UBR (unspecified bit rate)
An ATM service class that provides best-effort service. UBR offers no service guarantee, so you would use it for text data, image transfer, messaging, and distributing information that’s noncritical, where you don’t have to have a set response time or service guarantee.

UDP (User Datagram Protocol)
A minimal-functionality transport-layer protocol in the TCP/IP protocol suite.

UHF (ultrahigh frequency)
The portion of the electromagnetic spectrum that ranges from about 300MHz to about 3GHz and includes television and cellular radio frequencies.

UMTS (Universal Mobile Telecommunications System)
An evolving European standard for 3G mobile communications. The convergence of mobile and fixed networks, as well as terrestrial wireless and satellite-based networks.

unicasting
Sending streams from a single origination point directly to a single destination point.

Unicode
A coding scheme that assigns 16 bits per character (that is, 216), which translates to more than 65,000 possible characters.

uniform-spectrum random noise
Noise distributed over the spectrum in such a way that the power-per-unit bandwidth is constant. Also called white noise.

UNII (Unlicensed National Information Infrastructure)
Unlicensed radio bands that operate at 5.2GHz.

UNIX
A multiuser, open operating system developed by Bell Laboratories.

unlicensed bands
Radio bands that can be used to provide broadband access, without the need for obtaining a license. Networks that use unlicensed bands have a range of about 35 miles (55 kilometers) and offer throughput from 128Kbps to 10Mbps over shared media.

unshielded cable
A cable in which the signal carrying wire or circuit is not shielded to keep out electromagnetic noise that can potentially damage the data stream.

UPS (uninterruptible power supply)
A device that usually includes in an inverter, drawing its power from batteries, which generates an extremely “well-behaved” AC power signal for a PBX or other equipment. If a particularly heavy demand is anticipated, the system can be coupled with an auxiliary generator that is started when commercial power is interrupted.

upstream
The direction of transmission flow away from the user.

UPT (Universal Personal Telephony)
An evolving ITU-R standard for the provision of personal mobility across many different kinds of fixed and mobile networks.

UTP (unshielded twisted-pair cable)
A twisted-pair cable in which the two insulated wires that twist around each other are not surrounded by additional insulation.

UWC (Universal Wireless Communications)
Also called NADC, a wireless standard specified as IS-54 that was adopted in 1992. It has since been updated, and it is now called IS-136. It uses TDMA and TDD schemes.