Table of Contents




unnumbered acknowledgement.


Universal Asynchronous Receiver/Transmitter. Integrated circuit, attached to the parallel bus of a computer, used for serial communications. The UART translates between serial and parallel signals, provides transmission clocking, and buffers data sent to or from the computer.

UB Net/One

Ungermann-Bass Net/One. Routing protocol, developed by UB Networks, that uses hello packets and a path-delay metric, with end nodes communicating using the XNS protocol. There are a number of differences between the manner in which Net/One uses the XNS protocol and the usage common among other XNS nodes.


unspecified bit rate. QoS class defined by the ATM Forum for ATM networks. UBR allows any amount of data up to a specified maximum to be sent across the network, but there are no guarantees in terms of cell loss rate and delay. Compare with ABR, CBR, and VBR.


unspecified bit rate plus. UBR service complemented by ATM switches that use intelligent packet discard mechanisms such as EPD or TPD. See also EPD and TPD.


universal call model.


UniDirectional Link Protocol. Protocol used by inexpensive, receive-only antennas to receive data via satellite.


User Datagram Protocol. Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.


Unnumbered frame. One of three SDLC frame formats. See also I-frame and S-frame.


unnumbered information.


Universal I/O serial port (Cisco router).


UK Education and Research Networking Association.


Underwriters Laboratories. Independent agency within the United States that tests product safety.


Companding technique commonly used in North America. U-law is standardized as a 64-kbps CODEC in ITU-T G.711.


upper-layer protocol. Protocol that operates at a higher layer in the OSI reference model, relative to other layers. ULP is sometimes used to refer to the next-highest protocol (relative to a particular protocol) in a protocol stack.

unbalanced configuration

HDLC configuration with one primary station and multiple secondary stations.

Underwriters Laboratories

See UL.

Ungermann-Bass Net/One

See UB Net/One.


User-Network Interface. ATM Forum specification that defines an interoperability standard for the interface between ATM-based products (a router or an ATM switch) located in a private network and the ATM switches located within the public carrier networks. Also used to describe similar connections in Frame Relay networks. See also NNI, Q.920/Q.921, and SNI (Subscriber Network Interface).


Message sent to a single network destination. Compare with broadcast and multicast.

unicast address

Address specifying a single network device. Compare with broadcast address and multicast address. See also unicast.

UniDirectional Link Protocol

See UCM.

uninsured traffic

Traffic within the excess rate (the difference between the insured rate and maximum rate) for an ATM VCC. This traffic can be dropped by the network if congestion occurs. See also CLP, insured rate, and maximum rate.


Literally meaning one polarity, the fundamental electrical characteristic of internal signals in digital communications equipment. Contrast with bipolar.

unity gain

In broadband networks, the balance between signal loss and signal gain through amplifiers.

Universal Asynchronous Receiver/Transmitter

See UCM.

Universal Resource Locator

See URL.


Operating system developed in 1969 at Bell Laboratories. UNIX has gone through several iterations since its inception. These include UNIX 4.3 BSD (Berkeley Standard Distribution), developed at the University of California at Berkeley, and UNIX System V, Release 4.0, developed by AT&T.

UNIX-to-UNIX Copy Program


unnumbered frames

HDLC frames used for various control and management purposes, including link startup and shutdown, and mode specification.

unshielded twisted-pair

See UTP.

unspecified bit rate

See UBR.


usage parameter control. See traffic policing.

upper-layer protocol

See U-law.


Set of frequencies used to send data from a subscriber to the headend.

urban legend

Story, which may start with a grain of truth, that has been retold and end up on the Internet. Some legends that periodically make their rounds include "The Infamous Modem Tax," "Craig Shergold/Brain Tumor/Get Well Cards," and "The $250 Cookie Recipe." Urban Legends are conceptually similar to space junk that stays in orbit for years.


Universal Resource Locator. Standardized addressing scheme for accessing hypertext documents and other services using a browser. See also browser.

usage parameter control

See traffic policing.


Initiated in 1979, one of the oldest and largest cooperative networks, with over 10,000 hosts and a quarter of a million users. Its primary service is a distributed conferencing service called news.

User Datagram Protocol

See UDP.

User-Network Interface

See UNI.


Coordinated Universal Time. Time zone at zero degrees longitude. Formerly called Greenwich Mean Time (GMT) and Zulu time.


unshielded twisted-pair. Four-pair wire medium used in a variety of networks. UTP does not require the fixed spacing between connections that is necessary with coaxial-type connections. There are five types of UTP cabling commonly used: Category 1 cabling, Category 2 cabling, Category 3 cabling, Category 4 cabling, and Category 5 cabling. Compare with STP. See also EIA/TIA-586 and twisted pair.


P1024C data-link layer protocol.


UNIX-to-UNIX Copy Program. Protocol stack used for point-to-point communication between UNIX systems.


UNIX-to-UNIX encoding. Method of converting binary files to ASCII so that they can be sent over the Internet via e-mail. The name comes from its use by the UNIX operating system's uuencode command. See also uudecode.


UNIX-to-UNIX decode. Method of decoding ASCII files that were encoded using uuencode. See also uuencode.


Universal Voice Module.


Universal Voice Module-Channelized.


Universal Voice Module-Unchannelized.

Posted: Tue Sep 21 15:26:58 PDT 1999
Copyright 1989-1999©Cisco Systems Inc.