lls, as most can. Placing a video call to a customer using switched 56 service is much like making a long distance phone call. The caller dials two phone numbers, one for each channel. Placing a call using switched 56 is just as easy. One dials the telephone numbers associated with the channels that connected to the video communication system on the other end. Because signaling is in-band, the connection may take a minute or more to establish. Part of the delay is usually associated with the codec handshaking process, however. Three different serving arrangements exist for switched 56: the four-wire variety (INC USDC) that AT&T developed, a two-wire version that Northern Telecom developed (Datapath), and Circuit Switched Digital Capability (CSDC), which is yet another two-wire version that few LEC's offer. Two-wire switched 56 is transported at rates of 144 or 160 Kbps, and multiplexed using ping-pong modulation. Also called time- compression multiplexing (TCM), this technique supports transmissions from only one end of the connection at any given time, and simulates bi-directional (full- duplex) transmission by rapidly accepting signals from both ends on an alternating basis. Four-wire switched 56 provides a communications path for each end of the connection, with each pair of wires operating at only 56 Kbps. Two- and four-wire implementations of switched 56 are compatible. From a subscriber's point of view, there is really not much difference between the two except that four-wire service permits loops between the subscriber and the CO to span a greater distance. Switched 56 service terminates in a DSU, which connects to CPE via an RS-232 or V.35 interface. Customers must buy the proper type of DSU; they differ depending on whether the carrier provides two- or four-wire switched 56 service. The carrier should specify which type is required. In switched 56 applications, DSUs not only manage clocking and bipolar conversions, they also signal the destination of the call to the telephone company switch. DSU dialing 