B

Backbone A LAN or WAN that interconnects intermediate systems (bridges and/or routers).

Backplane The main bus that carries data within a device.

Balun (balanced-unbalanced) An impedance-matching device that connects a balanced line (such as a twisted-pair line) with an unbalanced line (such as a coaxial cable).

Bandwidth Measure of the information capacity of a transmission channel. Strictly speaking, bandwidth is the difference, expressed in hertz (Hz), between the highest and lowest frequencies of the channel.

Bandwidth-on-demand (see Dynamic bandwidth allocation)

Baseband Transmission scheme in which the entire bandwidth, or data-carrying capacity, of a medium (such as coaxial cable) is used to carry a single digital pulse, or signal, between multiple users. Because digital signals are not modulated, only one kind of data can be transmitted at a time. Contrast with broadband.

Basic Rate ISDN An a version of ISDN offering two 64 Kbps channels (B-channels) for speech or data and a 16 Kbps channel (D-channel) for signalling and control purposes. Aggregate data rate: (2x64)+16=144 Kbps.

Bastion Host A machine placed on the perimeter network to provide publicly available services. Although secured against attack, it is assumed to be compromised because it is exposed to the Internet.

Baud Rate The number of signal events per second occurring on a single communications channel. Often taken to mean bit rate, though not really accurate.

BBS Bulletin Board System - a software package that interacts with one or more dial-up lines to allow users to communicate with other users by reading and writing messages aswell as enabling them to download and upload files.

B-Channel The main type of channel in ISDN. It's a full duplex, 64 Kbps channel for sending data and voice. Basic Rate ISDN has two B-Channels and Primary Rate ISDN has between 6 and 30.

Beacon Token ring frame signaling that the ring is inoperative because of a serious hard error; defective cable or faulty nodes are possible causes.

BECN Backward Explicit Congestion Notification

BER (Basic Encoding Rule) Rule for encoding data units described in ANS.1; also, bit error rate, or the ratio of received bits that are in error.

BGP (Border Gateway Protocol) Protocol for communications between a router in one autonomous system and routers in other ASs.

Binary synchronous communication, or bisync Character-oriented data link protocol for half-duplex applications. Usually bisync.

BISDN (Broadband Integrated Services Digital Network) Communications standard designed to handle high bandwidth applications such as video over broadband. See ATM.

Bit BIinary digiT - has only two possible values 0 or 1.

Bit error rate Percentage of bits in a transmittal received in error.

BitNet Because It's Time NETwork - a network, separate from the Internet, of educational institutions. Becoming less commonly used.

Bit rate The number of bits travelling per second in a data stream.

BNC connector Standard connector to link IEEE 802.3 10BASE2 coaxial cable to a transceiver.

BOC Bell Operating Company (see RBOC).

Bonding An international standard for aggregating multiple data channels into a single logical connection. Very popular in videoconferencing applications.

Bookmark A method of marking a World Wide Web address (URL) that you wish to go back to. Known as a "hotlist" in Mosaic and "Favourites" in Microsoft Internet Explorer.

BootP Protocol that a network workstation uses on boot up to determine the IP address of its Ethernet interfaces.

Boot PROM (Programmable Read-Only Memory) Nonvolatile memory that contains information necessary for initializing a system. Boot PROM information can be transmitted over a network.

Bottlenecks Traffic slowdowns that result when too many network nodes try to access a single node, often a server node, at once.

Bounce To return undeliverable email to the sender. The term is sometimes used when a data packet is repeatedly sent between two routers (because of a routing problem) until it's time to live or hop count expires.

Boundary Function Capability of SNA subarea nodes -- encountered most often in IBM 3745 high-speed communications controllers -- to offer protocol support for attached peripheral nodes.

Boundary Routing System Architecture Software algorithms and methodology that enable a router at a central node of a wide area network to perform protocol-specific routing and bridging path table management on behalf of a router at a peripheral (leaf) node, greatly simplifying the router at the leaf node.

BPDU (Bridge Protocol Data Units) A packet to initiate communications between devices under a spanning-tree protocol. Compare PDU.

Bps bits per second

BRASICA Bridging Application-Specific Integrated Circuit.

BRI(Basic Rate Interface) The ISDN interface comprising two B channels and one 16 K bit/second D channel.

Bridge A device that interconnects local or remote networks no matter what higher level protocols (such as XNS* or TCP/IP) are involved. Bridges form a single logical network, centralizing network administration. They operate at the physical and link layers of the Open Systems Interconnect (OSI) reference model. See SRT (source routing transparent) bridge, and STA (Spanning Tree Algorithm). Contrast with router and gateway.

Bridge/router A device that can provide the functions of a bridge, router or both concurrently. A bridge/router can route one or more protocols, such as TCP/IP and/or XNS, and bridge all other traffic.

Broadband One of two methods used to transmit information around a LAN, the other being Baseband. Broadband uses modems to modulate the signal before putting it onto the LAN media. Multiple frequency channels are provided which operate independently of each carrying voice, data or video.This is usually done using Frequency Division Multiplexing (FDM). Alternate definition - Usually taken to mean "faster than commonly occuring networks", so the real meaning depends on what the most common network speeds are at the time. At the moment anything operating at speeds faster than 34Mbps is referred to as broadband.

Broadband ISDN A version of ISDN that works at Broadband speeds. This is different from Primary Rate ISDN which consists of a number of 64Kbps channels and is not a fully integrated service. The two main proposed Broadband ISDN rates are 150Mbps and 600Kbps.

Broadcast A message forwarded to all network destinations.

Broadcast Storm Multiple simultaneous broadcasts that typically absorb available network bandwidth and can cause network time-outs.

Buffer Area in a device for temporary storage of data in transit; can accommodate differences in processing speeds between devices by storing data blocks until they are ready to be processed by a slower device.

BUS (Broadcast and Unknown Server) It provides the broadcast function and resolution of unknown addresses for LAN Emulation which is connection-oriented.

Bypass Mode Operating mode on ring networks such as FDDI and token ring in which an interface has de-inserted from the ring.

Byte Order There are two main conventions for the ordering of bytes within multi-byte integers - "big-endian" (most significant byte first) and "little-endian" (least significant byte first). This is vendor dependent, for instance SUN machines use "big-endian" where as DEC machines use "little-endian". Obviously this can cause problem when sharing sets of data between machines. Software utilities such as "dd" in Unix can be used to "byte swap" (ie convert data produced using one convention for use on a machine that uses the other).