UA User Agent. An OSI application process that creates, submits, and takes delivery of messages on the user's behalf in the X.400 message handling system.

UCS (Uplink Control System). Software used to support the secure delivery of digitally compressed services.

UDP (User Datagram Protocol). A protocol that offers a limited service when messages are exchanged between network nodes in a network that uses IP. UDP is an alternative to TCP.

Umbrella cells. Cells used to reduce the number of handovers required to service fast moving mobile subscribers. An umbrella cell is a large cell that is superimposed over a series of smaller cells. It uses a different frequency to avoid interfering with these cells. When the network detects a fast moving mobile (indicated by its large number of handover requests), it passes control of the call over to the umbrella cell.

UMTS (Universal Mobile Telecommunications System). Part of the International Telecommunications Union’s ’IMT-2000’ vision of a global family of ’third-generation’ (3G) mobile communications systems. UMTS will allow the broadband, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to 2 Mbps. UMTS , is the planned standard for mobile users around the world by 2002.

UTRA: UMTS Terrestrial Radio Access. UTRA is the 3G IMT-2000 air interface standard adopted by ETSI.

UTRAN: UMTS Terrestrial Radio Access Network. UTRA is the 3G IMT-2000 radio access network adopted by ETSI.

UNI (User-Network Interface). An interface point between ATM end-users and a private ATM switch, or between a private ATM switch and the public-carrier ATM network, UNI is defined by physical and protocol specifications from the ATM Forum UNI documents. It is the standard adopted by the ATM Forum to define connections between users or end-stations and a local switch.

Uplink. The transmission of a radio signal up the hierarchy of infrastructure equipment, for example, from the mobile station (MS) to the base transceiver station (BTS), or from a base station controller (BSC) to a mobile services switching centre (MSC).

UHF (Ultra High Frequency). The RF spectrum
band of frequencies that extends from 300 MHz to 3 GHz.

User. A person who interacts with a user agent to use a resource.

User agent. Any software or device that can interpret content. For example, a browser.

USSD (Unstructured Supplementary Services Data). A GSM technology that sends text between a Mobile Station and an application program. It is supported by WAP and is similar to SMS; however transactions only occur during the session.

UTC (Universal Co-ordinated Time). This is the official world wide reference used for time measurement. UTC used to be known as Greenwich Mean Time (GMT).

UWCC: Universal Wireless Communications Consortium. An industry body established to promote IS-136.