or metallic foil that acts as the second wire in the circuit. As its name implies this outer braid acts as a shield for the inner conductor. Thus, it can help reduce the amount of interference. Outside this shield is the cable jacket. For local area networks coaxial cable can offer several advantages. Coaxial cable can run unboosted for longer distances than either shielded or unshielded twisted-pair cable. This means that it can run for longer distances between network nodes without a repeater being needed to reamplify the signal than twisted-pair cable can. Coaxial cable is available in different thicknesses. As a general rule, the thicker the cable, the more awkward it is to work with. This is an important point to remember especially if cable must be pulled through existing conduits and troughs that are limited in size. The largest diameter coaxial cable is specified for Ethernet backbone cables. Because it is stiff with shielding and because its jacket is a distinctive yellow in color, this type of coaxial cable has frequently been referred to as "thicknet." As is implied by its nickname, this type of coaxial cable can be too rigid to install easily in some situations. As a general rule of thumb, the more difficult network media is to install, the more expensive it will be to install. And, in fact, coaxial cable is more expensive to install than twisted-pair cable. In the past coaxial cable with an outside diameter of only .18 inch was used in Ethernet networks. Sometimes you will see this type of coaxial cable referred to as thinnet. It was especially useful for cable installations that required many twists and turns. Because it was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care must be taken to ensure that it is properly grounded. This is done by seeing that there is a solid electrical connection at both ends of the cable. Frequently, installers fail to see that this is done. As a result, poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems can result in electrical noise that interferes with transmittal of the signal on the networking media. For this reason, despite its small diameter, thinnet is no longer commonly used in Ethernet networks. Coaxial cable is less expensive than fiber-optic. CHARACTERISTICS: 10-100Mbps throughput, inexpensive, maximum cable length of 500 meters

Collision - In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical media. When a collision occurs, the data from each device impact - that is, the voltage pulses from each device are both present on the common bus wire at the same time - and thus the data from both devices is damaged. The network area within which data packets originate and collide is called a collision domain. Networking devices are able to tell when a collision has occurred because the strength of the signal on the networking media will double. When a collision occurs, each device that is transmitting will continue to transmit data for a short time. This is done to ensure that all devices see the collision. Once all devices on the network have seen that a collision has occurred, each will then invoke a backoff algorithm. After all devices on the network have backed off for a certain period of time, any device can attempt to gain access to the networking media once again. In other words, the devices that were involved in the collision do not have priority to transmit data when this resumes on the network. (see Collision Domain and Backoff)

Collision Domain - In Ethernet, the network area within which frames that have collided are propagated. Repeaters and hubs propagate collisions; LAN switches, bridges and routers do not. (see Collision)

CMIP - Common Management Information Protocol - OSI network management protocol created and standardized by ISO for the monitoring and control of heterogeneous networks. Currently CMIP is not implemented as much as SNMP, particularly in new installations. To obtain information about the network, CMIP uses a technique called MIB reporting. Using this technique, the central monitoring station waits for devices to report their current status to it. If concern about the amount of traffic is an issue on your LAN, CMIP may be a useful network management tool.

Common Mode Problem - Term used to describe problems involving either the hot or neutral wires and the safety ground wire on a power line. See normal mode. Common mode problems pose the greatest hazard over Normal Mode.

Concentrator - In Ethernet, where hubs act as multiport repeaters, they are sometimes referred to as concentrators. (see Hub)

Conductor - Any material with a low resistance to electrical current. Any material capable of carrying an electrical current. See Insulator.

Connectionless - Term used to describe data transfer without the existence of a virtual circuit. Compare with connection-oriented. See also virtual circuit. (see Ethernet)

Cross Talk - When electrical noise on the cable originates from signals on other wires in the cable, this is known as crosstalk. Interfering energy transferred from one circuit to another. (see Testing The Network)

CSU - Channel Service Unit - Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to together with DSU, as CSU/DSU.

Cut Sheet - A rough diagram indicating where cable runs are located and the numbers of rooms they lead to. (see Facility Audit)

Data - Logically grouped units of information that move between computer systems.

Data Encapsulation - The application, presentation, and session layers present data to the transport layer, where it is converted into segments. These transport layer segments are passed down to the network layer, where they gain header information and become packets. These network layer packets are passed down to the data link layer, where they gain additional information and become frames. Finally, these data link frames are passed to the physical layer, where they are converted to bits - voltage or light pulses representing binary ones and zeros. This process is similar to preparing a package for mailing - wrapping it, addressing it, and transporting it. The transport layer breaks up the data into segments, which include segment headers for ensuring that hosts at both ends can reliably communicate. The network layer adds a network header, which includes source and destination logical addresses. The data link layers adds frame trailers and headers, including source and destination physical addresses and information to allow access and connection to the next directly connected device on the networking medium.

DCE - Data Communications Equipment (EIA Expansion) or Data Circuit-terminating Equipment (ITU-T Expansion) - Refers to the end of the WAN provider’s side of the communication facility.The devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE.

Decorative Raceway - Type of wall-mounted channel with removable cover used to support horizontal cabling. Decorative raceway is big enough to hold two cables.

Degradation - See Signal Degradation

Digital Signal - Language of computers comprising only two states, on and off which are indicated by a series of voltage pulses.

DC - Direct Current - Electrical current that travels in only one direction. Direct current is generally used in electronic circuits.

Distance Vector Routing Algorithm - Network discovery for distance-vector routing involves exchange of routing tables; problems can include slow convergence. Class of routing algorithms that iterate on the number of hops in a route to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops, but are computationally simpler than link state routing algorithms. Also called Bellman-Ford routing algorithm. Distance vector-based routing algorithms pass periodic copies of a routing table from router to routers; each router receives a routing table from its direct neighbors. See also link state routing algorithm.

DNS - Domain Naming System - System used in the Internet for translating names of network nodes into addresses. See also authority zone.

Dot Address - Refers to the common notation for IP addresses in the form <a.b.c.d> where each number a represents, in decimal, 1 byte of the 4-byte IP address. Also called dotted notation or four-part dotted notation.

Dotted Notation - See Dot address

DSU - Data Service Unit - Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The DSU is also responsible for such functions as signal timing. Often referred to together with CSU, as CSU/DSU.

DTE - Data Terminal Equipment - Refers to the end of the user’s devices on a WAN link. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers.

Dynamic Routing - Routing that adjusts automatically to network topology or traffic changes. It occurs when routers send periodic routing update messages to each other. Each time such a message is received containing new information, the router recalculates the new best route and sends new update information out to other routers. Also called adaptive routing. RIP, IGRP, EIGRP, and OSPF are all examples of dynamic routing protocols. Dynamic routing eliminates the need for network administrators or vendors to manually enter information into routing tables. It is best used where bandwidth and large amounts of network traffic are not issues.

Expectational Acknowledgment - Type of acknowledgment scheme in which the acknowledgment number refers to the octet expected next, meaning that the acknowledgment number refers to the octet expected next. TCP uses this. TCP provides sequencing of segments with a forward reference acknowledgment. Each datagram is numbered before transmission. At the receiving station, TCP reassembles the segments into a complete message. If a sequence number is missing in the series, that segment is retransmitted. If segments are not acknowledged within a given time period, retransmission occurs. (see Sliding Window & Window Size)

Efficiency Audit - Allows you to determine that the network is performing to its potential. Like the operational audit, this audit is best performed once the network has begun to provide services to its clients. It will need to be performed using the network management tools. For your network's wiring system, a set of baseline measurements meeting IEEE and/or EIA/TIA standards should have been provided by the installer. To ensure that your network's cabling continues to operate efficiently, you should periodically measure its performance for comparison against this baseline. Other factors that should be included during in your network's efficiency audit include a cost analysis of the network, an analysis of the ease with which the network is able to retrieve information, and ability of the network to ensure integrity of its data. Also included should be an evaluation of the workforce in place to support the network. Are there enough people to maintain and troubleshoot the network? How quickly can they respond to and clear problems when they arise? Are they adequately trained? What additional training do they need? And finally, the efficiency audit should include an assessment of the network's clients and their capabilities in terms of using the network's hardware and software. Do they need additional training? What type of training is needed? How can such training be expected to benefit the network as a whole? (see Managing The Network)

EIA - Electronic Industries Association - Group that specifies electrical transmission standards. These standards were developed with the intent of identifying minimum requirements that would support multi-product and multi-vendor environments. Moreover, these standards were developed so that they allow for planning and installation of LAN systems without knowledge of the specific equipment that is to be installed. Thus, they allow the LAN designer room for options and expansion. The EIA/TIA standard addresses six elements of cabling for LAN systems. These are horizontal cabling, telecommunications closets, backbone cabling, equipment rooms, work areas, and entrance facilities. The EIA and TIA have developed numerous well-known communications standards, including EIA/TIA-232 and EIA/TIA-449. EIA/TIA has had the greatest impact on networking media standards. Specifically, it is the EIA/TIA-568, EIA/TIA-568B, and EIA/TIA-569 standards for technical performance of the networking media that have been and continue to be the most widely used. (see also TIA)

ESD - Electrostatic Discharge - A flow or spark of electricity that originates from a static source such as a carpet and arcs across a gap to another object.

EMI - Electromagnetic Interference - Interference by electromagnetic signals that can cause reduced data integrity and increased error rates on transmission channels. (see Signal Degradation)

Encapsulation - The wrapping of data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network is simply placed in the header used by the data link layer protocol of the other network.

Encoding - Process by which bits are represented by voltages.

ES - End System - Generally, an end-user device on a network.

Enhanced IGRP - Enhanced Interior Gateway Routing Protocol - Advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency, and combines the advantages of link state protocols with those of distance vector protocols. Compare with IGRP. See also IGP, OSPF, and RIP.

Ethernet - Ethernet network architecture is becoming the dominant one used. The access method used in an Ethernet architecture is called carrier sense multiple access collision detect or CSMA/CD. Ethernet is a broadcast transmission medium. This means that all devices on a network can see all data that passes along the networking media. However, not all the devices on the network will process the data. Only the device whose MAC address and IP address matches the destination MAC address and destination IP address carried by the data will copy the data. Once a device has verified the destination MAC and IP addresses carried by the data, it then check the data packet for errors. If it detects any errors, the data packet is discarded. When it discards the data, the destination device does not notify the source that it discarded the data. Nor does the destination device notify the source if the data was received in good condition. For this reason Ethernet is considered to be a connectionless network architecture. It is also why Ethernet is known as a best-effort delivery system. Baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10 Mbps. Ethernet is similar to, and follows the specifications set by the IEEE 802.3 series of standards.

Ethernet LAN Devices - Repeaters amplify and retime signals (layer 1 devices), Hubs concentrate signals, but typically act as collision domains (layer 1 devices), Bridges allow segmentation of networks based on layer 2 address information, Ethernet Switches allow concentration of signals with shared bandwidth (layer 2 and 3 devices), Routers allow packet forwarding and best path selection (layer 3 logical address devices).

Evaluation Report - Information gathered during a network evaluation is used to prepare an evaluation report which can become the basis for a request for change. In some instances the request for change may trigger responses from reviewers. Usually such responses change from a brief analysis to extensive investigation and analysis. Occasionally, such investigations may result in a return to an earlier point in the network's life cycle. At others, they could require a complete redesign of the network. If problems identified in the evaluation report are severe and far reaching enough, it could even mean a return to the study phase. In short, a request for change could result in a greatly modified network. (see Request For Change)

Extended Star Topology - Star topology where a central hub is connected by vertical cabling to other hubs that are dependent on it. If a simple star topology cannot provide enough coverage for the area to be networked, the star topology can be extended by using internetworking devices (repeaters) that do not result in attenuation of the signal. When this occurs, the resulting topology is referred to as an extended star topology. The topology that is used when more than one wiring closet is needed is the extended star topology. (see Repeaters, Hierarchical Star Topology)

EGP - Exterior Gateway Protocol - Internet protocol for exchanging routing information between autonomous systems. Documented in RFC 904. Not to be confused with the general term exterior gateway protocol. EGP is an obsolete protocol that has been replaced by BGP. See also BGP.

Facility Audit - Allows you to note where everything is. It should include the cabling, work stations, printers, and internetworking devices such as hubs, bridges, and routers. In short, it should provide detailed information about the location of all of the network's components. Ideally, all of this information should have been recorded on the cut sheets at the time the LAN was installed. It is a very good idea to transfer the information you recorded on the cut sheets to a set of the building's blueprints. (see Managing The Network)

Fiber-Optic - Physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is more expensive, but is not susceptible to electromagnetic interference, and is capable of higher data rates. Sometimes called optical fiber. It does not carry electrical impulses as other forms of networking media employing copper wire do. Instead, on fiber-optic cable, signals that signify bits are changed into beams of light. Fiber-optic cable used for networking is made up of two fibers encased in separate sheaths. If viewed in cross section, you would see that each fiber is surrounded by layers of reflective cladding, plastic coating, Kevlar (a protective material more commonly used in bulletproof vests), and an outer jacket. The outer jacket provides protection for the entire cable. Usually made of plastic, the outer jacket conforms to appropriate fire and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection for the fragile hair-thin glass fibers. Where buried fiber-optic cables are required, a stainless steel wire is sometimes included for added strength. The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When the core glass is surrounded by a cladding layer of glass or plastic with a low index of refraction, light can be trapped in the fiber core. This process is called total internal reflection, and it allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends. While fiber-optic cable has many benefits, its cost makes it hard to justify its extensive use in most network installations. Certainly, the cost of running fiber-optic cable to every desktop would be prohibitive! Therefore, today in most networks, the use of fiber is restricted to cabling runs between buildings and as the backbone between wiring hubs. ADVANTAGES: Not only is fiber-optic cable unaffected by electromagnetic interference, it is completely immune from crosstalk and radio frequency interference as well. Because of this freedom from internal and external noise, signals can go farther and faster when fiber-optic cable is used. Because it cannot carry electrical signals or power, fiber-optic is ideal for connecting buildings that might have different electrical grounds. Where long cable spans of copper cable between buildings could serve as an entry point for lightning strikes, fiber-optic cable used in the same type of situation will not. Moreover, like unshielded twisted pair cable, fiber-optic cable is small in diameter. In addition, fiber-optic cable is relatively flat much like a lamp cord. Therefore, a single conduit can accommodate several fiber-optic cables. Thus, it is ideal in older buildings with limited space. DISADVANTAGES: When compared with other types of networking media, fiber-optic cable is more expensive and more difficult to install. Because fiber-optic connectors are optical interfaces, they must be polished perfectly flat and free of scratches. Thus, installation is more difficult. Typically, even a trained installer requires several minutes to make each connection. This can quickly drive up the hourly cost of labor, and in large installations this cost can become prohibitive. CHARACTERISTICS: 100+ Mbps throughput, maximum cable length of 2 kilometers

File Server - See Server

Filter - Generally, a process or device that screens network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria. When using repeaters, filtering is not possible.

Fish Tape - Retractable coil of steel tape used to guide cable through a wall from above or below.

Flow Control - Technique for ensuring that a transmitting entity, such as a modem, does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed. In IBM networks, this technique is called pacing.

Frame - Logical grouping of information sent as a data link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control that surround the user data contained in the unit. The terms datagram, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Frame Relay - Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. Frame Relay uses high-quality digital facilities, simple framing with no error corrction mechanisms, and can send Layer 2 information very rapidly compared to other WAN protocols. By using a simplified framing with no error correction mechanisms, Frame Relay can send Layer 2 information much more rapidly than these other WAN protocols. See also X.25.

G.703/G.704 - ITU-T electrical and mechanical specifications for connections between telephone company equipment and DTE using BNC connectors and operating at E1 data rates.

Gateway - In the IP community, an older term referring to a routing device. Today, the term router is used to describe nodes that perform this function, and gateway refers to a special-purpose device that performs an application layer conversion of information from one protocol stack to another. If the source resides on a network with a different network number than that of the desired destination, and if the source does not know the MAC address of the destination, it will have to use the services of a router in order for its data to reach the destination. When a router is used in this manner, it is called a default gateway.

Ground - Electrically neutral contact point.

Ground Loop - Arrangement that exists when a multi-path connection exists between computers. Usually this occurs when computers are connected to each other through a ground wire and when computers are attached to the same network using twisted pair cable.

Gutter - Type of wall-mounted channel with removable cover used to support horizontal cabling. Gutter is big enough to hold several cables.

Hammer Drill - Tool resembling an oversized electric drill used for drilling into masonry. As it turns the bit, it hammers rapidly.

HDLC - High-Level Data Link Control - An IEEE standard. It might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations with minimal overhead Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. See also SDLC.

Header - Control information placed before data when encapsulating that data for network transmission. The data-link header of a frame contains the IP addresses of both the source and destination of the data.

Hierarchical Star Topology - Extended star topology where a central hub is connected by vertical cabling to other hubs that are dependent on it. See extended star topology. Because more complex equipment is located at the most central point in an extended star topology, sometimes it is referred to as a hierarchical star topology.

Highway - See Bus.

Hop - Term describing the passage of a data packet between two network nodes (for example, between two routers). See also hop count.

Hop Count - Routing metric used to measure the distance between a source and a destination. RIP uses hop count as its sole metric. See also hop and RIP.

HCC - Horizontal Cross-Connect - Wiring closet where the horizontal cabling connects to a patch panel which is connected by backbone cabling to the main distribution facility. According to EIA/TIA-568A, the maximum recommended length of a patch cord /jumper is 6 meters. Because the IDF is where the horizontal cabling connects to a patch panel in the wiring closet whose backbone cabling connects to the hub in the main distribution facility, the IDF is sometimes referred to as the horizontal cross-connect or HCC. (see IDF)

Horizontal Cabling - Runs from the telecommunications outlet to the horizontal cross-connect. This element includes the networking media that is run along a horizontal pathway, the telecommunications outlet or connector, the mechanical terminations in the wiring closet, and the patch cords or jumpers in the wiring closet. In short, horizontal cabling describes the networking media that is used in the area extending from the wiring closet to a work station. (see EIA/TIA-568B) (see IDF)

Host Address - See Host Number

Host Number - Part of an IP address that designates which node on the subnetwork is being addressed.

Hot Wire - Ungrounded lead wire that connects the transformer and electrical devices or appliances via an electrical outlet and power plug.

Hub - 1.) Generally, a term used to describe a device that serves as the center of a star-topology network. 2.) Hardware or software device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (where they repeat signals sent through them) or passive (where they do not repeat, but merely split, signals sent through them). 3.) In Ethernet and IEEE 802.3, an Ethernet multiport repeater, sometimes referred to as a concentrator. Power wiring hubs are vulnerable to surges because of their sensitive low voltage data lines.

IAB - Internet Architecture Board - Board of internetwork researchers who discuss issues pertinent to Internet architecture. Responsible for appointing a variety of Internet-related groups such as the IANA, IESG, and IRSG. The IAB is appointed by the trustees of the ISOC. See also IANA, IESG, IRSG, and ISOC.

IANA - Internet Assigned Numbers Authority - Organization operated under the auspices of the ISOC as a part of the IAB. IANA delegates authority for IP address-space allocation and domain-name assignment to the NIC and other organizations. IANA also maintains a database of assigned protocol identifiers used in the TCP/IP stack, including autonomous system numbers. See also IAB, ISOC, and NIC.

ICC - Intermediate Cross-Connect - IDF that connects the horizontal cross-connect to the main cross-connect. See horizontal cross-connect. See main cross-connect. (see IDF)

ICMP - Internet Control Message Protocol - Provides control and messaging capabilities. If a router receives a packet that it is unable to deliver to its ultimate destination, the router sends an ICMP "unreachable" message to the source. The message might be undeliverable because there is no known route to the destination. ICMP is implemented by all TCP/IP hosts. ICMP messages are carried in IP datagrams and are used to send error and control messages. ICMP uses the following types of defined messages. Others exist that are not included on this list: Destination Unreachable, Time to Live Exceeded, Parameter Problem, Source Quench, Redirect, Echo, Echo Reply, Timestamp, Timestamp Reply, Information Request, Information Reply, Address Request, Address Reply

IDF - Intermediate Distribution Facility - Secondary communications room for a building using a star networking topology. The IDF is dependent on the MDF. There is a special term for the type of cabling that connects wiring closets to each other when an extended star topology is used for an Ethernet LAN. EIA/TIA-568 refers to the type of cabling that connects wiring closets to each other as backbone cabling. Sometimes, to differentiate the backbone cabling from horizontal cabling you will see the term vertical cabling used. Because the IDF is where the horizontal cabling connects to a patch panel in the wiring closet whose backbone cabling connects to the hub in the main distribution facility, the IDF is sometimes referred to as the horizontal cross-connect or HCC. A second way of connecting an IDF to the central hub uses a first IDF interconnected to a second IDF. The second IDF is then connected to the MDF. In such instances, the IDF that connects to the work areas is called the horizontal cross-connect, and the IDF that connects the horizontal cross-connect to the MDF is called the intermediate cross-connect or ICC. Note that no work areas or horizontal wiring connects to the intermediate cross-connect when this type of hierarchical star topology is used. When this type of connection occurs, EIA/TIA-568 specifies that no more than one intermediate cross-connect can be passed through to reach the main cross-connect. To determine which IDFs will be intermediate cross connects, multiply each floor by its height as you move away from the MDF. Assuming that the backbone cabling runs will all be vertical, from the MDF to the IDF on the ninth floor, the distance would be say, sixteen feet. The distance from the MDF to the tenth floor would be thirty-two feet, and the distance to the eleventh floor IDF from the MDF would be forty-eight feet.

IEEE - Institute of Electrical and Electronics Engineers - Professional organization whose activities include the development of communications and network standards. IEEE LAN standards are the predominant LAN standards today. The IEEE has outlined cabling requirements in its 802.3 and 802.5 specifications for Ethernet and Token-Ring systems.

IETF - Internet Engineering Task Force - Task force consisting of over 80 working groups responsible for developing Internet standards. The IETF operates under the auspices of ISOC.

IGRP - Interior Gateway Routing Protocol - An IGP developed by Cisco to address the problems associated with routing in large, heterogeneous networks. IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90-second intervals that advertise networks for a particular autonomous system. The following are some key characteristics of IGRP: Its design emphasizes: Versatility to automatically handle indefinite, complex topologies. Flexibility for segments having different bandwidth and delay characteristics. Scalability to function in very large networks It uses a combination of variables to determine a composite metric. Variables IGRP uses include: Bandwidth, Delay, Load, Reliability, MTU. Compare with Enhanced IGRP. See also IGP, OSPF, and RIP.

Index of Refraction (n) - Light slows down when travelling through matter. The formula is vlight = c/n, where n is the index of refraction of a given material. The higher the n, the more light rays will bend upon entering another material at an angle. See refraction.

Interface - 1.) Connection between two systems or devices; 2.) In routing terminology, a network connection; 3.) In telephony, a shared boundary defined by common physical interconnection characteristics, signal characteristics, and meanings of interchanged signals; 4.) The boundary between adjacent layers of the OSI model.

Insulator - Any material with a high resistance to electrical current. (See conductor)

Internet - Term used to refer to the largest global internetwork, connecting tens of thousands of networks worldwide and having a "culture" that focuses on research and standardization based on real-life use. Many leading-edge network technologies come from the Internet community. The Internet evolved in part from ARPANET. At one time, called the DARPA Internet. Not to be confused with the general term internet.

InterNIC - International Network Information Center. Organization that serves the Internet community by supplying user assistance, documentation, training, and other services.

Interoperability - Ability of computing equipment manufactured by different vendors to communicate with one another successfully over a network.

IP - Internet Protocol - Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Documented in RFC 791. See IP address. An IP with 0’s in the host positions is reserved as the NETWORK ADDRESS. An IP with all 255’s in the host octets is reserved as the BROADCAST ADDRESS. IP datagram fields include Version Number and header length, type of service and total length of datagram, Time-To-Live, which upper-layer protocol has sent the datagram [TCP or UDP], header checksum, source and destination IP addresses, IP options [testing, debugging, security], and of course the data. IP provides connectionless, best-effort routing of datagrams [packets]. It is not concerned with their content; it is looking for ways to move the datagrams to their destination. The Internet protocols can be used to communicate across any set of interconnected networks. They are equally well-suited for both LAN and WAN communication. The Internet protocol suite includes not only Layer 3 and 4 specifications (such as IP and TCP), but also specifications for such common applications as e-mail, remote login, terminal emulation, and file transfer. (see ICMP, ARP, & RARP)

IP Address - 32-bit address assigned to hosts using TCP/IP. An IP address belongs to one of five classes (A, B, C, D, or E) and is written as 4 octets separated with periods (dotted decimal format). Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, while the host number is used to address an individual host within the network or subnetwork. A subnet mask is used to extract network and subnetwork information from the IP address. Also called an Internet address. CLASS A = 1 to 127, CLASS B = 128 to 191, CLASS C = 192 to 223. A CLASS A binary octet always starts with 0, CLASS B with 10, and CLASS C with 110.

ISDN - Integrated Services Digital Network - A set of digital services that transmits voice and data over existing phone lines. Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic. See also BISDN, BRI, N-ISDN, and PRI.

ISO - International Organization for Standardization - International organization that is responsible for a wide range of standards, including those relevant to networking. ISO developed the OSI reference model, a popular networking reference model.

ISOC - Internet Society - International nonprofit organization, founded in 1992, that coordinates the evolution and use of the Internet. In addition, ISOC delegates authority to other groups related to the Internet, such as the IAB. ISOC is headquartered in Reston, Virginia, U.S.A. See also IAB.

ITU-T - International Telecommunication Union Telecommunication Standardization Sector - Formerly the Committee for Internatiional Telegraph and Telephone (CCITT). An international organization that develops communication standards. See also CCITT, BISDN, G.703/G.704, V.24, V.35, X.21

Inventory Audit - Allows you to take stock of all of the network's hardware and software. Ideally, this information should be obtained when the hardware and software is purchased and before it is set up. This will save you time and effort and reduce the amount of inconvenience experienced by network end users. An inventory audit of the network's hardware should include the device's serial number, the type of device, and the name of the individual using the device. It should also list the settings on the various workstations and networking devices. Some network administrators find it useful to keep hardware inventory information directly attached to each networking device. Others prefer to store the information in either a written or computerized database where it is easily accessible to network support staff. An inventory audit of the network's software applications should include the types of software used, the number of users for each application as well as each application's operating requirements. During the inventory audit you should also make sure that the number of users for each software application does not surpass the number of licenses your site possesses. (see Manage The Network)

Journal - Record information about network problems and their solutions. It provides a record of your contributions to the system, and can later be used to justify requests for addition equipment, personnel, and training. The problems and solutions documented in the log can be useful tools to train additional troubleshooters for your network. It will also allow you to track trends that help in anticipating and proposing solutions to future problems either with situations or individuals

Jumper - 1.) Term used for patchcords found in a wiring closet. 2.) Electrical switch consisting of a number of pins and a connector that can be attached to the pins in a variety of different ways. Different circuits are created by attaching the connector to different pins.

Ladder Rack - Metal ladder that can be suspended from the ceiling. It is used to support cable.

LAN Adapter - See Adapter

LAN Design - For a LAN to be effective and serve the needs of its users, it should be implemented according to a planned series of systematic steps. The four steps are: study, selection and design, implementation, and operation.

Laser - Light amplification by stimulated emission of radiation. Analog transmission device in which a suitable active material is excited by an external stimulus to produce a narrow beam of coherent light that can be modulated into pulses to carry data. Networks based on laser technology are sometimes run over SONET.

Link State Routing Algorithm - Routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching each of its neighbors to all nodes in the internetwork. Link state algorithms create a consistent view of the network and are therefore not prone to routing loops, but they achieve this at the cost of relatively greater computational difficulty and more widespread traffic (compared with distance vector routing algorithms). Compare with distance vector routing algorithm. Link-state routing uses link state advertisements, a topological database, the SPF (shortest path first) algorithm, the resulting SPF tree, and finally a routing table of paths and ports to each network. Instead of simply passing routing tables, emphasizing hop counts to nearest neighbors (as in distance vector routing), link state routing shares a complex database of topology information amongst all routers in the network.

LAN - Local-Area Network - High-speed, low-error data network covering a relatively small geographic area. LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LAN technologies.

Logical Address - See Network address

Managing The Network - The first step in managing a network is to document it. There are five types of audits on the network. These are an inventory audit, a facility audit, an operational audit, an efficiency audit, and a security audit. The inventory and facility audits are the most critical. They should be done as quickly as possible, even before the network begins providing services to its customers if possible. Having the information provided by these audits at hand will let you troubleshoot problems more rapidly and effectively later, when they do occur. Information for the operational, efficiency, and security audits can and should be obtained after the network has begun to function, because they require data that can only be provided through monitoring and analysis of the network's behavior and performance. By monitoring the day-to-day operating of the network, you will be able to establish what is "normal" for your network. For instance, by tracking the information over a period of time, you will come to learn on an average how busy the network is. You will also discover when the peak traffic times are for the network as measured by the time of day, the time of week, and the time of month. You will learn what the network's most and least popular applications are and how they are being used. In some instances you will even be able to identify which network users are more prone to experience difficulties when working on the network. Logs should be kept of all this information. Later, when you notice something that might be a problem, you can compare it against this baseline information showing what normal network operation should be. (see Network Map)

MCC - Main Cross-Connect - Wiring closet that serves as the most central point in a star topology and where LAN backbone cabling connects to the Internet. (see MDF)

MDF - Main Distribution Facility - Primary communications room for a building. Central point of a star networking topology where patch panels, hub, and router are located. There is a special term for the type of cabling that connects wiring closets to each other when an extended star topology is used for an Ethernet LAN. EIA/TIA-568 refers to the type of cabling that connects wiring closets to each other is backbone cabling. Sometimes, to differentiate the backbone cabling from horizontal cabling you will see the term vertical cabling used. Because it connects the backbone cabling of the LAN to the Internet, the main distribution facility is sometimes referred to as the main cross-connect or MCC

MIB - Management Information Base - Database of network management information that is used and maintained by a network management protocol such as SNMP or CMIP. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.

MIB Collection - Polling technique used by the SNMP protocol to gather information needed to monitor the network.

MIB Reporting - Technique used by the CMIP protocol to obtain information needed to monitor the network. It is dependent upon network devices to initiate reports regarding their status to a central monitoring station on the network. (see CMIP)

Media - Plural of Medium. The various physical environments through which transmission signals pass. Common networking media include twisted-pair, coaxial and fiber-optic cable, and the atmosphere (through which microwave, laser, and infrared transmission occurs). Sometimes called physical media. Types of network media that employ copper wires include shielded twisted pair cable, unshielded twisted pair cable, and coaxial cable.

MOV - Metal Oxide Varistor - Voltage clamping surge suppressor capable of absorbing large currents without damage. Typically, they are used as wall mounted surge suppressors for individual work stations. Typically, a MOV can hold voltage surges on a 120 volt circuit to a level of approximately 330 volts. This type of surge suppressor has a limited lifetime dependent in part on heat and usage. A MOV may not be an effective means of protecting the networking device attached to it. This is because the ground also serves as the common reference point for data signals going into and out of the computer, dumping excess voltages into the power line near the computer can create problems. For these reasons, this type of surge suppressor is not the best choice for your network. Instead of installing individual surge suppressors at each work station, commercial quality surge suppressors should be used. These should be located at each power distribution panel rather than in close proximity to networking devices. By placing a commercial grade surge suppressor near the power panel, the impact on the network of voltage surges and spikes diverted to ground can be reduced. The problem of sags and brownouts cannot be addressed through the use of surge suppressors.

MAN - Metropolitan-Area Network - A network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN.

micron or µ - Unit of measure equal to one millionth of a meter or one thousandth of a millimeter. Sometimes the symbol µ is used instead of the word micron.

Multicast - Single packets copied by the network and sent to a specific subset of network addresses. These addresses are specified in the destination address field. Compare with broadcast and unicast.

Multiprotocol Routing - There are many different host systems all over the world, with many different routed protocols. Multiprotocol routing -- by which a router can pass traffic from Novell, Apple, DEC, IP and many other networks - allows these diverse hosts to communicate.

Near-End Cross Talk - See Testing The Network

Network Address - Network layer address referring to a logical, rather than a physical, network device. Also called a protocol address. This is the IP ADDRESS of the network, usually having a value of 0 for each of its host octets (EG: 159.82.0.0)

Network Addressing -

Network Analyzer - Network monitoring device that maintains statistical information regarding the status of the network and each device attached to it. More sophisticated versions using artificial intelligence can detect, define, and fix problems on the network. Also called a protocol analyzer, this device keeps track of statistical information much like a network monitor does. However, it offers a more sophisticated level of service than a network monitor can. In fact, in some instances these devices are so sophisticated and intelligent that they not only detect and identify problems such as bottlenecks, they fix them as well.

Network Architecture - The computing industry has determined that all network architectures must meet certain criteria. These are reliability, modularity, connectivity, ease of use, ease of modification, and ease of implementation. A network that is defined by a combination of existing standards and protocols is said to possess a particular network architecture. Because a network architecture uses standards and protocols, it is also a standard. That is to say, a network architecture defines the rules of a network and how the various components in that network can interact and communicate. ARCnet, Token Ring, and Ethernet comprise the three major networking architectures. ARCnet is no longer used. Increasingly, Ethernet network architecture is becoming the dominant one used. To be reliable, a network architecture must provide a means for error detection as well as facilities to correct the error. To satisfy the modularity criteria, a network architecture must be able to use a relatively small variety of mass-produced building blocks in a broad assortment of network devices. A network architecture must be able to incorporate a variety of hardware and software products in such a way that they can function in an interoperable and unified manner to meet the criteria of connectivity. To meet the ease of use criteria, a network architecture must perform in such a way that users need have no concern for or knowledge of the network's structure or implementation in order to take advantage of the communications facilities provided by the network. In order to satisfy the ease of modification criteria, a network architecture must allow the network to evolve and adapt as needs change or expand, or as new technologies emerge. Finally, a network architecture must provide for simplified implementation following industry networking standards, and it must allow for the installation of a variety of configurations that meet the needs of the network's users if it is to satisfy the ease of implementation criteria. Because network architecture is a combination of standards and protocols, it determines the topology or physical layout used in a local area network.

Network Audit - See Managing The Network

Networking - Connecting of any collection of computers, printers, routers, switches, and other devices for the purpose of communication over some transmission medium.

NFS - Network File System - As commonly used, a distributed file system protocol suite developed by Sun Microsystems that allows remote file access across a network. In actuality, NFS is simply one protocol in the suite. NFS protocols include NFS, RPC, XDR (External Data Representation), and others. These protocols are part of a larger architecture that Sun refers to as ONC.

NIC - Network Interface Card - Board that provides network communication capabilities to and from a computer system. The point at which a workstation or networking device connects to the networking media is called the NIC card. Also called an adapter.

Network Management Tools - See Operational Audit

Network Map - Once you have conducted the inventory and facility audits, use the information you have gathered to generate a network map. The map will include the physical locations and layout of all devices attached to the network and the applications that run on them. It should also include the IP and MAC addresses of each device. Finally, the network map should include the distances of each cabling run and between nodes on the network. The completed network map should be kept near the location selected for network administration and monitoring. In some instances when monitoring programs and devices report a problem with a network's physical components, they will indicate the location of a problem such as a break or short by providing you with its distance from where the monitoring device is located. In other instances the monitoring program will provide you with the address of the device or devices where a problem is occurring. Obviously, locating and solving the problem will be greatly facilitated if you have the information you need readily available. (see Managing The Network)

Network Monitors - See Network Analyzer, CMIP, and SNMP

Network Number - Part of an IP address that specifies the network to which the host belongs.

Network Testing - The basic testing of a network should proceed from layer 7 [using telnet to access various routers], to layer 3 [using ping, trace, and show ip route to learn logical address information about the network], and then to layers 3, 2, and 1 [using show interface to learn about specific port configurations and whether specific links are alive] and then on to layer 1 [specific medium tests]. (see Testing The Network)

Neutral Wire - Circuit wire that is connected to an earth ground at the power plant and at the transformer.

N-ISDN - Narrowband ISDN - Communication standards developed by the ITU-T for baseband networks. Based on 64-kbps B channels and 16 or 64-kbps D channels. Contrast with BISDN. See also BRI, ISDN, and PRI.

Node - Endpoint of a network connection or a junction common to two or more lines in a network. Nodes can be processors, controllers, or workstations. Nodes, which vary in routing and other functional capabilities, can be interconnected by links, and serve as control points in the network. Node is sometimes used generically to refer to any entity that can access a network, and is frequently used interchangeably with device.

Normal Mode - Term used to describe problems between the hot and neutral wires on a power line. See common mode.

NVRAM - These commands manage the contents of NVRAM: configure memory, erase startup-config, copy running-config startup-config, and show startup-config (see Router Configuration)

ONC - Open Network Computing - Distributed applications architecture designed by Sun Microsystems, currently controlled by a consortium led by Sun. The NFS protocols are part of ONC.

Operational Audit - Allows you to view the day-to-day activity on the LAN. It requires the use of specialized software and hardware. In addition to a network monitor, an operational audit may also include the use of devices such as a network analyzer, time domain reflectors, breakout boxes, power meters, and an oscillator. Devices such as network monitors and analyzers use specialized software to perform their functions. Together, all of this hardware and software allows the network administrator to keep track of network traffic by counting the number of packets sent, the number of times packets must be retransmitted, packet size, and how the network is being used. Simply stated, these devices and the software they use allow you to detect such events as shorts and breaks in the cable, EMI/RFI noise on the networking media, and network bottlenecks. Of the hardware management tools mentioned here, the ones used most frequently to provide information needed for operational, efficiency, and security audits are network monitors and analyzers. These devices are usually centrally located where they can be easily accessed by authorized support personnel. Vendors provide a variety of network software management tools that are designed to monitor the nodes on the network, monitor levels of network traffic, watch for network bottlenecks, keep track of software metering, and collect diagnostic information. Most of these applications support vendor specific types of information and follow one of two network management protocols. The two protocols are the simple network management protocol (SNMP) or the common management information protocol (CMIP). Both of these management protocols use a concept known as the management information base, or MIB. Simply put, an MIB contains information, tests, equations, and controls that all resources on a network conform to. Even though both SNMP and CMIP share the same mission and use the MIB concept, their methods of retrieving network information differ greatly. In some instances this may impact which protocol you choose to use in monitoring your network. (see Managing The Network)

OSPF - Open Shortest Path First - Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the ISIS protocol. See also Enhanced IGRP, IGP, IGRP,IS-IS, and RIP.

Oscillation - Secondary signal on top of the 60-Hz waveform. It has a magnitude that ranges from 15 % to 100 % of the normal voltage carried on the power line. The problem of oscillation can best be addressed by rewiring.

OSI Reference Model - Open System Interconnection reference model - Designed to help vendors create networks that would work compatibly and interoperably. By creating the OSI model, the ISO provided vendors with a set of standards. Network architectural model developed by ISO and ITUT-T The model consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The highest layer (the application layer) is closest to the user; the lowest layer (the physical layer) is closest to the media technology. The lowest layer is implemented in hardware. The next to lowest layer is implemented in hardware and software, while the upper five layers are implemented only in software. Each layer in a source host system uses its own layer protocol to communicate with its peer layer in the destination host system. In other words, like layers talk to each other. Implied in peer-to-peer communication is that each layer "below" the current layer provides services to the layer above it. The OSI reference model is used universally as a method for teaching and understanding network functionality. To remember the order in reverse: "All People Should Try New Diet Pepsi"

Patch Cords - See Patch Panel

Patch Panel - An assembly of pin locations and ports which can be mounted on a rack or wall bracket in the wiring closet. Patch panels act like switchboards that connect workstations cables to each other and to the outside. A patch panel is an interconnecting device through which horizontal cabling runs an be connected to other networking devices such as hubs and repeaters. More specifically, a patch panel is a gathering of pin locations and ports. Essentially, a patch panel acts as a switchboard where horizontal cabling coming from workstations can be connected to other workstations to form a local area network. In some instances the patch panel will also provide locations where devices can be connected to a wide area network or even to the Internet. To understand how a patch panel provides for the interconnection of horizontal cabling runs with other networking devices, let's look at how it is structured. Rows of pins much like those found in the RJ45 jack you have already worked with are located on one side of a patch panel. Just as they were on the RJ45 jack, these pins are color-coded. To make electrical connections to the pins, you'll use a punch tool to punch down the wires just as you did with the RJ45 jacks. And, as before, wire sequence is critical for optimal network performance. Therefore, when laying down the wires at the patch panel, make sure the colors of the wires correspond exactly to the colors indicated on the pin locations. Remember, different colored wires are not interchangeable. Ports are located on the opposite side of the patch panel. These ports resemble the ports found on the faceplates at the telecommunications outlets in the work area. Like those RJ45 ports, the ports on patch panels take the same size plug. Patch cords that connect to the ports allow for the interconnection of computers and other network devices such as hubs, repeaters, and routers that are also attached to the patch panel. In any local area network system, connectors are the weakest links. If not properly installed, connectors can create electrical noise. Poor connections can also cause intermittent electrical contact between wires and pins. When this occurs, transmittal of data on the network will be disrupted or will occur at a much slower rate. At the patch panel, it is important to lay down cable wires in ascending order by cable number. These are the cable numbers that were assigned to the cable when it was run from the work area to the wiring closet. cable numbers correspond to the numbers of the rooms in which the work stations attached to the cable at the opposite end are located. Laying the wires down in ascending order at the patch panel will allow for easy diagnosis and location of problems if they should arise later. Use the cut sheet you prepared earlier to lay down the wires at the patch panel. Later, you can label the patch panel. As you work, it is important to remember to keep the end of the cable centered above its pin locations. If you are not careful to do this, the wires can become skewed. This will slow the rate of data transmission later when your LAN is fully connected. To avoid exposing too much wire, be sure to keep the jacket within 1/4" of the pin locations you are working on. A good way to do this is to measure before you strip off the jacket. 1-1/2" to 2" should be sufficient to do the job. Remember, if too much wire is exposed, the rate of data transmission on the network will be slowed. As before, don't untwist the wire pairs more that necessary. Not only do untwisted wires transmit data more slowly, they can also lead to crosstalk. Depending on what type of patch panel is used, you will use either a 110 or a Krone punch tool. Check to see which you need to use before beginning work The punch tool has a spring-loaded actions. This allows the punch tool to do two jobs at the same time. As it pushes a wire between two metal pins skinning the sheath from the wires so that it makes an electrical connection with the pins, the punch tool's blade also cuts off any extra wire. HINT: Sometimes the punch tool will fail to make a clean cut. When that happens, simply twist the cut ends of the wires gently to remove them after they have been punched. When you use the punch tool, be sure to position it so that its blade is on the side away from where the wire enters each pin location. If you fail to take this precaution, you will cut the wire so that it falls short of where it should make an electrical connection. Patch panels can either be mounted on the wall in brackets, stand in racks, or reside in cabinets equipped with interior racks and doors. One of the most commonly used for mounting patch panels is the distribution rack. A distribution rack is nothing more than a simple skeletal frame for holding equipment such as patch panels, repeaters, hubs, and routers that are used in the wiring closet. They range in height between 39" and 74". The advantage of a distribution rack is that it allows easy access to both the front and the back of equipment such as patch panels. A floor plate is used to attach the distribution rack to the floor so that stability is ensured. While a few companies currently market a 23" wide rack, the standard since the 1940s has been the 19" rack. Described by EIA/TIA-568A as the horizontal cross connect. (see Wiring Closet)

Performance Evaluations - The first evaluation should occur after it has been in operation for a reasonable period of time. It should be based on information provided by the system's network management tools. Once they have been compiled, the results of the evaluation should be presented in the form of an evaluation report. The evaluation report will allow network management to see if the network is continuing to work as anticipated and needed by the organization. The purpose of the evaluation report is to reveal the network's strengths and weaknesses so they can be corrected if necessary. For example, logs maintained by a network analyzer could indicate a trend toward a slower rate of traffic on certain segments of the network. An updated audit of the network's hardware and software could reveal the additional of several new network devices running multimedia applications on those segments. When both sets of data are taken together and presented in an evaluation report, this information can be used by network management as the basis for formulating changes in the system and how it operates.

Ping - Ping is a layer 3 test that uses the ICMP protocol to verify the hardware connection and the logical address of the network layer (this is a very basic testing mechanism). (Allows you to verify address configuration in your internetwork)(see Trace and Telnet)

Pin Location - A color-coded slot on a patch panel. Cable wires are punched down using a punch tool to make an electrical connection that allows the network to function.

POP - Point Of Presence - The point of interconnection between the communication facilities provided by the telephone company and the building's main distribution facility. It is essential that the hub be located near the POP in order to facilitate wide area networking and connection to the Internet.

Polling - Access method in which a primary network device inquires, in an orderly fashion, whether secondaries have data to transmit. The inquiry occurs in the form of a message to each secondary that gives the secondary the right to transmit.

Port - 1.) Interface on an internetworking device (such as a router). The image shown here shows a cable plugged into the ports of two networking devices. ; 2.) In IP terminology, an upper-layer process that is receiving information from lower layers; 3.) To rewrite software or microcode so that it will run on a different hardware platform or in a different software environment than that for which it was originally designed; 4.) A female plug on a patch panel which accepts the same size plug as an RJ45 jack. Patch cords are used in these ports to cross connect computers wired to the patch panel. It is this cross connection which allows the LAN to function.

Port (or Socket) Numbers - An 8-bit number that identifies a socket. Both TCP and UDP use port (or socket) numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations crossing the network at the same time. Application software developers agree to use well-known port numbers that are defined in RFC 1700. For example, any conversation bound for the FTP application uses the standard port number 21. Conversations that do not involve an application with a well-known port number are assigned port numbers randomly chosen from within a specific range instead. These port numbers are used as source and destination addresses in the TCP segment. Some ports are reserved in both TCP and UDP, but applications might not be written to support them. Port numbers have the following assigned ranges: Numbers below 255 are for public applications. Numbers from 255 to 1023 are assigned to companies for salable applications. Numbers above 1023 are unregulated. A maximum of 254 different socket numbers can be assigned in an AppleTalk node.

PPP - Point-to-Point Protocol - Described by RFC 1661, PPP is two standards developed by the IETF. PPP contains a protocol field to identify the network-layer protocol. A successor to SLIP, PPP provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. See also SLIP.

PRI - Primary Rate Interface - ISDN interface to primary rate access. Primary rate access consists of a single 64-Kbps D channel plus 23 (T1) or 30 (E1) B channels for voice or data. Compare to BRI. See also BISDN, ISDN, and N-ISDN.

Protocol - Formal description of a set of rules and conventions that govern how devices on a network exchange information. A protocol is like a language. Protocols allow computers to communicate with each other. They transform information that people understand into a language that computers can understand. This computer language is comprised of 0s and 1s. When it is transmitted over the networking media, computer language is converted into a sequence of voltage pulses called digital signals.

Protocol Address - See Network Address

Protocol Analyzer - See Network Analyzer

Proxy ARP - Proxy Address Resolution Protocol - Variation of the ARP protocol in which an intermediate device (for example, a router) sends an ARP response on behalf of an end node to the requesting host. Proxy ARP can lessen bandwidth use on slow-speed WAN links. When a device on one subnetwork needs to find out the MAC address of a device on another subnetwork, if the source directs its question to the router, the router is in essence acting as a default gateway. This working through a third party is called proxy ARP.

Protocol Analyzer - See Network Analyzer

PTT - Post, Telephone, and Telegraph - Government agency that provides telephone services. PTTs exist in most areas outside North America and provide both local and long-distance telephone services.

Pull String - Strong, heavy string used to pull cable in multiple runs.

Punch Tool - Spring-loaded tool used for cutting and connecting wire in a jack or on a patch panel.

Raceway - Wall-mounted channel with a removable cover used to support horizontal cabling.

RFC - Request For Comments - Document series used as the primary means for communicating information about the Internet. Some RFCs are designated by the IAB as Internet standards. Most RFCs document protocol specifications such as Telnet and FTP, but some are humorous or historical. RFCs are available online from numerous sources.

RFI - Radio Frequency Interference - Radio frequencies that create noise that interferes with information being transmitted across unshielded copper cabling. (see Signal Degradation)

RAM - Random-Access Memory - Volatile memory that can be read and written by a microprocessor.

Refraction - The bending of waves, including light, when passing from one materials to another. See total internal reflection and index of refraction

RJ Connector - Registered Jack Connector - Standard connectors originally used to connect telephone lines. RJ connectors are now used for telephone connections and for 10BaseT and other types of network connections. RJ-11, RJ-12, and RJ-45 are popular types of RJ connectors.

Repeater - Device that regenerates and propagates electrical signals between two network segments. The purpose of repeaters in an extended star topology is to allow the network extend past the limits of the network media. You can’t filter when using repeaters. Repeaters take in weakened signals, clean them up, amplify them, and send them on their way along the network. By using repeaters, the distance over which a network can operate is extended. Because they work in tandem with the networking media, repeaters exist at the physical layer of the OSI model

Request For Change - When a network administrator believes that changes need to be implemented in a network, particularly those that will alter the way users interface with the network, the services that will be provided by that network, access to information housed in applications that run on the network, or those that will involve additional time, expense, and labor, he or she should draft a request for change. This document should be circulated for review. The individuals who review the request for change will vary from organization to organization. Ideally, the list of reviewers will include individuals in the organization who not only possess a degree of technical knowledge, but who are familiar with the types of services, applications, and work the organization must handle. (see Evaluation Report)

RARP - Reverse Address Resolution Protocol - Protocol in the TCP/IP stack that provides a method for finding IP addresses based on MAC addresses. Compare with ARP. Devices using RARP require that a RARP server be present on the network to answer RARP requests. Usually, several diskless workstations will be attached to the same network. Thus, if one diskless workstation on the network is unlikely to know what its IP address is, others will also be unlikely to possess this information and will be unable to provide it in response to RARP requests. Therefore, designated servers which do possess memory caches and disk storage capable of storing ARP tables are required to respond to RARP requests. Since the RARP request frame is sent out in broadcast mode, all devices on the network will see it. However, only a designated RARP server can respond to the RARP request. The designated RARP server replies by sending a RARP reply containing the IP address of the device that originated the RARP request. RARP relies on the presence of a RARP server with a table entry or other means to respond to these requests. On the local segment, RARP can be used to initiate a remote operating system load sequence.

Ring Topology - See Token Ring

Routed Protocols - Used between router to direct user traffic. Any network protocol that provides enough information in its network layer address to be forwarded from host to host based on the addressing scheme is a routed protocol. The most relevant example for us is IP.

Router - Network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. They are used to connect two or more networks having unique network numbers. Routers are internetworking devices that connect two or more networks and are used to pass data packets between networks based on network protocol information. In other words, routers work at layer 3, the network layer, of the OSI model. Routers can have more than one IP address, because they are attached to more than one network. Because subnetworks have unique network numbers, a router is also used to connect two or more subnetworks. When data, referred to as a frame, coming from another network reaches the router, the router performs the following functions. First, the router strips off the data link header carried by the frame. The network header contains the IP addresses of the source and destination of the data. This allows the router to examine the network layer to determine the destination network. Next, the router consults its routing tables to determine which of its ports it will need to send the data out on in order for it to reach its destination network. Before sending the data out the port, the router encapsulates the data in the appropriate data link frame. If the source resides on a network with a different network number than that of the desired destination, and if the source does not know the MAC address of the destination, it will have to use the services of a router in order for its data to reach the destination. When a router is used in this manner, it is called a default gateway. (also see ARP)

Router Commands & Prompts - The prompts for User EXEC mode, Privileged EXEC mode, and Global configuration mode look like this: Router>, Router#, and Router(config))# respectively.

Router Configuration - Routers may be configured from the console terminal through the console port; via modem using the auxiliary port; and from virtual terminals or a TFTP server once it is on a network. Internal configuration components include RAM [stores routing tables, ARP cache, and running configuration file]; NVRAM [stores the router’s backup configuration file; retains content even when powered down]; Flash [erasable, reprogrammable ROM, holds the operating system image]; ROM [contains power-on diagnostics, a bootstrap program, and operating system software]; and Interfaces [network connections through which packets enter and exit the router] (see NVRAM) The commands configure terminal, configure memory, and copy tftp running-config all load configuration information into RAM. The commands copy running-config tftp and copy running-config startup-config store the currently running configuration from RAM to a network tftp server and NVRAM, respectively. When configuring routers with Release 11.x methods, once you have typed show running-config and displayed a desired configuration, use copy running-config startup-config and copy running-config tftp to save changes to the backup. Two basic tasks when first configuring a router are password configuration and naming the router (router identification configuration). Flash, TFTP severs, or ROM (though not the full Cisco IOS) are all sources of Cisco IOS software, so if NVRAM is lacking boot system commands, routers can boot Cisco IOS software from these.

Router Modes - Privileged mode, which is used for copying and managing entire configuration files. Global configuration mode, which is used for one-line commands and commands that change the entire router. Other configuration modes, which are used for multiple command lines and detailed configurations. The router provides a host name, a banner, and interface descriptions to aid in identification. The EXEC interprets the commands you type and carries out the corresponding operations. You must log into the router before you can enter an EXEC command. There are two EXEC modes. The EXEC commands available in user mode are a subset of the EXEC commands available in privileged mode. From privileged mode, you can also access global configuration mode and specific configuration modes, some of which are listed here: Interface, Subinterface, Controller, Map-list, Map-class, Line, Router, IPX-router, and Route-map. If you type exit, the router will back out one level, eventually allowing you to log out. In general, typing exit from one of the specific configuration modes will return you to global configuration mode. Pressing Ctrl-Z leaves configuration mode completely and returns the router to privileged EXEC mode.

Router Status - Commands for examining a router’s status include: show version [RAM-IOS]; show processes CPU and show processes [RAM-programs]; show running-config [RAM-active configuration file]; show mem, show stacks, show buffers [RAM-Tables and Buffers]; show startup-config and show config [NVRAM], show flash [Flash], and show interfaces [Interfaces].

Router Startup & Setup - You use setup to bring up a minimal configuration. You should use various configuration-mode commands, rather than setup, for most router configuration tasks. The router initializes by first performing a hardware self-test, then loading a bootstrap, the operating system, and a configuration file. If the router cannot find a configuration file, the router enters setup mode. The router stores in NVRAM a backup copy of the new configuration from setup mode. When a Cisco router powers up, it performs a power-up self-test. During this self-test, the router executes diagnostics from ROM on all hardware modules. These diagnostics verify the basic operation of the CPU, memory, and network interface ports. After verifying the hardware functions, the router proceeds with software initialization. The startup routines must: Make sure that the router comes up with all its hardware tested. Find and load the Cisco IOS software that the router uses for its operating system. Find and apply the configuration statements about the router, including protocol functions and interface addresses. Global parameters include router name, passwords, routing protocols, and routed protocols. Interface parameters include type of port, IP addressees, and subnet masks.

Router User Interface - Routers may be configured from the console terminal through the console port; via modem using the auxiliary port; and from virtual terminals or a TFTP server once it is on a network.User mode allows you to check router status; privileged mode allows you to actually change the router’s configuration. The > is the user-mode prompt; the # is the privileged mode prompt. You must type a "?" to display the list of commonly used commands. To enter privileged mode from user mode, you must type "ena" [short for "enable"] and then a password. If you guess at the command syntax and arguments, the Cisco IOS context-sensitive help will highlight your errors and suggest corrections. Additionally, by adding a space and then the question mark "?" after a partial command, you will get hints about how to proceed.

Routing Metrics - The metrics most commonly used by routers include: Bandwidth - The data capacity of a link, Delay - Length of time required to move a packet from source to destination, Load - Amount of activity on a network resource such as a router or a link, Reliability - Usually refers to the error rate of each network link, Hop Count - Number of passages of a packet through the output port of one router, Ticks - Delay on a data link using IBM PC clock ticks, Cost - Arbitrary value, usually based on bandwidth or other measurements, that is assigned by the network administrator.

Routing Protocols - Routed protocols allow routers to direct user traffic, and routing protocols work between routers to maintain path tables. Routing protocols enable routers to internally draw a map of the entire Internet for the purposes of routing. Such maps become part of each router's routing table. (See IRP (IP), IGRP (IGP), EIGRP (EGP), and OSPF)

RIP - Routing Information Protocol - It is a distance-vector routing protocol. Hop count is used as the metric for path selection. The maximum allowable hop count is 15. Routing updates are broadcast every 30 seconds by default. An IGP supplied with UNIX BSD systems. The most common IGP in the Internet. RIP uses hop count as a routing metric. There is a maximum of 15 hops before it gives up. (See also Enhanced IGRP, hop count, IGP, IGRP, and OSPF)

Routing Table - Table stored in a router or some other internetworking device that keeps track of routes to particular network destinations and, in some cases, metrics associated with those routes.

Safety Ground Wire - Circuit wire that connects to a local earth ground and the chassis of an electrical appliance or device via an electrical outlet and plug. It is used to ensure that no voltage potential exists between the chassis of the electrical device and the earth ground.

Sag - Any decrease of below 80% in the normal voltage carried by a power line. A sag is sometimes referred to as a brownout because it’s a brownout that lasts for less than a second. Sags can best be addressed through the use of uninterruptable power supplies. The problem of sags and brownouts cannot be addressed through the use of surge suppressors. Sags and brownouts account for a large proportion of the power problems that affect networks and the devices attached to them.

Security Audit - Reviews what the security requirements of the network are and what kind of software and hardware security system best meets them. Only observation and experience in how the network and its clients use and access data will provide you with the information needed to complete this audit. Typical types of information that should be contained in this type of audit include noting which segments require limited authorized access or encryption of data, which devices, files, and directories require locks or password protection, which files should be provided with archived backup, how frequently backup procedures should be performed, and the type of virus protection is being utilized by the network. And probably most important, a security audit should provide information regarding what emergency and disaster procedures will be employed by the network. (see Managing The Network)

Segment - 1.) Section of a network that is bounded by bridges, routers, or switches; 2.) In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters.

Server - Node or software program that provides services to clients. See also back end, client, and front end.

Show CDP Interface - You use the command show cdp interface to display the values of the CDP timers, the interface status, and the encapsulation used by CDP for its advertisement and discovery frame transmission. Default values for timers set the frequency between CDP updates and for aging CDP entries. These timers are set automatically at 60 seconds and 180 seconds, respectively. If the device receives a more recent update or if this hold-time value expires, the device must discard the CDP entry.

Show CDP Neighbors - The command show cdp neighbors works at the data link layer to display the following information: neighbor device ID, local port type and number, decremental holdtime value in seconds, Neighbor’s device capability code, Hardware platform of neighbor, Neighbor’s remote port type and number.

STP - Shielded Twisted-Pair - Two-pair wiring medium used in a variety of network implementations. STP cabling has a layer of shielded insulation to reduce EMI. Combines the techniques of shielding, cancellation, and twisting of wires. As specified for use in Ethernet network installations, 100 ohm shielded twisted pair cable when properly installed, provides greater resistance to both electromagnetic interference and radio frequency interference without significantly increasing the weight or size of the cable. Another type of shielded twisted pair cable is specified for Token Ring installations. In this type of STP cable, known as 150 ohm STP, not only is the entire cable shielded to reduce electromagnetic interference and radio frequency interference, but each pair of twisted wires is shielded from each other in order to reduce crosstalk. Although the shielding used on 150 ohm shielded twisted pair cable is not part of the circuit as it is in coaxial cable, it still must be grounded at both ends. However, this type of shielded twisted pair cable requires an increased amount of insulation and a larger amount of shielding. These factors combine to considerably increase the size, weight, and cost of the cable. It also requires the installation of large wiring closets and large wiring ducts, luxuries that many older buildings cannot provide. Shielded twisted pair cable has all of the advantages and disadvantages of unshielded twisted pair cable. In addition, as you might expect, STP affords greater protection from all types of external interference than unshielded twisted pair cable. Moreover, generally speaking, shielded twisted pair cable is more expensive than unshielded twisted pair cable. Unlike coaxial cable, in shielded twisted pair cable, the shield is not part of the data circuit. Therefore the cable must only be grounded at one end. Usually, installers ground STP at either the wiring closet or the hub. This is not always easy to do, particularly if installers attempt to utilize older wiring hubs that were not designed to accommodate shielded twisted pair cable. In such cases improperly grounded STP can become a major source of problems, because it allows the shield to act like an antenna, absorbing electrical signals form other wires in the cable and from sources of electrical noise outside the cable. Finally, shielded twisted pair cable cannot be run as far unboosted as other networking media.

SGMP - Simple Gateway Monitoring Protocol - Network management protocol that was considered for Internet standardization and later evolved into SNMP. Documented in RFC 1028.

Shielding - In cable that employs shielding, each wire pair or group of wire pairs is surrounded by a metal braid or foil. This shielding acts as a barrier to any interfering signals. However, as with increasing the size of the conductors, using braid or foil covering increases the diameter of the cable, and it will increase the cost as well. Therefore, cancellation is the more commonly used technique to protect the wire from undesirable interference. (see Cancellation & Signal Degradation)

Signal - Electrical or light impulses representing data that pass along the networking media from one networking device to another for the purpose of communication.

Signal Degradation - Each wire in a cable can act like an antenna. When this happens, the wire actually absorbs electrical signals from other wires in the cable and from electrical sources outside the cable. If the resulting electrical noise reaches a high enough level, it can become difficult for NIC cards to discriminate the noise from the data signal. When electrical noise on the cable originates from signals on other wires in the cable, this is known as crosstalk. Internal copper wire factors such as opposition to the flow of electrons (resistance), opposition to changes in voltage (capacitance), and opposition to changes in current (inductance) can cause signals to degrade. External sources of electrical impulses that can attack the quality of electrical signals on the cable include lighting, electrical motors, and radio systems. One way to deal with degradation of the electrical signal is to increase the size of the conductors. Another way is to improve the type of insulating material used. However, such changes increase the size and cost of the cable faster than they improve its quality. Therefore, it is more typical for network designers to specify a cable of good quality and provide specifications for the maximum recommended cable length between nodes. Two techniques that cable designers have used successfully in dealing with EMI and RFI are shielding and cancellation. A complex electrical characteristic involving resistance, or the opposition to the flow of electrons, and reactance, or the opposition to changes in voltage and current, is known as impedance. For optimal network performance, it is important that the network media have a specific impedance in order to match the electrical components in the NIC cards. Unless the network media has the correct impedance, the signal will suffer signal loss and interference. (see EMI, RFI, Crosstalk, Shielding, Cancellation)

Signal Injector - Device used to measure attenuation of a signal on a network.

Signal Reference Ground - Reference point used by computing devices to measure and compare incoming digital signals to. When a computer attached to a network receives data in the form of digital signals, it must have some way of recognizing them. It does this by measuring and comparing the 3 to 5 volt signals it receives to a reference point called the signal reference ground. o function correctly, the signal reference ground must be close to a computer's digital circuits. Engineers have accomplished this by designing a ground plane into circuit boards. The computer's cabinet is used as a common point of connection for the circuit board ground planes to establish the signal reference ground. Signal reference ground establishes the zero volts. Ideally the signal reference ground should be completely isolated from the electrical ground. Isolation would keep AC power leakage and voltage spikes off the signal reference ground. However, engineers have not found it practical to isolate the signal reference ground in this manner. Instead, the chassis of a computing device serves as the signal reference ground, which also serves as the AC power line ground. Because there is a link between the signal reference ground and the power ground, problems with the power ground can lead to interference with the data system. Such interference can be difficult to detect and trace. Usually, however they stem from the fact that electrical contractors and installers don't care about the length of the neutral and ground wires that lead to each electrical outlet. Unfortunately, when these wires are long, they can act as an antenna for electrical noise. It is this noise that interferes with the digital signals a computer must be able to recognize. In order to avoid the problem of electrical noise described above, it is important to work closely with your electrical contractor and the power company. This will enable you to get the best and shortest electrical ground. One way to do this is to investigate the costs of getting a single power transformer dedicated to your LAN installation area. If you can afford this option, you will be able to control the attachment of other devices to your power circuit. Restricting how and where devices such as motors or high-current electrical heaters attach, you can eliminate much of the electrical noise generated by them. When working with your electrical contractor, you should ask that separate power distribution panels, known as breaker boxes, be installed for each office area. Because the neutral wires and ground wires from each outlet come together in the breaker box, taking this step will increase your chances of shortening the length of the signal ground. While installing individual power distribution panels for every cluster of computers will increase the up-front cost of our power wiring, it will reduce the length of the ground wires and limit several kinds of signal burying electrical noise.

Sliding Window - Refers to the fact that the window size is negotiated dynamically during the TCP session. The sliding part of sliding window refers to the fact that the window size is negotiated dynamically during the TCP session. A sliding window provides more efficient use of bandwidth by the hosts. (see Expectational Acknowledgment & Window Size)

SNMP - Simple Network Management Protocol - Network management protocol used almost exclusively in TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security. It is the most used and well-known of the network software management tools. Since it became a TCP/IP standard, the use of SNMP has increased. To retrieve network information, SNMP uses a technique called MIB collection. This means that it goes from one network device to another polling them as to their status. Then, it copies information regarding each device's status as well as each device's local MIB. One advantage of SNMP is that devices on the network do not have to be smart enough to report when a problem occurs. SNMP's polling takes care of that task for them. However, in large networks that have many devices and resources attached to them, SNMP's polling technique can be a disadvantage because it contributes significantly to network traffic. This can actually slow the network down. SNMP can help you determine if there is a problem with a particular network segment. (see Operational Audit)

SNMP2 - SNMP Version 2 - Version 2 of the popular network management protocol. SNMP2 supports centralized as well as distributed network management strategies, and includes improvements in the SMI, protocol operations, management architecture, and security.

Socket & Socket Numbers - See Port & Port Numbers

Software Management Tools - See Operational Audit & SNMP

Spike - Any power impulse lasting between .5 and 100 microseconds and possessing an amplitude over 100 % of peak power line voltage. (see MOV)

Standards - Sets of rules or procedures that are either widely used or officially specified and that they act as a type of blueprint, to ensure greater compatibility and interoperability between the various types of network technologies that were being produced by many companies around the world. (see IEEE, UL, EIA, TIA, ISO)

Star Topology - LAN topology in which end points on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links. The center of a star is a hub, repeater, or concentrator. Typically used in both Ethernet and Token Ring. In a star topology there is no need for external terminators, because the networking media is terminated where it connects to the hub and the work station. Most local area network designers consider a star topology to be the easiest to design and install. This is because the networking media is run directly out from a central hub to each work station area. Another advantage that a star topology provides is ease of maintenance. This is because the only area of concentration is located at the hub. In a star topology the layout used for the networking media is easy to modify and diagnosis of problems is easier to perform. Moreover, workstations can be easily added to a network employing star topology. If one run of networking media is broken or shorted, then only the device attached at that point is out of commission. However, the rest of the local area network will remain functional. In short, a star topology means greater reliability. While limiting one device per run of networking media can make diagnosis of problems easier, it also increased the amount of networking media required. This will result in increased costs in setting up a star topology local area network. And, while the hub can make maintenance easier, because all data has to pass through this central point, if the hub fails, the entire network will fail. Star Topology is used for horizontal cabling. The mechanical termination for each telecommunications outlet/connector is located at the patch panel in the wiring closet. In other words, every outlet is independently and directly wired to the patch panel. In a star topology because each of the horizontal cabling runs can radiate out from the hub much like the spokes of a wheel, this means a local area network using this type of topology could cover an 200 meter by 200 meter area. Advantages: Greater reliability, ease of designing, installing, and maintenance. Disadvantages: Uses a lot of network media (cable, Etc) and the network depends on the hub. (see Extended Star Topology)

Static Electricity - Unpredictable electrical charges in the atmosphere that interfere with radio reception, computer networking, and the like.

Static Route - Route that is explicitly configured and entered into the routing table. Static routes take precedence over routes chosen by dynamic routing protocols.

Stub Network - Network that has only a single connection to a router.

Subnet - See Subnetwork

Subnet Address - Portion of an IP address that is specified as the subnetwork by the subnet mask.

Subnet Mask - 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as mask.

Subnetwork - 1.) In IP networks, a network sharing a particular subnet address. Subnetworks are networks arbitrarily segmented by a network administrator in order to provide a multilevel, hierarchical routing structure while shielding the subnetwork from the addressing complexity of attached networks. Sometimes called a subnet. 2.) In OSI networks, a collection of ESs and ISs under the control of a single administrative domain and using a single network access protocol.

Surge - Any voltage increase above 110 % of the normal voltage carried by a power line. (see MOV)

Switch - Network device that filters, forwards, and floods frames based on the destination address of each frame. The switch operates at the data link layer of the OSI model (allows a connection to be established as necessary and terminated when there is no longer a session to support).

TCP - Transmission Control Protocol - TCP is a connection-oriented, reliable transport layer protocol that provides reliable full-duplex data transmission. It is responsible for breaking messages into segments, reassembling them at the destination station, and re-sending anything that is not received. TCP supplies a virtual circuit between end-user applications. TCP is part of the TCP/IP protocol stack. TCP field segments include: Source and destination ports [what services are being requested], sequence and acknowledgment numbers [keeping track of segments]; code bits [setup and termination of sessions]; window [number of octets that the sender is willing to accept]; checksum [of header and data fields, for error correction], and of course data [upper-layer protocol data].

TCP/IP Conceptual Layers - TCP/IP groups the OSI Application, presentation, and session layers into an "Application" Layer; the Transport layers map exactly; TCP/IP renames the OSI network layer as an "Internet" Layer; and TCP/IP calls the OSI data link and physical layers the "Network Interface" Layer.

TCP/IP - Transmission Control Protocol/Internet Protocol - Common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of worldwide internetworks. The TCP/IP protocol stack maps closely to the OSI reference model in the lower layers. All standard physical and data-link protocols are supported. TCP/IP information is transferred in a sequence of datagrams. One message may be transmitted as a series of datagrams that are reassembled into the message at the receiving location. The TCP/IP protocol stack has the following components: Protocols to support file transfer, e-mail, remote login, and other applications Reliable and unreliable transports Connectionless datagram deliver at the network layer ICMP provides control and message functions at the network layer. Several protocols operate at the TCP/IP Internet layer, which corresponds to the OSI network layer (see Network Layer of the OSI Model for details)

Telepole - Telescoping pole with a hook at one end. It is used to get cable across a ceiling or attic quickly.

Telnet - Telnet is a layer 7 test that verifies the application layer software between source and destination stations (this is the most complete test mechanism available). The Telnet application provides a virtual terminal so that you can connect with other hosts running TCP/IP. You can test to determine whether the remote router can be accessed. For example, your success running Telnet to connect from the router York to another router, Paris, provides a basic test of the network connecting the two. If you can remotely access another router through Telnet, then at least you know that one TCP/IP application can reach the remote router. A successful Telnet connection indicates that the upper-layer application (and the services of lower layers, as well) functions properly. If you can Telnet to one router but not to another router, it is likely that the Telnet failure is caused by specific addressing, naming, or access permission problems. These problems can exist on your router or on the router that failed as a Telnet target. (Allows you to verify address configuration in your internetwork)(see Ping and Trace)

Terminator - Device that provides electrical resistance at the end of a transmission line to absorb signals on the line, thereby keeping them from bouncing back and being received again by network stations.

Testing The Network - When testing your network you should follow these steps: 1) Break the system into logically conceived functional elements. 2) Note any symptoms. 3) Based on the symptoms you observe, determine what the most likely dysfunctional element is. 4) Use substitution or additional testing to discover if the likely element is in fact dysfunctional. 5) If the element suspected of being dysfunctional proves not to be the problem, proceed to the next most likely element you suspect. 6) When the dysfunctional element is found, repair it if possible. 7) If it is not possible to repair the dysfunctional element, replace it. The IEEE and the EIA/TIA have established standards that allow you to evaluate if your network is operating at an acceptable level after installation has been completed. Provided your network passes this test and has been certified as meeting the standards, you can use this measurement as an established baseline. Knowing this baseline measurement is important, because the need to test does not end just because your network installation has been certified as meeting the standards. You will want to continue testing your network on a periodic basis in order to ensure optimal network performance. You can do this by comparing recorded measurements taken when the system was known to be operating properly against current measurements. Any time there is a significant drop away from the baseline measurement, it is an indication that something is wrong with the network. Repeated testing of your network and comparison against its baseline will help you spot specific problems and allow you to track degradation cause by aging, poor maintenance practices, weather, or other factors. a common problem can effect all cables on a LAN. For this reason it is recommended that you use a cable tester to measure network performance. Cable testers are hand held devices that can be used to certify that the cable meets the required IEEE and EIA/TIA standards. Cable testers will vary in the types of testing functions they provide. Some can provide printouts. Others can be attached to a PC to create a data file. Little or no special training is required to use cable testers currently available on the market today. Most competent network administrators or installers find that the operating manuals provided by the manufacturers provide sufficient explanation. Cable testers come with a wide range of features and capabilities. Therefore, the list of things cable testers can measure provided here is intended to provide you with a general overview of the features that are available. You will need to determine what features will best meet your needs and make your selection accordingly. Generally speaking, cable testers can perform tests that measure the overall capability of a cable run. This includes determining cable distance, locating bad connections, providing wire maps for detecting crossed pairs, measuring signal attenuation, detecting near-end crosstalk, detecting split pairs, performing noise level tests, and tracing cable behind walls. It is important to measure the distance of the cable, because the overall length of the runs of cable can affect the ability of devices on the network to share the networking media. Cable that exceeds the maximum length specified by EIA/TIA-568A will cause signal degradation. Cable testers sometimes referred to as time domain reflectometers or TDRs measure the distance to open-ended or shorted cable. The TDR does this by sending an electrical pulse down the cable. The device then times the signal's reflection back from the end of the cable. This technique is called time-domain reflectometry. Distance readings provided using this technique can be expected to be accurate to within two feet. When unshielded twisted pair cable is used for a LAN installation, distance measurements can be used to determine if the connections at the patch panel and at the telecommunications outlets are good. To understand how this works, you must understand more about how the TDR works. When it measures distance on a cable, the TDR sends an electrical signal that is reflected when it encounters the most distant open connection. Now, imagine that this device is used to determine which connections in a cable run are faulty. Begin by attaching the device to the patch cord at the patch panel. If the TDR reports the distance to the patch panel instead of a more distance point, then you will know you have a connection problem. The same procedure can be used at the opposite end of the cable to measure through the RJ45 jack located at the telecommunications outlet. Cable testers use a feature called wire map to indicate which wire pairs connect to what pins on lugs and sockets. This test is used to show if an installer properly connected wires to a plug or jack or whether this was done in reverse order. When wires are connected in reversed order, this is referred to as crossed-pairs. Unique to UTP cable installations, this is a common problem. When crossed pairs are detected in UTP LAN cabling systems, the connection is not good. Where the crossed pairs have been detected, the wiring will have to be redone. A condition known as split pairs can only be detected through visual inspection or by detecting it through cross-talk measurements. Since the twisting in wire pairs shields them from external interference from signals passing along other wire pairs. However, this shielding effect can only occur if the wires in the pair are part of the same circuit. When wires become split, they are no longer part of the same circuit. They are part of different circuits. While a current can flow in a circuit and the system will appear to work, no shielding is in effect. Consequently, the signals are not protected. Eventually, near-end crosstalk will become a problem in such situations. Unfortunately, a wire map will not detect this condition, because in split pairs, a circuit is still present. Various factors can reduce the power of a signal as it passes through the copper wires used in unshielded twisted pair cables. This reduction in the power of the signal is called attenuation. Attenuation, you will recall, occurs because the signal (pulses representing bits of data) loses energy to the cable. A cable tester will measures the reduction in the power of a signal received from a device known as a signal injector. A signal injector is a small box about the size of a deck of playing cards. It attaches to the far end of the cable. Cable testers general measure attenuation at several frequencies. For Category 5 cable, cable testers generally measure up to 100 MHz. Check the EIA/TIA 568A specifications to see what amount of loss allowed for the type of cable used in your LAN. Several factors can contribute to near-end crosstalk. The most common cause of high levels of near-end crosstalk is crossed pairs. As mentioned earlier, these can be detected using the wire map feature of a cable tester. Near-end cross talk can also be caused by twisted pairs that have become untwisted when attached to cross-connect devices such as patch panels, patch cords that are untwisted, and cables that have been pulled too tightly around sharp corners, and as a result, pairs change position inside the cable jacket. If near-end crosstalk is detected, it is always wise to do a visual check of the horizontal cabling in order to rule out any of the possibilities. If nothing is found, then the problem is most likely caused by split pairs. A cable tester measures for near-end cross talk by measuring a series of frequencies up to 100 MHz. Generally speaking, the higher the numbers in the readings, the better. Lower numbers are indicative of problems on the network. any outside factors can contribute to interference on the networking media. Some examples of sources that can produce outside signals that can impose themselves on wire pairs in UTP cable include fluorescent lights, heaters, radios, electronic devices of all kinds, air cleaners, televisions, computers, motion sensors, radar, motors, switches, welders, and auto ignitions. Fortunately, signals produced by these outside sources often occupy specific frequencies. This enables an electrical noise level test to not only detect such outside interference, but to narrow the range of possible sources that produced it. When a cable tester is used to take a noise reading on the cable, all cables should be disconnected from the computer equipment. Generally speaking, high reading levels, indicate a problem. A simple way to locate the precise source is to try unplugging electrical devices until the source of the noise is found. However, be aware that this does not always work. (see Network Testing)

TFTP - Trivial File Transfer ProtocolSimplified version of FTP that allows files to be transferred from one computer to another over a network.

TFTP Server - A current copy of the configuration can be stored on a TFTP server. You use the copy running-config tftp command to store the current configuration in RAM on a network TFTP server. To do so, complete the following tasks: Step 1: Enter the copy running-config tftp command. Step 2: Enter the IP address of the host you want to store the configuration file. Step 3: Enter the name you would like to assign to the configuration file. Step 4: Confirm your choices by answering yes. You can configure the router by loading the configuration file stored on one of your network servers. To do so, complete the following tasks: Step 1: Enter configuration mode by entering the copy tftp running-config command. Step 2: At the system prompt, select a host or network configuration file. The network configuration file contains commands that apply to all routers and terminal servers on the network. The host configuration file contains commands that apply to one router in particular. Step 3: When prompted, enter the IP address of the remote host from which you are retrieving the configuration file. In this example, the router is configured from the TFTP server at IP address 131.108.2.155. When prompted, enter the name of the configuration file or accept the default name. The filename convention is UNIX based. The default filename is hostname-confg for the host file and network-confg for the network configuration file. If the TFTP server is running in the DOS environment, the server filenames are limited to eight characters plus a three-character extension (for example, router.cfg). Confirm the configuration filename and the server address that the system supplies.

Thinnet - Term used to define a thinner, less expensive version of the cable specified in the IEEE 802.3 10Base2 standard. Compare with Cheapernet. (see Coaxial)

TIA - Telecommunications Industry Association. - Organization that develops standards relating to telecommunications technologies. Together, the TIA and the EIA have formalized standards, such as EIA/TIA-232, for the electrical characteristics of data transmission.

TIR - Total Internal Reflection - The light guiding phenomenon on which optical fibers are based. TIR occurs when light rays are bouncing within a material with a higher index of refraction than its surroundings. For certain angles at which the rays bounce, the light becomes trapped in the high-index material. See refraction.

Tie-Wraps - Plastic ties used for holding cables together or for holding cables in place.

TDR - Time Domain Relfectometer - Device capable of sending signals through a network medium to check cable continuity, length, and other attributes. TDRs are used to find physical layer network problems. (see Testing The Network)

Time-Domain Reflectometry - Technique of sending an electrical signal down a cable and then timing the signal's reflection back from the end of the cable. (see Testing The Network)

Token Passing - Access method by which network devices access the physical medium in an orderly fashion based on possession of a small frame called a token. A free token is routed around the ring. As it passes around the ring, it polls devices on the network to see if they want to transmit data.

Token Ring - Token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Token Ring architecture uses token passing for its access method. Token Ring installations use 150-ohm STP cable. (see Token Passing & STP) Similar to IEEE 802.5.

Topology - Describes how a LAN is constructed. The physical layout of the local area network (arrangement of network nodes and media within an enterprise networking structure). (see Bus, Star, Token Ring, Tree, Complete, Irregular, and Intersecting Rings)

Trace - Trace uses Time-To-Live (TTL) values to generate messages from each router used along the path (this is very useful for locating path failures). (Allows you to verify address configuration in your internetwork)(see Ping and Telnet)

Trouble Report - The information recorded in each report should be divided into five general categories. The first category should be an ID number that has been assigned to the call. This will be useful in filing the information or entering it into a database. The second category should consist of preliminary information. It should include the name of the individual who reported the problem, the time the problem was reported, the method by which the problem was reported, whether the problem is related to previous calls reporting trouble and the ID numbers of those reports, the location of where the problem occurred, can the problem be replicated for network support staff, the time when the problem first appeared, whether anything was done differently or changed just before the problem occurred, and whether the problem is periodic or constant in nature. The third category should consist of information that network support staff gather at the site where the problem occurred. It should include comments by support staff regarding the PC environment such as power, temperature, humidity, and any others which suggest themselves, support staff observations about the problem or difficulty, and a listing of what actions were taken to remedy the problem. The fourth category should provide information that indicates whether the PC had to be taken into a repair area for further servicing, a listing of any actions taken, and the result of those actions. The final category that should appear on the trouble report should consist of the summary. The summary should list whether the problem was a hardware, software, or user problem, if the problem was software, what software, and if the problem was a hardware problem, what hardware was involved. Once you have collected all the information available about a problem, start listing possible causes. Based on the performance history you have at your disposal, you should be able to prioritize these causes from most likely to least likely. It will help if you keep in mind how data flows on your network while you do this. Using this list of possibilities, next begin to use your network's management tools to further identify the cause of the problem. Even if you are limited in the types of network management tools at your disposal, you can still successfully troubleshoot problems on your network. Instead of relying heavily on network management tools such as monitors and analyzers though, you will need to use what is called the replacement method. To understand how the replacement method works, when a problem occurs, assume you have one healthy entity and one non-functioning one. For example, if the information you've gathered has led you to believe that the problem lies in a particular workstation, get another one you know works. Start at the most basic level, the cabling. Switch the patch cords on the PCs. If the malfunctioning machine begins to work properly, and the healthy PC becomes sick, then you've found the problem on the first try. If nothing changes in the malfunctioning device, replace the patch cords to their original locations and move onto to the next most basic level, which in this case would be the NIC card. If you've tried everything, and still haven't been able to solve the problem, don't overlook the obvious. Lack of knowledge, experience, and proper management tools could be part of the problem. Once you've reached this conclusion, don't hesitate to call in the experts. The key to successfully troubleshooting problems on the network is to isolate the problem by systematically working your way through a simple hierarchy of troubleshooting procedures.

TTL - Time To Live - Field in an IP header that indicates how long a packet is considered valid.

TP -Twisted Pair - Transmission medium consisting of four or eight insulated wires arranged by pairs in a regular spiral pattern. The wires can be shielded or unshielded. Twisted pair is common in telephony applications and is most common in data networks. See also STP and UTP.

UDP (User Datagram Protocol) - A connection-less, "unreliable" transport layer protocol. UDP uses no windowing or acknowledgments. Application-layer protocols must provide for reliability if necessary. UDP is designed for applications that do not need to put sequences of segments together. Protocols that use UDP include TFTP, SNMP, Network File System (NFS), and Domain Name System (DNS). in theTCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768. Although UDP is responsible for transmitting messages, no software checking for segment delivery is provided at this layer, hence the description “unreliable.”

UL - Underwriters Laboratories - Independent agency within the United States that tests product safety. Cabling specifications issued by the Underwriters Laboratories are primarily concerned with safety standards, however, they also rate twisted pair networking media for performance. In addition it is the Underwriters Laboratories which established an identification program listing markings for shielded and unshielded twisted pair networking media in order to simplify the job of ensuring that materials used in LAN installations meet specification.

UPS - Uninterruptable Power Supply - Backup device designed to provide an uninterrupted power source in the event of a power failure. They are commonly installed on al file servers and wiring hubs. Generally speaking a UPS consists of batteries, a battery charger, and a power inverter. The function of the inverter is to convert low level direct current voltage of the batteries into the AC voltage normally supplied by the power line to networking devices. The battery charger is designed to keep the batteries in peak condition during periods when the power line system is functioning normally. Typically UPS devices that offer fewer features and cost less money will act as standby power systems only. This means that they monitor the power line. If and when a problem occurs, the UPS will switch over to the inverter powered by its batteries. The time needed for this switch to occur is referred to as the transfer time. Usually, the transfer time lasts for only a few milliseconds. Since the transfer time is of such short duration, this does not usually present a problem for most modern computers which are designed to coast on their own power supplies for at least a hundred milliseconds. UPS devices that offer more features and cost more money typically operate online. This means that they constantly supply power from inverters powered by their batteries. While they do this, their batteries continue charging from the power line. Because their inverters supplies freshly generated alternating current, such UPS devices have the added benefit of ensuring that no spikes from the power line reach networking devices it serves. If and when the AC power line goes down however, the UPS's batteries will smoothly switch from recharging to providing power to the inverter. Consequently, this type of UPS effectively reduces the transfer time needed to zero. In any event, a good UPS should be designed to communicate with the file server. This is important so the file server can be warned to shut down files when the UPS's battery power nears its end. Additionally, a good UPS will report when the server starts to run on battery power and supply this information to any work stations running on the network after the power outage has occurred.

UTP -Unshielded Twisted-Pair - Four-pair wire medium used in a variety of networks. UTP does not require the fixed spacing between connections that is necessary with coaxial-type connections. There are five types of UTP cabling commonly used: Category 1 cabling, Category 2 cabling, Category 3 cabling, Category 4 cabling, and Category 5 cabling. It is composed of pairs of wires. Each pair of wires is insulated from the other pairs. Unshielded twisted pair cable relies solely on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. In order to further reduce crosstalk between the pairs in unshielded twisted pair cable, the number of twists in the wire pairs varies from one to the other. In fact, like shielded twisted pair cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable. When used as a networking medium, unshielded twisted pair cable has four pairs of either 22 or 24 gauge copper wire. It is important to note that UTP used as a networking medium has an impedance of 100 ohms. This differentiates it from other types of twisted pair wiring such as that used for telephone wiring. Because UTP has an external diameter of approximately .17 inch, its small size can be advantageous during installation. Because it has such a small external diameter, UTP won't fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly if you are installing a network in an older building. Moreover, when unshielded twisted pair cable is installed using an RJ connector, a good, solid connection is practically guaranteed. Because of this, potential sources of network noise can be greatly reduced. UTP is the fastest copper based media. Because it can be used with most of the major networking architectures, UTP continues to grow in popularity. CHARACTERISTICS: Easy to install, 10-100Mbps throughput, least expensive per node, small RJ-45 connector, maximum cable length is 100 meters, more prone to electrical noise and interference than other types of networking media.

V.24 - ITU-T standard for a physical layer interface between DTE and DCE. V.24 is essentially the same as the EIA/TIA-232 standard.

V.35 - ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds up to 48 Kbps.

Vertical Cabling - See Backbone Cabling. (see IDF)

Vertical Terminal - See Telnet

WAN - Wide-Area Network - WANs operate over a large geographical area whereas LANs do not. WANs emphasize access over serial interfaces operating at lower speeds to ensure reliability, whereas LANs can operate reliably at very high speeds with multiple access. WANs operate at the OSI physical and data link layers. WAN standards are defined and managed by a number of recognized authorities, including ITU-T, ISO, EIA, & IETF. Three WAN physical layer standards commonly used are EIA/TIA-232, V.35, and X.21. Four WAN data link layer protocols are HDLC (High-Level Data Link Control), Frame Relay (a simplified version of HDLC framing), PPP (Point to Point protocol), and ISDN (Integrated Services Digital Network). The WAN network operates beyond the local LAN’s geographic scope. It uses the services of carriers such as Regional Bell operating companies (RBOCs), Sprint, and MCI. WANs use serial connections of various types to access bandwidth over wide-area geographies. By definition, the WAN connects devices separated by wide areas. WAN devices include: Routers - Which offer many services, including internetworking and WAN interface ports. Switches - Which connect to WAN bandwidth for voice, data, and video communication. Modems - Which interface voice-grade services; channel service units/digital service units(CSU/DSUs) that interface T1/E1 services; and Terminal Adapters/Network Termination 1 (TA/NT1s) that interface Integrated Services Digital Network (ISDN) services. Communication Servers - Which concentrate dialin and dial-out user communication. Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Frame Relay, SMDS, and X.25 are examples of WANs. WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and functional connections for wide-area networking services. These services are most often obtained from WAN service providers such as RBOCs, alternate carriers, and post, telephone, and telegraph (PTT) agencies. Three WAN physical layer standards commonly used are EIA/TIA-232, V.35, and X.21. WAN data link protocols describe how frames are carried between systems on a single data link. They include protocols designed to operate over dedicated point-to-point, multipoint, and multiaccess switched services such as Frame Relay. Four WAN data link layer protocols are HDLC (High-Level Data Link Control), Frame Relay (a simplified version of HDLC framing), PPP (Point to Point protocol), and ISDN (Integrated Services Digital Network).

WAN Devices - ROUTERS for internetworking and WAN interface ports, SWITCHES for sharing bandwidth, TERMINAL ADAPTERS to allow a variety of data connection services, COMMUNICATION SERVERS to concentrate dial-in and dial-out user communications, MODEMS, and CSU/DSUs.

Window Size - Refers to the number of messages that can be transmitted while awaiting an acknowledgment. It determines how much data the receiving station can accept at one time. With a window size of 1, each segment (each byte, actually) must be acknowledged before another segment (byte) is transmitted. This results in inefficient use of bandwidth by the hosts. (see Sliding Window & Expectational Acknowledgment)

Wire Map - Feature provided by most cable testers. Used to test twisted pair cable installations, it shows which wire pairs connect to what pins on the plugs and sockets.

Wiring Closet - Specially designed room used for wiring a data or voice network. Wiring closets serve as a central junction point for the wiring and wiring equipment that is used for interconnecting devices. EIA/TIA-569 specifies that there be a minimum of one wiring closet per floor and states that additional wiring closets should be provided for each area up to 1000 square meters when the floor area served exceeds 1000 square meters (10,000 square feet) or the horizontal cabling distance exceeds 90 meters (300 feet). Because a wiring closet serves as a central junction point for the wiring and wiring equipment that is used for connecting devices in a local area network, it is at the center point of a star topology. Typically, the equipment found in a wiring closet can include patch panels, wiring hubs, bridges, switches, and routers. In large networks it is not unusual to have more than one wiring closet. When this occurs, the topology is described as an extended star topology. Usually, when more than one wiring closet is required, one will be designated as the main distribution facility or MDF and all other wiring closets, referred to as intermediate distribution facilities or IDFs, will be dependent on it. In an Ethernet LAN using a star topology, the horizontal cabling runs coming from the work areas are usually terminated at a patch panel. Any location selected for a wiring closet must meet certain environmental requirements. Broadly speaking these environmental requirements include sufficient power and HVAC. In addition, the location selected should be one that is secure from unauthorized access and meets all applicable building and safety codes. More specifically, any room or closet selected to serve as a wiring closet should adhere to certain guidelines governing such things as the permissible types of walls, floors, and ceiling, acceptable temperature and humidity levels, locations and types of lighting and power outlets, requirements for room and equipment access, and specifications for cable access and support. (see Patch Panel)

X.21 - ITU-T standard for serial communications over synchronous digital lines. The X.21 protocol is used primarily in Europe and Japan.