ussed in Chapter 6), and integrated speech and video services will further increase its importance in the future. 2.10.1.7 ISDN The current telephone network is gradually developing into ISDN, in which all information is transmitted in digital form from end to end. With the help of some hardware and software updating, modern digital telephone exchanges are able to provide ISDN service. The main hardware modifica- tion required is the replacement of analog subscriber interface units with digital ones, as shown in Figure 2.18. The ordinary two-wire subscriber loop of the telephone network is upgraded to the basic rate access of ISDN by an NT on the subscriber premises and by a basic rate interface unit and ISDN software in the local exchange. The bidirectional data rate in the subscriber loop is 160 Kbps, which carries 144 Kbps of user data and additional framing information. With the help of Introduction to Telecommunications Network Engineering Ordinary 2W subscriber line with bidirectional 160 Kbps digital transmission ISDN- exchange Network basic terminal Max. 8 subscriber interface equipment, two of which can communicate at the same time Routing of B-channels, 64 Kbps, is independent. D-channel, 16 Kbps, is used for signaling. Total information rate is 144 Kbps, which makes 160 Kbps when framing information is added. Figure 2.18 ISDN basic rate interface. framing information, the receiving end is able to distinguish different channels from the data stream. User data contain two independent 64-Kbps circuit- switched user channels, B channels, and a 16-Kbps signaling channel, the D channel. Subscribers may use user channels, B channels at 64 Kbps, for ordi- nary speech transmission, data, facsimile, or videoconferencing connections. Subscribers may use both B channels independently at the same time and dial them up independently, for example, using one of these channels for a telephone call and another for an Internet connection. For Internet surfing B channels can be combined to provide a single 128-Kbps data r