ining channels (called bearer channels) can be used exclusively for voice. Because a separate channel is used for signaling, greater functionality is possible with ISDN. The bearer channels are sometimes referred to as B-channels, and the data channel is sometimes referred to as the D-channel. One of the benefits of ISDN is that it can support normal voice calls and ISDN digital calls. In the early 1990s, ISDN was considered to be the next big thing: it was supposed to revolutionize phone and data service. There are two types of ISDN links: BRI is short for Basic Rate Interface. A BRI is an ISDN link composed of two 64 Kbps bearer channels and one 16 Kbps data channel. PRI is short for Primary Rate Interface. A PRI is an ISDN T1 link composed of twenty-three 64 Kbps bearer channels, and one 64 Kbps data channel. PRIs are used a lot for connecting PBX systems, or at ISPs for dial-up lines. While PRI circuits are used today for voice, BRI circuits, which were commonly used for data, have been widely replaced by cheaper alternatives such as DSL. IXC stands for interexchange carrier. An IXC is a telephone company that supplies connections between local exchanges provided by local exchange carriers. Con- necting between LATAs may involve IXCs. J-carrier The J-carrier hierarchy is much closer to the US T-carrier hierarchy (in terms of speed) than the European hierarchy, though the values change as the rates get faster. While J-carrier circuits may still be seen, most of the circuits I've worked on in Japan have actually been E1s or T1s. The Japanese J-carrier hierarchy is shown in Table 20-3. Table 20-3. Japanese J-carrier hierarchy Designator Transmission rate Voice channels 64 Kbps Telecom Glossary Designator Transmission rate Voice channels 1.544 Mbps 6.312 Mbps 32.064 Mbps 397.200 Mbps 5,760 LATA (pronounced lat-ah) is short for local access and transport area. LATAs are geographically defined areas in which a telecom provider can provide local service. The Regional Bell Operating Companies (RBOCs), for example, we