Unspecified Bit Rate. A service class in ATM that is allocated the remaining bandwidth after all prioritized services get their bandwidths. UBR is used for best-effort services such as Web-browsing, e-mail, and file transfer.
Also see ATM.

User Datagram Protocol. A Layer 4 protocol (of the OSI model) that defines data transport without any guarantee for delivery. An alternative to TCP in delivering real-time multimedia streaming.
Also see TCP and OSI model.

Ultra Fast IrDA. An IrDA standard for short-range, point-to-point, and half-duplex communication using infrared that provides maximum data rate of 100 Mbps. A.k.a. 100M-Ir.
Also see IrDA and infrared.

Ultra High Frequency. Frequency band in the range of 300 MHz - 3 GHz, corresponding to wavelength from 1 m to 100 mm. UHF is used in television broadcast and cellular communications. ISM band that is used in wireless LAN also falls within UHF spectrum.

Ultra Low Frequency. Frequency band in the range of 300 Hz - 3 kHz, corresponding to wavelength from 1000 km to 100 km. This band overlaps audio frequencies. Because of its very long wavelength, there is no antenna and transmitter operating at this band.

Unlicensed Mobile Access. A 3GPP specification that provides access to GSM-based voice and data services over license-exempt spectrum technologies such as Bluetooth and Wi-Fi. UMA facilitates voice routing using broadband IP access connection between a Bluetooth/Wi-Fi access point and a UMA network controller. Using UMA, an operator can allow users to roam and handover between mobile (cellular) network and public/private wireless PAN/LAN using a dual-mode handset. Example: BT (British Telecom) Fusion project that caters to residential subscribers.

Universal Mobile Telecommunications System. Refers to one version of 3G technology that offers theoretical bit rates of up to 384 kbps in high mobility, rising as high as 2 Mbps in stationary/nomadic user environments. 3G/UMTS uses WCDMA technology and operates in radio spectrum allocated by the ITU for IMT-2000 mobile communications.
Also see 3G.

User to Network Interface. The interface between end user and the network or the switch. Used in ATM and frame relay.
Also see NNI.

The transmission of data (packet) from a transmitter (server) to a receiver (workstation).
Compare with broadcast and multicast.

Unlicensed National Information Infrastructure. UNII band refers to license-exempt frequency band in the 5 GHz range. 802.11a wireless LAN standard operates at this frequency band to avoid interference as in the crowded ISM band. But the higher the frequency, the shorter the distance it can cover.
Also see ISM band. 

The wireless link that is used by a user station to transmit information (signal) to a wireless base station, access point, or satellite.
Contrast with downlink.

The transfer of a content from a client to a server. An FTP program is used to upload files from a client computer to an FTP server over the Internet.
Contrast with download. Also see FTP.

Universal Plug and Play. A set of protocols that enable a network device from any vendor to be seamlessly discovered and connected to a network. UPnP is developed and promoted by Microsoft and UPnP Forum.

The stream of information from a client (user) computer to an ISP or the Internet. This term is often referred to in DSL or cable Internet access service.
Contrast with downstream.

Uniform Resource Locator. An address that refers to a location on the Internet. Example: http://www.conniq.com is the URL of the home page of this Web site.

Universal Serial Bus. A high speed computer interface for connection to peripherals. Up to 127 peripherals can be connected to a USB interface.
USB is plug-and-play, that's a USB device is automatically detected by the Operating System once it is plugged. USB supports hot plugging, that's a USB device can be connected to or disconnected from the computer without having to reboot the computer.
USB data rate is 12 Mbps in USB 1.1 specification and 480 Mbps in USB 2.0 specification. The difference between USB 1.1 and USB 2.0 is not in the hardware, but in the Operating System and USB device specific application.
Since its inception, USB has replaced serial, parallel, PS/2, SCSI, and even FireWire as the favorite interface for connecting computers to peripherals.

USB On-The-Go. The new USB interface standard that enables two portable or mobile devices such as a camera and a printer to connect directly without a PC, via USB mini-connectors. USB OTG is characterized by smaller size, lower power consumption, and dual-role capability that lets a USB device to act as either host or peripheral.
Without USB OTG, a USB device always takes a peripheral role that is controlled by a PC as the host during a USB connection.
See picture.

Unshielded Twisted Pair. UTP is a type of twisted pair cable that has no protective shields. Because UTP is cheaper and gives enough quality and bandwidth in typical network environment, it is the most popular cable type in telephone local loop and LAN.
See picture. Also see twisted pair.

Ultra Wide Band. FCC defines UWB as any radio technology with a spectrum that occupies greater than 20 percent of the center frequency or a minimum of 500MHz and allocates unlicensed radio spectrum from 3.1 GHz to 10.6 GHz.
UWB is used for point-to-point, short-range, very fast data transfer between multimedia devices.
Also see WiMedia.