their connect LANs in the same city. WANs are wide area networks, of course. networks, often heterogeneous, which cover many sites, Long-distance carriers compete with offerings in the spanning large corporations and sometimes continents. following specific categories: Five common topologies exist for point-to-point 1. Voice Grade Service provides leased private analog networks: star, ring, net, tree, and irregular. Two topol- lines. logies, which do not use a radio band, have been developed for broadcast: ring, where the ends of the 2. Digital Data Service (DDS) provides leased digital channel are connected, and bus, where they are not. lines used for data only at speeds ranging from 2400 bps to 19.2 kbps or, in some cases, 64 kbps. The DSO Service provides 64-kbps digital lines, each Voice traffic characteristics providing one DSO channel. Some carriers provide a clear channel capability, meaning that the carrier To understand voice communications, we have to explore does not require any portion of the channel for the history of voice networks. When AT&T was divested control signaling. The channel is "clear," and the of its local phone companies on January 1, 1984, two entire 64 kbps band width can be used by the sub- distinct types of telephone companies were legally scriber. Otherwise, only 56 kbps of the 64 kbps can defined. be used. 3. Fractional T1 lines are the FT1 lines described earlier 1. Local Exchange Carriers (LECs). These consist of the 23 Bell Operating Companies (BOCs) that in this chapter. Some vendors allow the individual were created by the divestiture, the former inde- FTI components to be accessed as if they were pendent telephone companies such as GTE and private voice grade lines or DDS lines while others Contel, and about 1500 small-town telephone provide only T1 access. companies. 4. T1 Service provides lines that support voice, data, or 2. Interexchange Carriers (IECs or sometimes IXCs), video at DS1 speed. Service may be channelized or unchannelized. Channelized means that it is divide