g noise, and we can never develop essen- tially higher rate voice-band modems. The next step after voice-band modems is ISDN, which provides a two or four times higher data rate and speeds the call establishment process. Other options for even higher data rate access are DSL and cable modems, which we discuss later in this chapter. 6.4.2 We introduced ISDN in Chapter 2 as a new generation telephone network. Now we look at it again from the data service point of view. As we saw, the full capacity of digital telecommunications network is not utilized by voice- band modems. If we rarely need higher data rate service, it may be attractive to use a circuit-switched telecommunications service to provide the connec- tion only when it is needed. The ISDN provides switched end-to-end digital n X 64-Kbps circuits that we can use for voice or data. Figure 6.15 presents an example of an interconnection when ISDN basic rate interfaces (BRIs), 2 X 64 Kbps, are available at both ends of the circuit. 64 Kbps ISDN-Basic Transmission Basic rate interface, BRI, 2B+D: Network rate interface network Two bearer channels (B) at 64 Kbps terminal and one signaling channel (D) at 16 Kbps, 2B+D, totally 144 Kbps. The framing information increases data rate on a subscriber line to telephone interface: 160 Kbps. U-interface exchange Maximum 8 subscriber devices Office A may be connected to NT and (group 4) Ordinary 2-wire two of them may communicate at subscriber line the same time with bidirectional 160 Kbps digital transmission Primary rate interface, PRI: European PRI: 30 bearer channels, one signaling channel and framing, all at 64 Kbps make up data rate of ISDN- 2.048 Mbps. Framing structure is Basic the same as in ordinary 2 Mbps PCM. telephone interface exchange American PRI: 23B+D plus framing, interface 1.544 Mbps. (group 4) Office B Figure 6.15 Basic rate interface and ISDN connection. Data Communications The basic rate interface provides two independent 64-Kbps circuits, and the routing of one B-channel is independent of the routi