line network, except that it makes use of the PSTN rather than physically dedicated leased lines. In customer-based VPNs, carriers install gateways, routers, and other VPN equipment on the customer premises. This is preferred when customers want to have control over all aspects of security. In network-based VPNs, the carrier houses all the necessary equipment at a POP near the customer's location. Customers that want to take advantage of the carrier's VPN economies of scale prefer this type of VPN. VPN gateway A device that enables VPNs to set up and maintain secure tunnels through the Internet. VPWS (virtual private wire service) A type of VPN that emulates a point-to-point link and provides a single service that is perceived by its user as an unshared link or circuit of the chosen service. VPWS provides a mesh of point-to-point customer edge-to-customer edge Layer 2 connections over a packet-switched network. VR (virtual reality) A computer-based application that provides a human-computer inter- face such that the computer and its devices create a sensory environment called a virtual world. The sensory environment is dynamically controlled by actions of the individual so that the envi- ronment appears real. VRC (vertical redundancy check) See parity check. VSAT (very-small-aperture terminal) An earth station with a small antenna, usually 2 feet (0.6 m) or less. VSF-OFCDM (Variable-Spreading-Factor Orthogonal Frequency Code Division Multiplexing) A form of multiplexing used in 4G wireless. It increases downlink speeds by using multiple radio frequencies to send the same data stream. VT (virtual tributary) A data structure designed for the transport and switching of sub-STS-1 network services such as DS-1, DS-1C, and DS-2. All network services below DS-3 are mapped into VTs, and VTs are multiplexed into the SPE of an STS-1. Walsh Code Walsh Code An algorithm that generates statistically unique sets of numbers for use in encryption and cellular communications. It is an orthogonal code used to uniquely define indi-