B (bidirectional) frame
The part of the MPEG video compression process in which both past and future pictures/frames are used as references. B frames typically result in the most compression.

B/D link (bridge/diagonal link)
A link that interconnects two mated pairs of STP. It carries signaling messages beyond the initial point of entry to the signaling network, toward the intended destination.

backbone
A central network that connects several other, usually lower-bandwidth, networks so that those networks can pass data to each other. The backbone network is usually composed of a high-capacity communications medium, such as fiber optic or microwave.

backplane
The board that contains a bus.

bandpass filter
A circuit that is designed to allow a single band of frequencies to pass, neither of the cut-off frequencies being zero or infinity.

bandwidth
The range of frequencies, expressed in Hertz, that can pass over a given transmission channel. Also, the difference between the lowest and highest frequency carried. The bandwidth determines the rate at which information can be transmitted through a circuit. The greater the bandwidth, the more bits per second that can be carried.

bandwidth exchange
An organization or a facility that functions as an exchange where bandwidth is the commodity. Some exchanges act to bring together buyers and sellers of bandwidth, and facilitate contract negotiations and transactions; other exchanges actually switch traffic in real-time based on changes in bandwidth prices throughout the course of the day.

bandwidth-on-demand
A concept in wide area networking in which the user can access additional WAN bandwidth as the application warrants. It enables users to pay for only the bandwidth they use, when they use it.

Barker Code
An 11-bit chipping code. The one-bits are encoded as a particular sequence of 1s and 0s, and the zero-bits are the inverse of that sequence.

base station controller
An intermediate device in the cellular system that controls a group of base-station transceivers,

baseband signaling
Transmission of a digital or analog signal at its original frequencies (that is, a signal in its original form, not changed by modulation).

batch processing
A processing method in which a program or programs process data with little or no operator interaction.

baud
A unit of signaling speed.

Baudot code
A data code that uses a 5-bit structure that was used on vintage teleprinters (e.g., Telex).

beacon frame
A frame sent by a Token Ring adapter indicating that it has detected a serious problem. An adapter sending such frames is said to be beaconing.

BECN (Backward Explicit Congestion Notification)
A bit in the Frame Relay header that the network uses to inform the transmitter of network congestion.

BER (bit error rate)
In data communications testing, the ratio between the total number of bits transmitted in a given message and the number of bits in that message received in error. A measure of the quality of a data transmission, usually expressed as number referred to a power of 10 (e.g., 1 bit error in 105 bits transmitted, or 1 in 100,000).

best-effort QoS
QoS that is not guaranteed but that is as good as possible under the circumstances.

beta test
The stage at which a new product is tested under actual usage conditions.

BGP (Border Gateway Protocol)
A gateway protocol that allows routers to communicate with each other. BGP is an exterior routing protocol used between autonomous systems and is of concern to service providers and other large or complex organizations.

binary
A base-two system of numbers; the binary digits are 0 and 1.

bipolar
The predominant signaling method used for digital transmission services, such as DDS and T1, in which the signal carrying the binary values successfully alternates between positive and negative polarities. One values are represented by the signal amplitude at either polarity, and no-value “spaces” are at zero amplitude.

bit
The smallest unit of information in a digital device. In binary notation, either the characters 0 or 1.

bit duration
The time it takes one encoded bit to pass a point on the transmission medium. In serial communications, a relative unit of time measurement used for comparison of delay times, where the data rate of a transmission channel can vary.

bit errors
Missing video elements or synchronization problems or complete loss of the picture.

bitmap
A pixel-by-pixel description of an image. Each pixel is a separate element.

bit-oriented protocol
A communications protocol or transmission procedure in which control information in encoded in fields of one or more bits.

bit rate
The speed at which bits are transmitted, usually expressed in bps.

blocking
A network is said to be blocking if there are connection sets that will prevent some additional desired connections from being set up between unused ports, even with rearrangement of existing connections.

Blowfish
A 64-bit block code that has key lengths of 32 to 448 bits. Blowfish is used in more than 100 products, and it is viewed as one of the best available algorithms.

Bluetooth
A very low-cost chip that gives a device a short-range wireless capability. Personal digital assistants, laptops, cell phones, and any other intelligent appliance embedded with a Bluetooth chip are capable of communicating and linking with each other wirelessly.

BNC connector
A locking type of connector. Thinnet, a form of Ethernet, uses BNC connectors.

board
The circuit card on which integrated circuits are mounted.

bps (bits per second)
A measure of the amount of transmission capacity available. For example, a 10Gbps backbone can support 10 billion bits per second. Bytes are used as a measure of storage, so a 10GB hard drive is not the same thing as a 10Gbps communications link. Abbreviated as Kbps for thousands of bits per second; Mbps for millions of bits per second; Gbps for billions of bits per second; Tbps for trillions of bits per second; Pbps for 1,000Tbps; and Ebps for 1 billion Gbps.

BRI (Basic Rate Interface)
In ISDN, the interface to the basic rate, which is 2B+D: two 64Kbps information-carrying channels plus one 16Kbps signaling channel. Also called Basic Rate Access (BRA).

bridge
An attaching device that connects two LAN segments to allow the transfer of information from one LAN segment to the other. Bridges operate by filtering packets according to their destination addresses. Most bridges automatically learn where these addresses are located and, thus, are called learning bridges. A bridge works at OSI Layer 2 and is transparent to upper-layer devices and protocols.

broadband
(1) A multichannel, high-bandwidth transmission line. (2) According to the ITU-T, any transmission rate over 2Mbps. (3) Typically, the technology of CATV transmission, as applied to data communications; employs coaxial cable as the transmission medium and radio frequency carrier signals in the 5MHz to 1,000MHz range.

Broadband ISDN
A standard that was envisioned for use with advanced applications. SDH/SONET and ATM were both born out of the Broadband ISDN standard and a desire to be able to deliver advanced applications.

broadcast
A transmission to multiple receiving locations simultaneously. A broadcast can be made, for example, over a multipoint line to all terminals that share the line, or over a radio or television channel to all receivers tuned to that channel.

broadcast storm
A pathological network condition in which an increasing and insupportable number of broadcast packets are generated.

brouter
A device that can transparently bridge protocols as well as route them. It is a hybrid of a bridge and a router.

brownout operation
An operation in response to heavy demand, in which main system voltages are lowered and power is not lost but reduced. Although conventional networking equipment is relatively immune to brownouts, the computer controlling the system is very sensitive to voltage variations and could fail under these conditions. Most equipment today has the capability to cope with these reductions, or a heavy-duty power supply can be furnished. A UPS can be installed to ensure continued service during prolonged outages and to regulate power.

BSC (binary synchronous communications)
A half-duplex, character-oriented data communications protocol originated by IBM in 1964. It includes control character and procedures for controlling the establishment of a valid connection and the transfer of data. Also called bisync.

buffer
A storage device that is used to compensate for a difference in rate of data flow, or time of occurrence events, when transmitting data from device to another.

buffered network
A real-time store-and-forward message-switching network, with computers at the switching points, which act as buffers for the packets.

buffered repeater
A hybrid device halfway between a repeater and a bridge. Entire packets are received and retransmitted (as with a bridge) but no address filtering is implemented (as with a repeater).

bundled
A pricing strategy in which a service provider or manufacturer includes all products?hardware, software, services, training, and the like?in a single price.

burst
In data communications, a sequence of signals counted as one unit in accordance with some specific criterion or measure.

burst switching
A switching method to switch digitized voice and data characters in an integrated fashion.

bus
(1) A physical transmission path or channel. Typically an electric connection, with one or more conductors, wherein all attached devices receive all transmissions at the same time. (2) A LAN topology, such as used in Ethernet and the token bus, where all network nodes listen to all transmissions, selecting certain ones based on address identification. Involves some type of contention-control mechanism for accessing the bus transmission medium.

bus topology
A network architecture in which all the nodes are connected to a shared cable.

bypass
To establish a communication link without using the facilities of the local exchange carrier (that is, the telephone company).

byte
The amount of storage required to represent one alphanumeric character, or 8 bits. Bytes are used as a measure of storage, as in a 2GB hard drive. This is different from the measurement for transmission capacity, which is expressed in bits per second (for example, a 10Gbps backbone).