# ISDN carriers

ISDN is transmitted over standard T1 and E1 carriers. T1 and E1 trunks are typically four-wire digital transmission links. T1 trunks are used mainly in the United States, Canada, Hong Kong, and Japan. E1 trunks are used in most of the rest of the world.

Data on a trunk is transmitted in channels. Each channel carries information digitized at 64000 bits per second (b/s).

## Primary rate interface (PRI)

For primary rate
ISDN, T1 trunks carry 24 channels. E1 trunks carry 32 channels. The channels are usually used as follows:

- On a T1 trunk, 23 of the 24 channels carry data: voice, audio, data and/or video signals. These channels are called bearer channels (B channels).

- On an E1 trunk, 30 of the 32 channels are B channels.

- On a T1 or E1 trunk, one channel carries signaling information for all B channels. This is called the D channel.

The following illustration shows a T1 trunk (standard configuration):

In setups with multiple T1 ISDN trunks, a non-facility associated signaling (NFAS) configuration is sometimes used. In this configuration, the D channel on one of the ISDN trunks carries signaling for all channels on several other trunks. This leaves channel 24 free on each of the other trunks to be used as another B channel.

The following illustration shows a sample NFAS configuration:

Note: NFAS configurations are supported only on T1 trunks. For more information about NFAS, see Non-facility associated signaling (NFAS).

## Basic rate interface (BRI)

ISDN is also transmitted over BRI
trunks with four-wire digital transmission links. BRI trunks are used mainly in Europe and Asia and transmit data in 3 channels.

The three channels are usually used as follows:

- Two of the channels are B channels, carrying voice, audio, data and/or video signals at 64000 b/s.

- One of the channels is a D channel, carrying signaling information for the B channels at 16000 b/s.

The following illustration shows a BRI trunk (standard configuration):