Ethernet specification using 50-ohm thin coaxial cable and a signaling rate of 10-Mbps baseband.

10Base5

Ethernet specification using standard (thick) 50-ohm baseband coaxial cable and a signaling rate of 10-Mbps baseband.

10BaseFL

Ethernet specification using fiber-optic cabling and a signaling rate of 10-Mbps baseband, and FOIRL.

10BaseT

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, and a signaling rate of 10-Mbps baseband.

10Broad36

Ethernet specification using broadband coaxial cable and a signaling rate of 10 Mbps.

100BaseFX

Fast Ethernet specification using two strands of multimode fiber-optic cable per link and a signaling rate of 100-Mbps baseband. A 100BaseFX link cannot exceed 400 meters in length.

100BaseT

Fast Ethernet specification using UTP wiring and a signaling rate of 100-Mbps baseband. 100BaseT sends link pulses out on the wire when there is no data traffic present.

100BaseT4

Fast Ethernet specification using four pairs of Category 3, 4, or 5 UTP wiring and a signaling rate of 100-Mbps baseband. The maximum length of a 100BaseT4 segment is 100 meters.

100BaseTX

Fast Ethernet specification using two pairs of UTP or STP wiring and 100-Mbps baseband signaling. One pair of wires is used to receive data; the other is used to transmit. A 100BaseTX segment cannot exceed 100 meters in length.

100BaseX

100-Mbps baseband Fast Ethernet specification based on the IEEE 802.3 standard. 100BaseX refers to the whole 100Base family of standards for Fast Ethernet.

80/20 rule

General network standard that 80 percent of traffic on a given network is local (destined for targets in the same workgroup); and not more than 20 percent of traffic requires internetworking.

AAL (ATM adaptation layer)

Service-dependent sublayer of the Data Link layer. The function of the AAL is to accept data from different applications and present it to the ATM layer in 48-byte ATM segments.

AARP (AppleTalk Address Resolution Protocol)

The protocol that maps a data-link address to an AppleTalk network address.

ABR (area border router)

Router located on the border of an OSPF area, which connects that area to the backbone network. An ABR would be a member of both the OSPF backbone and the attached area. It maintains routing tables describing both the backbone topology and the topology of the other area.

access list

A sequential list of statements in a router configuration that identify network traffic for various purposes, including traffic and route filtering.

accounting

Cisco command option that, when applied to an interface, makes the router keep track of the number of bytes and packets sent between each pair of network addresses.

acknowledgment

Notification sent from one network device to another to acknowledge that a message or group of messages has been received. Sometimes abbreviated ACK. Opposite of NACK.

active hub

A multiport device that repeats and amplifies LAN signals at the Physical layer.

active monitor

A network device on a Token Ring that is responsible for managing ring operations. The active monitor ensures that tokens are not lost, or that frames do not circulate indefinitely on the ring.

address

A numbering convention used to identify a unique entity or location on a network.

address mapping

Technique that allows different protocols to operate together by associating addresses from one format with those of another.

address mask

A string of bits, which, when combined with an address, describes which portion of an address refers to the network or subnet and which part refers to the host. See also subnet mask.

address resolution

A technique for resolving differences between computer addressing schemes. Address resolution most often specifies a method for mapping network layer addresses to Data Link layer addresses. See also address mapping.

Address Resolution Protocol

See ARP.

administrative distance

A rating of the preferability of a routing information source. Administrative distance is expressed as a value between 0 and 255. The higher the value, the lower the preference.

advertising

A process in which a router sends routing or service updates at frequent intervals so that other routers on the network can maintain lists of usable routes or services.

algorithm

A specific process for arriving at a solution to a problem.

AMI (alternate mark inversion)

The line-code type that is used on T1 and E1 circuits. In this code, zeros are represented by 01 during each bit cell, and ones are represented by 11 or 00, alternately, during each bit cell.

ANSI (American National Standards Institute)

An organization of representatives of corporate, government, and other entities that coordinates standards-related activities, approves U.S. national
standards, and develops positions for the United States in international standards organizations.

APaRT (automated packet recognition/translation)

 Technology that enables a server to be attached to CDDI or FDDI without necessitating the reconfiguration of applications or network protocols. APaRT recognizes specific data link layer encapsulation packet types; when these packet types are transferred to another medium, they are translated into the native format of the destination device.

AppleTalk

A suite of communications protocols developed by Apple Computer for allowing communication among their devices over a network.

Application layer

Layer 7 of the OSI reference model. This layer provides services to end-user application processes such as electronic mail, file transfer, and terminal emulation.

ARP (Address Resolution Protocol)

Internet protocol used to map an IP address to a MAC address.

ASBR (autonomous system boundary router)

An ASBR is an ABR connecting an OSPF autonomous system to a non-OSPF network. ASBRs run two protocols: OSPF and another routing protocol. ASBRs must be located in a nonstub OSPF area.

asynchronous transmission

Describes digital signals that are transmitted without precise clocking or synchronization.

ATM (Asynchronous Transfer Mode)

An international standard for cell relay suitable for carrying multiple service types (such as voice, video, or data) in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing latency.

ATM adaptation layer

See AAL.

ATM Forum

International organization founded in 1991 by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint to develop and promote standards-based implementation agreements for ATM technology.

AUI (attachment unit interface)

An interface between an MAU and a NIC (network interface card) described in the IEEE 802.3 specification. AUI often refers to the physical port to which an AUI cable attaches.

auto-discovery

A mechanism used by many network management products, including CiscoWorks, to build a map of a network.

autonomous system

A group of networks under a common administration that share in a common routing strategy. Sometimes abbreviated AS.

B channel (Bearer channel)

An ISDN term meaning a full-duplex, 64-Kbps channel used to send user data.

B8ZS (binary 8-zero substitution)

The line-code type that is used on T1 and E1 circuits. With B8ZS, a special code is substituted whenever eight consecutive zeros are sent over the link. This code is then interpreted at the remote end of the connection.

backoff

The retransmission delay used by contention-based MAC protocols such as Ethernet, after a network node determines that the physical medium is already in use.

bandwidth

The difference between the highest and lowest frequencies available for network signals. The term may also describe the throughput capacity of a network link or segment.

baseband

A network technology in which a single carrier frequency is used. Ethernet is a common example of a baseband network technology.

baud

Unit of signaling speed equal to the number of separate signal elements transmitted in one second. Baud is synonymous with bits per second (bps), as long as each signal element represents exactly one bit.

bearer channel

See B channel.

BECN (backward explicit congestion notification)

A Frame Relay network facility that allows switches in the network to advise DTE devices of congestion. The BECN bit is set in frames traveling in the opposite direction of frames encountering a congested path.

best-effort delivery

Describes a network system that does not use a system of acknowledgment to guarantee reliable delivery of information.

BGP (Border Gateway Protocol)

An interdomain path-vector routing protocol. BGP exchanges reachability information with other BGP systems. It is defined by RFC 1163.

binary

A numbering system in which there are only two digits, ones
and zeros.

bit stuffing

A 0 insertion and deletion process defined by HDLC. This technique ensures that actual data never appears as flag characters.

BNC connector

Standard connector used to connect coaxial cable to an MAU or line card.

BOOTP (Bootstrap Protocol)

Part of the TCP/IP suite of protocols, used by a network node to determine the IP address of its network interfaces, in order to boot from a network server.

BPDU (Bridge Protocol Data Unit)

 A Layer 2 protocol used for communication among bridges.

bps

Bits per second.

BRI (Basic Rate Interface)

ISDN interface consisting of two B channels and one D channel for circuit-switched communication. ISDN BRI can carry voice, video, and data.

bridge

Device that connects and forwards packets between two network segments that use the same data-link communications protocol. Bridges operate at the Data Link layer of the OSI reference model. A bridge will filter, forward, or flood an incoming frame based on the MAC address of the frame.

broadband

A data transmission system that multiplexes multiple independent signals onto one cable. Also, in telecommunications, any channel with a bandwidth greater than 4 KHz. In LAN terminology, a coaxial cable using analog signaling.

broadcast

Data packet addressed to all nodes on a network. Broadcasts are identified by a broadcast address that matches all addresses on the network.

broadcast address

Special address reserved for sending a message to all stations. At the Data Link layer, a broadcast address is a MAC destination address of all 1s.

broadcast domain

The group of all devices that will receive the same broadcast frame originating from any device within the group. Because routers do not forward broadcast frames, broadcast domains are typically bounded by routers.

buffer

A memory storage area used for handling data in transit. Buffers are used in internetworking to compensate for differences in processing speed between network devices or signaling rates of segments. Bursts of packets can be stored in buffers until they can be handled by slower devices.

bus

Common physical path composed of wires or other media, across which signals are sent from one part of a computer to another.

bus topology

A topology used in LANs. Transmissions from network stations propagate the length of the medium and are then received by all other stations.

byte

A series of consecutive binary digits that are operated upon as a
unit, usually eight bits.

cable

Transmission medium of copper wire or optical fiber wrapped in a protective cover.

cable range

A range of network numbers on an extended AppleTalk network. The cable range value can be a single network number or a contiguous sequence of several network numbers. Nodes assign addresses within the cable range values provided.

CAM

Content-addressable memory.

carrier

Electromagnetic wave or alternating current of a single frequency, suitable for modulation by another, data-bearing signal.

Carrier Detect

See CD.

carrier sense multiple access with collision detection

See CSMA/CD.

Category 5 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 5 cabling can transmit data at speeds up to 100 Mbps.

CCITT (Consultative Committee for International Telegraphy and Telephony)

International organization responsible for the development of communications standards. Now called the ITU-T.
See also ITU-T.

CCO (Cisco Connection Online)

 Self-help resource for Cisco customers. Available 24 hours a day, seven days a week at http://www.cisco.com. The CCO family includes CCO Documentation, CCO Open Forum, CCO CD-ROM, and the TAC (Technical Assistance Center).

CD (Carrier Detect)

Signal that indicates whether an interface
is active.

CDDI (Copper Distributed Data Interface)

The implementation of FDDI protocols over STP and UTP cabling. CDDI transmits over distances of approximately 100 meters, providing data rates of 100 Mbps. CDDI uses a dual-ring architecture to provide redundancy.

CDP (Cisco Discovery Protocol)

 Used to discover neighboring Cisco devices, and used by network management software. The CiscoWorks network management software takes advantage of CDP.

cell

The basic data unit for ATM switching and multiplexing. A cell consists of a five-byte header and 48 bytes of payload. Cells contain fields in their headers that identify the data stream to which they belong.

CHAP (Challenge Handshake Authentication Protocol)

Security feature used with PPP encapsulation, which prevents unauthorized access by identifying the remote end. The router or access server determines whether that user is allowed access.

checksum

Method for checking the integrity of transmitted data. A checksum is an integer value computed from a sequence of octets taken through a series of arithmetic operations. The value is recomputed at the receiving end and compared for verification.

CIDR (classless interdomain routing)

Technique supported by BGP4 and based on route aggregation. CIDR allows routers to group routes together in order to cut down on the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. With CIDR, IP addresses and their subnet masks are written as four octets, separated by periods, followed by a forward slash and a two-digit number that represents the subnet mask.

CIR (committed information rate)

The rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics.

circuit switching

A system in which a dedicated physical path must exist between sender and receiver for the entire duration of a call. Used heavily in telephone networks.

CiscoWorks

Network management package that provides a graphical view of a network, collects statistical information about a network, and offers various network management components.

client

Node or software program, or front-end device, that requests services from a server.

CLNS (Connectionless Network Service)

An OSI network layer service, for which no circuit need be established before data can be transmitted. Routing of messages to their destinations is independent of other messages.

CMU SNMP

A free command-line SNMP management package that comes in source code form. Originally developed at the Carnegie Mellon University, and available at http://www.net.cmu.edu/projects/snmp/.

collision

In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device cause an increase in voltage when they meet on the physical media, and are damaged.

collision domain

A group of nodes such that any two or more of the nodes transmitting simultaneously will result in a collision.

congestion

Traffic in excess of network capacity.

connectionless

Term used to describe data transfer without the prior existence of a circuit.

console

A DTE device, usually consisting of a keyboard and display unit, through which users interact with a host.

contention

Access method in which network devices compete for permission to access the physical medium. Compare with circuit switching and token passing.

cost

A value, typically based on media bandwidth or other measures, that is assigned by a network administrator and used by routing protocols to compare various paths through an internetwork environment. Cost values are used to determine the most favorable path to a particular destination—
the lower the cost, the better the path.

count to infinity

A condition in which routers continuously increment the hop count to particular networks. Often occurs in routing algorithms that are slow to converge. Usually, some arbitrary hop count ceiling is imposed to limit the extent of this problem.

CPE (customer premises equipment)

Terminating equipment, such as terminals, telephones, and modems, installed at customer sites and connected to the telephone company network.

CRC (cyclic redundancy check)

An error-checking technique in which the receiving device performs a calculation on the frame contents and compares the calculated number to a value stored in the frame by the sending node.

CSMA/CD (carrier sense multiple access collision detect)

Media-access mechanism used by Ethernet and IEEE 802.3. Devices use CSMA/CD to check the channel for a carrier before transmitting data. If no carrier is sensed, the device transmits. If two devices transmit at the same time, the collision is detected by all colliding devices. Collisions delay retransmissions from those devices for a randomly chosen length of time.

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.

D channel

Data channel. Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel.

DAS (dual attachment station)

Device that is attached to both the primary and the secondary FDDI rings. Provides redundancy for the FDDI ring. Also called a Class A station. See also SAS.

datagram

Logical unit of information sent as a network layer unit over a transmission medium without prior establishment of a circuit.

Data Link layer

Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The Data Link layer is concerned with physical addressing, network topology, access to the network medium, error detection, sequential delivery of frames, and flow control. The Data Link layer is divided into two sublayers: the MAC sublayer and the LLC sublayer.

DCE (data circuit-terminating equipment)

The devices and connections of a communications network that represent the network end of the user-to-network interface. The DCE provides a physical connection to the network and provides a clocking signal used to synchronize transmission between DCE and DTE devices. Modems and interface cards are examples of DCE devices.

DDR (dial-on-demand routing)

Technique whereby a router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end-stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adapter or modem.

de facto standard

A standard that exists because of its widespread use.

de jure standard

Standard that exists because of its development or approval by an official standards body.

DECNet

Group of communications products (including a protocol suite) developed and supported by Digital Equipment Corporation. DECNet/OSI (also called DECNet Phase V) is the most recent iteration and supports both OSI protocols and proprietary Digital protocols. Phase IV Prime supports inherent MAC addresses that allow DECNet nodes to coexist with systems running other protocols that have MAC address restrictions. See also DNA.

dedicated line

Communications line that is indefinitely reserved for transmissions, rather than switched as transmission is required. See also leased line.

default gateway

Another term for default router. The router that a host will use to reach another network when it has no specific information about how to reach that network.

default route

A routing table entry that is used to direct packets when there is no explicit route present in the routing table.

delay

The time between the initiation of a transaction by a sender and the first response received by the sender. Also, the time required to move a packet from source to destination over a network path.

demarc

The demarcation point between telephone carrier equipment and CPE.

demultiplexing

The separating of multiple streams of data that have been multiplexed into a common physical signal for transmission, back into multiple output streams. Opposite of multiplexing.

destination address

Address of a network device to receive data.

DHCP (Dynamic Host Configuration Protocol)

Provides a mechanism for allocating IP addresses dynamically so that addresses can be reassigned instead of belonging to only one host.

Dijkstra algorithm

Dijkstra's algorithm is a graph algorithm used to find the shortest path from one node on a graph to all others. Used in networking to determine the shortest path between routers.

discovery mode

Method by which an AppleTalk router acquires information about an attached network from an operational router and then uses this information to configure its own addressing information.

distance vector routing algorithm

Class of routing algorithms that use the number of hops in a route to find a shortest path to a destination network. Distance vector routing algorithms call for each router to send its entire routing table in each update to each of its neighbors. Also called Bellman-Ford routing algorithm.

DLCI (data-link connection identifier)

A value that specifies a virtual circuit in a Frame Relay network.

DNA (Digital Network Architecture)

Network architecture
that was developed by Digital Equipment Corporation. DECNet is the collective term for the products that comprise DNA (including communications protocols).

DNIC (Data Network Identification Code)

Part of an X.121 address. DNICs are divided into two parts: the first specifying the country in which the addressed PSN is located and the second specifying the PSN itself. See also X.121.

DNS (Domain Name System)

System used in the Internet for translating names of network nodes into addresses.

DSP (domain specific part)

Part of an ATM address. A DSP is comprised of an area identifier, a station identifier, and a selector byte.

DTE (data terminal equipment)

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, routers and multiplexers.

DUAL (Diffusing Update Algorithm)

Convergence algorithm used in EIGRP. DUAL provides constant loop-free operation throughout a route computation by allowing routers involved in a topology change to synchronize at the same time, without involving routers that are unaffected by the change.

DVMRP (Distance Vector Multicast Routing Protocol)

DVMRP is an internetwork gateway protocol that implements a typical dense mode IP multicast scheme. Using IGMP, DVMRP exchanges routing datagrams with its neighbors.

dynamic routing

Routing that adjusts automatically to changes in network topology or traffic patterns.

E1

Wide-area digital transmission scheme used in Europe that carries data at a rate of 2.048 Mbps.

EIA/TIA-232

Common Physical layer interface standard, developed by EIA and TIA, that supports unbalanced circuits at signal speeds of up to 64 Kbps. Formerly known as RS-232.

EIGRP (Enhanced IGRP)

A multiservice routing protocol supporting IPX, AppleTalk, and IP. BGP is used for interconnecting networks and defining strict routing policies.

encapsulation

The process of attaching a particular protocol header to a unit of data prior to transmission on the network. For example, a frame of Ethernet data is given a specific Ethernet header before network transit.

endpoint

Device at which a virtual circuit or virtual path begins or ends.

enterprise network

A privately maintained network connecting most major points in a company or other organization. Usually spans a large geographic area and supports multiple protocols and services.

entity

Generally, an individual, manageable network device. Sometimes called an alias.

error control

Technique for detecting and correcting errors in
data transmissions.

Ethernet

Baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use the CSMA/CD method of media access control and run over a variety of cable types at 10 Mbps. Ethernet is similar to the IEEE 802.3 series of standards.

EtherTalk

Apple Computer's data-link product that allows an AppleTalk network to be connected by Ethernet cable.

EtherWave

A product from Netopia (formerly Farallon) used to connect AppleTalk devices with LocalTalk connectors to Ethernet networks. They are an alternative to LocalTalk-to-EtherTalk routers.

explorer packet

Generated by an end-station trying to find its way through a SRB network. Gathers a hop-by-hop description of a path through the network by being marked (updated) by each bridge that it traverses, thereby creating a complete topological map.

Fast Ethernet

Any of a number of 100-Mbps Ethernet specifications. Fast Ethernet offers a speed increase ten times that of the 10BaseT Ethernet specification, while preserving such qualities as frame format, MAC mechanisms, and MTU. Such similarities allow the use of existing 10BaseT applications and network management tools on Fast Ethernet networks. Based on an extension to the IEEE 802.3 specification. Compare with Ethernet. See also 100BaseFX; 100BaseT; 100BaseT4; 100BaseTX; 100BaseX; IEEE 802.3.

FDDI (Fiber Distributed Data Interface)

LAN standard, defined by ANSI X3T9.5, specifying a 100-Mbps token-passing network using fiber-optic cable, with transmission distances of up to 2 km. FDDI uses a dual-ring architecture to provide redundancy. Compare with CDDI.

FECN (forward explicit congestion notification)

A facility in a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action as appropriate.

file transfer

Category of popular network applications that features movement of files from one network device to another.

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 or routes based on the established criteria.

firewall

Router or other computer designated as a buffer between public networks and a private network. A firewall router uses access lists and other methods to ensure the security of the private network.

Flash memory

Nonvolatile storage that can be electrically erased and reprogrammed as necessary.

flash update

Routing update sent asynchronously when a change in the network topology occurs.

flat addressing

A system of addressing that does not incorporate a hierarchy to determine location.

flooding

Traffic-passing technique used by switches and bridges in which traffic received on an interface is sent out all of the interfaces of that device except the interface on which the information was originally received.

flow control

Technique for ensuring that a transmitting device, such as a modem, does not overwhelm a receiving device with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend transmission until it has processed the data in the buffers.

forwarding

The process of sending a frame or packet toward its destination.

fragment

Piece of a larger packet that has been broken down to smaller units.

fragmentation

Process of breaking a packet into smaller units
when transmitting over a network medium that is unable to support a transmission unit the original size of the packet.

frame

Logical grouping of information sent as a Data Link layer unit over a transmission medium. Sometimes refers to the header and trailer, used for synchronization and error control, which surround the user data contained in the unit. The terms cell, 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 over a single physical interface.
Frame Relay is more efficient than X.25, for which it is generally considered a replacement.

Frame Relay Cloud

A generic term used to refer to a collective Frame Relay network. For Frame Relay carrier customers, it generally refers to the carrier's entire Frame Relay network. It's referred to as a “cloud” because the network layout is not visible to the customer.

frequency

Number of cycles, measured in hertz, of an alternating current signal per unit of time.

FTP (File Transfer Protocol)

An application protocol, part
of the TCP/IP protocol stack, used for transferring files between hosts
on a network.

full duplex

Capability for simultaneous data transmission and receipt of data between two devices.

full mesh

A network topology in which each network node has either a physical circuit or a virtual circuit connecting it to every other network node.

gateway

In the IP community, an older term referring to a routing device. Today, the term router is used to describe devices 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.

GB

Gigabyte. Approximately 1,000,000,000 bytes.

Gb

Gigabit. Approximately 1,000,000,000 bits.

GBps

Gigabytes per second.

Gbps

Gigabits per second.

giants

Ethernet frames over the maximum frame size.

GNS (Get Nearest Server)

Request packet sent by a client on an IPX network to locate the nearest active server of a particular type. An IPX network client issues a GNS request to solicit either a direct response from a connected server or a response from a router that tells it where on the internetwork the service can be located. GNS is part of the IPX SAP.

half-duplex

Capability for data transmission in only one direction at a time between a sending station and a receiving station.

handshake

Sequence of messages exchanged between two or more network devices to ensure transmission synchronization.

hardware address

See MAC address.

HDLC (High-level Data Link Control)

Bit-oriented synchronous Data Link layer protocol developed by ISO and derived from SDLC. HDLC specifies a data encapsulation method for synchronous serial links and includes frame characters and checksums in its headers.

header

Control information placed before data when encapsulating that data for network transmission.

Hello packet

Multicast packet that is used by routers for neighbor discovery and recovery. Hello packets also indicate that a client is still operating on the network.

Hello protocol

Protocol used by OSPF and other routing protocols for establishing and maintaining neighbor relationships.

hierarchical addressing

A scheme of addressing that uses a logical hierarchy to determine location. For example, IP addresses consist of network numbers, subnet numbers, and host numbers, which IP routing algorithms use to route the packet to the appropriate location.

holddown

State of a routing table entry in which routers will neither advertise the route nor accept advertisements about the route for a specific length of time (known as the holddown period).

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 metric.

host

A computer system on a network. Similar to the term node except that host usually implies a computer system, whereas node can refer to any networked system, including routers.

host number

Part of an IP address that designates which node is being addressed. Also called a host address.

hub

A term used to describe a device that serves as the center of a star topology network; or, an Ethernet multiport repeater, sometimes referred to as a concentrator.

ICMP (Internet Control Message Protocol)

A network layer Internet protocol that provides reports of errors and other information about IP packet processing. ICMP is documented in RFC 792.

IEEE (Institute of Electrical and Electronics Engineers)

A professional organization among whose activities are the development of communications and networking standards. IEEE LAN standards are the most common LAN standards today.

IEEE 802.3

IEEE LAN protocol for the implementation of the Physical layer and the MAC sublayer of the Data Link layer. IEEE 802.3 uses CSMA/CD access at various speeds over various physical media.

IEEE 802.5

IEEE LAN protocol for the implementation of the Physical layer and MAC sublayer of the Data Link layer. Similar to Token Ring, IEEE 802.5 uses token-passing access over STP cabling.

IGP (Interior Gateway Protocol)

A generic term for an Internet routing protocol used to exchange routing information within an autonomous system. Examples of common Internet IGPs include IGRP, OSPF, and RIP.

InARP (Inverse Address Resolution Protocol)

 A basic Frame Relay protocol that allows routers on the Frame network to learn the protocol addresses of other routers.

interface

A connection between two systems or devices; or in routing terminology, a network connection.

Internet

Term used to refer to the global internetwork that evolved from the ARPANET, that now connects tens of thousands of networks worldwide.

Internet protocol

Any protocol that is part of the TCP/IP protocol stack. See TCP/IP.

internetwork

Collection of networks interconnected by routers and other devices that functions (generally) as a single network.

internetworking

General term used to refer to the industry that has arisen around the problem of connecting networks together. The term may be used to refer to products, procedures, and technologies.

Inverse ARP (Inverse Address Resolution Protocol)

Method
of building dynamic address mappings in a Frame Relay network. Allows
a device to discover the network address of a device associated with a
virtual circuit.

IP (Internet Protocol)

Network layer protocol in the TCP/IP stack offering a connectionless datagram service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Documented in RFC 791.

IP address

A 32-bit address assigned to hosts using the TCP/IP suite of protocols. An IP address is written as four octets separated by dots (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 often used with the address to extract network and subnetwork information from the IP address.

IPX (Internetwork Packet Exchange)

NetWare network layer (Layer 3) protocol used for transferring data from servers to workstations. IPX is similar to IP in that it is a connectionless datagram service.

IPXCP (IPX Control Protocol)

The protocol that establishes and configures IPX over PPP.

IPXWAN

A protocol that negotiates end-to-end options for new links on startup. When a link comes up, the first IPX packets sent across are IPXWAN packets negotiating the options for the link. When the IPXWAN options have been successfully determined, normal IPX transmission begins, and no more IPXWAN packets are sent. Defined by RFC 1362.

ISDN (Integrated Services Digital Network)

Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.

ISL (Inter-Switch Link)

Cisco's protocol for trunking VLANs over
Fast Ethernet.

ITU-T (International Telecommunication Union Telecommunication Standardization Sector)

International
body dedicated to the development of worldwide standards for telecommunications technologies. ITU-T is the successor to CCITT.

jabbers

Long, continuous frames exceeding 1518 bytes that prevent all stations on the Ethernet network from transmitting data. Jabbering violates CSMA/CD implementation by prohibiting stations from transmitting data.

jam pattern

Initiated by Ethernet transmitting station when a collision is detected during transmission.

KB

Kilobyte. Approximately 1,000 bytes.

Kb

Kilobit. Approximately 1,000 bits.

KBps

Kilobytes per second.

Kbps

Kilobits per second.

keepalive interval

Period of time between keepalive messages sent by a network device.

keepalive message

Message sent by one network device to inform another network device that it is still active.

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 the most widely used LAN technologies.

LANE (LAN Emulation)

Technology that allows an ATM network to function as a LAN backbone. In this situation LANE provides multicast and broadcast support, address mapping (MAC-to-ATM), and virtual circuit management.

LAPB (Link Access Procedure, Balanced)

The Data Link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC.

LAPD (Link Access Procedure on the D channel)

ISDN Data Link layer protocol for the D channel. LAPD was derived from the LAPB protocol and is designed to satisfy the signaling requirements of ISDN basic access. Defined by ITU-T Recommendations Q.920 and Q.921.

LAPF

Data link standard for Frame Relay.

late collision

Collision that is detected only after a station places a complete frame of the network.

latency

The amount of time elapsed between the time a device requests access to a network and the time it is allowed to transmit; or, amount of time between the point at which a device receives a frame and the time that frame is forwarded out the destination port.

LCP (Link Control Protocol)

A protocol used with PPP, which establishes, configures, and tests data-link connections.

leased line

Transmission line reserved by a communications carrier for the private use of a customer. A leased line is a type of dedicated line.

LEC (LAN Emulation Client)

Performs data forwarding, address resolution, and other control functions for a single end system within a single ELAN. Each LEC has a unique ATM address, and is associated with one or more MAC addresses reachable through that ATM address.

LECS (LAN Emulation Configuration Server)

Assigns LANE clients to ELANs by directing them to the LES that corresponds to the ELAN. There can be logically one LECS per administrative domain, which serves all ELANs within that domain.

LES (LAN Emulation Server)

Implements the control function for an ELAN. There can be only one logical LES per ELAN. It has a unique ATM address.

link

Network communications channel consisting of a circuit or transmission path and all related equipment between a sender and a receiver. Most often used to refer to a WAN connection. Sometimes called a line or a transmission link.

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 require that routers maintain a consistent view of the network and are therefore not prone to routing loops.

LLC (Logical Link Control)

Higher of two Data Link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow
control, framing, and MAC-sublayer addressing. The most common
LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented types.

LMI (Local Management Interface)

A set of enhancements to the basic Frame Relay specification. LMI includes support for keepalives, a multicast mechanism; global addressing, and a status mechanism.

load balancing

In routing, the ability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Load balancing increases the utilization of network segments, thus increasing total effective network bandwidth.

local loop

A line from the premises of a telephone subscriber to the telephone company central office.

LocalTalk

Apple Computer's proprietary baseband protocol that operates at the Dat Link and Physical layers of the OSI reference model. LocalTalk uses CSMA/CA and supports transmissions at speeds of 230.4 Kbps.

loop

A situation in which packets never reach their destination, but are forwarded in a cycle repeatedly through a group of network nodes.

MAC (Media Access Control)

Lower of the two sublayers of the Data Link layer defined by the IEEE. The MAC sublayer handles access to shared media.

MAC address

Standardized Data Link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 48 bits long and are controlled by the IEEE. Also known as a hardware address, a MAC-layer address, or a physical address.

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.

Mb

Megabit. Approximately 1,000,000 bits.

Mbps

Megabits per second.

media

The various physical environments through which transmission signals pass. Common network media include cable (twisted-pair, coaxial, and fiber optic) and the atmosphere (through which microwave, laser, and infrared transmission occurs). Sometimes referred to as physical media.

Media Access Control

See MAC.

mesh

Network topology in which devices are organized in a segmented manner with redundant interconnections strategically placed between network nodes.

message

Application layer logical grouping of information, often composed of a number of lower-layer logical groupings such as packets.

MIB (Management Information Base)

Database for network management information; it is used and maintained by a network management protocol such as SNMP.

MSAU (multistation access unit)

A wiring concentrator to which all end stations in a Token Ring network connect. Sometimes abbreviated MAU.

multiaccess network

A network that allows multiple devices to connect and communicate by sharing the same medium, such as a LAN.

multicast

A single packet copied by the network and sent to a specific subset of network addresses. These addresses are specified in the Destination Address field.

multicast address

A single address that refers to multiple network devices. Sometimes called a group address.

multiplexing

A technique that allows multiple logical signals to be transmitted simultaneously across a single physical channel.

mux

A multiplexing device. A mux combines multiple input signals for transmission over a single line. The signals are demultiplexed, or separated, before they are used at the receiving end.

NACK (negative acknowledgment)

A response sent from a receiving device to a sending device indicating that the information received contained errors.

name resolution

The process of associating a symbolic name with a network location or address.

NAT (Network Address Translation)

A technique for reducing the need for globally unique IP addresses. NAT allows an organization with addresses that may conflict with others in the IP address space, to connect to the Internet by translating those addresses into unique ones within the globally routable address space.

NBMA (nonbroadcast multiaccess)

Term describing a multiaccess network that either does not support broadcasting (such as X.25) or in which broadcasting is not feasible.

NBP (Name Binding Protocol)

AppleTalk transport level protocol that translates a character string name into the DDP address of the corresponding socket client.

NCP (Network Control Protocol)

Protocols that establish and configure various network layer protocols. Used for AppleTalk over PPP.

NDS (NetWare Directory Services)

A feature added in NetWare 4.0 as a replacement for individual bindaries. NDS allows NetWare
and related resources to be grouped in a tree hierarchy to better provide central administration.

NetBIOS (Network Basic Input/Output System)

An application programming interface used by applications on an IBM LAN to request services from lower-level network processes such as session establishment and termination, and information transfer.

netmask

A number, usually used as a bit-mask, to separate an address into its network portion and host portion.

NetWare

A network operating system developed by Novell, Inc. Provides remote file access, print services, and numerous other distributed network services.

network

Collection of computers, printers, routers, switches, and other devices that are able to communicate with each other over some transmission medium.

network interface

Border between a carrier network and a privately owned installation.

Network layer

Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The Network layer is the layer at which routing takes place.

NLSP (NetWare Link Services Protocol)

Link-state routing protocol for IPX based on IS-IS.

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 their functional capabilities, can be interconnected by links, and serve as control points in the network.

NVRAM (nonvolatile RAM)

RAM that retains its contents when a device is powered off.

ODI

Novell's Open Data-link Interface.

OSI reference model (Open System Interconnection reference model)

A network architectural framework developed by ISO and ITU-T. The model describes seven layers, each of which specifies a particular network. The lowest layer, called the Physical layer, is closest to the media technology. The highest layer, the Application layer, is closest to the user. The OSI reference model is widely used as a way of understanding network functionality.

OSPF (Open Shortest Path First)

A link-state, hierarchical IGP routing algorithm, which includes features such as least-cost routing, multipath routing, and load balancing. OSPF was based on an early version of the IS-IS protocol.

out-of-band signaling

Transmission using frequencies or channels outside the frequencies or channels used for transfer of normal data. Out-of-band signaling is often used for error reporting when normal channels are unusable for communicating with network devices.

packet

Logical grouping of information that includes a header containing control information and (usually) user data. Packets are most often used to refer to network layer units of data. The terms datagram, frame, message, and segment are also used to describe logical information groupings at various layers of the OSI reference model, and in various technology circles. See also PDU.

packet analyzer

A software package (also sometimes including specialized hardware) used to monitor network traffic. Most packet analyzer packages will also do packet decoding, making the packets easier for humans to read.

packet burst

Allows multiple packets to be transmitted between Novell clients and servers in response to a single read or write request. It also allows NCP connections to greatly improve throughput by reducing the number of acknowledgments.

packet starvation effect

On Ethernet, when packets experience latencies up to 100 times the average, or completely starve out due to 16 collisions. Occurs as a result of the CSMA/CD implementation.

PAP (Password Authentication Protocol)

 Authentication protocol that allows PPP peers to authenticate one another. The remote router attempting to connect to the local router is required to send an authentication request. Unlike CHAP, PAP passes the password and host name or username in the clear (unencrypted). PAP does not itself prevent unauthorized access, but merely identifies the remote end. The router or access server then determines if that user is allowed access. PAP is supported only on PPP lines.

partial mesh

Term describing a network in which devices are organized in a mesh topology, with some network nodes organized in a full mesh, but with others that are only connected to one or two other nodes in the network. A partial mesh does not provide the level of redundancy of a full mesh topology, but is less expensive to implement. Partial mesh topologies are generally used in the peripheral networks that connect to a fully meshed backbone. See also full mesh; mesh.

PDU (protocol data unit)

The OSI term for a packet.

Physical layer

Layer 1 of the OSI reference model; it corresponds with the Physical control layer in the SNA model. The Physical layer defines the specifications for activating, maintaining, and deactivating the physical link between end systems.

ping (packet internet groper)

ICMP echo message and its reply. Often used in IP networks to test the reachability of a network device.

poison reverse updates

Routing updates that explicitly indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates. Poison reverse updates are sent to defeat large routing loops.

port

1. Interface on an internetworking device (such as a router). 2. In IP terminology, an upper-layer process that receives information from lower layers. Ports are numbered, and each numbered port is associated with a specific process. For example, SMTP is associated with port 25. A port number is also known as a well-known address. 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.

PPP (Point-to-Point Protocol)

A successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP. See also CHAP; LCP; NCP; PAP; SLIP.

Presentation layer

Layer 6 of the OSI reference model. This layer ensures that information sent by the Application layer of one system will be readable by the Application layer of another. The Presentation layer is also concerned with the data structures used by programs and therefore negotiates data transfer syntax for the Application layer.

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.

protocol

Formal description of a set of rules and conventions that govern how devices on a network exchange information.

protocol stack

Set of related communications protocols that operate together and, as a group, address communication at some or all of the seven layers of the OSI reference model. Not every protocol stack covers each layer of the model, and often a single protocol in the stack will address a number of layers at once. TCP/IP is a typical protocol stack.

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. See also ARP.

PVC (permanent virtual circuit)

Permanently established virtual circuits save bandwidth in situations where certain virtual circuits must exist all the time, such as during circuit establishment and tear down.

Q.921

ITU (International Telecommunication Union) standard document for ISDN Layer 2 (Data Link layer).

Q.931

ITU (International Telecommunication Union) standard document for ISDN Layer 3.

query

Message used to inquire about the value of some variable or
set of variables.

queue

A backlog of packets stored in buffers and waiting to be forwarded over a router interface.

RAM (random-access memory)

Volatile memory that can be read and written by a computer.

reassembly

The putting back together of an IP datagram at the destination after it has been fragmented either at the source or at an intermediate node. See also fragmentation.

reload

The event of a Cisco router rebooting, or the command that causes the router to reboot.

reverse path forwarding

If a packet server receives a packet through different interfaces from the same source, the server drops all packets
after the first.

reverse Telnet

Using a router to connect to a serial device, frequently a modem, in order to connect out. For example, telnetting to a special port on an access router in order to access a modem to dial out. Called “reverse” because it's the opposite of the router's usual function, to accept calls into the modem.

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.

ring

Connection of two or more stations in a logically circular topology. Information is passed sequentially between active stations. Token Ring, FDDI, and CDDI are based on this topology.

ring topology

Network topology that consists of a series of repeaters connected to one another by unidirectional transmission links to form a single closed loop. Each station on the network connects to the network
at a repeater.

RIP (Routing Information Protocol)

A routing protocol for TCP/IP networks. The most common routing protocol in the Internet. RIP uses hop count as a routing metric.

RMON (Remote monitor)

A set of SNMP standards used to collect statistical network information. RMON is divided into groups, with each additional group providing more statistical information.

ROM (read-only memory)

Nonvolatile memory that can be read, but not written, by the computer.

routed protocol

Protocol that carries user data so it can be routed by a router. A router must be able to interpret the logical internetwork as specified by that routed protocol. Examples of routed protocols include AppleTalk, DECNet, and 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.

routing

Process of finding a path to a destination host.

routing metric

Method by which a routing algorithm determines preferability of one route over another. This information is stored in routing tables. Metrics include bandwidth, communication cost, delay,
hop count, load, MTU, path cost, and reliability. Sometimes referred to simply as a metric.

routing protocol

Protocol that accomplishes routing through the implementation of a specific routing algorithm. Examples of routing protocols include IGRP, OSPF, and RIP.

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.

routing update

Message sent from a router to indicate network reachability and associated cost information. Routing updates are typically sent at regular intervals and after a change in network topology. Compare with flash update.

RSRB (remote source-route bridging)

Equivalent to an SRB over WAN links.

RTMP (Routing Table Maintenance Protocol)

The protocol used by AppleTalk devices to communicate routing information. Structurally similar to RIP.

runts

Ethernet frames that are smaller than 64 bytes.

SAP (service access point)

1. Field defined by the IEEE 802.2 specification that is part of an address specification. Thus, the destination plus the DSAP define the recipient of a packet. The same applies to the SSAP. 2. Service Advertising Protocol. IPX protocol that provides a means of informing network routers and servers of the location of available network resources and services.

SAS (single attachment station)

Device attached to the primary ring of an FDDI ring. Also known as a Class B station.  See also DAS.

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. 3. Term used in the TCP specification to describe a single Transport layer unit of information.

serial transmission

Method of data transmission in which the bits of a data character are transmitted sequentially over a single channel.

session

1. Related set of communications transactions between two or more network devices. 2. In SNA, a logical connection that enables two NAUs to communicate.

Session layer

Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between Presentation layer entities. Corresponds to the data flow control layer of the SNA model. See also Application layer; Data Link layer; Network layer; Physical layer; Presentation layer; Transport layer.

sliding window flow control

Method of flow control in which a receiver gives a transmitter permission to transmit data until a window is full. When the window is full, the transmitter must stop transmitting until the receiver acknowledges some of the data, or advertises a larger window. TCP, other transport protocols, and several Data Link layer protocols use this method of flow control.

SLIP (Serial Line Internet Protocol)

Uses a variation of TCP/IP to make point-to-point serial connections. Succeeded by PPP.

SNAP (Subnetwork Access Protocol)

Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system. SNAP specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks.

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.

SNMP Manager

Software used to manage network devices via SNMP. Often includes graphical representation of the network and individual devices, and the ability to set and respond to SNMP traps.

SNMP Trap

A threshold of some sort which, when reached, causes the SNMP managed device to notify the SNMP Manager. This allows for immediate notification, instead of having to wait for the SNMP Manager
to poll again.

socket

Software structure operating as a communications endpoint within a network device.

SONET (Synchronous Optical Network)

High-speed synchronous network specification developed by Bellcore and designed to run on optical fiber.

source address

Address of a network device that is sending data.

spanning tree

Loop-free subset of a network topology. See also Spanning Tree Protocol.

Spanning Tree Protocol

Developed to eliminate loops in the network. The Spanning Tree Protocol ensures a loop-free path by placing one of the bridge ports in “blocking mode,” preventing the forwarding of packets.

SPF (shortest path first algorithm)

Routing algorithm that sorts routes by length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm.

SPIDs (Service Profile Identifiers)

These function as addresses for B channels on ISDN BRI circuits. When call information is passed over the D channel, the SPIDs are used to identify which channel is being referred to. SPIDs are usually some variant of the phone number for the channel.

split-horizon updates

Routing technique in which information about routes is prevented from being advertised out the router interface through which that information was received. Split-horizon updates are used to prevent routing loops.

SPX (Sequenced Packet Exchange)

Reliable, connection-oriented protocol at the Transport layer that supplements the datagram service provided by IPX.

SR/TLB (source-route translational bridging)

Method of bridging that allows source-route stations to communicate with transparent bridge stations, using an intermediate bridge that translates between the
two bridge protocols.

SRB (source-route bridging)

Method of bridging in Token Ring networks. In an SRB network, before data is sent to a destination, the entire route to that destination is predetermined in real time.

SRT (source-route transparent bridging)

IBM's merging of SRB and transparent bridging into one bridging scheme, which requires no translation between bridging protocols.

standard

Set of rules or procedures that are either widely used or officially specified.

star topology

LAN topology in which endpoints 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. Compare with bus topology, ring topology, and tree topology.

static route

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

subinterface

A virtual interface defined as a logical subdivision of a physical interface.

subnet address

Portion of an IP address that is specified as the subnetwork by the subnet mask. See also IP address; subnet mask; subnetwork.

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. See also address mask; IP address.

subnetwork

1. In IP networks, a network sharing a particular subnet address. 2. 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.

SVC (switched virtual circuit)

Virtual circuit that can be established dynamically on demand, and which is torn down after a transmission is complete. SVCs are used when data transmission is sporadic.

switch

1. 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. 2. General term applied to an electronic or mechanical device that allows a connection to be established as necessary and terminated when there is no longer a session to support.

T1

Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps through the telephone-switching network, using AMI or B8ZS coding. Compare with E1. See also AMI; B8ZS.

TCP (Transmission Control Protocol)

Connection-oriented Transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.

TCP/IP (Transmission Control Protocol/Internet Protocol)

Common name for the suite of protocols developed by the U.S. DoD in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.

TDR (time-domain reflectometer)

A TDR test is used to measure the length of a cable, or the distance to a break. This is accomplished by sending a signal down a wire, and measuring how long it takes for an echo of the signal to bounce back.

TEI (Terminal Endpoint Identifier)

Field in the LAPD address that identifies a device on an ISDN interface.

TFTP (Trivial File Transfer Protocol)

Simplified version of FTP that allows files to be transferred from one computer to another over a network.

three-way handshake

The three required packets to set up a TCP connection. It consists of a SYN packet, acknowledged by a SYN+ACK packet, which is finally acknowledged by an ACK packet. During this handshake, sequence numbers are exchanged.

throughput

Rate of information arriving at, and possibly passing through, a particular point in a network system.

timeout

Event that occurs when one network device expects to hear from another network device within a specified period of time, but does not. A timeout usually results in a retransmission of information or the termination of the session between the two devices.

token

Frame that contains only control information. Possession of the token allows a network device to transmit data onto the network. See also token passing.

token passing

Method by which network devices access the physical medium based on possession of a small frame called a token. Compare this method to circuit switching and contention.

Token Ring

Token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to IEEE 802.5. See also IEEE 802.5; ring topology; token passing.

TokenTalk

Apple Computer's data-link product that allows an AppleTalk network to be connected by Token Ring cables.

transparent bridging

Bridging scheme used in Ethernet and IEEE 802.3 networks. Allows bridges to pass frames along one hop at a time, based on tables that associate end nodes with bridge ports. Bridges are transparent to network end nodes.

Transport layer

Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The Transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control.

tree topology

A LAN topology that resembles a bus topology. Tree networks can contain branches with multiple nodes. In a tree topology, transmissions from a station propagate the length of the physical medium, and are received by all other stations.

twisted-pair

Relatively low-speed transmission medium consisting of two insulated wires arranged in a regular spiral pattern. The wires can be shielded or unshielded. Twisted-pair is common in telephony applications and is increasingly common in data networks.

UDP (User Datagram Protocol)

Connectionless Transport layer protocol in the TCP/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.

unicast

Regular IP packet sent from a single host to a single host.

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.

VCC (virtual channel connection)

Logical circuit for carrying data between two end points in an ATM network.

virtual circuit

Logical circuit created to ensure reliable communication between two network devices. A virtual circuit is defined by a VPI/VCI pair, and can be either permanent or switched. Virtual circuits are used in Frame Relay and X.25. In ATM, a virtual circuit is called a virtual channel. Sometimes abbreviated VC.

VLAN (virtual LAN)

Group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

VLSM (Variable-length Subnet Masking)

Ability to specify a different length subnet mask for the same network number at different locations in the network. VLSM can help optimize available address space.

VTY (Virtual Terminal)

VTYs work like physical terminal ports on routers so they can be managed across a network, usually via Telnet.

WAN (wide area network)

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. Compare with LAN and MAN.

wildcard mask

32-bit quantity used in conjunction with an IP address to determine which bits in an IP address should be matched and ignored when comparing that address with another IP address. A wildcard mask is specified when defining access list statements.

X.121

ITU-T standard describing an addressing scheme used in X.25 networks. X.121 addresses are sometimes called IDNs (International Data Numbers).

X.21

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

X.25

ITU-T standard that defines how connections between DTE
and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a Data Link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25.

ZIP broadcast storm

Occurs when a route advertisement without a corresponding zone triggers the network with a flood of Zone Information Protocol requests.

zone
In AppleTalk, a logical group of network devices.

Zone Information Protocol (ZIP)
A protocol used in AppleTalk to communicate information about AppleTalk zone names and cable ranges.

Zone Information Table (ZIT)
A table of zone name to cable range mappings in AppleTalk. These tables are maintained in each AppleTalk router.