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10.1 INTRODUCTION TO ISDN AND DSS1 233Figure 10.1-1. ISDN users served by a local ISDN exchange. TE, Terminal equipment; NT,network terminal; DSL, digital subscriber line; ET, exchange terminal.terminal equipment (TE) units of various types (telephones, facsimile machines,data processing equipment, etc.). The NT is attached to an exchange termination(ET) in the local exchange by a digital subscriber line (DSL). The line has anumber of time-division multiplexed B-channels and one D-channel. The B-channels are used for circuit-mode communications (PCM encoded speech or voicebandmodem data, or 64-kb/s digital data). The D-channel is used for DSS1 signalingmessages and for packet-mode data communications.In most countries, the DSL, NT, and ET are considered to be internal parts of thetelecommunication network. Therefore, the network-to-user interface specified inITU-T is the S/T interface (between NT and the TEs). In the United States, theDSL and NT are considered to be outside the network, and the network-to-user interfaceis the U interface. We distinguish the following two DSL types.Basic Access. A basic access DSL is intended for residential or small businessuse. In the United States, it is a two-wire circuit with two bidirectional B-channels(64 kb/s) and one bidirectional D-channel that operates at 16 kb/s (2B þ D). The bitrate in each direction is 2 64 þ 16 þ 48 (for overhead functions) ¼ 192 kb/s.Physically, the DSL is an ordinary two-wire subscriber line. Circuitry in the ETand NT enables the line to transfer both bit streams simultaneously (full duplex operation).Since a call requires one B-channel, this DSL allows two simultaneous calls,and up to eight TEs can be connected to the NT.Other countries use a variety of basic access DSLs. They can be two- or four-wirecircuits with one (B þ D) or two (2B þ D) B-channels.At the user’s premises, the TEs are attached to the NT by a bidirectional passivebus. Each terminal has access to the B-channels and the D-channel of the DSL.