DSO is the smallest designation, and is the rate required for a single phone line. The hierarchy is shown in Table 19-1. Chapter 19: Telecom Nomenclature Table 19-1. Digital signal hierarchy Designator Carrier Transmission rate Voice channels 64 Kbps 1.544 Mbps 6.312 Mbps 44.736 Mbps 274.176 Mbps 4,032 E-carrier The E-carrier hierarchy is similar to the U.S. T-carrier hierarchy (described later), though the speeds are slightly different, as is the signaling. The European E- carrier hierarchy is shown in Table 19-2. Table 19-2. European E-carrier hierarchy Designator Transmission rate Voice channels 64 Kbps 2.048 Mbps 8.448 Mbps 34.368 Mbps 139.268 Mbps 1,920 565.148 Mbps 7,680 ISDN stands for Integrated Services Digital Network. ISDN is a form of digital transmission for voice and data. Unlike normal POTS lines, or channelized T1 services-which use the voice path for signaling-ISDN uses a separate channel called the data channel for signaling, SO the remaining channels (called bearer channels) can be used exclusively for voice. Because a separate channel is used for signaling, greater functionality is possible when using 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. Telecom Glossary PRI is short for Primary Rate Interface. A PRI is an ISDN T1 link composed of 23 64-Kbps bearer channels, and one 64-Kbps data channel. PRIs are used a lot when 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 s