Glossary
A
ABR 1. Area Border Router. Router located on the border of one or more OSPF areas that connects those areas to the backbone network. 2. available bit rate. QoS class for ATM networks. ABR provides a feedback mechanism that lets traffic sources adapt their transmissions to changing network conditions to facilitate low cell loss and the fair sharing of available bandwidth. ABR provides no guarantees in terms of cell loss or delay. Compare with CBR, UBR, and VBR.
access layer One of three layers in a hierarchical network topology, provides users on local segments access to the internetwork. Compare with core layer and distribution layer. See also hierarchical network design.
access server See remote-access server.
accuracy The amount of useful traffic that is correctly transmitted on a network, relative to total traffic. Accuracy is negatively affected by the bit error rate (BER), cell error rate (CER), or the number of frame errors compared to the total number of bytes transmitted. See also BER and CER.
ACL access control list. List kept by a router or firewall to control access to or from the router for several services (for example, to prevent packets with a certain IP address from leaving a particular interface on the router).
adaptability The ease with which a network design and implementation can adapt to network faults, changing traffic patterns, additional business or technical requirements, and other changes.
Address Resolution Protocol See ARP.
administrative distance Rating of the trustworthiness of a route in a routing table. A route with a low administrative distance is preferred over routes with higher administrative distances.
ADSL asymmetric digital subscriber line. One of many DSL technologies. ADSL is designed to deliver more bandwidth downstream (from the provider to the customer site) than upstream. See also DSL and HDSL.
affordability A common network design goal that specifies the importance of containing the costs associated with developing and implementing a network design, including the purchasing and operating of network equipment and services.
AFP Apple Filing Protocol. Application and presentation layer protocol that enables users to share data files and application programs that reside on a file server.
agent In network management, a process that resides in a managed device and reports the values of specified variables to management stations.
Apple Filing Protocol See AFP.
AppleTalk Series of communications protocols designed by Apple Computer that featured ease of use, dynamic addressing, and simplified resource discovery.
application layer Layer 7 of the OSI reference model. This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation) that are outside the OSI model. The application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity.
area Logical set of network segments and their attached devices. Areas are usually connected to other areas via routers.
ARP Address Resolution Protocol. Internet protocol used to map an IP address to a MAC address. Defined in RFC 826. Compare with RARP. See also proxy ARP.
asymmetric digital subscriber line See ADSL.
asymmetric encryption An encryption technique in which a different key is used to encrypt a message than is used to decrypt the message. Compare with symmetric encryption.
asynchronous routing A function of a remote-access server that provides Layer 3 routing functionality to connect LANs via an asynchronous serial WAN link.
ATM Asynchronous Transfer Mode. International standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells.
authentication In security, the verification of the identity of a person or process.
authorization Securing a network by specifying which areas of the network (applications, devices, and so forth) a user is allowed to access.
autonomous system Collection of networks or areas under a common administration sharing a common routing strategy.
availability The amount of time a network is available to users, often expressed as a percent uptime, or as a mean time between failure (MTBF) and mean time to repair (MTTR). See also MTBF and MTTR.
B
backbone A network that connects many other networks and acts as the primary path for traffic between those networks.
backup links Physical redundant connections between network devices.
backward explicit congestion notification See BECN.
bandwidth domain In a LAN, the set of devices that share and compete for bandwidth. Bandwidth domains are bounded by switches, bridges, or routers. A hub or repeater does not bound a bandwidth domain. Also called a collision domain on Ethernet networks.
baseline Characterization of the normal traffic flow and performance of a network, used as input to a new or enhanced design for the network.
beacon Frame sent by a wireless access point during normal operation. Wireless clients listen to beacon frames to locate access points.
BECN backward explicit congestion notification. Bit set by a Frame Relay network in frames traveling in the opposite direction of frames encountering a congested path. Compare with FECN.
BER bit error rate. Ratio of received bits that contain errors to the total number of received bits.
BERT bit error rate tester. Device that determines the BER on a given communications channel.
BGP Border Gateway Protocol. Interdomain routing protocol that exchanges reachability information with other BGP systems. BGP Version 4 (BGP4) is the predominant interdomain routing protocol used on the Internet.
BIND Berkeley Internet Name Domain. Implementation of DNS developed and distributed by the University of California at Berkeley (United States).
bit error rate tester See BERT.
BOOTP Bootstrap Protocol. Protocol used by a network node to determine the IP address of its interfaces to achieve network booting. See also DHCP.
Border Gateway Protocol See BGP.
BPDU bridge protocol data unit. Spanning Tree Protocol frame that is sent out at configurable intervals to exchange information among bridges in a network.
bridge Device that connects and passes frames between two network segments. Bridges operate at the data link layer (Layer 2) of the OSI reference model. A bridge filters, forwards, or floods an incoming frame based on the MAC destination address of the frame.
bridge protocol data unit See BPDU.
broadcast Message that is sent to all nodes on a network. Compare with multicast and unicast.
broadcast address Special address reserved for sending a message to all nodes. At Layer 2, a broadcast address is a MAC destination address of all 1s (FF:FF:FF:FF:FF:FF in hexadecimal). At Layer 3, for IP, a broadcast address is 255.255.255.255. Compare with multicast address and unicast address.
broadcast domain The set of all devices that receives broadcast frames originating from any device within the set. Broadcast domains are bounded by routers (which do not forward broadcast frames). A switch or hub does not bound a broadcast domain.
broadcast storm Undesirable network event in which many broadcasts are sent in quick succession across numerous network segments. A broadcast storm uses substantial network bandwidth, causes extra processing at network nodes, and can cause network timeouts.
building network Multiple LANs within a building, usually connected to a building-backbone network.
bursty traffic Network traffic characterized by short intervals of intense activity with lulls between the intervals.
C
cable modem A modem that operates over the coaxial cable that is used by cable TV providers. Because the coaxial cable provides greater bandwidth than telephone lines, a cable modem offers much faster access than an analog modem.
caching Form of replication in which information learned during a previous transaction is used to process later transactions.
campus network A set of LAN segments and building networks in a geographical area that is a few miles in diameter.
capacity The data-carrying capability of a circuit or network, measured in bits per second (bps). Because this book focuses on digital transmission technologies, it uses the terms capacity and bandwidth synonymously.
carrier sense multiple access with collision avoidance See CSMA/CA.
carrier sense multiple access with collision detection See CSMA/CD.
CBR constant bit rate. QoS class for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery. Compare with ABR, UBR, and VBR.
CD Carrier Detect. Signal that indicates whether an interface is active. Also, a signal generated by a modem indicating that a call has been connected.
CDP Cisco Discovery Protocol. Device-discovery protocol that runs on Cisco-manufactured equipment, including routers, remote-access servers, and switches. Using CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN or on the remote side of a WAN.
CDV cell delay variation. In ATM, component of cell transfer delay (CTD) that is induced by buffering and cell scheduling. CDV is a QoS delay parameter associated with CBR and VBR service.
cell Basic data unit for ATM switching and multiplexing. Cells contain identifiers that specify the data stream to which they belong. Each cell consists of a 5-byte header and 48 bytes of payload.
CER cell error ratio. In ATM, the ratio of transmitted cells that have errors to the total cells sent in a transmission for a specific period of time.
Challenge Handshake Authentication Protocol See CHAP.
channel aggregation A process wherein a device can automatically bring up multiple circuits as bandwidth requirements increase.
CHAP Challenge Handshake Authentication Protocol. Security feature supported on links using PPP encapsulation that identifies the remote end of a PPP session using a handshake protocol and a variable challenge value that is unique and unpredictable. Compare with PAP.
CIDR classless interdomain routing. Technique supported by BGP4 and other routing protocols based on route summarization (aggregation). CIDR allows routers to group routes together to cut down on the quantity of routing information carried by the core routers.
CIR committed information rate. Rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics for a Frame Relay service.
circuit Communications path between two or more points.
Cisco Discovery Protocol See CDP.
Cisco IOS Cisco Internetwork Operating System. Cisco software that provides common functionality, scalability, and security for Cisco products. Cisco IOS Software supports a wide variety of protocols, media, services, and platforms.
classless interdomain routing See CIDR.
client Node or software program that requests services from a server.
client/server Distributed-computing network systems in which transaction responsibilities are divided into two parts: client and server. Clients rely on servers for services such as file storage, printing, and processing power.
CLR cell loss ratio. In ATM, the ratio of discarded cells to cells that are successfully transmitted. CLR can be set as a QoS parameter when a connection is set up.
coaxial cable Cable consisting of a single inner wire conductor that is surrounded by a flexible, tubular insulating layer that is surrounded by a tubular conductive shield. Also known simply as coax cable.
codec coder-decoder. Device that typically uses pulse-code modulation to transform analog signals into a digital bit stream and digital signals back into analog.
collision In Ethernet, the result of two nodes transmitting simultaneously.
collision domain In Ethernet, the network area within which frames that have collided are propagated. Repeaters and hubs propagate collisions; LAN switches, bridges, and routers do not. See also bandwidth domain.
committed information rate See CIR.
Common Open Policy Service See COPS.
compression The running of data through an algorithm that reduces the space required to store or the bandwidth required to transmit the data.
congestion A condition whereby network traffic has reached or is approaching network capacity.
connection admission control Set of actions taken by an ATM switch during connection setup that determines whether a connection’s requested QoS violates the QoS guarantees for established connections.
connection-oriented Data transfer that requires the establishment of a virtual circuit.
connectionless Data transfer without the existence of a virtual circuit.
convergence Speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology.
COPS Common Open Policy Service. An IETF protocol that defines a client/server model for supporting policy control with QoS-reservation protocols such as RSVP.
core layer The high-speed backbone of an internetwork in a hierarchical topology. The core layer should be highly reliable and adapt to changes quickly. Compare with access layer and distribution layer. See also hierarchical network design.
cost Arbitrary value, typically based on hop count, media bandwidth, a configurable parameter, or other measures, that is used by routing protocols to determine the most favorable path to a particular destination. The lower the cost, the better the path. See also routing metric.
count to infinity Problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically, some arbitrary hop-count limit is imposed to prevent this problem.
CRC cyclic redundancy check. Error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.
CSMA/CA carrier sense multiple access with collision avoidance. Media-access mechanism wherein devices try to avoid collisions by waiting a random period of time following a transmission before starting their own transmission. Devices sense the medium to determine if another device is already transmitting. If no transmission is sensed, a device can transmit. If the medium is busy, devices defer their transmissions for a random period of time. CSMA/CA is used by IEEE 802.11 wireless networks.
CSMA/CD carrier sense multiple access with collision detection. Media-access mechanism wherein devices determine if another device is already transmitting before starting their own transmissions. If no transmission is sensed for a specific period of time, a device can transmit. If multiple devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmission from those devices for some random length of time. CSMA/CD is used by Ethernet and IEEE 802.3.
CSU channel service unit. Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to together with DSU, as CSU/DSU.
CTD cell transfer delay. In ATM, the elapsed time between a cell-exit event at the source UNI and the corresponding cell-entry event at the destination UNI for a particular connection.
custom queuing Cisco IOS routing feature that assigns different amounts of queue space to different protocols and handles queues in a round-robin fashion. Custom queuing assures each specified traffic type a fixed portion of available bandwidth. Compare with priority queuing.
cut-through switching Frame-switching approach that streams data through a switch so that the leading edge of a frame exits the switch at the output port before the frame finishes entering the input port. A device using cut-through switching forwards frames as soon as the destination address is looked up and the outgoing port determined. Compare with store-and-forward switching.
cyclic redundancy check See CRC.
D
Data Encryption Standard See DES.
data link layer Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer.
data store An area in a network where application layer data resides. A data store can be a server, a set of servers, a mainframe, a tape backup unit, a digital video library, or any device or component of an internetwork where large quantities of data are stored. Sometimes called a data sink.
data terminal equipment See DTE.
datagram Logical grouping of information sent as a network layer unit over a transmission medium without prior establishment of a virtual circuit.
DCE data communications equipment or data circuit-terminating equipment. Devices and connections of a communications network that comprise the network end of the user-to-network interface. Modems and interface cards are examples of DCE. Compare with DTE.
DDR dial-on-demand routing. Technique whereby a router can automatically initiate and close a circuit-switched session as transmitting stations demand.
decapsulation The reverse application of encapsulation. Decapsulation unwraps data from a protocol header that was added by a device or process before transmitting the data. Compare with encapsulation.
decryption The reverse application of an encryption algorithm to encrypted data, thereby restoring the data to its original, unencrypted state. See also encryption.
default gateway See default router.
default route Routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table.
default router IP address of a router configured on an end station to allow the station to get to the rest of the internetwork. Also called default gateway.
delay 1. Time between the initiation of a transaction by a sender and the first response received by the sender. 2. Time required to move a frame from source to destination over a given path.
delay variation The amount of time average delay varies. See also jitter.
demultiplexing Separating of multiple input streams that were multiplexed into a common physical signal back into multiple output streams. See also multiplexing.
denial of service A security attack where an intruder disables a network service by flooding it with requests, making it unusable by legitimate users.
dense-mode PIM One of two PIM operational modes. With dense-mode PIM, packets are forwarded on all outgoing interfaces until pruning occurs. Receivers are densely populated, and it is assumed that many downstream networks want to receive and will probably use multicast datagrams that are forwarded to them. See also PIM, prune, and sparse-mode PIM.
DES Data Encryption Standard. Standard cryptographic algorithm developed by the U.S. National Bureau of Standards (now called the National Institute of Standards and Technology, or NIST).
DHCP Dynamic Host Configuration Protocol. Provides a mechanism for allocating IP addresses dynamically to minimize configuration and allow addresses to be reused when hosts no longer need them.
dial-on-demand routing See DDR.
differentiated services codepoint See DSCP.
Diffusing Update Algorithm See DUAL.
digital signal level 1 See DS-1.
digital signature String of bits appended to a message that provides authentication and data integrity.
digital subscriber line See DSL.
discontiguous subnet An IP subnet that is made up of two or more physical networks that are separated by routers.
distance learning A training method wherein students attend a class from remote sites using videoconferencing or other digital or analog video and audio techniques.
Distance Vector Multicast Routing Protocol See DVMRP.
distance vector routing algorithm Class of routing algorithms that calls for each router to send its routing table in periodic update packets to its neighbors. Compare with link-state routing algorithm.
distribution layer Connects network services to the access layer in a hierarchical topology, and implements policies regarding security, traffic loading, and routing. Compare with access layer and core layer. See also hierarchical network design.
DNS Domain Name System. System used in the Internet for translating names of network nodes into addresses.
DS-1 digital signal level 1. Framing specification used in transmitting digital signals at 1.544 Mbps on a WAN circuit.
DSCP differentiated services codepoint. A 6-bit field in the IP header used for packet classification purposes and indicating how packets should be forwarded. Replaces the outdated IP precedence field.
DSL digital subscriber line. Public network technology that delivers high bandwidth over conventional telephone wiring at limited distances. See also ADSL and HDSL.
DSU data service unit. Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. Often referred to together with CSU, as CSU/DSU. See also CSU.
DTE data terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, internetworking devices, and multiplexers. Compare with DCE.
DUAL Diffusing Update Algorithm. Convergence algorithm used in EIGRP that provides loop-free operation. DUAL allows routers involved in a topology change to synchronize, while not involving routers that are unaffected by the change. See also Enhanced Interior Gateway Routing Protocol.
DVMRP Distance Vector Multicast Routing Protocol. Multicast routing protocol, largely based on RIP. Packets are forwarded on all outgoing interfaces until pruning occurs. See also prune.
Dynamic Host Configuration Protocol (DHCP) See DHCP.
dynamic password Security mechanism that incorporates a dynamically generated password that can be used only once.
E
E1 Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate of 2.048 Mbps. E1 lines can be leased for private use from common carriers.
efficiency A measure of how much overhead is required to produce a certain amount of data throughput on a network. Overhead includes frame headers and trailers, acknowledgments, media access methods, and flow-control mechanisms.
EIGRP See Enhanced Interior Gateway Routing Protocol.
encapsulation Wrapping of data in a particular protocol header.
encryption Application of a specific algorithm to alter the appearance of data, making it incomprehensible to those who are not authorized to see the information. See also decryption.
encryption key A code used by an encryption algorithm to scramble and unscramble data.
Enhanced Interior Gateway Routing Protocol Advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency, and combines the advantages of link-state protocols with those of distance vector protocols.
enterprise network Large and diverse internetwork connecting most major points in an organization. An enterprise network typically consists of building and campus networks, remote-access services, and one or more WANs.
Ethernet LAN technology invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10 Mbps. Ethernet is similar to IEEE 802.3.
F
Fast Ethernet Any of several 100-Mbps Ethernet specifications. Fast Ethernet offers a speed increase ten times that of the original IEEE 802.3 specification, while preserving such qualities as frame format, MAC mechanisms, and frame size.
FDDI Fiber Distributed Data Interface. LAN standard specifying a 100-Mbps token-passing network using fiber-optic cable and a dual-ring architecture to provide redundancy.
FECN forward explicit congestion notification. Bit set by a Frame Relay network to inform a device receiving the frame that congestion was experienced in the path from source to destination. Compare with BECN.
Fiber Distributed Data Interface See FDDI.
fiber-optic cable Physical medium capable of conducting modulated light transmission. Fiber-optic cable is not susceptible to electromagnetic interference and is capable of high data rates.
fiber-optic interrepeater link See FOIRL.
FIFO first in, first out. Method of sending traffic through a device whereby the first packet received is the first packet transmitted. Does not support prioritization.
File Transfer Protocol See FTP.
filter Generally, a process or device that screens network traffic for certain characteristics, such as a source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.
firewall Router, software, appliance, or remote-access server designated as a buffer between connected networks. A firewall uses access lists and other methods to ensure the security of a network.
flat network design A network design that has little or no hierarchy or modularity and is generally only appropriate for small shared or switched LANs.
flooding Traffic-passing technique used by switches and bridges, in which traffic received on an interface is sent out all interfaces except the interface on which the information was received.
flow Stream of data traveling between two endpoints across a network.
flow control Technique for ensuring that a transmitting entity does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend transmission until the data in the buffers has been processed.
FOIRL fiber-optic interrepeater link. Fiber-optic signaling methodology for transmitting Ethernet frames on fiber-optic cables, based on the IEEE 802.3 fiber-optic specification.
forward explicit congestion notification See FECN.
forwarding Process of sending a frame toward its ultimate destination by way of an internetworking device.
fragmentation Process of breaking a packet into smaller units when transmitting over a network medium that cannot support the original size of the packet. See also reassembly.
frame Logical grouping of information sent as a data link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control, that surround the user data contained in the unit.
Frame Relay Industry-standard, switched data link layer protocol that handles multiple virtual circuits between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. See also X.25.
FTP File Transfer Protocol. Application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes. FTP is defined in RFC 959.
full duplex Capability for simultaneous data transmission between a sending station and a receiving station. Compare with half duplex.
full mesh Term describing a network in which devices are organized in a mesh topology, with each network node having either a physical circuit or a virtual circuit connecting it to every other network node. See also mesh and partial mesh.
G–H
Gigabit Ethernet 1000-Mbps LAN technologies specified in IEEE 802.3z. Gigabit Ethernet offers a speed increase 100 times that of the original IEEE 802.3 specification, while preserving such qualities as frame format, MAC mechanisms, and frame size.
group address See multicast address.
H.320 Suite of international standard specifications for videoconferencing over circuit-switched media such as ISDN, fractional T1, or switched-56 lines.
H.323 Extension of H.320 that enables videoconferencing over LANs and other packet-switched networks, and video over the Internet.
half duplex Capability for data transmission in only one direction at a time between a sending station and a receiving station. Compare with full duplex.
handshaking Process whereby two protocol entities synchronize during connection establishment.
hardware address See MAC address.
hash Resulting string of bits from a hash function. See also message digest.
hash function Any well-defined procedure or mathematical function that converts a large, possibly variable-sized piece of data into a small hash code, or hash. The hash is generally a single integer that serves as an index into an array or as an integrity check of the input. The function is not reversible to produce the original input.
HDLC High-Level Data Link Control. Synchronous data link layer protocol for WAN links that specifies framing and error control.
HDSL high-data-rate digital subscriber line. One of many DSL technologies. HDSL delivers 1.544 Mbps of bandwidth each way over two copper twisted pairs. See also DSL and ADSL.
hello packet Packet that is used by networking devices for neighbor discovery and recovery, and to indicate that a device is still operating.
helper address Address configured on a router interface to which broadcasts received on that interface will be sent, commonly used to forward DHCP requests.
hierarchical network design A technique for designing scalable campus and enterprise network topologies using a layered, modular model. See also access layer, core layer, and distribution layer.
hierarchical routing A model for distributing knowledge of a network topology among internetwork routers. With hierarchical routing, no single router needs to understand the complete topology. See also route summarization.
high-data-rate digital subscriber line See HDSL.
High-Level Data Link Control See HDLC.
holddown State into which a route is placed so routers will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). Holddown is used to flush bad information about a route from all routers in the network. A route is typically placed in holddown when a link to that route fails.
hop Term describing the passage of a data packet between two network nodes (for example, between two routers).
hop count Routing metric used to measure the distance between a source and a destination in number of routers or hops between the source and destination.
host A system on an IP network.
HSRP Hot Standby Router Protocol. Provides high network availability and transparent network topology changes. HSRP creates a Hot Standby router group with a lead router that services all packets sent to the Hot Standby address. The lead router is monitored by other routers in the group, and if it fails, one of these standby routers inherits the lead position and the Hot Standby group address. Hosts use the Hot Standby address as the address of their default gateway.
hub 1. Generally, a term used to describe a device or network that serves as the center of a star or hub-and-spoke topology. 2. In Ethernet and IEEE 802.3, an Ethernet multiport repeater.
hub-and-spoke topology A topology that consists of one central network and a set of remote networks each with one connection to the central network and no direct connections to each other. Traffic between remote networks goes through the hub network.
I
IANA Internet Assigned Numbers Authority. Organization operated by ICANN that manages IP address-space allocation, domain-name assignment, and autonomous system number assignment. IANA also maintains a database of assigned protocol identifiers used in the TCP/IP protocol stack.
ICANN Internet Corporation for Assigned Names and Numbers. A nonprofit corporation with global participants dedicated to keeping the Internet secure, stable, and interoperable, tasked with delegating authorization for naming and numbering on the Internet.
ICMP Internet Control Message Protocol. Network layer TCP/IP protocol that reports errors and provides other information relevant to IP packet processing. Documented in RFC 792.
IEEE Institute of Electrical and Electronics Engineers. Professional organization whose activities include the development of communications and network standards. IEEE LAN standards are the predominant LAN standards today.
IEEE 802.1D IEEE specification that describes an algorithm that prevents bridging loops by creating a spanning tree.
IEEE 802.1p IEEE LAN protocol for supporting QoS on LANs. Specifies mechanisms in bridges to expedite the delivery of time-critical traffic and to limit the extent of high-bandwidth multicast traffic within a bridged LAN. IEEE 802.1p is now a standard part of IEEE 802.1D.
IEEE 802.1Q IEEE LAN protocol for supporting VLANs across various media.
IEEE 802.2 IEEE LAN protocol that specifies an implementation of the LLC sublayer of the data link layer. See also LLC.
IEEE 802.3 IEEE LAN protocol that specifies an implementation of the physical layer and the MAC sublayer of the data link layer. IEEE 802.3 uses CSMA/CD access at a variety of speeds over a variety of physical media. IEEE 802.3 is similar to Ethernet.
IETF Internet Engineering Task Force. Task force consisting of numerous working groups responsible for developing Internet and TCP/IP standards.
IGMP Internet Group Management Protocol. Used by IP hosts to report their multicast group memberships to an adjacent multicast router. Defined in RFC 1112.
IGRP Interior Gateway Routing Protocol. An interior routing protocol developed by Cisco to address the problems associated with routing in large, heterogeneous networks. Compare with Enhanced Interior Gateway Routing Protocol.
Institute of Electrical and Electronics Engineers See IEEE.
Integrated Services Digital Network See ISDN.
Integrated Services working group See ISWG.
integrity Keeping data unchanged and safe from tampering as it traverses the network.
interface 1. Connection between two systems or devices. 2. A port on a device.
Interior Gateway Routing Protocol See IGRP.
Intermediate System-to-Intermediate System See IS-IS.
International Organization for Standardization See ISO.
Internet Term used to refer to the largest global internetwork, connecting hundreds of thousands of networks worldwide using the TCP/IP protocol stack.
Internet Assigned Numbers Authority See IANA.
Internet Control Message Protocol See ICMP.
Internet Corporation for Assigned Names and Numbers See ICANN.
Internet Engineering Task Force See IETF.
Internet Group Management Protocol See IGMP.
Internet service provider See ISP.
Internet telephony Generic term used to describe various approaches to running voice traffic over IP networks, in particular, the Internet. See also VoIP.
internetwork Collection of networks interconnected by routers.
Internetwork Packet Exchange See IPX.
internetworking General term used to refer to the industry and technologies devoted to connecting networks together.
IP Internet Protocol. Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, differentiated services, forwarding packets, and fragmentation and reassembly. Defined in RFC 791.
IP address A unique address assigned to an interface using TCP/IP. An IPv4 32-bit address is written as four octets separated by periods (dotted decimal format). Each address consists of a network number, an optional subnetwork number, and a host number. An IPv6 128-bit address is written in hexadecimal.
IP multicast Routing technique that allows IP traffic to be propagated from one source to several destinations. Rather than sending one packet to each destination, one packet is sent to a multicast group identified by a single IP destination group address.
IPsec IP Security Protocol. A set of open standards that provides data confidentiality, data integrity, and authentication between participating peers at the IP layer. VPNs use IPsec.
IPv4 Internet Protocol (IP), version 4.
IPv6 Internet Protocol (IP), version 6. Replacement for IPv4.
IPX Internetwork Packet Exchange. Novell NetWare network layer (Layer 3) protocol used for transferring data between servers and workstations.
ISDN Integrated Services Digital Network. Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.
IS-IS Intermediate System-to-Intermediate System. OSI link-state hierarchical routing protocol based on DECnet Phase V routing.
ISL Inter-Switch Link. Cisco-proprietary protocol that maintains VLAN information as traffic flows between switches and routers.
ISO International Organization for Standardization. International organization that is responsible for a wide range of standards, including those relevant to networking. ISO developed the OSI reference model, a popular networking reference model. Because International Organization for Standardization has different acronyms in different languages (IOS in English, OIN in French for Organisation Internationale de Normalisation), its founders decided to give it a short, all-purpose name. They chose ISO, derived from the Greek isos, meaning equal.
ISOC Internet Society. International nonprofit organization, founded in 1992, that coordinates the evolution and use of the Internet.
ISP Internet service provider. Company that provides Internet access to other companies and individuals.
ISWG Integrated Services working group. Subset of the IETF dedicated to defining QoS mechanisms for the Internet and other TCP/IP-based networks.
J–K–L
jitter Communication line distortion caused by the variation of a signal from its reference timing positions. See also delay variation.
keepalive message Message sent by one network device to inform another network device that the virtual circuit between the two is still active.
Kerberos An authentication system that provides user-to-host security for application layer protocols such as FTP and Telnet.
L2TP Layer 2 Tunneling Protocol. An IETF standard for tunneling private data over public networks.
LAN Local-area network. High-speed, low-error data network covering a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area.
late collision An Ethernet collision that occurs after the first 64 bytes of a frame. Compare with legal collision. See also collision and CSMA/CD.
latency 1. A delay, a period between the initiation of something and the occurrence. 2. Delay between the time a device requests access to a network and the time it is granted permission to transmit. 3. Delay between the time a forwarding device receives a frame and the time that frame is forwarded out the destination port.
Layer 2 Tunneling Protocol See L2TP.
Layer 3 switch Switch that filters and forwards packets based on MAC addresses and network addresses. Also referred to as multilayer switch.
leased line Transmission line reserved by a communications carrier for the private use of a customer.
legal collision An Ethernet collision that occurs within the first 64 bytes of a frame. Compare with late collision. See also collision and CSMA/CD.
link Network communications channel consisting of a circuit or transmission path and all related equipment between a sender and a receiver.
link-state advertisement See LSA.
link-state routing algorithm Routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching its neighbors to all nodes in its area or areas. Compare with distance vector routing algorithm.
LLC Logical Link Control. Higher of the two data link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow control, framing, and the network layer (Layer 3) service interface. Includes both connectionless and connection-oriented services. Also known as IEEE 802.2.
LLC2 Logical Link Control, type 2. Connection-oriented LLC.
load balancing In routing, the capability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Sometimes called load sharing to indicate that most protocols do not evenly balance traffic.
LSA link-state advertisement. Multicast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.
M
MAC Media Access Control. Lower of the two sublayers of the data link layer defined by the IEEE. The MAC sublayer handles access to shared media, such as whether token passing or contention will be used.
MAC address Standardized data link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network. MAC addresses are 6 bytes long and include a 3-byte vendor code that is controlled by the IEEE. Also referred to as a hardware address, MAC layer address, or physical address. Compare with network address.
MAN metropolitan-area network. A network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN.
manageability The ease with which a network can be managed and monitored, including the management of the network’s performance, faults, configuration, security, and accounting capabilities.
Management Information Base See MIB.
maximum cell delay variation See MCDV.
maximum cell transfer delay See MCTD.
maximum transmission unit See MTU.
MBS maximum burst size. Parameter defined for ATM traffic management. MCR is defined for VBR transmissions.
MCDV maximum cell delay variation. In an ATM network, the maximum two-point CDV objective across a link or node for the specified service category.
MCR minimum cell rate. Parameter defined for ATM traffic management. MCR is defined for ABR transmissions.
MCTD maximum cell transfer delay. In an ATM network, the sum of the MCDV and the fixed delay component across the link or node.
mean time between failure See MTBF.
media Plural of medium. The various physical environments through which transmission signals pass. Common network media include twisted-pair, coaxial, and fiber-optic cable; and the atmosphere (through which wireless microwave, laser, and infrared transmission occurs).
mesh Network topology in which devices are organized with many, often redundant, interconnections strategically placed between network nodes. See also full mesh and partial mesh.
message digest Value returned by a hash function. Also referred to as hash.
metropolitan-area network See MAN.
MIB Management Information Base. Database of network management information that is used and maintained by a network management protocol such as SNMP. The value of a MIB object can be changed or retrieved using SNMP commands, usually through a network management system (NMS). MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.
MPEG Moving Picture Experts Group. Standard for compressing video.
MPLS See Multiprotocol Label Switching.
MPPP Multilink PPP. Method of splitting, recombining, and sequencing datagrams across multiple PPP circuits. Sometimes abbreviated as MLP or simply MP.
MTBF mean time between failure. The average time that elapses between network or system failures.
MTTR mean time to repair. The average amount of time it takes to fix a network or system when it fails.
MTU maximum transmission unit. Maximum frame size, in bytes, that a particular interface or medium can handle.
multicast Message that is sent to a subset of nodes on a network. Compare with broadcast and unicast.
multicast address Single address that refers to multiple network nodes. Also referred to as group address. Compare with broadcast address and unicast address.
multicast group Dynamically determined group of IP hosts identified by a single IP multicast address.
multicast routing protocol Routing protocol used to route multicast packets. See also DVMRP and PIM.
multichassis MPPP Extends MPPP support across multiple routers and remote-access servers. Multichassis MPPP enables multiple routers and access servers to operate as a single, large dialup pool, with a single network address and access number. Multichassis MPPP correctly handles packet fragmenting and reassembly when a user connection is split between two physical access devices. Sometimes abbreviated MMP. See also MPPP.
multihoming Attaching a host or network to multiple physical network segments.
multilayer switch Switch that filters and forwards packets based on MAC addresses and network addresses. Also referred to as Layer 3 switch.
multimode fiber Optical fiber with a relatively large core that supports more than one propagation mode. Typically used for communication over shorter distances, such as within a building or on a campus, with data rates of 10 Mbps to 10 Gbps. Compare with single-mode fiber.
multiplexing Scheme that allows multiple logical signals to be transmitted simultaneously across a single physical channel. Compare with demultiplexing.
Multiprotocol Label Switching Switching method that forwards IP traffic using a label. This label instructs routers in the network where to forward packets based on pre-established IP routing information.
N
NAT Network Address Translation. Mechanism for reducing the need for globally unique IP addresses. NAT allows an organization with addresses that are not globally unique to connect to the Internet by translating those addresses into globally routable addresses.
NBMA nonbroadcast multiaccess. Term describing a multiaccess network that does not inherently support broadcasting; for example, ATM.
NetBIOS Network Basic Input/Output System. API used by applications on a LAN to request services from lower-level network processes. These services include session establishment and termination, and information transfer. NetBIOS is used by Windows operating systems primarily.
NetFlow A Cisco optimization technique that identifies traffic flows and speeds the forwarding of traffic for a flow. When a flow is identified, the switching, security, QoS, and traffic-measurement services required for the flow are used to build an entry in a NetFlow cache. Subsequent packets in the flow are handled via a single streamlined task that references the cache. Also refers to NetFlow export records, which provide details of the traffic in a network.
NetWare Distributed network-operating system developed by Novell. Provided transparent remote file access and numerous other distributed network services.
network address Network layer address referring to a logical, rather than a physical, network device. Used by the network layer. Compare with MAC address.
Network Address Translation See NAT.
Network Information Center See NIC.
network interface card See NIC.
network layer Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The network layer is the layer at which routing occurs.
network management system See NMS.
network utilization See utilization.
Network-to-Network Interface See NNI.
Next Hop Resolution Protocol See NHRP.
NFS Network File System. A distributed file-system protocol suite developed by Sun Microsystems that allows remote file access across a network.
NHRP Next Hop Resolution Protocol. Protocol used by routers to dynamically discover the MAC address of other routers and hosts connected to an NBMA network. These systems can then directly communicate without requiring traffic to use an intermediate hop, thus increasing performance in ATM, Frame Relay, and other environments.
NIC 1. network interface card. Board that provides network-communication capabilities for a computer system. 2. Network Information Center. Organization that historically served the Internet community by supplying addressing, naming, documentation, training, and other services.
NMS network management system. System responsible for managing a network. An NMS is generally a powerful and well-equipped computer such as an engineering workstation. NMSs communicate with agents to help keep track of network statistics and resources.
NNI Network-to-Network Interface. ATM standard that defines the interface between two ATM switches that are both located in a private network, or are both located in a public network.
nonbroadcast multiaccess See NBMA.
O–P
OC Optical Carrier. Series of physical protocols (OC-1, OC-2, OC-3, and so on) defined for SONET optical signal transmissions. OC signal levels put STS frames onto fiber-optic lines at a variety of speeds. The base rate is 51.84 Mbps (OC-1); each signal level thereafter operates at a speed divisible by that number (thus, OC-3 runs at 155.52 Mbps). See also SONET and STS-1.
offered load The sum of all the data all network nodes have ready to send at a particular time.
Open Shortest Path First See OSPF.
OSI reference model Open System Interconnection reference model. Network architectural model that consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The OSI reference model is used universally as a method for teaching and understanding network functionality. See application layer, data link layer, network layer, physical layer, presentation layer, session layer, and transport layer.
OSPF Open Shortest Path First. Link-state, hierarchical interior routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF is an open, nonproprietary protocol.
packet Logical grouping of information that includes a header containing control information and (usually) user data. Packets are most often used to refer to network layer units of data.
PAP Password Authentication Protocol. Authentication protocol that allows PPP peers to authenticate one another. Unlike CHAP, PAP passes the password and hostname or username in clear text (unencrypted). Compare with CHAP.
partial mesh Term describing a network in which devices are organized in a mesh topology without requiring that every device have a direct connection to every other device. Compare with full mesh. See also mesh.
PBX private branch exchange. Digital or analog telephone switchboard located on a subscriber premises and used to connect private and public telephone networks.
PCR peak cell rate. Parameter for ATM traffic management. In CBR transmissions, a source can emit cells at the PCR at any time and for any duration and the negotiated QoS commitments should pertain.
phantom router See virtual router.
physical address See MAC address.
physical layer Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
PIM Protocol Independent Multicast. Multicast routing architecture that allows the addition of IP multicast routing on existing IP networks. PIM does not require a specific unicast routing protocol and can be operated in two modes: dense mode and sparse mode. See also dense-mode PIM and sparse-mode PIM.
ping 1. ICMP echo message and its reply. Used in IP networks to test the reachability of a network device. 2. Generic term for an echo mechanism in any protocol stack.
plesiochronous transmission Term describing digital signals that are sourced from different clocks of comparable accuracy and stability.
Point-to-Point Protocol See PPP.
poison-reverse updates Routing updates that explicitly indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates. Poison-reverse updates are sent to defeat routing loops.
port 1. Interface on an internetworking device (such as a router or switch). 2. In TCP/IP terminology, a transport layer process that sends and receives information from lower layers. Ports are numbered, and each numbered port is associated with a specific process.
PPP Point-to-Point Protocol. Protocol that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP was designed to work with several network layer protocols, such as IP, IPv6, IPX, and AppleTalk.
pps packets per second. A measure of how quickly a switch or router can forward data.
precedence Outdated field in an IP header that indicated the priority of a packet. Replaced by the DSCP field. Precedence helped a router determine which packet to send when several packets were queued for transmission to the same output interface. See also DSCP.
presentation layer Layer 6 of the OSI reference model. This layer ensures that information sent by the application layer of one system is readable by the application layer of another.
priority queuing Cisco IOS routing feature in which frames in an interface output queue are prioritized based on various characteristics such as packet and interface type. Compare with custom queuing.
private branch exchange See PBX.
private key Digital code used to decrypt/encrypt information and provide digital signatures. This key should be kept secret by the owner; it has a corresponding public key.
propagation delay Time required for data to travel over a network, from its source to its ultimate destination.
protocol analyzer Hardware or software device offering various network troubleshooting features, including protocol-specific packet decodes, specific preprogrammed troubleshooting tests, and traffic generation.
Protocol Independent Multicast See PIM.
proxy ARP Proxy Address Resolution Protocol. Variation of the ARP protocol, in which an intermediate device (for example, a router) sends an ARP response on behalf of an end node to the requesting host. Defined in RFC 1027.
prune In an IP multicast environment, the process wherein a router detects that there are no group members on one of its interfaces and stops sending a multicast stream out that interface.
public key A digital code used to encrypt/decrypt information and verify digital signatures. This key can be made widely available; it has a corresponding private key.
pull In client/server information-dissemination applications, requesting data from another computer or application. The web is based on pull technology in which a client uses a browser to request (pull) a web page. Compare with push.
push In client/server information-dissemination applications, sending data to a client without the client requesting it. Increasingly, companies are using push technologies to deliver customized data to users without the users explicitly requesting it. An example is customized news and stock information that is delivered on a daily basis. Compare with pull.
Q–R
QoS quality of service. 1. Methodology for prioritizing traffic on an internetwork, usually to reduce delay for specific types of traffic such as VoIP. 2. Measure of performance for a transmission system that reflects its transmission quality and service availability.
queue 1. Generally, an ordered list of elements waiting to be processed. 2. In routing, a backlog of packets waiting to be forwarded over a router interface.
RADIUS Remote Authentication Dial-In User Service. Protocol and database for authenticating users, tracking connection times, and authorizing services permitted to users. A remote-access server acts as a client of a RADIUS server.
RARP Reverse Address Resolution Protocol. Protocol in the TCP/IP stack that provides a method for a diskless station to determine its IP address when its MAC address is known. Compare with ARP.
Real-Time Control Protocol See RTCP.
Real-Time Transport Protocol See RTP.
reassembly The putting back together of an IP datagram at the destination after it has been fragmented either at the source or at an intermediate node. See also fragmentation.
recoverability How quickly a network or computer system can recover from a problem.
redistribution Allowing routing information discovered through one routing protocol to be distributed in the update messages of another routing protocol.
redundancy The duplication of devices, services, or connections so that, if a failure occurs, the redundant devices, services, or connections can perform the work of those that failed.
reliability The extent to which a network or computer system provides dependable, error-free service.
remote access Analog and digital dial-in and dial-out technologies for reaching remote networks via remote-access servers.
Remote Authentication Dial-In User Service See RADIUS.
remote-access server Communications server that connects remote nodes or LANs to an internetwork. Generally supports standard terminal services, such as Telnet, and remote-node, protocol-translation, and asynchronous-routing services.
remote-node services A function of a remote-access server that allows PCs, Macs, and X Window terminals to connect to a remote network and access network services as if they were directly connected to the network.
rendezvous point Router specified in sparse-mode PIM implementations to track membership in multicast groups and to forward messages to known multicast group addresses. See also sparse-mode PIM.
repeater Physical layer device that regenerates and propagates electrical signals between two network segments.
repetitive pattern suppression See RPS.
resiliency Capability of a network to withstand failures and still maintain network operation.
resource discovery The processes and protocols that network users and applications employ to find network resources such as file, naming, and print servers.
Resource Reservation Protocol See RSVP.
response time The amount of time between a request for some network service and a response to the request.
Reverse Address Resolution Protocol See RARP.
reverse-path forwarding See RPF.
RFC Request For Comments. Document series written by the IETF as the primary means for communicating information about the Internet and the TCP/IP protocols. RFCs are available online from numerous sources, including http://www.rfc-editor.org.
RIP Routing Information Protocol. Interior distance vector routing protocol supplied with UNIX BSD systems and widely used in the early years of the Internet. Defined in RFC 1058 and RFC 2453.
RMON Remote Monitoring. MIB agent specifications developed by the IETF that define functions for the remote monitoring of networked devices to facilitate statistics gathering, problem determination, and reporting.
route Path through an internetwork.
route summarization Consolidation of advertised addresses in routing protocols that causes a single summary route to be advertised instead of many individual routes.
router Network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Routers also perform network layer services on the network.
Routing Information Protocol See RIP.
routing metric Method by which a routing algorithm determines that one route is better than another. This information is stored in routing tables. Metrics include bandwidth, communication cost, delay, hop count, load, MTU, path cost, and reliability. Sometimes referred to simply as a metric. See also cost.
routing table Table stored in a router or some other internetworking device that keeps track of routes to particular network destinations and, in some cases, metrics associated with those routes.
Routing Table Maintenance Protocol See RTMP.
routing update Message sent from a router to indicate network reachability and associated cost information. Routing updates are typically sent at regular intervals and after a change in network topology.
RPF reverse-path forwarding. Multicasting technique in which a multicast datagram is forwarded out all but the receiving interface, if the receiving interface is the one used to forward unicast datagrams to the source of the multicast datagram.
RPS repetitive pattern suppression. An option for WAN data circuits that replaces repeating strings of data by a single occurrence of the string and a code that indicates to the far end how many repetitions of the string were in the original data.
RSVP Resource Reservation Protocol. A protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (bandwidth, jitter, maximum burst, and so on) of the packet streams they want to receive.
RTCP Real-Time Control Protocol. Protocol that monitors the QoS of an RTP connection and conveys information about the ongoing session. See also RTP.
RTMP Routing Table Maintenance Protocol. Distance vector routing protocol developed by Apple Computer for use with the AppleTalk protocol suite. RTMP was similar in behavior to RIP.
RTP Real-Time Transport Protocol. IETF protocol that provides end-to-end network-transport functions for applications transmitting real-time data, such as audio, video, or simulation data, over multicast or unicast network services. RTP provides services such as payload type identification, sequence numbering, time-stamping, and data delivery.
RTT round-trip time. Time required for a network communication to travel from the source to the destination and back. RTT includes the time required for the destination to process the message from the source and generate a reply.
runt frame An Ethernet frame that is shorter than 64 bytes.
S
SAP Service Advertising Protocol. IPX protocol that provided a means of informing network clients about available network resources and services via routers and servers.
scalability Capacity of a network to keep pace with changes and growth.
SCR sustainable cell rate. Parameter for ATM traffic management. For VBR connections, SCR determines the long-term average cell rate that can be transmitted. See also VBR.
SDH Synchronous Digital Hierarchy. European standard that defines a set of rate and format standards that are transmitted using optical signals over fiber. SDH is similar to SONET, with a basic SDH rate of 51.84 Mbps, designated as STS-1.
segment 1. A single network that is based on a particular Layer 2 protocol and is bounded by repeaters, bridges, or switches. 2. Term used in the TCP specification to describe a single transport layer unit of information.
Sequenced Packet Exchange See SPX.
server Node or software program that provides services to clients.
Service Advertising Protocol See SAP.
session layer Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities.
shielded twisted-pair See STP.
shortest path first algorithm See SPF.
Simple Mail Transfer Protocol See SMTP.
Simple Network Management Protocol See SNMP.
simulation The process of using software and mathematical models to analyze the behavior of a system without requiring that an actual system be built.
single-mode fiber Fiber-optic cabling with a narrow core that allows light to enter only at a single angle. Such cabling has higher bandwidth than multimode fiber but requires a light source with a narrow spectral width (for example, a laser). See also multimode fiber.
SMTP Simple Mail Transfer Protocol. Internet protocol providing email services.
SNA Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM for communication between terminals and mainframes.
SNMP Simple Network Management Protocol. Network management protocol for TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security.
SONET Synchronous Optical Network. High-speed synchronous network specification developed by Bellcore and designed to run on optical fiber. STS-1 is the basic building block of SONET. Approved as an international standard in 1988. See also STS-1.
spanning tree Loop-free subset of a network topology.
Spanning Tree Protocol Bridge protocol that uses the spanning-tree algorithm, enabling a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange BPDU messages with other bridges to detect loops, and then remove the loops by shutting down selected bridge interfaces.
spanning-tree algorithm Algorithm used by the Spanning Tree Protocol to create a spanning tree.
sparse-mode PIM One of two PIM operational modes. Sparse-mode PIM tries to constrain data distribution so that a minimal number of routers in a network receive irrelevant data. Packets are sent only if they are explicitly requested at the rendezvous point. In sparse mode, receivers are widely distributed, and the assumption is that downstream networks do not necessarily use the multicast datagrams that are sent to them. See also dense-mode PIM, PIM, and rendezvous point.
SPF Shortest Path First algorithm. Routing algorithm that iterates on length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra’s algorithm.
split-horizon updates Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.
spoofing 1. Scheme used by routers to cause a host to treat an interface as if it were up and supporting a session. The router spoofs replies to keepalive messages from the host to convince the host that the session still exists. Spoofing is useful in routing environments such as DDR, in which a circuit-switched link is taken down when there is no traffic to be sent across it to save toll charges. See also DDR. 2. Action of a packet illegally claiming to be from an address from which it was not actually sent. Spoofing is designed to foil network security mechanisms such as filters and access lists.
SPX Sequenced Packet Exchange. Reliable, connection-oriented protocol that supplements the datagram service provided by IPX in the NetWare protocol suite.
SQL Structured Query Language. International standard language for defining and accessing relational databases.
SSL Secure Sockets Layer. Encryption technology for the web used to provide secure transactions such as the transmission of credit card numbers for electronic commerce applications.
static route Route that is explicitly configured and entered into a routing table.
store-and-forward switching Frame-switching technique in which frames are completely processed before being forwarded out the appropriate port. This processing includes calculating the CRC and checking the destination address. In addition, frames must be temporarily stored until network resources (such as an unused link) are available to forward the frame. Compare with cut-through switching.
STP shielded twisted-pair. Two- or four-pair wiring medium used in a variety of networks. STP cabling has a layer of shielded insulation to reduce noise and interference. Compare with UTP.
STS-1 Synchronous Transport Signal level 1. Basic building-block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps. See also SONET.
subinterface A virtual interface on a single physical interface.
subnet address Portion of an IP address that is specified as the subnetwork by the subnet mask.
subnet mask 32-bit address mask used in IP to indicate where the network portion of the address ends and the hosts portion starts.
subnetwork In IP networks, a network sharing a particular subnet address. Subnetworks are networks that are arbitrarily segmented by a network administrator to provide a multilevel, hierarchical routing structure while shielding the subnetwork from the addressing complexity of attached networks.
sustainable cell rate See SCR.
switch 1. Network device that filters, forwards, and floods frames based on the MAC destination address of each frame. A switch operates at the data link layer of the OSI model. 2. A generic term for an electronic or mechanical device that connects devices or networks and relays data between devices or networks.
symmetric encryption Encryption method that provides data confidentiality. When two end stations use symmetric encryption, they must agree on the algorithm to use and on the encryption key they will share. Compare with asymmetric encryption.
Synchronous Digital Hierarchy See SDH.
Synchronous Optical Network See SONET.
T
T1 Digital WAN facility provided by telephone companies in the United States. T1 transmits DS-1-formatted data at 1.544 Mbps. T1 lines can be leased for private use.
TACACS Terminal Access Controller Access Control System. Authentication protocol that provides remote access authentication and related services, such as event logging. User passwords are administered in a central database rather than in individual routers, providing a scalable network-security solution.
tandem switching system See TSS.
TCP Transmission Control Protocol. Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.
TDM 1. time-division multiplexer. A device that implements time-division multiplexing. 2. time-division multiplexing. Technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots.
TDR time-domain reflectometer. Device capable of sending signals through a network medium to check cable continuity and other attributes. TDRs are used to find physical layer network problems.
Telnet Standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connection, enabling users to log in to remote systems and use resources as if they were connected to a local system. Telnet is defined in RFC 854.
Terminal Access Controller Access Control System See TACACS.
throughput Rate of information arriving at, and possibly passing through, a particular point in a network system.
time domain reflectometer See TDR.
time-division multiplexer See TDM.
time-division multiplexing See TDM.
token Frame that contains control information. Possession of the token allows a network device to transmit data onto the network.
top-down network design A network-design methodology that calls for analyzing business and technical requirements and developing a logical design, including a topology and protocols, before selecting products and devices to implement the physical design.
topology Logical arrangement of network nodes and media within a networking structure.
traceroute Program available on many systems that traces the path a packet takes to a destination. It is mostly used to debug routing problems between hosts.
traffic shaping Use of queues to limit surges that can congest a network. Data is buffered and then sent into the network in regulated amounts to ensure that the traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is used in ATM, Frame Relay, and other types of networks.
Transmission Control Protocol See TCP.
transparent bridging Bridging scheme often used in Ethernet and IEEE 802.3 networks in which bridges pass frames along one hop at a time based on tables associating end nodes with bridge ports. Transparent bridging is so named because the presence of bridges is transparent to network end nodes.
transport layer Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits; transport fault detection and recovery; and information flow control.
TTL Time To Live. Field in an IP header that indicates how long a packet is considered valid.
TSS tandem switching system. An intermediate switch that interconnects circuits from the switch of one telephone company central office to the switch of a second central office in the same exchange.
twisted pair A commonly used transmission medium consisting of 22- to 26-gauge insulated copper wire. Can be either shielded (STP) or unshielded (UTP).
type of service 1. An outdated byte-sized field in an IP header that indicated precedence and type of service. Replaced by the DSCP field. See also precedence and DSCP. 2. Outdated 4-bit subfield within the type-of-service byte in an IP header that helped a router select a routing path when multiple paths were available. A source node could specify whether low delay, high throughput, high reliability, or low monetary cost is desired.
U
UBR unspecified bit rate. QoS class for ATM networks. UBR allows any amount of data up to a specified maximum to be sent across the network, but there are no guarantees in terms of cell loss rate and delay. Compare with ABR, CBR, and VBR.
UDP User Datagram Protocol. Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.
UNI User-Network Interface. ATM specification that defines an interoperability standard for the interface between ATM-based products (a router or an ATM switch) located in a private network and the ATM switches located within the public carrier networks.
unicast Message that is sent to a single network node. Compare with broadcast and multicast.
unicast address Address specifying a single network node. Compare with broadcast address and multicast address.
UNIX Operating system developed in 1969 at Bell Laboratories. UNIX has gone through several iterations since its inception. These include UNIX 4.3 BSD (Berkeley Standard Distribution), developed at the University of California at Berkeley, and UNIX System V, Release 4.0, developed by AT&T.
unshielded twisted-pair See UTP.
UPC usage parameter control. In ATM, the set of actions taken by the network to monitor and control traffic at the end-system access point.
usability The ease with which network users can access a network and its services, including the ease of network addressing, naming, and resource discovery.
usage parameter control See UPC.
user community A set of network users that employ a particular application or set of applications and have similar network-design goals.
User Datagram Protocol See UDP.
utilization The percent of total available capacity in use on a network or circuit.
UTP unshielded twisted-pair. Two or four-pair wire medium used in a variety of networks. Lacks shielding and is subject to electrical noise and interference. Compare with STP.
V
VAD voice activity detection. A technology that compresses voice traffic by not sending packets in the absence of speech. Other types of traffic can use the extra bandwidth saved.
variable-length subnet masking See VLSM.
variance 1. In statistics, a measurement of how widely data disperses from the mean. 2. In Cisco routers, a routing feature that allows IGRP and EIGRP to load balance traffic across multiple paths that do not have the same bandwidth, but whose bandwidth varies by some small amount that is configurable.
VBR variable bit rate. QoS class for ATM networks. VBR is subdivided into a Real Time (RT) class and Non-Real Time (NRT) class. VBR (RT) is used for connections in which there is a fixed timing relationship between samples. VBR (NRT) is used for connections in which there is no fixed timing relationship between samples but that still need a guaranteed QoS. Compare with ABR, CBR, and UBR.
videoconferencing Conducting a conference between two or more participants at different sites by using networking devices and protocols to transmit digital audio and video data. Generally each participant has a video camera, microphone, and equipment to transform analog signals into a digital bit stream for traversal across a LAN or WAN.
virtual circuit Logical circuit created to ensure reliable communication between two network devices.
virtual private network See VPN.
virtual private networking See VPN.
virtual router In HSRP, the third, nonphysical router that is created when two or more physical routers share the same virtual IP address and virtual MAC address. See also HSRP.
VLAN virtual LAN. Group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire when they are located on several different LAN segments.
VLAN trunk A single physical link that supports more than one VLAN.
VLAN Trunking Protocol See VTP.
VLSM variable-length subnet masking. Capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space.
voice activity detection See VAD.
VoIP Voice over IP. Protocols and products that enable the transmission of telephone calls over IP networks.
VPN 1. virtual private network. A network that implements virtual private networking. 2. virtual private networking. Set of processes and protocols that enables an organization to securely interconnect sites that are part of a private network via a public network, such as a service provider’s network or the Internet.
VTP VLAN Trunking Protocol. A Cisco switch-to-switch and switch-to-router VLAN management protocol that exchanges VLAN configuration changes as they are made to a network.
W
WAN wide-area network. Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers.
WDM wavelength-division multiplexing. A type of multiplexing developed for use on fiber-optic cables. WDM modulates each of several data streams onto a different part of the light spectrum.
web Short for the World Wide Web. A large network of Internet servers that provides hypertext and other services to client applications, such as web browsers.
WFQ weighted fair queuing. Congestion-management algorithm that identifies conversations (in the form of traffic flows), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations.
window A protocol data structure that stores outgoing data and generally allows a sender to send a set of packets before an acknowledgment arrives.
windowing Using windows as a flow-control mechanism when sending data.
wire speed The theoretical maximum throughput of a network or circuit.
wiring closet Specially designed room used for wiring a data or voice network. Wiring closets serve as a central junction point for the wiring and wiring equipment that is used for interconnecting devices.
X
X.25 International standard that defines how a connection between a DTE and DCE is maintained for remote terminal access and computer communications in packet-switched networks.
xDSL Group term that refers to the different varieties of digital subscriber line, such as ADSL and HDSL. See DSL, ADSL, and HDSL.
X Window System Distributed, network-transparent, device-independent, multitasking windowing and graphics system originally developed by MIT for communication between X terminals and UNIX workstations.