|  | 5.4 | ISDN Services: BRI and PRI
| 5.4.1 | ISDN BRI and ISDN PRI

There are two ISDN services: BRI and PRI. The ISDN BRI service offers two 8-bit B channels and one 2-bit D channel, often referred to as 2B+D, as shown in the figure. ISDN BRI delivers a total bandwidth of a 144-kbps line into three separate channels (8000 frames per second * (2*8-bit B channel+2 bit D channel)=8000*18 = 144kbps). BRI B channel service operates at 64 kbps (8000 frames per second* 8-bit B channel) and is meant to carry user data and voice traffic. ISDN provides great flexibility to the network designer because of its ability to use each of the B channels for separate voice and/or data applications. For example, a long document can be downloaded from the corporate network over one ISDN 64-kbps B channel while the other B channel is being used to connect to browse a Web page. The third channel, the D channel, is a 16-kbps (8000 frames per second * 2 bit D channel) signaling channel used to carry instructions that tell the telephone network how to handle each of the B channels. BRI D channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. The D channel signaling protocol occurs at Layers 1 through 3 of the OSI reference model. Terminals cannot transmit into the D channel unless they first detect a specific number of ones (indicating no signal) corresponding to a preestablished priority. If the TE detects a bit in the echo (E) channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures that only one terminal can transmit its D message at one time. This technique is similar and has the same effect as collision detection in Ethernet LANs. After successful D message transmission, the terminal has its priority reduced by requiring it to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than nonsignaling information. ISDN PRI service offers 23 8-bit channels and 1 8-bit D channel plus 1 framing bit in North America and Japan, yielding a total bit rate of 1.544 Mbps (8000 frames per second * (23 * 8-bit B channels + 8-bit D channel + 1 bit framing) = 8000*8*24.125 = 1.544 Mbps) (the PRI D channel runs at 64 kbps). ISDN PRI in Europe, Australia, and other parts of the world provides 30 8-bit B channels plus one 8-bit D channel plus one 8-bit Framing channel, for a total interface rate of 2.048 Mbps (8000 frames per second* (30*8-bit B channels + 8-bit D channel + 8-bit Framing channel = 8000*8*32 =2.048 Mbps). In the T1/E1 and higher data rate frames the B channels are strung together like boxcars in a freight train. Like boxcars in a switchyard the B channels are rearranged and moved to other frames as they traverse the Public Switched Telephone Network (PTSN) until they reach their destination. This path through the switch matrix establishes a synchronous link between the two endpoints. This allows continuous voice communications without pauses, dropped data, or degradation. ISDN takes advantage of this digital transmission structure for the transfer of digital data.
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