ccessible by the user. It is used solely for signaling purposes-that is, it cannot carry user data. The pri- mary reason for this is service protection: In the PRI, the D-Channel is used to control the 23 B-Channels and, therefore, requires significantly more bandwidth than the BRI D-Channel. Furthermore, the PRI stan- dards allow multiple PRIs to share a single D-Channel, which makes the D-Channel's operational consistency all the more critical. The functional devices and reference points are not appreciably dif- ferent from those of the BRI. The local loop is still identified as the U ref- erence point. In addition to an NT1, we now add an NT2, which is a service distribution device, usually a PBX, which allocates the PRI's 24 channels to customers. This makes sense since PRIs are typically installed at businesses that employ PBXs for voice distribution. The S/T reference point is now divided; the S reference point sits between the Figure 6-2 The ISDN Primary Rate Interface (PRI) Up to 24 Channels (signaling channel implied). Switch 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 NT2 and TEs, while the T reference point is found between the NT1 and the NT2. PRI service also has the ability to provision B-Channels as super-rate channels to satisfy the bandwidth requirements of higher bit rate ser- vices. These are called "H-Channels," and are provisioned as shown in Table 6-1. Table 6-1 Channel Bandwidth H-Channels 384 Kbps (6B) 1.472 Mbps (23B) 1.536 Mbps (24B) 1.920 Mbps (30B) PBX Applications The PRI's marketplace is the business community, and its primary advantage is pair gain-that is, to conserve copper pairs by multiplexing the traffic from multiple user channels onto a shared, four-wire circuit. Inasmuch as a PRI can deliver the equivalent of 23 voice channels to a location over a single circuit, it is 