Page 545

Glossary

513

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.

20/80 rule

This rule means that 20 percent of what the user performs on

the network is local, whereas up to 80 percent crosses the network segmen-
tation points to get to network services.

UART

The Universal Asynchronous Receiver/Transmitter: A chip that

governs asynchronous communications. Its primary function is to buffer
incoming data, but it also buffers outbound bits.

U reference point

Reference point between a TE1 and an ISDN network.

The U reference point understands ISDN signaling techniques and uses a
two-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 acknowledgements 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 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.

www.sybex.com