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Glossary

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TUD

Trunk Up-Down: A protocol used in ATM networks for the moni-

toring of trunks. Should a trunk miss a given number of test messages being
sent by ATM switches to ensure trunk line quality, TUD declares the trunk
down. When a trunk reverses direction and comes back up, TUD recognizes
that the trunk is up and returns the trunk to service.

tunneling

A method of avoiding protocol restrictions by wrapping

packets from one protocol in another protocol's packet and transmitting this
encapsulated packet over a network that supports the wrapper protocol. See
also: encapsulation.

U reference point

Reference point between a TE1 and an ISDN network.

The U reference point understands ISDN signaling techniques and uses a
2-wire connection.

UDP

User Datagram Protocol: A connectionless transport layer protocol in

the TCP/IP protocol stack that simply allows datagrams to be exchanged
without acknowledgments or delivery guarantees, requiring other protocols
to handle error processing and retransmission. UDP is defined in RFC 768.

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 for-

warding. 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.

user mode

Cisco IOS EXEC mode that allows an administrator to per-

form very few commands. You can only verify statistics in user mode; you
cannot see or change the router or switch configuration.

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

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