5.1 ISDN Architecture
5.1.2 ISDN services and channelized E1 and T1

Connectivity to ISDN is provided by physical PRI and BRI interfaces. A single PRI or BRI interface provides a multiplexed bundle of B and D channels .

The ISDN bearer service channel—B channel—carries voice or data, usually in frame format. The D channel is the ISDN out-of-band signaling channel, which carries the user-network messages, such as call setup and teardown.

ISDN-BRI specifies the following:

  • There are two 64-kilobit-per-second (Kbps) bearer channels and one 16-Kbps data channel service. BRI connects to an NT1 (Network Termination 1) for four-wire connection.

  • Framing and synchronization are at 48 Kbps.

  • The total speed is two B channels at 64 Kbps each (128), one D channel at 16 Kbps, plus framing and synchronization at 48 Kbps. The total comes to 192 Kbps (128 + 16 + 48 = 192).

  • It is intended to be used at small concentration points.

  • It is referred to as a digital signal level zero (DS0) interface.

ISDN-PRI specifies the following:

  • There are 23 or 30 B channels, at 64 Kbps each.

  • There is one D channel, at 64 Kbps.

  • Framing and synchronization use 8 Kbps (North America T1) or 64 Kbps (European E1).

  • Total speed is 1.544 Mbps (T1) or 2.048 Mbps (E1)

Note: In an E1 PRI, there are actually 32 channels: 30 B channels, one D channel, and one synchronization channel. Also, in Europe, the D channel is carried in timeslot 16; in the United States, it is in timeslot 24. Table displays the relationship between the digital signal level, speed, T designation, and number of channels.

In some cases, a DS0 can carry only 56 Kbps, usually because of legacy telco equipment or a signaling method called robbed-bit signaling (RBS). RBS and other signaling methods are discussed later in this chapter in the "ISDN Primary Rate Interface" section.

In Europe, the equivalent of a T1 facility is an E1. E1 has 32 64-Kbps channels for a total of 2.048 Mbps.