gloss_U

U

undervoltage.

In an undervoltage condition, a lower-than-usual power-line voltage lasts from several seconds to several hours.

uninterruptible power supply (UPS).

A UPS is a power conditioning and supply system that affords protection against short- term power outages. A UPS rectifies the incoming AC line voltage to DC, which is then applied to batteries. An inverter, driven by DC power, supplies AC voltage for equipment that requires conditioned power. During outages, the converter is driven by battery power.

Unix.

Unix is a 32-bit multitasking, multiuser operating system. Versions of Unix are available for nearly every type of computer platform. Unix was initially popular in universities and research labs, but it is now the basis of many corporate applications.

unshielded twisted pair (UTP).

UTP is a pair of insulated copper wires, twisted around each other. UTP is classified into several levels of wire quality suitable for different transmission speeds (see "Category").

user agent (UA).

In X.400 mail systems, the user agent is the client component that provides the X.400 envelope, headers, and addressing. The user agent sends the messages to the X.400 mail server, or Message Transfer Agent, which then routes the messages to their destinations.

User Datagram Protocol (UDP).

UDP is the connectionless transport protocol within the TCP/IP suite. Because it does not add overhead, as the connection-oriented TCP does, UDP is typically used with network-management applications and SNMP.