ISDN Channels

In ISDN, the local loop carries only digital signals that represent signaling information and user data. The local loop typically carries voice, data, or video. The term channel, unlike other techniques, means a unidirectional conduit that carries signaling and user information. Three basic channels are defined by the standards:

• D channel (Delta channel, signaling channel) The D channel is a packet-switched channel that carries signaling and control information for B channels between the customer premises equipment (CPE) and the network. It can also be used for user data. Typically, the D channel provides signaling for one or more ISDN access points. When providing signaling for more than one ISDN access point, the D channel saves equipment and resources (see "NFAS Groups" in Chapter 12, "ISDN BRI Troubleshooting," Scenario 4). It operates at 16 or 64 kbps, depending on the implementation.

• B channel (Bearer channel) The B channel carries user data, including data, voice, and video. It operates at digital signal level 0 (DS-0) rates (64 kbps). It can be used for both circuit and packet-switching . The B channel also provides additional services depending on the signaling information from the D channel.

• H channel (Hybrid channel) The standard defines H _0, H _10, H ₁₁ and H ₁₂ , which operate at rates of 384 kbps, 1.472 Mbps, 1.536 Mbps, and 1.920 Mbps respectively. The H channel is a logical grouping of six B channels, and the H ₁₁ is equivalent to four H channels. The H channels are used for user information that requires higher speeds such as high quality video, high-speed data, and facsimile .

NOTE

D, B, and H channels were defined in the first ISDN standard and are known as narrowband ISDN (NISDN) . A later set of standards called communication services was defined to provide data rates for up to 622 Mbps. These services were designed for multimedia services and are known as Broadband ISDN (BISDN) . The focus of this book is on NISDN.

All types of channels share one physical medium. B and D channels define the BRI, as shown in Figure 9-1. For the BRI, the D channel uses time-division multiplexing (TDM) technology to provide the signaling. The same applies for the PRI, but unlike BRI, PRI in the U.S. and Canada uses 23 B channels (from number 1 to number 23 and one D channel (number 24). For its practical use, the D channel can be the following:

0 = Idle

1 = Propose

2 = Busy

3 = Reserved

4 = Restart

5 = Maint_Pend

Figure 9-1. ISDN BRI and PRI Channels

There is a predefined time slot for every channel, which is typical of synchronous TDM. See Chapter 3, "The Cloud" for more information on synchronous TDM. The PRI is defined differently for European and Asian countries .

The ISDN network architecture is a roadmap that provides detail sufficient to guide ongoing network planning, design, and implementation. At the same time, the ISDN layered architecture matches the suggested layer-by-layer approach to troubleshooting in this book.