In the Integrated Services Digital Network (ISDN), there are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information.


The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service.


The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel using a T-1 line or 30 B-channels and 1 D-channel using an E1 line. Thus, a Primary Rate Interface user on a T-1 line can have up to 1.544 Mbps service or up to 2.048 Mbps service on an E1 line. PRI uses the Q.931 protocol over the D-channel.

The Primary Rate Interface channels are carried on a T-carrier system line (in the U.S., Canada, and Japan) or an E-carrier line (in other countries) and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office.


E1 is similar to the T1. T1 is the North American term whereas the E1 is the European term for the transmission (digital). The data rate of E1 is about 2 mega bits per second. It has 32 channels at the speed of 64 Kbps. It is important to know that 2 channels among the 32 are already reserved. One channel is used for signaling while the other channel is used for controlling. The difference between T1 and E1 lies in the number of channels here. The speed remains the same. There may be inter – connection between the E1 and T1 lines. This is interconnected because it is used for international purpose.


E1 connections have 32 channels in total.

Typically, for E1 CCS and CAS protocols, 30 channels are used as B-channels and 2 channels are used as D-channels. For this reason, the frame structure of an E1 PRI connection is said to be 30B+2D where the 2 D-channels are located on timeslots 0 and 16 and the remaining channels are used as B-channels. Hence;

Channel 0          –           D-channel (Used mainly for clocking and synchronisation)
Channel 1-15    –           B-channel (Voice)
Channel 16        –           D-channel (Signalling)
Channel 17-31  –           B-channel (Voice)


T1 connections have 24 channels in total.

Typically, for T1 ISDN (CCS) protocols, 23 channels are used as B-channels and 1 channel is used as a D-channel. For this reason, the frame structure of a T1 PRI connection is said to be 23B+1D where the D-channel is located on timeslot 23 (channels numbered from 0 to 23) and the remaining channels are used as B-channels. Hence;

Channel 0-22     –           B-Channel (Voice)
Channel 23         –           D-Channel (Signalling)

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