that uses TDMA and TDD schemes. V.11 An ITU-T standard that describes electrical characteristics for balanced double-current interchange circuits for general use with integrated circuit equipment. V.21 An ITU-T standard that describes 300bps modems for use in the PSTN. V.24 An ITU-T definition for interchange circuits between data terminal equipment and data circuit-terminating equipment. V.25 An ITU-T standard that describes automatic calling and/or answering equipment on the general switched network. V.29 ITU-T standard that describes 9,600bps modems for use in the PSTN. V.35 An ITU-T standard that describes data transmission at 56Kbps that uses balanced trans- mission methods through a 34-pin physical interface. VAD (voice activity detection) A technique that reduces the amount of information needed to re-create voice at the destination end by removing silent periods and redundant infor- mation found in human speech. VAD also helps with compression. validation An attempt to find errors by executing a program in a given environment. value-added carrier A company that sells services of a value-added network. It can be a PTT, a subsidiary, or an independent company. VAN (value-added network) services Telecommunication services provided over public or private networks that, in some way, add value to the basic carriage, usually through the application of computerized intelligence (e.g., reservation systems, bulletin boards, informa- tion services). VAR (value-added reseller) A provider that deals with distribution and sales. VAS (value-added service) A communications facility that uses communications common carrier networks for transmission and that provides enhanced extra data features with separate equipment. Such extra features, including store-and-forward message switching, terminal inter- facing, and host interfacing, are common. VBR (variable bit rate) An ATM service class for network traffic that is typically from bursty data transfer applications, such as client/server and LAN-to-LAN interconnection. VBR of