T1: A Survival Guide

Appendix A. Access Aggregation with cT1 andISDN PRI

When have people been half as rotten as what the panderers to the people dangle before crowds?

—Carl Sandburg, The People, Yes

Most of this book has been about T1 as a single data pipe for network access. With the explosion of Internet service providers and dial-up access, though, T1 also became a high-density solution for dial-in POPs. Each T1 has 24 voice channels and can support 23 or 24 dial-in users, depending on the technology chosen. Furthermore, T1 is digital technology that allows higher transmission speeds and lower noise. New 56-kbps modems require that ISPs use digital pipes to their dial-in servers so that only one digital/analog conversion is required.

Two main technologies are used for this purpose. Channelized T1 (cT1) is a regular T1 that is broken into 24 small 64-kbps pipes. Creating and tearing down phone circuits requires signaling. After bit-robbing, the maximum available throughput is 56 kbps per channel. ISPs may also choose to use ISDN for dial-in POPs. In addition to the familiar 128-kbps Basic Rate Interface (BRI), ISDN has been specified at T1 speeds as the Primary Rate Interface (PRI). ISDN PRI provides 24 64-kbps channels, but only 23 are used for data transmission. The remaining channel is used for signaling messages. ISPs often use ISDN PRIs to provide ISDN services, and large businesses use ISDN PRIs for dial-on-demand backup services. Generally, ISDN PRI is priced so that if more than eight ...