UART (Universal Asynchronous Receiver/Transmitter) - A device, usually an integrated circuit chip, that performs the parallel-to-serial conversion of digital data to be transmitted and the serial-to-parallel conversion of digital data that has been transmitted.

UAS (Unavailable Seconds) - a measurement of signal quality. Unavailable seconds start accruing when ten consecutive severely errored seconds occur.

UBR (Unspecified Bit Rate) - a type of traffic that is not considered time-critical (e.g., ARP messages, pure data), allocated whatever bandwidth is available at any given time. UBR traffic is given a "best effort" priority in an ATM network with no guarantee of successful transmission.

UDP (User Datagram Protocol) - the TCP/IP transaction protocol used for applications such as remote network management and name-service access; this lets users assign a name, such as "RVAX*2,S," to a physical or numbered address.

UL Listed - Tested and listed by the Underwriters Laboratories, Inc.

Unassigned Cells - a generated cell identified by a standardized virtual path identifier (VPI) and virtual channel identifier (VCI) value, which does not carry information from an application using the ATM Layer service.

Undercurrent Relay - Relay which operates when the current being monitored falls below a specified value.

Undervoltage Alarm - Alarm given when the monitored voltage falls below a specified level.

Undervoltage Protection - Protection of the power supply and/or connected equipment against excessively low input voltage.

Undervoltage Relay - Relay which operates when voltage being monitored falls below a specified value.

Underwriters Laboratories, Inc. - A non-profit laboratory which examines and tests devices, materials and systems for their safety, not for their satisfactory operation.

UNI (User-to-Network Interface) - the physical and electrical demarcation point between the user and the public network service provider.

UNI 3.0/3.1 - the User-to-Network Interface standard set forth by the ATM Forum that defines how private customer premise equipment interacts with private ATM switches.

Unicasting - The transmit operation of a single PDU by a source interface where the PDU reaches a single destination. 

Unity Power Factor - The condition when voltage and current are in phase, as with a load consisting of resistance only (incandescent lights, strip heaters, etc.).

UPC (Usage Parameter Control) - the mechanism that ensures that traffic on a given connection does not exceed the contracted bandwidth of the connection. UPC is responsible for policing or enforcement. UPC is sometimes confused with congestion management, to which it is functionally related on the CellPath 300 (see congestion management).

Uplink - Represents the connectivity from a border node to an upnode.

Upload - To transmit a file of data from your computer to another computer.

Upnode - The node that represents a border node's outside neighbor in the common peer group. The upnode must be a neighboring peer of one of the border node's ancestors. 

UPS (Uninterruptible Power Supply) - A system designed to provide power, without delay, during any period when the normal power supply is incapable of performing acceptably.

UPS Load - A load that can tolerate an interruption of 20 milliseconds or less in the transition from commercial to standby power when commercial power fails. A UPS load that must be transferred in less than 4 milliseconds is called a no break UPS load.

Useful Life - The period between the early high failure rate expected with new components and the later high failure rate due to component wear.

USITA (United States Independent Telephone Association) - The old name for an association now called the United States Telephone Association (USTA).

USTA (United States Telephone Association) - The largest trade association of telephone companies, its primary role is to ensure no regulation or legislation is passed that is unfavorable to the telephone industry.

USTSA (United States Telecommunications Suppliers Association) - An affiliate of the United States Independent Telephone Association.

UTOPIA (Universal Test & Operations Interface for ATM) - Refers to an electrical interface between the TC and PMD sublayers of the PHY layer.

UTP (Unshielded Twisted Pair) - a cable that consists of two or more insulated conductors in which each pair of conductors are twisted around each other. There is no external protection and noise resistance comes solely from the twists.