Glossary C

B

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.

BackboneFast

A method whereby, if the switch receives an inferior BPDU on a root port, the switch will begin figuring out who the new root bridge is in less time than normal spanning-tree convergence. This accelerates spanning-tree convergence after the failure of a non-directly connected network link.

back end

A node or software program supplying services to a front end. See also: 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.' Contrast with: broadband.

Basic Management Setup

Used with Cisco routers when in setup mode. Provides only enough management and configuration to get the router working so someone can telnet into the router and configure it.

baud

Synonymous with bits per second (bps), if each signal element represents 1 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 to: 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.

bearer service

Used by service providers to provide DS0 service to ISDN customers. A DS0 is one 64KB channel. An ISDN bearer service provides either two DS0s, called two bearer channels, for a Basic Rate Interface (BRI), or 24 DS0s, called a Primary Rate Interface (PRI).

BECN

Backward Explicit Congestion Notification: 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. Contrast with: FECN.

best-efforts network

A network where traffic is forwarded with no QoS features and delivery is not guaranteed in any way.

BGP

Border Gateway Protocol: This protocol has had four revisions. Version 4 of the interdomain routing protocol is most commonly used on the Internet. BGP4 supports CIDR and uses route-counting mechanisms to decrease the size of routing tables. See also: CIDR.

bidirectional shared tree

A method of shared tree multicast forwarding. This method enables group members to receive data from the source or the RP, whichever is closer. See also: RP (rendezvous point).

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 enables 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/SDH-based transmission circuits, supplying data rates from 155Mbps through 622Mbps and beyond. 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 to: byte-oriented protocol.

BoD

Bandwidth on Demand: This function enables an additional B channel to be used to increase the amount of bandwidth available for a particular connection.

Boot ROM

Used in routers to put the router into bootstrap mode. Bootstrap mode then boots the device with an operating system. The ROM can also hold a small Cisco IOS.

border gateway

A router that facilitates communication with routers in different autonomous systems.

border router

Typically defined within Open Shortest Path First (OSPF) as a router that connected an area to the backbone area. However, a border router can be a router that connects a company to the Internet as well. See also: OSPF.

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 to: PRI. See also: BISDN.

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.

bridge ID

Used to find and elect the root bridge in a layer 2 switched internetwork. The bridge ID is a combination of the bridge priority and base MAC address.

bridging

A layer 2 process to block or forward frames based on MAC layer addresses. Bridges are lower speed, lower port density switches.

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 to: directed broadcast.

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.

brute force attack

A type of attack that bombards the resource with attempted connections until successful. In the most common brute force attack, different passwords are repeatedly tried until a match that is then used to compromise the network is found.

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.'

buffer overflow

When a buffer has no room for additional frames or packets, it either drops new arrivals or clears out some additional space by dropping selective data already stored. The term for either of these processes is buffer overflow.

bursting

Some technologies, including ATM and Frame Relay, are considered burstable. This means that user data can exceed the bandwidth normally reserved for the connection; however, this cannot exceed the port speed. An example of this is a 128Kbps Frame Relay CIR on a T1- depending on the vendor, it might be possible to send more than 128Kbps for a short time.

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 servers: 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: LANE, LEC, LECS, and LES.

bus fabric

A switch-fabric that uses an interrupt-driven internal bus.

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. Contrast with: ring topology and star topology.

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 to: bit-oriented protocol.