# ISDN carriers

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

Data on a trunk is transmitted in channels. Each channel carries information digitized at 64,000 bits per second (b/s). This topic describes the:

- Primary rate interface (PRI)

- Basic rate interface (BRI)

## Primary rate interface (PRI)

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

- On a T1 trunk, 23 of the 24 channels carry data: voice, audio, data and video signals. These channels are called bearer channels (B channels). On an E1 trunk, 30 of the 32 channels are bearer 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 often 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 as shown in the following illustration:

Note: NFAS configurations are supported only on T1 trunks. For more information about NFAS, see the Dialogic® NaturalAccess™ ISDN Messaging API Developer's Manual.

## 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 three channels.

The three channels are usually used as follows:

- Two of the channels are B channels, carrying data: voice, audio, data and 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: