log local loop. In ISDN implementations, local-switch interfaces must be modified to support a digital local loop. Instead of using analog frequency modula- tion to represent voice or data traffic carried over the local loop, ISDN digitizes the traffic at the origination point, either in the voice set itself or in an adjunct device known as a terminal adapter (TA). The digital local loop then uses time-division multiplexing to create multiple chan- nels over which the digital information is transported, which provide for a wide variety of truly integrated services. The Basic Rate Interface (BRI) There are two well-known implementations of ISDN. The most common (and the one intended primarily for residence and small-business appli- cations) is called the Basic Rate Interface, or BRI. In BRI, the two-wire local loop supports a pair of 64 Kbps digital channels-known as B-Channels-as well as a 16 Kbps D-Channel, which is primarily used Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Chapter 6 for signaling but can also be used by the customer for low-speed (up to 9.6 Kbps) packet data. The B-Channels can be used for voice and data, and in some implementations can be bonded together to create a single 128 Kbps channel for videoconferencing or other higher bandwidth appli- cations. Figure 6-1 shows the layout of a typical ISDN BRI implementation. In this diagram, the LE is the local exchange, or switch. The NT1 is the net- work termination device that serves as the demarcation point between the customer and the service provider; among other things, it converts the two-wire local loop to a four-wire interface on the customer's premises. The TE1 (terminal equipment, type 1) is an ISDN-capable device such as an ISDN telephone. This simply means that the phone is a digital device and, therefore, is capable of performing the voice d