ng of the other chan- nel. This allows residential users to have two independent telephone connec- tions via one two-wire subscriber line, or alternatively one line for telephone and the other for a simultaneous connection to the Internet. Network termi- nals provided by network operators contain one analog interface and a PCM codec for ordinary analog telephone. The provision of a 64-Kbps end-to-end digital connection by ISDN also allows quicker and better quality Group 4 facsimile transmission. The BRI of ISDN 2B+D (2 <insert> 64 Kbps + 16 Kbps) is designed to replace the present analog subscriber telephone interface in the future. However, ISDN is a circuit-switched technology, in which the user fee is based on the call duration and its data rate is quite moderate for Internet access. When new access technologies have evolved, many residential customers prefer to keep ordinary analog telephone and order higher data rate DSL or cable modem access for data services instead of ISDN. In corporate networks, many B-channels are required and these are provided by the primary rate interface (PRI) that has the structure of 30B+D (30 X 64 Kbps + 64 Kbps) or 23B+D. The PRI utilizes 2.048- or 1.544- Mbps transmission in the local-access network and it is able to support many simultaneous (ISDN) telephone calls. This interface is used for PABX con- nections to the public network and rarely for data connections. The frame structures at 1.544 and 2 Mbps were explained in Chapter 4. 6.4.3 The access technologies discussed earlier do not utilize all of the potential capacity of the symmetrical twisted cable pair of a subscriber loop. A family of technologies, known as DSL, or digital subscriber line, has been developed to increase the data transmission rate over ordinary local loops to the order of a few megabits per second and it is simultaneously available for ordinary tele- phone service. This is far beyond the capacity of ISDN subscriber lines. The ISDN data channels are expensive dial-up circuits that are switched by ISDN