Top-Down Network Design, Second Edition

486 autonomous system

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.

available bit rate. See ABR.

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. See capacity.

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.

Banyan VINES. See VINES.

baseline. Characterization of the normal traffic flow and performance of a

network, used as input to a new or enhanced design for the network.

Basic Rate Interface. See BRI.

beacon. 1. Frame from a Token Ring or FDDI device indicating a serious

problem with the ring, such as a broken cable. 2. 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.

Broadcast and Unknown Server 487

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. See BER.

bit error rate tester. See BERT.

BOOTP. Bootstrap Protocol. Protocol used by a network node to determine the

IP address of its interfaces in order to achieve network booting.

Border Gateway Protocol. See BGP.

BPDU. bridge protocol data unit. Spanning Tree Protocol hello packet that is

sent out at configurable intervals to exchange information among bridges in a

network.

BRI. Basic Rate Interface. ISDN interface composed of two bearer (B) channels

for user data and one data (D) channel used for signaling. Compare with PRI.

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. Generally, a broadcast address is a MAC destination address of all ones

(FF:FF:FF:FF:FF:FF in hexadecimal). Compare with multicast address and

unicast address.

Broadcast and Unknown Server. See BUS.