CCDP: Cisco Internetwork Design Study Guide:Glossary

Table of Contents

A&B bit signaling: Used in T1 transmission facilities and sometimes called 24th channel signaling. Each of the 24 T1 subchannels in this procedure uses one bit of every sixth frame to send supervisory signaling information.
AAL: ATM Adaptation Layer: A service-dependent sublayer of the data-link layer, which accepts data from other applications and brings it to the ATM layer in 48-byte ATM payload segments. CS and SAR are the two sublayers that form AALs. Currently, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5. AALs are differentiated by the source-destination timing they use, whether they are CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transmission. See also: AAL1, AAL2, AAL3/4, AAL5, CS, and SAR, ATM, and ATM layer.
AAL1: ATM Adaptation Layer 1: One of four AALs recommended by the ITU-T, it is used for connection-oriented, time-sensitive services that need constant bit rates, such as isochronous traffic and uncompressed video. See also: AAL.
AAL2: ATM Adaptation Layer 2: One of four AALs recommended by the ITU-T, it is used for connection-oriented services that support a variable bit rate, such as voice traffic. See also: AAL.
AAL3/4: ATM Adaptation Layer 3/4: One of four AALs (a product of two initially distinct layers) recommended by the ITU-T, supporting both connectionless and connection-oriented links. Its primary use is in sending SMDS packets over ATM networks. See also: AAL.
AAL5: ATM Adaptation Layer 5: One of four AALs recommended by the ITU-T, it is used to support connection-oriented VBR services primarily to transfer classical IP over ATM and LANE traffic. This least complex of the AAL recommendations uses SEAL, offering lower bandwidth costs and simpler processing requirements but also providing reduced bandwidth and error-recovery capacities. See also: AAL.
AARP: AppleTalk Address Resolution Protocol: The protocol in an Apple-Talk stack that maps data-link addresses to network addresses.
AARP probe packets: Packets sent by the AARP to determine whether a given node ID is being used by another node in a nonextended AppleTalk network. If the node ID is not in use, the sending node appropriates that node’s ID. If the node ID is in use, the sending node will select a different ID and then send out more AARP probe packets. See also: AARP.
ABM: Asynchronous Balanced Mode: When two stations can initiate a transmission, ABM is an HDLC (or one of its derived protocols) communication technology that supports peer-oriented, point-to-point communications between both stations.
ABR: Area Border Router: An OSPF router that is located on the border of one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF backbone area.
access list: A set of test conditions kept by routers that determines “interesting traffic” to and from the router for various services on the network.
access method: The manner in which network devices approach gaining access to the network itself.
access server: Also known as a network access server, it is a communications process connecting asynchronous devices to a LAN or WAN through network and terminal emulation software, providing synchronous or asynchronous routing of supported protocols.
acknowledgement: Verification sent from one network device to another signifying that an event has occurred. May be abbreviated as ACK. Contrast with: NAK.
ACR: allowed cell rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled using congestion control measures, the ACR varies between the minimum cell rate (MCR) and the peak cell rate (PCR). See also: MCR and PCR.
active monitor: The mechanism used to manage a Token Ring. The network node with the highest MAC address on the ring becomes the active monitor and is responsible for management tasks such as preventing loops and ensuring tokens are not lost.
address mapping: By translating network addresses from one format to another, this methodology permits different protocols to operate inter-changeably.
address mask: A bit combination descriptor identifying which portion of an address refers to the network or subnet and which part refers to the host. Sometimes simply called the mask. See also: subnet mask.
address resolution: The process used for resolving differences between computer addressing schemes. Address resolution typically defines a method for tracing network layer (Layer 3) addresses to data-link layer (Layer 2) addresses. See also: address mapping.
adjacency: The relationship made between defined neighboring routers and end nodes, using a common media segment, to exchange routing information.
administrative distance: A number between 0 and 225 that expresses the value of trustworthiness of a routing information source. The lower the number, the higher the integrity rating.
administrative weight: A value designated by a network administrator to rate the preference given to a network link. It is one of four link metrics exchanged by PTSPs to test ATM network resource availability.
ADSU: ATM Data Service Unit: The terminal adapter used to connect to an ATM network through an HSSI-compatible mechanism. See also: DSU.
advertising: The process whereby routing or service updates are transmitted at given intervals, allowing other routers on the network to maintain a record of viable routes.
AEP: AppleTalk Echo Protocol: A test for connectivity between two Apple-Talk nodes where one node sends a packet to another and receives an echo, or copy, in response.
AFI: Authority and Format Identifier: The part of an NSAP ATM address that delineates the type and format of the IDI section of an ATM address. See also: IDI and NSAP.
AFP: AppleTalk Filing Protocol: A presentation-layer protocol, supporting AppleShare and Mac OS File Sharing, that permits users to share files and applications on a server.
AIP: ATM Interface Processor: Supporting AAL3/4 and AAL5, this inter-face for Cisco 7000 series routers minimizes performance bottlenecks at the UNI. See also: AAL3/4 and AAL5.
algorithm: A set of rules or process used to solve a problem. In networking, algorithms are typically used for finding the best route for traffic from a source to its destination.
alignment error: An error occurring in Ethernet networks, in which a received frame has extra bits; that is, a number not divisible by eight. Alignment errors are generally the result of frame damage caused by collisions.
all-routes explorer packet: An explorer packet that can move across an entire SRB network, tracing all possible paths to a given destination. Also known as an all-rings explorer packet. See also: explorer packet, local explorer packet, and spanning explorer packet.
AM: Amplitude Modulation: A modulation method that represents information by varying the amplitude of the carrier signal. See also: modulation.
AMI: Alternate Mark Inversion: A line-code type on T1 and E1 circuits that shows zeros as “01” during each bit cell, and ones as “11” or “00,” alternately, during each bit cell. The sending device must maintain ones density in AMI but not independently of the data stream. Also known as binary-coded, alternate mark inversion. Contrast with: B8ZS. See also: ones density.
amplitude: An analog or digital waveform’s highest value.
analog transmission-Signal: messaging whereby information is represented by various combinations of signal amplitude, frequency, and phase.
ANSI: American National Standards Institute: The organization of corporate, government, and other volunteer members that coordinates standardsrelated activities, approves U.S. national standards, and develops U.S. positions in international standards organizations. ANSI assists in the creation of international and U.S. standards in disciplines such as communications, networking, and a variety of technical fields. It publishes over 13,000 standards, for engineered products and technologies ranging from screw threads to networking protocols. ANSI is a member of the IEC and ISO. See also: IEC and ISO.
anycast: An ATM address that can be shared by more than one end system, allowing requests to be routed to a node that provides a particular service.
AppleTalk: Currently in two versions, the group of communication protocols designed by Apple Computer for use in Macintosh environments. The earlier Phase 1 protocols support one physical network with only one network number that resides in one zone. The later Phase 2 protocols support more than one logical network on a single physical network, allowing networks to exist in more than one zone. See also: zone.
application layer: Layer 7 of the OSI reference network model, supplying services to application procedures (such as electronic mail or file transfer) that are outside the OSI model. This layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery. See also: data-link layer, network layer, physical layer, presentation layer, session layer, and transport layer.
ARA: AppleTalk Remote Access: A protocol for Macintosh users establishing their access to resources and data from a remote AppleTalk location.
area: A logical, rather than physical, set of segments (based on either CLNS, DECnet, or OSPF) along with their attached devices. Areas are commonly connected to others using routers to create a single autonomous system. See also: autonomous system.
ARM: Asynchronous Response Mode: An HDLC communication mode using one primary station and at least one additional station, in which transmission can be initiated from either the primary or one of the secondary units.
ARP: Address Resolution Protocol: Defined in RFC 826, the protocol that traces IP addresses to MAC addresses. See also: RARP.
ASBR: Autonomous System Boundary Router: An area border router placed between an OSPF autonomous system and a non-OSPF network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area. See also: ABR, non-stub area, and OSPF.
ASCII: American Standard Code for Information Interchange: An eight-bit code for representing characters, consisting of seven data bits plus one parity bit.
ASN.1: Abstract Syntax Notation One: An OSI language used to describe types of data that is independent of computer structures and depicting methods. Described by ISO International Standard 8824.
ASP: AppleTalk Session Protocol: A protocol employing ATP to establish, maintain, and tear down sessions, as well as sequence requests. See also: ATP.
AST: Automatic Spanning Tree: A function that supplies one path for spanning explorer frames traveling from one node in the network to another, supporting the automatic resolution of spanning trees in SRB networks. AST is based on the IEEE 802.1 standard. See also: IEEE 802.1 and SRB.
asynchronous transmission: Digital signals sent without precise timing, usually with different frequencies and phase relationships. Asynchronous transmissions generally enclose individual characters in control bits (called start and stop bits) that show the beginning and end of each character. Contrast with: isochronous transmission and synchronous transmission.
ATCP: AppleTalk Control Program: The protocol for establishing and configuring AppleTalk over PPP, defined in RFC 1378. See also: PPP.
ATDM: Asynchronous Time-Division Multiplexing: A technique for sending information, it differs from normal TDM in that the time slots are assigned when necessary rather than preassigned to certain transmitters. Contrast with: FDM, statistical multiplexing, and TDM.
ATG: Address Translation Gateway: The mechanism within Cisco DECnet routing software that enables routers to route multiple, independent DECnet networks and to establish a user-designated address translation for chosen nodes between networks.
ATM: Asynchronous Transfer Mode: The international standard, identified by fixed-length 53-byte cells, for transmitting cells in multiple service systems, such as voice, video, or data. Transit delays are reduced because the fixed-length cells permit processing to occur in the hardware. ATM is designed to maximize the benefits of high-speed transmission media, such as SONET, E3, and T3.
ATM ARP server: A device that supplies logical subnets running classical IP over ATM with address-resolution services.
ATM endpoint: The initiating or terminating connection in an ATM network. ATM endpoints include servers, workstations, ATM-to-LAN switches, and ATM routers.
ATM Forum: The international organization founded jointly by Northern Telecom, Sprint, Cisco Systems, and NET/ADAPTIVE in 1991 to develop and promote standards-based implementation agreements for ATM technology. The ATM Forum broadens official standards developed by ANSI and ITU-T and creates implementation agreements before official standards are published.
ATM layer: A sublayer of the data-link layer in an ATM network that is service independent. To create standard 53-byte ATM cells, the ATM layer receives 48-byte segments from the AAL and attaches a 5-byte header to each. These cells are then sent to the physical layer for transmission across the physical medium. See also: AAL.
ATMM: ATM Management: A procedure that runs on ATM switches, managing rate enforcement and VCI translation. See also: ATM and VCI.
ATM user-user connection: A connection made by the ATM layer to supply communication between at least two ATM service users, such as ATMM processes. These communications can be uni- or bidirectional, using one or two VCCs, respectively. See also: ATM layer and ATMM.
ATP: AppleTalk Transaction Protocol: A transport-level protocol that enables reliable transactions between two sockets, where one requests the other to perform a given task and to report the results. ATP fastens the request and response together, assuring a loss-free exchange of request-response pairs.
attenuation: In communication, weakening or loss of signal energy, typically caused by distance.
AURP: AppleTalk Update-based Routing Protocol: A technique for encapsulating AppleTalk traffic in the header of a foreign protocol that allows the connection of at least two noncontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) to create an AppleTalk WAN. The connection made is called an AURP tunnel. By exchanging routing information between exterior routers, the AURP maintains routing tables for the complete AppleTalk WAN. See also: AURP tunnel and exterior router.
AURP tunnel: A connection made in an AURP WAN that acts as a single, virtual link between AppleTalk internetworks separated physically by a foreign network such as a TCP/IP network. See also: AURP.
authority zone: A portion of the domain-name tree associated with DNS for which one name server is the authority. See also: DNS.
automatic call reconnect: A function that enables automatic call rerouting away from a failed trunk line.
autonomous confederation: A collection of self-governed systems that depend more on their own network accessibility and routing information than on information received from other systems or groups.
autonomous switching: The ability of Cisco routers to process packets more quickly by using the ciscoBus to switch packets independently of the system processor.
autonomous system (ASA: group of networks under mutual administration that share the same routing methodology. Autonomous systems are subdivided by areas and must be assigned an individual 16-bit number by the IANA. See also: area and IANA.
autoreconfiguration: A procedure executed by nodes within the failure domain of a Token Ring, wherein nodes automatically perform diagnostics, trying to reconfigure the network around failed areas.
B8ZS: Binary 8-Zero Substitution: A line-code type, interpreted at the remote end of the connection, that uses a special code substitution whenever eight consecutive zeros are transmitted over the link on T1 and E1 circuits. This technique assures ones density independent of the data stream. Also known as bipolar 8-zero substitution. Contrast with: AMI. See also: ones density.
backbone: The basic portion of the network that provides the primary path for traffic sent to and initiated from other networks.
back end: A node or software program supplying services to a front end. See also: client, front end, and server.
bandwidth: The gap between the highest and lowest frequencies employed by network signals. More commonly, it refers to the rated throughput capacity of a network protocol or medium.
baseband: A feature of a network technology that uses only one carrier frequency, for example Ethernet. Also named narrowband. Compare with: broadband.
baud: Synonymous with bits per second (bps), if each signal element represents one bit. It is a unit of signaling speed equivalent to the number of separate signal elements transmitted per second.
B channel: Bearer channel: A full-duplex, 64Kbps channel in ISDN that transmits user data. Compare with: D channel, E channel, and H channel.
beacon: An FDDI device or Token Ring frame that points to a serious problem with the ring, such as a broken cable. The beacon frame carries the address of the station thought to be down. See also: failure domain.
BECN: Backward Explicit Congestion Notification: BECN is the bit set by a Frame Relay network in frames moving away from frames headed into a congested path. A DTE that receives frames with the BECN may ask higher-level protocols to take necessary flow-control measures. Compare with: FECN.
BGP4: BGP Version 4: Version 4 of the interdomain routing protocol most commonly used on the Internet. BGP4 supports CIDR and uses route-counting mechanisms to decrease the size of routing tables. See also: CIDR.
binary: A two-character numbering method that uses ones and zeros. The binary numbering system underlies all digital representation of information.
BIP: Bit Interleaved Parity: A method used in ATM to monitor errors on a link, sending a check bit or word in the link overhead for the previous block or frame. This allows bit errors in transmissions to be found and delivered as maintenance information.
BISDN: Broadband ISDN: ITU-T standards created to manage high-bandwidth technologies such as video. BISDN presently employs ATM technology along SONET-based transmission circuits, supplying data rates between 155Mbps and 622Mbps and beyond. Contrast with N-ISDN. See also: BRI, ISDN, and PRI.
bit-oriented protocol: Regardless of frame content, the class of data-link layer communication protocols that transmits frames. Bit-oriented protocols, as compared with byte-oriented, supply more efficient and trustworthy, full-duplex operation. Compare with: byte-oriented protocol.
border gateway: A router that facilitates communication with routers in different autonomous systems.
BPDU: Bridge Protocol Data Unit: A Spanning-Tree Protocol initializing packet that is sent at definable intervals for the purpose of exchanging information among bridges in networks.
BRI: Basic Rate Interface: The ISDN interface that facilitates circuit-switched communication between video, data, and voice; it is made up of two B channels (64Kbps each) and one D channel (16Kbps). Compare with: PRI. See also: BISDN, ISN.
bridge: A device for connecting two segments of a network and transmitting packets between them. Both segments must use identical protocols to communicate. Bridges function at the data-link layer, Layer 2 of the OSI reference model. The purpose of a bridge is to filter, send, or flood any incoming frame, based on the MAC address of that particular frame.
broadband: A transmission methodology for multiplexing several independent signals onto one cable. In telecommunications, broadband is classified as any channel with bandwidth greater than 4kHz (typical voice grade). In LAN terminology, it is classified as a coaxial cable on which analog signaling is employed. Also known as wideband. Contrast with: baseband.
broadcast: A data frame or packet that is transmitted to every node on the local network segment (as defined by the broadcast domain). Broadcasts are known by their broadcast address, which is a destination network and host address with all the bits turned on. Also called local broadcast. Compare with: directed broadcasts.
broadcast domain: A group of devices receiving broadcast frames initiating from any device within the group. Because they do not forward broadcast frames, broadcast domains are generally surrounded by routers.
broadcast storm: An undesired event on the network caused by the simultaneous transmission of any number of broadcasts across the network segment. Such an occurrence can overwhelm network bandwidth, resulting in time-outs.
buffer: A storage area dedicated to handling data while in transit. Buffers are used to receive/store sporadic deliveries of data bursts, usually received from faster devices, compensating for the variations in processing speed. Incoming information is stored until everything is received prior to sending data on. Also known as an information buffer.
bus topology-A: linear LAN architecture in which transmissions from various stations on the network are reproduced over the length of the medium and are accepted by all other stations. Compare with: ring, star, and tree topologies.
bus: Any physical path, typically wires or copper, through which a digital signal can be used to send data from one part of a computer to another.
BUS: broadcast and unknown server: In LAN emulation, the hardware or software responsible for resolving all broadcasts and packets with unknown (unregistered) addresses into the point-to-point virtual circuits required by ATM. See also: LEC, LECS, LES, and LANE.
BX.25: AT&T’s use of X.25. See also: X.25.
bypass mode: An FDDI and Token Ring network operation that deletes an interface.
bypass relay: A device that enables a particular interface in the Token Ring to be closed down and effectively taken off the ring.
byte-oriented protocol: Any type of data-link communication protocol that, in order to mark the boundaries of frames, uses a specific character from the user character set. These protocols have generally been superseded by bit-oriented protocols. Compare with: bit-oriented protocol.
cable range: In an extended AppleTalk network, the range of numbers allotted for use by existing nodes on the network. The value of the cable range can be anywhere from a single to a sequence of several touching network numbers. Node addresses are determined by their cable range value.
CAC: Connection Admission Control: The sequence of actions executed by every ATM switch while connection setup is performed in order to determine if a request for connection is violating the guarantees of QoS for established connections. Also, CAC is used to route a connection request through an ATM network.
call admission control: A device for managing of traffic in ATM networks, determining the possibility of a path containing adequate bandwidth for a requested VCC.
call priority: In circuit-switched systems, the defining priority given to each originating port; it specifies in which order calls will be reconnected. Additionally, call priority identifies which calls are allowed during a bandwidth reservation.
call set-up time: The length of time necessary to effect a switched call between DTE devices.
CBR: Constant Bit Rate: An ATM Forum QoS class created for use in ATM networks. CBR is used for connections that rely on precision clocking to guarantee trustworthy delivery. Compare with: ABR and VBR.
CD: Carrier Detect: A signal indicating that an interface is active or that a connection generated by a modem has been established.
CDP: Cisco Discovery Protocol: Cisco’s proprietary protocol that is used to tell a neighbor Cisco device about the type of hardware, software version, and active interfaces that the Cisco device is using. It uses a SNAP frame between devices and is not routable.
CDVT: Cell Delay Variation Tolerance: A QoS parameter for traffic management in ATM networks specified when a connection is established. The allowable fluctuation levels for data samples taken by the PCR in CBR transmissions are determined by the CDVT. See also: CBR and PCR.
cell: In ATM networking, the basic unit of data for switching and multiplexing. Cells have a defined length of 53 bytes, including a 5-byte header that identifies the cell’s data stream and 48 bytes of payload. See also: cell relay.
cell payload scrambling: The method by which an ATM switch maintains framing on some medium-speed edge and trunk interfaces (T3 or E3 circuits). Cell payload scrambling rearranges the data portion of a cell to maintain the line synchronization with certain common bit patterns.
cell relay: A technology that uses small packets of fixed size, known as cells. Their fixed length enables cells to be processed and switched in hardware at high speeds, making this technology the foundation for ATM and other high-speed network protocols. See also: cell.
Centrex: A local exchange carrier service, providing local switching that resembles that of an on-site PBX. Centrex has no on-site switching capability. Therefore, all customer connections return to the CO. See also: CO.
CER: Cell Error Ratio: The ratio in ATM of transmitted cells having errors to the total number of cells sent in a transmission within a certain span of time.
channelized E1: Operating at 2.048Mpbs, an access link that is sectioned into 29 B channels and one D channel, supporting DDR, Frame Relay, and X.25. Compare with: channelized T1.
channelized T1: Operating at 1.544Mbps, an access link that is sectioned into 23 B channels and one D channel of 64Kbps each, where individual channels or groups of channels connect to various destinations, supporting DDR, Frame Relay, and X.25. Compare with: channelized E1.
CHAP: Challenge Handshake Authentication Protocol: Supported on lines using PPP encapsulation, it is a security feature that identifies the remote end, helping keep out unauthorized users. After CHAP is performed, the router or access server determines whether a given user is permitted access. It is a newer, more secure protocol than PAP. Compare with: PAP.
checksum: A test for ensuring the integrity of sent data. It is a number calculated from a series of values taken through a sequence of mathematical functions, typically placed at the end of the data from which it is calculated, and then recalculated at the receiving end for verification. Compare with: CRC.
choke packet: When congestion exists, it is a packet sent to inform a transmitter that it should decrease its sending rate.
CIDR: Classless Interdomain Routing: A method supported by classless routing protocols, such as OSPF and BGP4, based on the concept of ignoring the IP class of address, permitting route aggregation and VLSM that enable routers to combine routes in order to minimize the routing information that needs to be conveyed by the primary routers. It allows a group of IP networks to appear to other networks as a unified, larger entity. In CIDR, IP addresses and their subnet masks are written as four dotted octets, followed by a forward slash and the numbering of masking bits (a form of subnet notation shorthand). See also: BGP4.
CIP: Channel Interface Processor: A channel attachment interface for use in Cisco 7000 series routers that connects a host mainframe to a control unit. This device eliminates the need for an FBP to attach channels.
CIR: Committed Information Rate: Averaged over a minimum span of time and measured in bps, a Frame Relay network’s agreed-upon minimum rate of transferring information.
Cisco FRADCisco: Frame Relay access device: A Cisco product that supports Cisco IPS Frame Relay SNA services, connecting SDLC devices to Frame Relay without requiring an existing LAN. May be upgraded to a fully functioning multiprotocol router. Can activate conversion from SDLC to Ethernet and Token Ring, but does not support attached LANs. See also: FRAD.
CiscoFusion: Cisco’s name for the internetworking architecture under which its Cisco IOS operates. It is designed to “fuse” together the capabilities of its disparate collection of acquired routers and switches.
Cisco IOS software: Cisco Internet Operating System software. The kernel of the Cisco line of routers and switches that supplies shared functionality, scalability, and security for all products under its CiscoFusion architecture. See also: CiscoFusion.
CiscoView: GUI-based management software for Cisco networking devices, enabling dynamic status, statistics, and comprehensive configuration information. Displays a physical view of the Cisco device chassis and provides device-monitoring functions and fundamental troubleshooting capabilities. May be integrated with a number of SNMP-based network management platforms.
classical IP over ATM: Defined in RFC 1577, the specification for running IP over ATM that maximizes ATM features. Also known as CIA.
CLP: Cell Loss Priority: The area in the ATM cell header that determines the likelihood of a cell being dropped during network congestion. Cells with CLP = 0 are considered insured traffic and are not apt to be dropped. Cells with CLP = 1 are considered best-effort traffic that may be dropped during congested episodes, delivering more resources to handle insured traffic.
CLR: Cell Loss Ratio: The ratio of discarded cells to successfully delivered cells in ATM. CLR can be designated a QoS parameter when establishing a connection.
CO: Central Office: The local telephone company office where all loops in a certain area connect and where circuit switching of subscriber lines occurs.
collapsed backbone: A nondistributed backbone where all network segments are connected to each other through an internetworking device. A collapsed backbone can be a virtual network segment at work in a device such as a router, hub, or switch.
collision: The effect of two nodes sending transmissions simultaneously in Ethernet. When they meet on the physical media, the frames from each node collide and are damaged. See also: collision domain.
collision domain: The network area in Ethernet over which frames that have collided will spread. Collisions are propagated by hubs and repeaters, but not by LAN switches, routers, or bridges. See also: collision.
configuration register: A 16-bit configurable value stored in hardware or software that determines how Cisco routers function during initialization. In hardware, the bit position is set using a jumper. In software, it is set by spec-ifying specific bit patterns used to set startup options, configured using a hexadecimal value with configuration commands.
congestion: Traffic that exceeds the network’s ability to handle it.
congestion avoidance: To minimize delays, the method an ATM network uses to control traffic entering the system. Lower-priority traffic is discarded at the edge of the network when indicators signal it cannot be delivered, thus using resources efficiently.
congestion collapse: The situation that results from the retransmission of packets in ATM networks where little or no traffic successfully arrives at destination points. It usually happens in networks made of switches with ineffective or inadequate buffering capabilities combined with poor packet discard or ABR congestion feedback mechanisms.
connectionless: Data transfer that occurs without the creating of a virtual circuit. No overhead, best-effort delivery, not reliable. Contrast with: connection-oriented. See also: virtual circuit.
connection-oriented: Data transfer method that sets up a virtual circuit before any data is transferred. Uses acknowledgments and flow control for reliable data transfer. Contrast with: connectionless. See also: virtual circuit.
control direct VCC: One of three control connections defined by Phase I LAN Emulation; a bi-directional virtual control connection (VCC) established in ATM by an LEC to an LES. See also: control distribute VCC.
control distribute VCC: One of three control connections defined by Phase 1 LAN Emulation; a unidirectional virtual control connection (VCC) set up in ATM from an LES to an LEC. Usually, the VCC is a point-tomultipoint connection. See also: control direct VCC.
convergence: The process required for all routers in an internetwork to update their routing tables and create a consistent view of the network, using the best possible paths. No user data is passed during a convergence time.
cost: Also known as path cost, an arbitrary value, based on hop count, bandwidth, or other calculation, that is typically assigned by a network administrator and used by the routing protocol to compare different routes through an internetwork. Routing protocols use cost values to select the best path to a certain destination: the lowest cost identifies the best path. See also: routing metric.
count to infinity: A problem occurring in routing algorithms that are slow to converge where routers keep increasing the hop count to particular networks. To avoid this problem, various solutions have been implemented into each of the different routing protocols. Some of those solutions include defining a maximum hop count (defining infinity), route poisoning, poison reverse, and split-horizon.
CPCS: Common Part Convergence Sublayer: One of two AAL sublayers that is service-dependent, it is further segmented into the CS and SAR sublayers. The CPCS prepares data for transmission across the ATM network; it creates the 48-byte payload cells that are sent to the ATM layer. See also: AAL and ATM layer.
CPE: Customer Premises Equipment: Items such as telephones, modems, and terminals installed at customer locations and connected to the telephone company network.
crankback: In ATM, a correction technique used when a node somewhere on a chosen path cannot accept a connection setup request, blocking the request. The path is rolled back to an intermediate node, which then uses GCAC to attempt to find an alternate path to the final destination.
CRC: Cyclical Redundancy Check: A methodology that detects errors, whereby the frame recipient makes a calculation by dividing frame contents with a prime binary divisor and compares the remainder to a value stored in the frame by the sending node. Contrast with: checksum.
CSMA/CD: Carrier Sense Multiple Access Collision Detect: A technology defined by the Ethernet IEEE 802.3 committee. Each device senses the cable for a digital signal before transmitting. Also, CSMA/CD allows all devices on the network to share the same cable, but one at a time. If two devices transmit at the same time, a frame collision will occur and a jamming pattern will be sent; the devices will stop transmitting, wait a predetermined amount of time, and then try to transmit again.
CSU: Channel Service Unit: A digital mechanism that connects end-user equipment to the local digital telephone loop. Frequently referred to along with the data service unit as CSU/DSU. See also: DSU.
CTD: Cell Transfer Delay: For a given connection in ATM, the time period between a cell exit event at the source user-network interface (UNI) and the corresponding cell entry event at the destination. The CTD between these points is the sum of the total inter-ATM transmission delay and the total ATM processing delay.
cut-through frame switching: A frame-switching technique that flows data through a switch so that the leading edge exits the switch at the output port before the packet finishes entering the input port. Frames will be read, processed, and forwarded by devices that use cut-through switching as soon as the destination address of the frame is confirmed and the outgoing port is identified.
data direct VCC: A bidirectional point-to-point virtual control connection (VCC) set up between two LECs in ATM and one of three data connections defined by Phase 1 LAN Emulation. Because data direct VCCs do not guarantee QoS, they are generally reserved for UBR and ABR connections. Compare with: control distribute VCC and control direct VCC.
datagram: A logical collection of information transmitted as a network layer unit over a medium without a previously established virtual circuit. IP datagrams have become the primary information unit of the Internet. At various layers of the OSI reference model, the terms cell, frame, message, packet, and segment also define these logical information groupings.
data-link control layer: Layer 2 of the SNA architectural model, it is responsible for the transmission of data over a given physical link and compares somewhat to the data-link layer of the OSI model.
data-link layer: Layer 2 of the OSI reference model, it ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error notification, ordered delivery of frames, and flow control. The IEEE has further segmented this layer into the MAC sublayer and the LLC sublayer. Also known as the link layer. Can be compared somewhat to the data-link control layer of the SNA model. See also: application layer, LLC, MAC, network layer, physical layer, presentation layer, session layer, and transport layer.
DCC: Data Country Code: Developed by the ATM Forum, one of two ATM address formats designed for use by private networks. Compare with: ICD.
DCE: data communications equipment (as defined by the EIA) or data circuit-terminating equipment (as defined by the ITU-T): The mechanisms and links of a communications network that make up the network portion of the user-to-network interface, such as modems. The DCE supplies the physical connection to the network, forwards traffic, and provides a clocking signal to synchronize data transmission between DTE and DCE devices. Compare with: DTE.
D channel: 1. Data channel: A full-duplex, 16Kbps (BRI) or 64Kbps (PRI) ISDN channel. Compare with: B channel, E channel, and H channel. 2. In SNA, anything that provides a connection between the processor and main storage with any peripherals.
DDP: Datagram Delivery Protocol : Used in the AppleTalk suite of protocols as a connectionless protocol that is responsible for sending datagrams through an internetwork.
DDR: dial-on-demand routing: A technique that allows a router to automatically initiate and end a circuit-switched session per the requirements of the sending station. By mimicking keepalives, the router fools the end station into treating the session as active. DDR permits routing over ISDN or telephone lines via a modem or external ISDN terminal adapter.
default route: The static routing table entry used to direct frames whose next hop is not spelled out in the dynamic routing table.
delay: The time elapsed between a sender’s initiation of a transaction and the first response they receive. Also, the time needed to move a packet from its source to its destination over a path. See also: latency.
demarc: The demarcation point between the customer premises equipment (CPE) and the telco’s carrier equipment.
demodulation: A series of steps that return a modulated signal to its original form. When receiving, a modem demodulates an analog signal to its original digital form (and, conversely, modulates the digital data it sends into an analog signal). See also: modulation.
demultiplexing: The process of converting a single multiplex signal, comprising more than one input stream, back into separate output streams. See also: multiplexing.
designated bridge: In the process of forwarding a frame from a segment to the route bridge, the bridge with the lowest path cost.
designated router: An OSPF router that creates LSAs for a multiaccess network and is required to perform other special tasks in OSPF operations. Multiaccess OSPF networks that maintain a minimum of two attached routers identify one router that is chosen by the OSPF Hello protocol, which makes possible a decrease in the number of adjacencies necessary on a multi-access network. This in turn reduces the quantity of routing protocol traffic and the physical size of the database.
destination address: The address for the network devices that will receive a packet.
directed broadcast: A data frame or packet that is transmitted to a specific group of nodes on a remote network segment. Directed broadcasts are known by their broadcast address, which is a destination subnet address with all the bits turned on. Compare with: local broadcasts.
discovery mode: Also known as dynamic configuration, this technique is used by an AppleTalk interface to gain information from a working node about an attached network. The information is subsequently used by the interface for self-configuration.
distance-vector routing algorithm: In order to find the shortest path, this group of routing algorithms repeats on the number of hops in a given route, requiring each router to send its complete routing table with each update, but only to its neighbors. Routing algorithms of this type tend to generate loops, but they are fundamentally simpler than their link-state counterparts. See also: link-state routing algorithm and SPF.
DLCI: Data-Link Connection Identifier: Used to identify virtual circuits in a Frame Relay network.
DNS: Domain Name System: Used to resolve host names to IP addresses.
DSAP: Destination Service Access Point: The service access point of a network node, specified in the destination field of a packet. See also: SSAP and SAP.
DSR: Data Set Ready: When a DCE is powered up and ready to run, this EIA/TIA-232 interface circuit is also engaged.
DSU: Data Service Unit: This device is used to adapt the physical interface on a data terminal equipment (DTE) mechanism to a transmission facility such as T1 or E1 and is also responsible for signal timing. It is commonly grouped with the channel service unit and referred to as the CSU/DSU. See also: CSU.
DTE: data terminal equipment: Any device located at the user end of a user-network interface serving as a destination, a source, or both. DTE includes devices such as multiplexers, protocol translators, and computers. The connection to a data network is made through data channel equipment (DCE) such as a modem, using the clocking signals generated by that device. See also: DCE.
DTR: data terminal ready: An activated EIA/TIA-232 circuit communicating to the DCE the state of preparedness of the DTE to transmit or receive data.
DUAL: Diffusing Update Algorithm: Used in Enhanced IGRP, this convergence algorithm provides loop-free operation throughout an entire route’s computation. DUAL grants routers involved in a topology revision the ability to synchronize simultaneously, while routers unaffected by this change are not involved. See also: Enhanced IGRP.
DVMRP: Distance Vector Multicast Routing Protocol: Based primarily on the Routing Information Protocol (RIP), this Internet gateway protocol implements a common, condensed-mode IP multicast scheme, using IGMP to transfer routing datagrams between its neighbors. See also: IGMP.
DXI: Data Exchange Interface: Described in RFC 1482, DXI defines the effectiveness of a network device such as a router, bridge, or hub to act as an FEP to an ATM network by using a special DSU that accomplishes packet encapsulation.
dynamic routing-Also: known as adaptive routing, this technique automatically adapts to traffic or physical network revisions.
E1: Generally used in Europe, a wide-area digital transmission scheme carrying data at 2.048Mbps. E1 transmission lines are available for lease from common carriers for private use.
E.164: 1. Evolved from standard telephone numbering system, the standard recommended by ITU-T for international telecommunication numbering, particularly in ISDN, SMDS, and BISDN. 2. Label of field in an ATM address containing numbers in E.164 format.
E channel-Echo: channel: A 64Kbps ISDN control channel used for circuit switching. Specific description of this channel can be found in the 1984 ITU-T ISDN specification, but was dropped from the 1988 version. See also: B, D, and H channels.
edge device: A device that enables packets to be forwarded between legacy interfaces (such as Ethernet and Token Ring) and ATM interfaces based on information in the data-link and network layers. An edge device does not take part in the running of any network layer routing protocol; it merely uses the route description protocol in order to get the forwarding information required.
EEPROM: Electronically Erasable Programmable Read-Only Memory: Programmed after their manufacture, these nonvolatile memory chips can be erased if necessary using electric power and reprogrammed. See also: EPROM, PROM.
EFCI: Explicit Forward Congestion Indication: A congestion feedback mode permitted by ABR service in an ATM network. The EFCI may be set by any network element that is in a state of immediate or certain congestion. The destination end system is able to carry out a protocol that adjusts and lowers the cell rate of the connection based on value of the EFCI. See also: ABR.
EIGRP: See: Enhanced IGRP.
EIP: Ethernet Interface Processor: A Cisco 7000 series router interface processor card, supplying 10Mbps AUI ports to support Ethernet Version 1 and Ethernet Version 2 or IEEE 802.3 interfaces with a high-speed data path to other interface processors.
ELAN: Emulated LAN: An ATM network configured using a client/server model in order to emulate either an Ethernet or Token Ring LAN. Multiple ELANs can exist at the same time on a single ATM network and are made up of an LAN Emulation Client (LEC), an LAN Emulation Server (LES), a broadcast and unknown server (BUS), and an LAN Emulation Configuration Server (LECS). ELANs are defined by the LANE specification. See also: LANE, LEC, LECS, and LES.
ELAP: EtherTalk Link Access Protocol: In an EtherTalk network, the link-access protocol constructed above the standard Ethernet data-link layer.
encapsulation: The technique used by layered protocols in which a layer adds header information to the protocol data unit (PDU) from the layer above. As an example, in Internet terminology, a packet would contain a header from the physical layer, followed by a header from the network layer (IP), followed by a header from the transport layer (TCP), followed by the application protocol data.
encryption: The conversion of information into a scrambled form that effectively disguises it to prevent unauthorized access. Every encryption scheme uses some well-defined algorithm, which is reversed at the receiving end by an opposite algorithm in a process known as decryption.
Enhanced IGRP: Enhanced Interior Gateway Routing Protocol: An advanced routing protocol created by Cisco, combining the advantages of link-state and distance-vector protocols. Enhanced IGRP has superior convergence attributes, including high operating efficiency. See also: IGP, OSPF, and RIP.
enterprise network: A privately owned and operated network that joins most major locations in a large company or organization.
EPROM: Erasable Programmable Read-Only Memory: Programmed after their manufacture, these nonvolatile memory chips can be erased if necessary using high-power light and reprogrammed. See also: EEPROM, PROM.
ESF: Extended Superframe: Made up of 24 frames with 192 bits each, with the 193rd bit providing other functions including timing. This is an enhanced version of SF. See also: SF.
Ethernet: A baseband LAN specification created by the Xerox Corporation and then improved through joint efforts of Xerox, Digital Equipment Corporation, and Intel. Ethernet is similar to the IEEE 802.3 series standard and, using CSMA/CD, operates over various types of cables at 10Mbps. Also called DIX (Digital/Intel/Xerox) Ethernet. See also: 10BaseT, Fast Ethernet, and IEEE.
EtherTalk: A data-link product from Apple Computer that permits Apple-Talk networks to be connected by Ethernet.
excess rate: In ATM networking, traffic exceeding a connection’s insured rate. The excess rate is the maximum rate less the insured rate. Depending on the availability of network resources, excess traffic can be discarded during congestion episodes. Compare with: maximum rate.
expansion: The procedure of directing compressed data through an algorithm, restoring information to its original size.
expedited delivery: An option that can be specified by one protocol layer, communicating either with other layers or with the identical protocol layer in a different network device, requiring that identified data be processed faster.
explorer packet: An SNA packet transmitted by a source Token Ring device to find the path through a source-route-bridged network.
failure domain: The region in which a failure has occurred in a Token Ring. When a station gains information that a serious problem, such as a cable break, has occurred with the network, it sends a beacon frame that includes the station reporting the failure, its NAUN, and everything between. This defines the failure domain. Beaconing then initiates the procedure known as autoreconfiguration. See also: autoreconfiguration and beacon.
fallback: In ATM networks, this mechanism is used for scouting a path if it isn’t possible to locate one using customary methods. The device relaxes requirements for certain characteristics, such as delay, in an attempt to find a path that meets a certain set of the most important requirements.
FastEthernet: Any Ethernet specification with a speed of 100Mbps. FastEthernet is ten times faster than 10BaseT, while retaining qualities like MAC mechanisms, MTU, and frame format. These similarities make it possible for existing 10BaseT applications and management tools to be used on FastEthernet networks. FastEthernet is based on an extension of IEEE 802.3 specification (IEEE 802.3u). Compare with: Ethernet. See also: 100BaseT, 100BaseTX, and IEEE.
fast switching: A Cisco feature that uses a route cache to speed packet switching through a router. Contrast with: process switching.
FDM: Frequency-Division Multiplexing: A technique that permits information from several channels to be assigned bandwidth on one wire based on frequency. See also: TDM, ATDM, and statistical multiplexing.
FDDI: Fiber Distributed Data Interface: A LAN standard, defined by ANSI X3T9.5, that can run at speeds up to 200Mbps and uses token-passing media access on fiber-optic cable. For redundancy, FDDI can use a dual-ring architecture.
FECN: Forward Explicit Congestion Notification: A bit set by a Frame Relay network that informs the DTE receptor that congestion was encountered along the path from source to destination. A device receiving frames with the FECN bit set can ask higher-priority protocols to take flow-control action as needed. See also: BECN.
FEIP: FastEthernet Interface Processor: An interface processor employed on Cisco 7000 series routers, supporting up to two 100Mbps 100BaseT ports.
firewall: A barrier purposefully erected between any connected public networks and a private network, made up of a router or access server or several routers or access servers, that uses access lists and other methods to ensure the security of the private network.
flash memory: Developed by Intel and licensed to other semiconductor manufacturers, it is nonvolatile storage that can be erased electronically and reprogrammed, physically located on an EEPROM chip. Flash memory permits software images to be stored, booted, and rewritten as needed. Cisco routers and switches use flash memory to hold the IOS by default. See also: EPROM, EEPROM.
flooding: When traffic is received on an interface, it is then transmitted to every interface connected to that device with exception of the interface from which the traffic originated. This technique can be used for traffic transfer by bridges and switches throughout the network.
flow control: A methodology used to ensure that receiving units are not overwhelmed with data from sending devices. Pacing, as it is called in IBM networks, means that when buffers at a receiving unit are full, a message is transmitted to the sending unit to temporarily halt transmissions until all the data in the receiving buffer has been processed and the buffer is again ready for action.
FRAD: Frame Relay access device: Any device affording a connection between a LAN and a Frame Relay WAN. See also: Cisco FRAD, FRAS.
fragment: Any portion of a larger packet that has been intentionally segmented into smaller pieces. A packet fragment does not necessarily indicate an error and can be intentional. See also: fragmentation.
fragmentation: The process of intentionally segmenting a packet into smaller pieces when sending data over an intermediate network medium that cannot support the larger packet size.
frame: A logical unit of information sent by the data-link layer over a transmission medium. The term often refers to the header and trailer, employed for synchronization and error control, that surround the data contained in the unit.
Frame Relay: A more efficient replacement of the X.25 protocol (an unrelated packet relay technology that guarantees data delivery). Frame Relay is an industry-standard, shared-access, best-effort, switched data-link layer encapsulation that services multiple virtual circuits and protocols between connected mechanisms.
Frame Relay bridging: Defined in RFC 1490, this bridging method uses the identical spanning-tree algorithm as other bridging operations but permits packets to be encapsulated for transmission across a Frame Relay network.
FRAS: Frame Relay access support: A feature of Cisco IOS software that enables SDLC, Ethernet, Token Ring, and Frame Relay-attached IBM devices to be linked with other IBM mechanisms on a Frame Relay network. See also: FRAD.
frequency: The number of cycles of an alternating current signal per time unit, measured in Hertz (cycles per second).
FSIP: Fast Serial Interface Processor: The Cisco 7000 routers’ default serial interface processor, it provides four or eight high-speed serial ports.
FTP: File Transfer Protocol: The TCP/IP protocol used for transmitting files between network nodes, it supports a broad range of file types and is defined in RFC 959. See also: TFTP.
full duplex: The capacity to transmit information between a sending station and a receiving unit at the same time. See also: half duplex.
full mesh: A type of network topology where every node has either a physical or a virtual circuit linking it to every other network node. A full mesh supplies a great deal of redundancy but is typically reserved for network backbones because of its expense. See also: partial mesh.
GNS: Get Nearest Server: On an IPX network, a request packet sent by a customer for determining the location of the nearest active server of a given type. An IPX network client launches a GNS request to get either a direct answer from a connected server or a response from a router disclosing the location of the service on the internetwork to the GNS. GNS is part of IPX and SAP. See also: IPX and SAP.
GRE: Generic Routing Encapsulation: A tunneling protocol created by Cisco with the capacity for encapsulating a wide variety of protocol packet types inside IP tunnels, thereby generating a virtual point-to-point connection to Cisco routers across an IP network at remote points. IP tunneling using GRE permits network expansion across a single-protocol backbone environment by linking multiprotocol subnetworks in a single-protocol backbone environment.
guard band: The unused frequency area found between two communications channels, furnishing the space necessary to avoid interference between the two.
half duplex: The capacity to transfer data in only one direction at a time between a sending unit and receiving unit. See also: full duplex.
handshake: Any series of transmissions exchanged between two or more devices on a network to ensure synchronized operations.
H channel: High-speed channel: A full-duplex, ISDN primary rate channel operating at a speed of 384Kbps. See also: B, D, and E channels.
HDLC: High-Level Data-Link Control: Using frame characters, including checksums, HDLC designates a method for data encapsulation on synchronous serial links and is the default encapsulation for Cisco routers. HDLC is a bit-oriented synchronous data-link layer protocol created by ISO and derived from SDLC. However, most HDLC vendor implementations (including Cisco’s) are proprietary. See also: SDLC.
helper address: The unicast address specified, which instructs the Cisco router to change the client’s local broadcast request for a service into a directed unicast to the server.
hierarchical addressing: Any addressing plan employing a logical chain of commands to determine location. IP addresses are made up of a hierarchy of network numbers, subnet numbers, and host numbers to direct packets to the appropriate destination.
HIP: HSSI Interface Processor: An interface processor used on Cisco 7000 series routers, providing one HSSI port that supports connections to ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.
holddown: The state a route is placed in so that routers can neither advertise the route nor accept advertisements about it for a defined time period. Holddown is used to surface bad information about a route from all routers in the network. A route is generally placed in holddown when one of its links fails.
hop: The movement of a packet between any two network nodes. See also: hop count.
hop count: A routing metric that calculates the distance between a source and a destination. RIP employs hop count as its sole metric. See also: hop and RIP.
HSCI: High-Speed Communication Interface: Developed by Cisco, a single-port interface that provides full-duplex synchronous serial communications capability at speeds up to 52Mbps.
HSRP: Hot Standby Router Protocol: A protocol that provides high network availability and provides nearly instantaneous hardware failover without administrator intervention. It generates a Hot Standby router group, including a lead router that lends its services to any packet being transferred to the Hot Standby address. If the lead router fails, it will be replaced by any of the other routers—the standby routers—that monitor it.
HSSI: High-Speed Serial Interface: A network standard physical connector for high-speed serial linking over a WAN at speeds of up to 52Mbps.
ICD: International Code Designator: Adapted from the subnetwork model of addressing, this assigns the mapping of network layer addresses to ATM addresses. HSSI is one of two ATM formats for addressing created by the ATM Forum to be utilized with private networks. See also: DCC.
ICMP: Internet Control Message Protocol: Documented in RFC 792, it is a network layer Internet protocol for the purpose of reporting errors and providing information pertinent to IP packet procedures.
IEEE: Institute of Electrical and Electronics Engineers: A professional organization that, among other activities, defines standards in a number of fields within computing and electronics, including networking and communications. IEEE standards are the predominant LAN standards used today throughout the industry. Many protocols are commonly known by the reference number of the corresponding IEEE standard.
IEEE 802.1: The IEEE committee specification that defines the bridging group. The specification for STP (Spanning-Tree Protocol) is IEEE 802.1d. The STP uses SPA (spanning-tree algorithm) to find and prevent network loops in bridged networks. The specification for VLAN trunking is IEEE 802.1q. Compare to: ISL.
IEEE 802.3: The IEEE committee specification that defines the Ethernet group, specifically the original 10Mbps standard. Ethernet is a LAN protocol that specifies physical layer and MAC sublayer media access. IEEE 802.3 uses CSMA/CD to provide access for many devices on the same network. FastEthernet is defined as 802.3u, and Gigabit Ethernet is defined as 802.3q. See also: CSMA/CD.
IEEE 802.5: IEEE committee that defines Token Ring media access.
IGMP: Internet Group Management Protocol: Employed by IP hosts, the protocol that reports their multicast group memberships to an adjacent multicast router.
IGP: Interior Gateway Protocol: Any protocol used by the Internet to exchange routing data within an independent system. Examples include RIP, IGRP, and OSPF.
ILMI: Integrated (or Interim) Local Management Interface. A specification created by the ATM Forum, designated for the incorporation of network-management capability into the ATM UNI. Integrated Local Management Interface cells provide for automatic configuration between ATM systems. In LAN emulation, ILMI can provide sufficient information for the ATM end station to find an LECS. In addition, ILMI provides the ATM NSAP (Network Service Access Point) prefix information to the end station.
in-band management-In-band: management is the management of a network device “through” the network. Examples include using Simple Network Management Protocol (SNMP) or Telnet directly via the local LAN. Compare with: out-of-band management.
insured burst: In an ATM network, it is the largest, temporarily permitted data burst exceeding the insured rate on a PVC and not tagged by the traffic policing function for being dropped if network congestion occurs. This insured burst is designated in bytes or cells.
interarea routing: Routing between two or more logical areas. Contrast with: intra-area routing. See also: area.
interface processor: Any of several processor modules used with Cisco 7000 series routers. See also: AIP, CIP, EIP, FEIP, HIP, MIP, and TRIP.
Internet: The global “network of networks,” whose popularity has exploded in the last few years. Originally a tool for collaborative academic research, it has become a medium for exchanging and distributing information of all kinds. The Internet’s need to link disparate computer platforms and technologies has led to the development of uniform protocols and standards that have also found widespread use within corporate LANs. See also: TCP/IP and MBONE.
internet: Before the rise of the Internet, this lowercase form was shorthand for “internetwork” in the generic sense. Now rarely used. See also: internetwork.
Internet protocol: Any protocol belonging to the TCP/IP protocol stack. See also: TCP/IP.
internetwork: Any group of private networks interconnected by routers and other mechanisms, typically operating as a single entity.
internetworking: Broadly, anything associated with the general task of linking networks to each other. The term encompasses technologies, procedures, and products. When you connect networks to a router, you are creating an internetwork.
intra-area routing: Routing that occurs within a logical area. Contrast with: interarea routing.
Inverse ARP: Inverse Address Resolution Protocol: A technique by which dynamic mappings are constructed in a network, allowing a device such as a router to locate the logical network address and associate it with a permanent virtual circuit (PVC). Commonly used in Frame Relay to determine the far-end node’s TCP/IP address by sending the Inverse ARP request to the local DLCI.
IP: Internet Protocol: Defined in RFC 791, it is a network layer protocol that is part of the TCP/IP stack and allows connectionless service. IP furnishes an array of features for addressing, type-of-service specification, fragmentation and reassembly, and security.
IP address: Often called an Internet address, this is an address uniquely identifying any device (host) on the Internet (or any TCP/IP network). Each address consists of four octets (32 bits), represented as decimal numbers separated by periods (a format known as “dotted-decimal”). Every address is made up 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 addresses an individual host within the network or subnetwork. The network and subnetwork information is extracted from the IP address using the subnet mask. There are five classes of IP addresses (A–E), which allocate different numbers of bits to the network, subnetwork, and host portions of the address. See also: CIDR, IP, and subnet mask.
IPCP: IP Control Program: The protocol used to establish and configure IP over PPP. See also: IP and PPP.
IP multicast-A: technique for routing that enables IP traffic to be reproduced from one source to several endpoints or from multiple sources to many destinations. Instead of transmitting only one packet to each individual point of destination, one packet is sent to a multicast group specified by only one IP endpoint address for the group.
IPX: Internetwork Packet Exchange: Network layer protocol (Layer 3) used in Novell NetWare networks for transferring information from servers to workstations. Similar to IP and XNS.
IPXCP: IPX Control Program: The protocol used to establish and configure IPX over PPP. See also: IPX and PPP.
IPXWAN: Protocol used for new WAN links to provide and negotiate line options on the link using IPX. After the link is up and the options have been agreed upon by the two end-to-end links, normal IPX transmission begins.
ISDN: Integrated Services Digital Network: Offered as a service by telephone companies, a communication protocol that allows telephone networks to carry data, voice, and other digital traffic. See also: BISDN, BRI, and PRI.
isochronous transmission-Asynchronous: data transfer over a synchronous data-link, requiring a constant bit rate for reliable transport. Compare with: asynchronous transmission and synchronous transmission.
ITU-T: International Telecommunication Union Telecommunication Standardization Sector: This is a group of engineers that develops worldwide standards for telecommunications technologies.
LAN: Local Area Network: Broadly, any network linking two or more computers and related devices within a limited geographical area (up to a few kilometers). LANs are typically high-speed, low-error networks within a company. Cabling and signaling at the physical and data-link layers of the OSI are dictated by LAN standards. Ethernet, FDDI, and Token Ring are among the most popular LAN technologies. Compare with: MAN and WAN.
LANE: LAN emulation: The technology that allows an ATM network to operate as a LAN backbone. To do so, the ATM network is required to provide multicast and broadcast support, address mapping (MAC-to-ATM), SVC management, in addition to an operable packet format. Additionally, LANE defines Ethernet and Token Ring ELANs. See also: ELAN.
LAN switch: A high-speed, multiple-interface transparent bridging mechanism, transmitting packets between segments of data-links, usually referred to specifically as an Ethernet switch. LAN switches transfer traffic based on MAC addresses. Multilayer switches are a type of high-speed, special-purpose, hardware-based router. See also: multilayer switch, cut-through packet switching, and store-and-forward packet switching.
LAPB: Link Accessed Procedure, Balanced: A bit-oriented data-link layer protocol that is part of the X.25 stack and has its origin in SDLC. See also: SDLC and X.25.
LAPD: Link Access Procedure on the D channel. The ISDN data-link layer protocol used specifically for the D channel and defined by ITU-T Recommendations Q.920 and Q.921. LAPD evolved from LAPB and is created to comply with the signaling requirements of ISDN basic access.
latency: Broadly, the time it takes a data packet to get from one location to another. In specific networking contexts, it can mean either 1) the time elapsed (delay) between the execution of a request for access to a network by a device and the time the mechanism actually is permitted transmission, or 2) the time elapsed between when a mechanism receives a frame and the time that frame is forwarded out of the destination port.
Layer-3 switch: See also: multilayer switch.
LCP: Link Control Protocol: The protocol designed to establish, configure, and test data-link connections for use by PPP. See also: PPP.
leaky bucket: An analogy for the basic cell rate algorithm (GCRA) used in ATM networks for checking the conformance of cell flows from a user or network. The bucket’s “hole” is understood to be the prolonged rate at which cells can be accommodated, and the “depth” is the tolerance for cell bursts over a certain time period. See also: GCRA.
learning bridge: A bridge that transparently builds a dynamic database of MAC addresses and the interfaces associated with each address. Transparent bridges help to reduce traffic congestion on the network.
LE ARPLAN: Emulation Address Resolution Protocol: The protocol providing the ATM address that corresponds to a MAC address.
LEC: LAN Emulation Client: Software providing the emulation of the link layer interface that allows the operation and communication of all higher-level protocols and applications to continue. The LEC client runs in all ATM devices, which include hosts, servers, bridges, and routers. The LANE client is responsible for address resolution, data transfer, address caching, inter-facing to the emulated LAN, and driver support for higher-level services. See also: ELAN and LES.
LECS: LAN Emulation Configuration Server: An important part of emulated LAN services, providing the configuration data that is furnished upon request from the LES. These services include address registration for Integrated Local Management Interface (ILMI) support, configuration support for the LES addresses and their corresponding emulated LAN identifiers, and an interface to the emulated LAN. See also: LES and ELAN.
LES: LAN Emulation Server: The central LANE component that provides the initial configuration data for each connecting LEC. The LES typically is located on either an ATM-integrated router or a switch. Responsibilities of the LES include configuration and support for the LEC, address registration for the LEC, database storage and response concerning ATM addresses, and interfacing to the emulated LAN See also: ELAN, LEC, and LECS.
link-state routing algorithm: A routing algorithm that allows each router to broadcast or multicast information regarding the cost of reaching all its neighbors to every node in the internetwork. Link-state algorithms provide a consistent view of the network and are therefore not vulnerable to routing loops. However, this is achieved at the cost of somewhat greater difficulty in computation and more widespread traffic (compared with distance-vector routing algorithms). See also: distance-vector routing algorithm.
LLAP: LocalTalk Link Access Protocol: In a LocalTalk environment, the data-link-level protocol that manages node-to-node delivery of data. This protocol provides node addressing and management of bus access, and it also controls data sending and receiving to assure packet length and integrity.
LLC: Logical Link Control: Defined by the IEEE, the higher of two data-link layer sublayers. LLC is responsible for error detection (but not correction), flow control, framing, and software-sublayer addressing. The predominant LLC protocol, IEEE 802.2, defines both connectionless and connection-oriented operations. See also: data-link layer and MAC.
LMI: An enhancement to the original Frame Relay specification. Among the features it provides are a keepalive mechanism, a multicast mechanism, global addressing, and a status mechanism.
LNNI: LAN Emulation Network-to-Network Interface: In the Phase 2 LANE specification, an interface that supports communication between the server components within one ELAN.
local explorer packet: In a Token Ring SRB network, a packet generated by an end system to find a host linked to the local ring. If no local host can be found, the end system will produce one of two solutions: a spanning explorer packet or an all-routes explorer packet.
LocalTalk: Utilizing CSMA/CD, in addition to supporting data transmission at speeds of 230.4Kbps, LocalTalk is Apple Computer’s proprietary baseband protocol, operating at the data-link and physical layers of the OSI reference model.
LSA: link-state advertisement: Contained inside of link-state packets (LSPs), these advertisements are usually multicast packets, containing information about neighbors and path costs, that are employed by link-state protocols. Receiving routers use LSAs to maintain their link-state databases and, ultimately, routing tables.
LUNI: LAN Emulation User-to-Network Interface: Defining the interface between the LAN Emulation Client (LEC) and the LAN Emulation Server, LUNI is the ATM Forum’s standard for LAN Emulation on ATM networks. See also: LES and LECS.
MAC: Media Access Control: The lower sublayer in the data-link layer, it is responsible for hardware addressing, media access, and error detection of frames. See also: data-link layer and LLC.
MAC address: A data-link layer hardware address that every port or device needs in order to connect to a LAN segment. These addresses are used by various devices in the network for accurate location of logical addresses. MAC addresses are defined by the IEEE standard and their length is six characters, typically using the burned-in address (BIA) of the local LAN inter-face. Variously called hardware address, physical address, burned-in address, or MAC-layer address.
MacIP: In AppleTalk, the network layer protocol encapsulating IP packets in Datagram Delivery Protocol (DDP) packets. MacIP also supplies substitute ARP services.
MAN: Metropolitan-Area Network: Any network that encompasses a metropolitan area; that is, an area typically larger than a LAN but smaller than a WAN. See also: LAN and WAN.
Manchester encoding: A method for digital coding in which a mid-bit-time transition is employed for clocking, and a 1 (one) is denoted by a high voltage level during the first half of the bit time. This scheme is used by Ethernet and IEEE 802.3.
maximum burst: Specified in bytes or cells, the largest burst of information exceeding the insured rate that will be permitted on an ATM permanent virtual connection for a short time and will not be dropped even if it goes over the specified maximum rate. Compare with: insured burst. See also: maximum rate.
maximum rate: The maximum permitted data throughput on a particular virtual circuit, equal to the total of insured and uninsured traffic from the traffic source. Should traffic congestion occur, uninsured information may be deleted from the path. Measured in bits or cells per second, the maximum rate represents the highest throughput of data the virtual circuit is ever able to deliver and cannot exceed the media rate. Compare with: excess rate. See also: maximum burst.
MBS: Maximum Burst Size: In an ATM signaling message, this metric, coded as a number of cells, is used to convey the burst tolerance.
MBONE: multicast backbone: The multicast backbone of the Internet, it is a virtual multicast network made up of multicast LANs, including point-to-point tunnels interconnecting them.
MCDV: Maximum Cell Delay Variation: The maximum two-point CDV objective across a link or node for the identified service category in an ATM network. The MCDV is one of four link metrics that are exchanged using PTSPs to verify the available resources of an ATM network. Only one MCDV value is assigned to each traffic class.
MCLR: Maximum Cell Loss Ratio: The maximum ratio of cells in an ATM network that fail to transit a link or node compared with the total number of cells that arrive at the link or node. MCDV is one of four link metrics that are exchanged using PTSPs to verify the available resources of an ATM network. The MCLR applies to cells in VBR and CBR traffic classes whose CLP bit is set to zero. See also: CBR, CLP, and VBR.
MCR: Minimum Cell Rate: A parameter determined by the ATM Forum for traffic management of the ATM networks. MCR is specifically defined for ABR transmissions and specifies the minimum value for the allowed cell rate (ACR). See also: ACR and PCR.
MCTD: Maximum Cell Transfer Delay: In an ATM network, the total of the maximum cell delay variation and the fixed delay across the link or node. MCTD is one of four link metrics that are exchanged using PNNI topology state packets to verify the available resources of an ATM network. There is one MCTD value assigned to each traffic class. See also: MCDV.
MIB: Management Information Base: Used with SNMP management software to gather information from remote devices. The management station can poll the remote device for information, or the MIB running on the remote station can be programmed to send information on a regular basis.
MIP: Multichannel Interface Processor: The resident interface processor on Cisco 7000 series routers, providing up to two channelized T1 or E1 connections by serial cables connected to a CSU. The two controllers are capable of providing 24 T1 or 30 E1 channel groups, with each group being introduced to the system as a serial interface that can be configured individually.
mips: millions of instructions per second: A measure of processor speed.
MLP: Multilink PPP: A technique used to split, recombine, and sequence datagrams across numerous logical data-links.
MMP: Multichassis Multilink PPP: A protocol that supplies MLP support across multiple routers and access servers. MMP enables several routers and access servers to work as a single, large dial-up pool with one network address and ISDN access number. MMP successfully supports packet fragmenting and reassembly when the user connection is split between two physical access devices.
modem: modulator-demodulator: A device that converts digital signals to analog and vice-versa so that digital information can be transmitted over analog communication facilities, such as voice-grade telephone lines. This is achieved by converting digital signals at the source to analog for transmission and reconverting the analog signals back into digital form at the destination. See also: modulation and demodulation.
modem eliminator: A mechanism that makes possible a connection between two DTE devices without modems by simulating the commands and physical signaling required.
modulation: The process of modifying some characteristic of an electrical signal, such as amplitude (AM) or frequency (FM), in order to represent digital or analog information. See also: AM.
MOSPF: Multicast OSPF: An extension of the OSPF unicast protocol that enables IP multicast routing within the domain. See also: OSPF.
MPOA: Multiprotocol over ATM: An effort by the ATM Forum to standardize how existing and future network-layer protocols such as IP, Ipv6, AppleTalk, and IPX run over an ATM network with directly attached hosts, routers, and multilayer LAN switches.
MTU: maximum transmission unit: The largest packet size, measured in bytes, that an interface can handle.
multicast: Broadly, any communication between a single sender and multiple receivers. Unlike broadcast messages, which are sent to all addresses on a network, multicast messages are sent to a defined subset of the network addresses; this subset has a group multicast address, which is specified in the packet’s destination address field. See also: broadcast, directed broadcast.
multicast address: A single address that points to more than one device on the network by specifying a special non-existent MAC address specified in that particular multicast protocol. Identical to group address. See also: multicast.
multicast send VCCA: two-directional point-to-point virtual control connection (VCC) arranged by an LEC to a BUS, it is one of the three types of informational link specified by phase 1 LANE. See also: control distribute VCC and control direct VCC.
multilayer switch: A highly specialized, high-speed, hardware-based type of LAN router, the device filters and forwards packets based on their Layer 2 MAC addresses and Layer 3 network addresses. It’s possible that even Layer 4 can be read. Sometimes called a Layer 3 switch. See also: LAN switch.
multiplexing: The process of converting several logical signals into a single physical signal for transmission across one physical channel. Contrast with: demultiplexing.
NAK: negative acknowledgment: A response sent from a receiver, telling the sender that the information was not received or contained errors. Compare with: acknowledgment.
NAT: Network Address Translation: An algorithm instrumental in minimizing the requirement for globally unique IP addresses, permitting an organization whose addresses are not all globally unique to connect to the Internet, regardless, by translating those addresses into globally routable address space.
NBP: Name Binding Protocol: In AppleTalk, the transport-level protocol that interprets a socket client’s name, entered as a character string, into the corresponding DDP address. NBP gives AppleTalk protocols the capacity to discern user-defined zones and names of mechanisms by showing and keeping translation tables that map names to their corresponding socket addresses.
neighboring routers: Two routers in OSPF that have interfaces to a common network. On networks with multiaccess, these neighboring routers are dynamically discovered using the Hello protocol of OSPF.
NetBEUI: NetBIOS Extended User Interface: An improved version of the NetBIOS protocol used in a number of network operating systems including LAN Manager, Windows NT, LAN Server, and Windows for Workgroups, implementing the OSI LLC2 protocol. NetBEUI formalizes the transport frame not standardized in NetBIOS and adds more functions. See also: OSI.
NetBIOS: Network Basic Input/Output System: The API employed by applications residing on an IBM LAN to ask for services, such as session termination or information transfer, from lower-level network processes.
NetView: A mainframe network product from IBM, used for monitoring SNA (Systems Network Architecture) networks. It runs as a VTAM (Virtual Telecommunications Access Method) application.
NetWare: A widely used NOS created by Novell, providing a number of distributed network services and remote file access.
network layer: In the OSI reference model, it is Layer 3—the layer in which routing is implemented, enabling connections and path selection between two end systems. See also: application layer, data-link layer, physical layer, presentation layer, session layer, and transport layer.
NFS: Network File System: One of the protocols in Sun Microsystems’ widely used file system protocol suite, allowing remote file access across a network. The name is loosely used to refer to the entire Sun protocol suite, which also includes RPC, XDR (External Data Representation), and other protocols.
NHRP: Next Hop Resolution Protocol: In a nonbroadcast multiaccess (NBMA) network, the protocol employed by routers in order to dynamically locate MAC addresses of various hosts and routers. It enables systems to communicate directly without requiring an intermediate hop, thus facilitating increased performance in ATM, Frame Relay, X.25, and SMDS systems.
NHS: Next Hop Server: Defined by the NHRP protocol, this server maintains the next-hop resolution cache tables, listing IP-to-ATM address maps of related nodes and nodes that can be reached through routers served by the NHS.
NIC: network interface card: An electronic circuit board placed in a computer. The NIC provides network communication to a LAN.
NLSP: NetWare Link Services Protocol: Novell’s link-state routing protocol, based on the IS-IS model.
NMP: Network Management Processor: A Catalyst 5000 switch processor module used to control and monitor the switch.
non-stub area: In OSPF, a resource-consuming area carrying a default route, intra-area routes, interarea routes, static routes, and external routes. Non-stub areas are the only areas that can have virtual links configured across them and exclusively contain an anonymous system boundary router (ASBR). Compare with: stub area. See also: ASBR and OSPF.
NRZ: Nonreturn to Zero: One of several encoding schemes for transmitting digital data. NRZ signals sustain constant levels of voltage with no signal shifting (no return to zero-voltage level) during a bit interval. If there is a series of bits with the same value (1 or 0), there will be no state change. The signal is not self-clocking. See also: NRZI.
NRZI: Nonreturn to Zero Inverted: One of several encoding schemes for transmitting digital data. A transition in voltage level (either from high to low or vice-versa) at the beginning of a bit interval is interpreted as a value of 1; the absence of a transition is interpreted as a 0. Thus, the voltage assigned to each value is continually inverted. NRZI signals are not self-clocking. See also: NRZ.
NVRAM: Non-Volatile RAM: Random-access memory that keeps its contents intact while power is turned off.
OC: Optical Carrier: A series of physical protocols, designated as OC-1, OC-2, OC-3, and so on, for SONET optical signal transmissions. OC signal levels place STS frames on a multimode fiber-optic line at various speeds, of which 51.84Mbps is the lowest (OC-1). Each subsequent protocol runs at a speed divisible by 51.84. See also: SONET.
100BaseT: Based on the IEEE 802.3u standard, 100BaseT is the Fast-Ethernet specification of 100Mbps baseband that uses UTP wiring. 100BaseT sends link pulses (containing more information than those used in 10BaseT) over the network when no traffic is present. See also: 10BaseT, FastEthernet, and IEEE 802.3.
100BaseTX: Based on the IEEE 802.3u standard, 100BaseTX is the 100Mbps baseband FastEthernet specification that uses two pairs of UTP or STP wiring. The first pair of wires receives data; the second pair sends data. To ensure correct signal timing, a 100BaseTX segment cannot be longer than 100 meters.
ones density: Also known as pulse density, this is a method of signal clocking. The CSU/DSU retrieves the clocking information from data that passes through it. For this scheme to work, the data needs to be encoded to contain at least one binary 1 for each eight bits transmitted. See also CSU and DSU.
OSI: Open System Interconnection: International standardization program designed by ISO and ITU-T for the development of data networking standards that make multivendor equipment interoperability a reality.
OSI reference model: Open System Interconnection reference model: A conceptual model defined by the International Organization for Standardization (ISO), describing how any combination of devices can be connected for the purpose of communication. The OSI model divides the task into seven functional layers, forming a hierarchy with the applications at the top and the physical medium at the bottom, and it defines the functions each layer must provide. See also: application layer, data-link layer, network layer, physical layer, presentation layer, session layer, and transport layer.
OSPF: Open Shortest Path First: A link-state, hierarchical IGP routing algorithm derived from an earlier version of the IS-IS protocol, whose features include multipath routing, load balancing, and least-cost routing. OSPF is the suggested successor to RIP in the Internet environment. See also: enhanced IGRP, IGP, and IP.
out-of-band management: Management “outside” of the network’s physical channels. For example, using a console connection not directly interfaced through the local LAN or WAN or a dial-in modem. Compare to: in-band management.
out-of-band signaling: Within a network, any transmission that uses physical channels or frequencies separate from those ordinarily used for data transfer. For example, the initial configuration of a Cisco Catalyst switch requires an out-of-band connection via a console port.
packet: In data communications, the basic logical unit of information transferred. A packet consists of a certain number of data bytes, wrapped or encapsulated in headers and/or trailers that contain information about where the packet came from, where it’s going, and so on. The various protocols involved in sending a transmission add their own layers of header information, which the corresponding protocols in receiving devices then interpret.
packet switch: A physical device that makes it possible for a communication channel to share several connections, its functions include finding the most efficient transmission path for packets.
packet switching: A networking technology based on the transmission of data in packets. Dividing a continuous stream of data into small units— packets—enables data from multiple devices on a network to share the same communication channel simultaneously but also requires the use of precise routing information. Contrast with: circuit switching.
PAP: Password Authentication Protocol: In Point-to-Point Protocol (PPP) networks, a method of validating connection requests. The requesting (remote) device must send an authentication request, containing a password and ID, to the local router when attempting to connect. Unlike the more secure CHAP (Challenge Handshake Authentication Protocol), PAP sends the password unencrypted and does not attempt to verify whether the user is authorized to access the requested resource; it merely identifies the remote end. See also: CHAP.
parity checking: A method of error-checking in data transmissions. An extra bit (the parity bit) is added to each character or data word so that the sum of the bits will be either an odd number (in odd parity) or an even number (even parity).
partial mesh: A type of network topology in which some network nodes form a full mesh (where every node has either a physical or a virtual circuit linking it to every other network node), but others are attached to only one or two nodes in the network. A typical use of partial-mesh topology is in peripheral networks linked to a fully meshed backbone. See also: full mesh.
PCR: Peak Cell Rate: As defined by the ATM Forum, the parameter specifying, in cells per second, the maximum rate at which a source may transmit.
PDN: Public Data Network: Generally for a fee, a PDN offers the public access to computer communication network operated by private concerns or government agencies. Small organizations can take advantage of PDNs, aiding them in creating WANs without investing in long-distance equipment and circuitry.
PGP: Pretty Good Privacy: A popular public-key/private-key encryption application offering protected transfer of files and messages.
physical layer: The lowest layer—Layer 1—in the OSI reference model, it is responsible for converting data packets from the data-link layer (Layer 2) into electrical signals. Physical-layer protocols and standards define, for example, the type of cable and connectors to be used, including their pin assignments and the encoding scheme for signaling 0 and 1 values. See also: application layer, data-link layer, network layer, presentation layer, session layer, and transport layer.
ping: packet Internet groper: A Unix-based Internet diagnostic tool, consisting of a message sent to test the accessibility of a particular device on the IP network. The acronym (from which the “full name” was formed) reflects the underlying metaphor of submarine sonar. Just as the sonar operator sends out a signal and waits to hear it echo (“ping”) back from a submerged object, the network user can ping another node on the network and wait to see if it responds.
pleisochronous: Nearly synchronous, except that clocking comes from an outside source instead of being embedded within the signal as in synchronous transmissions.
PLP: Packet Level Protocol: Occasionally called X.25 Level 3 or X.25 Protocol, a network-layer protocol that is part of the X.25 stack.
PNNI: Private Network-Network Interface: An ATM Forum specification for offering topology data used for the calculation of paths through the network, among switches and groups of switches. It is based on well-known link-state routing procedures and allows for automatic configuration in networks whose addressing scheme is determined by the topology.
point-to-multipoint connection: In ATM, a communication path going only one way, connecting a single system at the starting point, called the “root node,” to systems at multiple points of destination, called “leaves.” See also: point-to-point connection.
point-to-point connection: In ATM, a channel of communication that can be directed either one way or two ways between two ATM end systems. See also: point-to-multipoint connection.
poison reverse updates: These update messages are transmitted by a router back to the originator (thus ignoring the split-horizon rule) after route poisoning has occurred. Typically used with DV routing protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates). See also: route poisoning.
polling: The procedure of orderly inquiry, used by a primary network mechanism, to determine if secondary devices have data to transmit. A message is sent to each secondary, granting the secondary the right to transmit.
POP: 1. Point Of Presence: The physical location where an interexchange carrier has placed equipment to interconnect with a local exchange carrier. 2. Post Office Protocol (currently at version 3): A protocol used by client e-mail applications for recovery of mail from a mail server.
PPP: Point-to-Point Protocol: The protocol most commonly used for dialup Internet access, superseding the earlier SLIP. Its features include address notification, authentication via CHAP or PAP, support for multiple protocols, and link monitoring. PPP has two layers: the Link Control Protocol (LCP) establishes, configures, and tests a link; and then any of various Network Control Programs (NCPs) transport traffic for a specific protocol suite, such as IPX. See also: CHAP, PAP, and SLIP.
presentation layer: Layer 6 of the OSI reference model, it defines how data is formatted, presented, encoded, and converted for use by software at the application layer. See also: application layer, data-link layer, network layer, physical layer, session layer, and transport layer.
PRI: Primary Rate Interface: A type of ISDN connection between a PBX and a long-distance carrier, which is made up of a single 64Kbps D channel in addition to 23 (T1) or 30 (E1) B channels. See also: ISDN.
priority queueing: A routing function in which frames temporarily placed in an interface output queue are assigned priorities based on traits such as packet size or type of interface.
process switching: As a packet arrives on a router to be forwarded, it’s copied to the router’s process buffer, and the router performs a lookup on the Layer 3 address. Using the route table, an exit interface is associated with the destination address. The processor forwards the packet with the added new information to the exit interface, while the router initializes the fast-switching cache. Subsequent packets bound for the same destination address follow the same path as the first packet.
PROM: programmable read-only memory: ROM that is programmable only once, using special equipment. Compare with: EPROM.
propagation delay: The time it takes data to traverse a network from its source to its destination.
protocol: In networking, the specification of a set of rules for a particular type of communication. The term is also used to refer to the software that implements a protocol.
protocol stack: A collection of related protocols.
PSE: Packet Switch Exchange: The X.25 term for a switch.
PSN: packet-switched network: Any network that uses packet-switching technology. Also known as packet-switched data network (PSDN). See also: packet switching.
PSTN: Public Switched Telephone Network: Colloquially referred to as “plain old telephone service” (POTS). A term that describes the assortment of telephone networks and services available globally.
PVC: permanent virtual circuit: In a Frame Relay network, a logical connection, defined in software, that is maintained permanently. Compare with: SVC. See also: virtual circuit.
PVP: permanent virtual path: A virtual path made up of PVCs. See also: PVC.
PVP tunneling: permanent virtual path tunneling: A technique that links two private ATM networks across a public network using a virtual path; wherein the public network transparently trunks the complete collection of virtual channels in the virtual path between the two private networks.
QoS: Quality of Service: A set of metrics used to measure the quality of transmission and service availability of any given transmission system.
queue: Broadly, any list of elements arranged in an orderly fashion and ready for processing, such as a line of people waiting to enter a movie theater. In routing, it refers to a backlog of information packets waiting in line to be transmitted over a router interface.
RARP: Reverse Address Resolution Protocol: The protocol within the TCP/ IP stack that maps MAC addresses to IP addresses. See also: ARP.
rate queue: A value, assigned to one or more virtual circuits, that specifies the speed at which an individual virtual circuit will transmit data to the remote end. Every rate queue identifies a segment of the total bandwidth available on an ATM link. The sum of all rate queues should not exceed the total available bandwidth.
RCP: Remote Copy Protocol: A protocol for copying files to or from a file system that resides on a remote server on a network, using TCP to guarantee reliable data delivery.
redistribution: Command used in Cisco routers to inject the paths found from one type of routing protocol into another type of routing protocol. For example, networks found by RIP can be inserted into an IGRP network.
redundancy: In internetworking, the duplication of connections, devices, or services that can be used as a backup in the event that the primary connections, devices, or services fail.
reload: An event or command that causes Cisco routers to reboot.
RIF: Routing Information Field: In source-route bridging, a header field that defines the path direction of the frame or token. If the Route Information Indicator (RII) bit is not set, the RIF is read from source to destination (left to right). If the RII bit is set, the RIF is read from the destination back to the source, so the RIF is read right to left. It is defined as part of the Token Ring frame header for source-routed frames, which contains path information.
ring: Two or more stations connected in a logical circular topology. In this topology, which is the basis for Token Ring, FDDI, and CDDI, information is transferred from station to station in sequence.
ring topology: A network logical topology comprising a series of repeaters that form one closed loop by connecting unidirectional transmission links. Individual stations on the network are connected to the network at a repeater. Physically, ring topologies are generally organized in a closed-loop star. Compare with: bus topology and star topology.
RIP: Routing Information Protocol: The most commonly used interior gateway protocol in the Internet. RIP employs hop count as a routing metric. See also: Enhanced IGRP, IGP, OSPF, and hop count.
routed protocol: Routed protocols (such as IP and IPX) are used to transmit user data through an internetwork. By contrast, routing protocols (such as RIP, IGRP, and OSPF) are used to update routing tables between routers.
route poisoning-Used: by various DV routing protocols in order to over-come large routing loops and offer explicit information about when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates). Typically, this is accomplished by setting the hop count to one more than maximum. See also: poison reverse.
route summarization: In various routing protocols, such as OSPF, EIGRP, and IS-IS, the consolidation of publicized subnetwork addresses so that a single summary route is advertised to other areas by an area border router.
router: A network-layer mechanism, either software or hardware, using one or more metrics to decide on the best path to use for transmission of network traffic. Sending packets between networks by routers is based on the information provided on network layers. Historically, this device has sometimes been called a gateway.
routing: The process of forwarding logically addressed packets from their local subnetwork toward their ultimate destination. In large networks, the numerous intermediary destinations a packet might travel before reaching its destination can make routing very complex.
routing domain: Any collection of end systems and intermediate systems that operate under an identical set of administrative rules. Every routing domain contains one or several areas, all individually given a certain area address.
routing metric: Any value that is used by routing algorithms to determine whether one route is superior to another. Metrics include such information as bandwidth, delay, hop count, path cost, load, MTU, reliability, and communication cost. Only the best possible routes are stored in the routing table, while all other information may be stored in link-state or topological databases. See also: cost.
routing protocol: Any protocol that defines algorithms to be used for updating routing tables between routers. Examples include IGRP, RIP, and OSPF.
routing table-A: table kept in a router or other internetworking mechanism that maintains a record of only the best possible routes to certain network destinations and the metrics associated with those routes.
RP: Route Processor: Also known as a supervisory processor, a module on Cisco 7000 series routers that holds the CPU, system software, and most of the memory components used in the router.
RSP: Route/Switch Processor: A processor module combining the functions of RP and SP used in Cisco 7500 series routers. See also: RP and SP.
RTS: Request To Send: An EIA/TIA-232 control signal requesting permission to transmit data on a communication line.
sampling rate: The rate at which samples of a specific waveform amplitude are collected within a specified period of time.
SAP: 1. Service Access Point: A field specified by IEEE 802.2 that is part of an address specification. 2. Service Advertising Protocol: The Novell NetWare protocol that supplies a way to inform network clients of resources and services availability on network, using routers and servers. See also: IPX.
SCR: Sustainable Cell Rate: An ATM Forum parameter used for traffic management, it is the long-term average cell rate for VBR connections that can be transmitted.
SDLC: Synchronous Data-Link Control: A protocol used in SNA data-link layer communications. SDLC is a bit-oriented, full-duplex serial protocol that is the basis for several similar protocols, including HDLC and LAPB. See also: HDLC and LAPB.
seed router: In an AppleTalk network, the router that is equipped with the network number or cable range in its port descriptor. The seed router specifies the network number or cable range for other routers in that network section and answers to configuration requests from nonseed routers on its connected AppleTalk network, permitting those routers to affirm or modify their configurations accordingly. Every AppleTalk network needs at least one seed router physically connected to each network segment.
server: Hardware and software that provide network services to clients.
session layer: Layer 5 of the OSI reference model, responsible for creating, managing, and terminating sessions between applications and overseeing data exchange between presentation layer entities. See also: application layer, data-link layer, network layer, physical layer, presentation layer, and transport layer.
SF: super frame: A super frame (also called a D4 frame) consists of 12 frames with 192 bits each, and the 193rd bit providing other functions including error checking. SF is frequently used on T1 circuits. A newer version of the technology is Extended Super Frame (ESF), which uses 24 frames. See also: ESF.
signaling packet: An informational packet created by an ATM-connected mechanism that wants to establish connection with another such mechanism. The packet contains the QoS parameters needed for connection and the ATM NSAP address of the endpoint. The endpoint responds with a message of acceptance if it is able to support the desired QoS, and the connection is established. See also: QoS.
silicon switching: A type of high-speed switching used in Cisco 7000 series routers, based on the use of a separate processor (the Silicon Switch Processor, or SSP). See also: SSE.
sliding window: The method of flow control used by TCP, as well as several data-link layer protocols. This method places a buffer between the receiving application and the network data flow. The “window” available for accepting data is the size of the buffer minus the amount of data already there. This window increases in size as the application reads data from it and decreases as new data is sent. The receiver sends the transmitter announcements of the current window size, and it may stop accepting data until the window increases above a certain threshold.
SLIP: Serial Line Internet Protocol: An industry standard serial encapsulation for point-to-point connections that supports only a single routed protocol, TCP/IP. SLIP is the predecessor to PPP. See also: PPP.
SMDS: Switched Multimegabit Data Service: A packet-switched, datagram-based WAN networking technology offered by telephone companies that provides high speed.
SMTP: Simple Mail Transfer Protocol: A protocol used on the Internet to provide electronic mail services.
SNA: System Network Architecture: A complex, feature-rich, network architecture similar to the OSI reference model but with several variations; created by IBM in the 1970s and essentially composed of seven layers.
SNAP: Subnetwork Access Protocol: SNAP is a frame used in Ethernet, Token Ring, and FDDI LANs. Data transfer, connection management, and QoS selection are three primary functions executed by the SNAP frame.
socket: 1. A software structure that operates within a network device as a destination point for communications. 2. In AppleTalk networks, an entity at a specific location within a node; AppleTalk sockets are conceptually similar to TCP/IP ports.
SONET: Synchronous Optical Network: The ANSI standard for synchronous transmission on fiber-optic media, developed at Bell Labs. It specifies a base signal rate of 51.84Mbps and a set of multiples of that rate, known as Optical Carrier levels, up to 2.5Gbps.
SP: Switch Processor: Also known as a ciscoBus controller, it is a Cisco 7000 series processor module acting as governing agent for all CxBus activities.
span: A full-duplex digital transmission line connecting two facilities.
SPAN: Switched Port Analyzer: A feature of the Catalyst 5000 switch, offering freedom to manipulate within a switched Ethernet environment by extending the monitoring ability of the existing network analyzers into the environment. At one switched segment, the SPAN mirrors traffic onto a predetermined SPAN port, while a network analyzer connected to the SPAN port is able to monitor traffic from any other Catalyst switched port.
spanning explorer packet: Sometimes called limited-route or single-route explorer packet, it pursues a statically configured spanning tree when searching for paths in a source-route bridging network. See also: all-routes explorer packet, explorer packet, and local explorer packet.
spanning tree: A subset of a network topology, within which no loops exist. When bridges are interconnected into a loop, the bridge, or switch, cannot identify a frame that has been forwarded previously, so there is no mechanism for removing a frame as it passes the interface numerous times. Without a method of removing these frames, the bridges continuously forward them—consuming bandwidth and adding overhead to the network. Spanning trees prune the network to provide only one path for any packet. See also: Spanning-Tree Protocol and spanning-tree algorithm.
spanning-tree algorithm (STA): An algorithm that creates a spanning tree using the Spanning-Tree Protocol (STP). See also: spanning tree and Spanning-Tree Protocol.
Spanning-Tree Protocol (STP): The bridge protocol (IEEE 802.1d) that enables a learning bridge to dynamically avoid loops in the network topology by creating a spanning tree using the spanning-tree algorithm. Spanning-tree frames called bridge protocol data units (BPDUs) are sent and received by all switches in the network at regular intervals. The switches participating in the spanning tree don’t forward the frames; instead, they’re processed to determine the spanning-tree topology itself. Cisco Catalyst series switches use STP 802.1d to perform this function. See also: BPDU, learning bridge, MAC address, spanning tree, and spanning-tree algorithm.
SPF: shortest path first algorithm: A routing algorithm used to decide on the shortest-path spanning tree. Sometimes called Dijkstra’s algorithm and frequently used in link-state routing algorithms. See also: link-state routing algorithm.
SPID: Service Profile Identifier: A number assigned by service providers or local telephone companies and assigned by administrators to a BRI port. SPIDs are used to determine subscription services of a device connected via ISDN. ISDN devices use SPID when accessing the telephone company switch that initializes the link to a service provider.
split horizon: Useful for preventing routing loops, a type of distance-vector routing rule where information about routes is prevented from leaving the router interface through which that information was received.
spoofing: 1. In dial-on-demand routing (DDR), where a circuit-switched link is taken down to save toll charges when there is no traffic to be sent, spoofing is a scheme used by routers that causes a host to treat an interface as if it were functioning and supporting a session. The router pretends to send “spoof” replies to keepalive messages from the host in an effort to convince the host that the session is up and running. See also: DDR. 2. The illegal act of sending a packet labeled with a false address, in order to deceive network security mechanisms such as filters and access lists.
spooler: A management application that processes requests submitted to it for execution in a sequential fashion from a queue. A good example is a print spooler.
SPX: Sequenced Packet Exchange: A Novell NetWare transport protocol that augments the datagram service provided by network layer (Layer 3) protocols, it was derived from the Switch-to-Switch Protocol of the XNS protocol suite.
SQE: Signal Quality Error: In an Ethernet network, a message sent from a transceiver to an attached machine that the collision-detection circuitry is working.
SRB: Source-Route Bridging: Created by IBM, the bridging method used in Token-Ring networks. The source determines the entire route to a destination before sending the data and includes that information in route information fields (RIF) within each packet. Contrast with: transparent bridging.
SRT: source-route transparent bridging: A bridging scheme developed by IBM, merging source-route and transparent bridging. SRT takes advantage of both technologies in one device, fulfilling the needs of all end nodes. Translation between bridging protocols is not necessary. Compare with: SR/TLB.
SR/TLB: source-route translational bridging: A bridging method that allows source-route stations to communicate with transparent bridge stations aided by an intermediate bridge that translates between the two bridge protocols. Used for bridging between Token Ring and Ethernet. Compare with: SRT.
SSAP: Source Service Access Point: The SAP of the network node identified in the Source field of the packet. See also: DSAP and SAP.
SSE: Silicon Switching Engine: The software component of Cisco’s silicon switching technology, hard-coded into the Silicon Switch Processor (SSP). Silicon switching is available only on the Cisco 7000 with an SSP. Silicon-switched packets are compared to the silicon-switching cache on the SSE. The SSP is a dedicated switch processor that offloads the switching process from the route processor, providing a fast-switching solution, but packets must still traverse the backplane of the router to get to the SSP and then back to the exit interface.
star topology: A LAN physical topology with endpoints on the network converging at a common central switch (known as a hub) using point-to-point links. A logical ring topology can be configured as a physical star topology using a unidirectional closed-loop star rather than point-to-point links. That is, connections within the hub are arranged in an internal ring. See also: bus topology and ring topology.
startup range: If an AppleTalk node does not have a number saved from the last time it was booted, then the node selects from the range of values from 65280 to 65534.
static route: A route whose information is purposefully entered into the routing table and takes priority over those chosen by dynamic routing protocols.
statistical multiplexing: Multiplexing in general is a technique that allows data from multiple logical channels to be sent across a single physical channel. Statistical multiplexing dynamically assigns bandwidth only to input channels that are active, optimizing available bandwidth so that more devices can be connected than with other multiplexing techniques. Also known as statistical time-division multiplexing or stat mux.
STM-1: Synchronous Transport Module Level 1. In the European SDH standard, one of many formats identifying the frame structure for the 155.52Mbps lines that are used to carry ATM cells.
store-and-forward packet switching: A technique in which the switch first copies each packet into its buffer and performs a cyclical redundancy check (CRC). If the packet is error-free, the switch then looks up the destination address in its filter table, determines the appropriate exit port, and sends the packet.
STP: 1. Shielded Twisted Pair: A two-pair wiring scheme, used in many network implementations, that has a layer of shielded insulation to reduce EMI. 2. Spanning-Tree Protocol.
stub area: An OSPF area carrying a default route, intra-area routes, and interarea routes, but no external routes. Configuration of virtual links cannot be achieved across a stub area, and stub areas are not allowed to contain an ASBR. See also: non-stub area, ASBR, and OSPF.
stub network: A network having only one connection to a router.
STUN: Serial Tunnel: A technology used to connect an HDLC link to an SDLC link over a serial link.
subarea: A portion of an SNA network made up of a subarea node and its attached links and peripheral nodes.
subarea node: An SNA communications host or controller that handles entire network addresses.
subchannel: A frequency-based subdivision that creates a separate broad-band communications channel.
subinterface: One of many virtual interfaces available on a single physical interface.
subnet: See: subnetwork.
subnet address: The portion of an IP address that is specifically identified by the subnet mask as the subnetwork. See also: IP address, subnetwork, and subnet mask.
subnet mask: Also simply known as mask, a 32-bit address mask used in IP to identify the bits of an IP address that are used for the subnet address. Using a mask, the router does not need to examine all 32 bits, only those selected by the mask. See also: address mask and IP address.
subnetwork: 1. Any network that is part of a larger IP network and is identified by a subnet address. A network administrator segments a network into subnetworks in order to provide a hierarchical, multilevel routing structure, and at the same time protect the subnetwork from the addressing complexity of networks that are attached. Also known as a subnet. See also: IP address, subnet mask, and subnet address. 2. In OSI networks, the term specifically refers to a collection of ESs and ISs controlled by only one administrative domain, using a solitary network connection protocol.
SVC: switched virtual circuit: A dynamically established virtual circuit, created on demand and dissolved as soon as transmission is over and the circuit is no longer needed. In ATM terminology, it is referred to as a switched virtual connection. See also: PVC.
switch: 1. In networking, a device responsible for multiple functions such as filtering, flooding, and sending frames. It works using the destination address of individual frames. Switches operate at the data-link layer of the OSI model. 2. Broadly, any electronic/mechanical device allowing connections to be established as needed and terminated if no longer necessary.
switched LAN: Any LAN implemented using LAN switches. See also: LAN switch.
synchronous transmission: Signals transmitted digitally with precision clocking. These signals have identical frequencies and contain individual characters encapsulated in control bits (called start/stop bits) that designate the beginning and ending of each character. See also: asynchronous transmission and isochronous transmission.
T1: Digital WAN that uses 24 DS0s at 64K each to create a bandwidth of 1.536Mbps, minus clocking overhead, providing 1.544Mbps of usable bandwidth.
T3: Digital WAN that can provide bandwidth of 44.763Mbps.
tag switching: Based on the concept of label swapping, where packets or cells are designated to defined-length labels that control the manner in which data is to be sent, tag switching is a high-performance technology used for forwarding packets. It incorporates data-link layer (Layer 2) switching and network layer (Layer 3) routing and supplies scalable, high-speed switching in the network core.
tagged traffic: ATM cells with their cell loss priority (CLP) bit set to 1. Also referred to as discard-eligible (DE) traffic. Tagged traffic can be eliminated in order to ensure trouble-free delivery of higher priority traffic, if the network is congested. See also: CLP.
TCP: Transmission Control Protocol: A connection-oriented protocol that is defined at the transport layer of the OSI reference model. Provides reliable delivery of data.
TCP/IP: Transmission Control Protocol/Internet Protocol. The suite of protocols underlying the Internet. TCP and IP are the most widely known protocols in that suite. See also: IP and TCP.
TDM: time division multiplexing: A technique for assigning bandwidth on a single wire, based on preassigned time slots, to data from several channels. Bandwidth is allotted to each channel regardless of a station’s ability to send data. See also: ATDM, FDM, and multiplexing.
TE1: A device with a four-wire, twisted-pair digital interface is referred to as terminal equipment type one. Most modern ISDN devices are of this type.
TE: terminal equipment: Any peripheral device that is ISDN-compatible and attached to a network, such as a telephone or computer.
telco: A common abbreviation for the telephone company.
Telnet: The standard terminal emulation protocol within the TCP/IP protocol stack. Method of remote terminal connection, enabling users to log in on remote networks and use those resources as if they were locally connected. Telnet is defined in RFC 854.
10BaseT: Part of the original IEEE 802.3 standard, 10BaseT is the Ethernet specification of 10Mbps baseband that uses two pairs of twisted-pair, Category 3, 4, or 5 cabling—using one pair to send data and the other to receive. 10BaseT has a distance limit of about 100 meters per segment. See also: Ethernet and IEEE 802.3.
terminal adapter: A hardware interface between a computer without a native ISDN interface and an ISDN line. In effect, a device to connect a standard async interface to a non-native ISDN device, emulating a modem.
terminal emulation: The use of software, installed on a PC or LAN server, that allows the PC to function as if it were a “dumb” terminal directly attached to a particular type of mainframe.
TFTP: Conceptually, a stripped-down version of FTP, it’s the protocol of choice if you know exactly what you want and where it’s to be found. TFTP doesn’t provide the abundance of functions that FTP does. In particular, it has no directory-browsing abilities; it can do nothing but send and receive files.
token: A frame containing only control information. Possessing this control information gives a network device permission to transmit data onto the network. See also: token passing.
token bus: LAN architecture that is the basis for the IEEE 802.4 LAN specification and employs token passing access over a bus topology. See also: IEEE.
token passing: A method used by network devices to access the physical medium in a systematic way based on possession of a small frame called a token. Contrast with: circuit switching. See also: token.
Token Ring: IBM’s token-passing LAN technology. It runs at 4Mbps or 16Mbps over a ring topology. Defined formally by IEEE 802.5. See also: ring topology and token passing.
transparent bridging: The bridging scheme used in Ethernet and IEEE 802.3 networks, it passes frames along one hop at a time, using bridging information stored in tables that associate end-node MAC addresses within bridge ports. This type of bridging is considered transparent because the source node does not know it has been bridged, because the destination frames are sent directly to the end node. Contrast with: SRB.
transport layer: Layer 4 of the OSI reference model, used for reliable communication between end nodes over the network. The transport layer provides mechanisms used for establishing, maintaining, and terminating virtual circuits, transport fault detection and recovery, and controlling the flow of information. See also: application layer, data-link layer, network layer, physical layer, presentation layer, and session layer.
TRIP: Token Ring Interface Processor: A high-speed interface processor used on Cisco 7000 series routers. The TRIP provides two or four ports for interconnection with IEEE 802.5 and IBM media with ports set to speeds of either 4Mbps or 16Mbps set independently of each other.
TTL: Time To Live: A field in an IP header, indicating the length of time a packet is valid.
TUD: Trunk Up-Down: A protocol used in ATM networks for the monitoring of trunks. Should a trunk miss a given number of test messages being sent by ATM switches to ensure trunk line quality, TUD declares the trunk down. When a trunk reverses direction and comes back up, TUD recognizes that the trunk is up and returns the trunk to service.
tunneling: A method of avoiding protocol restrictions by wrapping packets from one protocol in another protocol’s packet and transmitting this encapsulated packet over a network that supports the wrapper protocol. See also: encapsulation.
UDP: User Datagram Protocol: A connectionless transport layer protocol in the TCP/IP protocol stack that simply allows datagrams to be exchanged without acknowledgements or delivery guarantees, requiring other protocols to handle error processing and retransmission. UDP is defined in RFC 768.
unnumbered frames: HDLC frames used for control-management purposes, such as link startup and shutdown or mode specification.
VBR: Variable Bit Rate: A QoS class, as defined by the ATM Forum, for use in ATM networks that is subdivided into real time (RT) class and non-real time (NRT) class. RT is employed when connections have a fixed-time relationship between samples. Conversely, NRT is employed when connections do not have a fixed-time relationship between samples, but still need an assured QoS.
VCC: Virtual Channel Connection: A logical circuit that is created by VCLs. VCCs carry data between two endpoints in an ATM network. Sometimes called a virtual circuit connection.
VIP: 1. Versatile Interface Processor: An interface card for Cisco 7000 and 7500 series routers, providing multilayer switching and running the Cisco IOS software. The most recent version of VIP is VIP2. 2. Virtual IP: A function making it possible for logically separated switched IP workgroups to run Virtual Networking Services across the switch ports of a Catalyst 5000.
virtual circuit: Abbreviated VC, a logical circuit devised to assure reliable communication between two devices on a network. Defined by a virtual path connection (VPC)/virtual path identifier (VCI) pair, a virtual circuit can be permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25. Known as virtual channel in ATM. See also: PVC and SVC.
virtual ring: In an SRB network, a logical connection between physical rings, either local or remote.
VLAN: Virtual LAN: A group of devices on one or more logically segmented LANs (configured by use of management software), enabling devices to communicate as if attached to the same physical medium, when they are actually located on numerous different LAN segments. VLANs are based on logical instead of physical connections and thus are tremendously flexible.
VLSM: variable-length subnet mask: Helps optimize available address space and specify a different subnet mask for the same network number on various subnets. Also commonly referred to as “subnetting a subnet.”
WinSock: Windows Socket Interface: A software interface that makes it possible for an assortment of applications to use and share an Internet connection. The WinSock software consists of a Dynamic Link Library (DLL) with supporting programs such as a dialer program that initiates the connection.
workgroup switching: A switching method that supplies high-speed (100Mbps) transparent bridging between Ethernet networks as well as high-speed translational bridging between Ethernet and CDDI or FDDI.
X.25: An ITU-T packet-relay standard that defines communication between DTE and DCE network devices. X.25 uses a reliable data-link layer protocol called LAPB. X.25 also uses PLP at the network layer. X.25 has mostly been replaced by Frame Relay.
ZIP: Zone Information Protocol: A session-layer protocol used by Apple-Talk to map network numbers to zone names. NBP uses ZIP in the determination of networks containing nodes that belong to a zone. See also: ZIP storm and zone.
ZIP storm: A broadcast storm occurring when a router running AppleTalk reproduces or transmits a route for which there is no corresponding zone name at the time of execution. The route is then forwarded by other routers downstream, thus causing a ZIP storm. See also: broadcast storm and ZIP.
zone: A logical grouping of network devices in AppleTalk. See also: ZIP.

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