fers guaranteed service delivery. VBR-RT (real-time) is designed for real-time voice and videoconfer- encing applications, and VBR-NRT (non-real-time) is for mission-critical data applications. VC (virtual circuit) A series of logical, rather than physical, connections between sending and receiving devices. With a virtual circuit, two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination. VC (virtual container) A data structure designed for the transport and switching of sub- STM-0 network services such as CEPT-1. All network services below E-3 are mapped into VCs, and VCs are multiplexed into the SPE of an STM-1. VCI (virtual channel identifier) In ATM, the part of a cell header that identifies the chan- nel associated with the cell. Glossary VDSL (Very-High-Bit-Rate DSL) VDSL (Very-High-Bit-Rate DSL) A transmission medium that provides a maximum span of about 5,000 feet (1,500 m) over a single twisted-pair cable. Over this distance, VDSL can pro- vide a rate of up to 13Mbps downstream, and with a shorter distance of 1,000 feet (300 m), it can provide up to 52Mbps downstream, which is enough capacity to facilitate DTV. VDSL provides 1.5Mbps to 2.3Mbps upstream. VDSL2 (VDSL version 2) A broadband wireline medium whose key applications are the next generation of TV-VOD, DTV, HDTV, and forms of interactive multimedia Internet access. VDSL2 has two bandwidth options: LR-VDSL2-12MHz (the "long reach" alternative) and SR-VDSL2-30MHz (the "short reach" alternative). VDT (video dial tone) A U.S. term that defines the capability of a network access provider to offer video access and carriage directly to or from subscribers. vertical blanking interval Unused lines in each field of a TV signal. Some of these lines may be used for captions and specialized signal and cable service. VF (virtual fiber) A wireless solution to the "first-mile" problem of delivering high-speed access to the end user. The use of VF as a last-mile so