t A circuit formed by two insulated electrical conductors. One wire is used for the transmission of information, and the other wire acts as a return to complete the circuit. type approval An administrative procedure of technical tests and vetting that is applied to items of telecommunication equipment before they can be sold or interconnected with the public network. Also known as homologation. UBR (unspecified bit rate) An ATM service class that provides best-effort service. UBR offers no service guarantee, so it is used for text data, image transfer, messaging, and distribution of information that is noncritical, where it is not necessary 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. UHDV (Ultra High Definition Video) A digital video format proposed by NHK of Japan. UHDV provides a resolution that is 16 times greater than that of HDTV. UHF (ultrahigh frequency) The portion of the electromagnetic spectrum that ranges from about 300MHz to about 3GHz and includes television and cellular radio frequencies. Glossary UMTS (Universal Mobile Telecommunications System) UMTS (Universal Mobile Telecommunications System) An evolving European stan- dard for 3G mobile communications. UMTS represents the convergence of mobile and fixed net- works as well as terrestrial wireless and satellite-based networks. UMTS TDD A 3G mobile standard that uses TDD and is designed to work in a single, unpaired frequency band. UMTS TDD uses the TD-CDMA scheme. UNI (User-to-Network Interface) A type of network interface that defines how customers can access their providers' networks. It shields the client from network complexities. unicasting Sending streams from a single origination point directly to a single destination point. Unicode A coding scheme that assigns 16 bits per character (i.e., 216), which translates to more than 65,000 possible characters. uniform-spectrum random noise Noise distributed over the spectrum in such a w