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Glossary

unicast

Used for direct host-to-host communication. Communication is

directed to only one destination and is originated only from one source.

unidirectional shared tree

A method of shared tree multicast forwarding.

This method allows only multicast data to be forwarded from the RP.

unnumbered frames

HDLC frames used for control-management pur-

poses, such as link startup and shutdown or mode specification.

UTP

unshielded twisted-pair: Copper wiring used in small-to-large net-

works to connect host devices to hubs and switches. Also used to connect
switch to switch or hub to hub.

VBR

Variable Bit Rate: A QoS class, as defined by the ATM Forum, for use

in ATM networks that is subdivided into real time (RT) class and non-real
time (NRT) class. RT is employed when connections have a fixed-time rela-
tionship between samples. Conversely, NRT is employed when connections
do not have a fixed-time relationship between samples, but still need an
assured QoS.

VCC

Virtual Channel Connection: A logical circuit that is created by

VCLs. VCCs carry data between two endpoints in an ATM network. Some-
times called a virtual circuit connection.

VIP

1) Versatile Interface Processor: An interface card for Cisco 7000 and

7500 series routers, providing multilayer switching and running the Cisco
IOS software. The most recent version of VIP is VIP2. 2) Virtual IP: A func-
tion making it possible for logically separated switched IP workgroups to
run Virtual Networking Services across the switch ports of a Catalyst 5000.

virtual circuit

Abbreviated VC, a logical circuit devised to assure reliable

communication between two devices on a network. Defined by a virtual path
connection (VPC)/virtual path identifier (VCI) pair, a virtual circuit can be
permanent (PVC) or switched (SVC). Virtual circuits are used in Frame
Relay and X.25. Known as virtual channel in ATM. See also: PVC and SVC.

virtual ring

In an SRB network, a logical connection between physical

rings, either local or remote.

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