 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. BGP4, the latest version of BGP, provides mechanisms for supporting (CIDR) and allowing aggregation of routes, includ- ing aggregation of autonomous system paths. BGP MPLS VPN As described in RFC 2547, a VPN in which BGP is used to distribute VPN routing information across the provider's backbone and MPLS is used to forward VPN traffic from one VPN site to another. 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 T-1, in which the signal carrying the binary values successfully alternates between pos- bipolar violation itive and negative polarities. The 1 values are represented by the signal amplitude at either polar- ity, and no-value "spaces" are at 0 amplitude. bipolar violation An encoding method used on T-1 circuits that inserts two successive 1s of the same voltage. The device receiving the signal interprets the bipolar violation as a timing mark, which keeps the transmitting and receiving devices synchronized. B-ISDN (Broadband ISDN) A standard that was envisioned for use with advanced applica- tions. SDH/SONET and ATM were both born out of the B-ISDN standard and a desire to deliver advanced applications. bit The smallest unit of information in a digital device. In binary notation, a bit is either the character 0 or the character 1. bit duration The time it takes an 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, synchronization problems, or complete loss of picture. bitmap A pixel-by-pixel description of an image in which each pixel is a separate element. bit-orie