# ISDN carriers

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

Data on a T1 or E1 trunk is transmitted in channels. Each channel carries information digitized at 64000 bits per second (bps).

## Primary-rate ISDN channels

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/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.

T1 trunk (standard configuration)

## NFAS channel usage

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 configuration leaves channel 24 free on each of the other trunks to be used as another B channel.

Sample NFAS configuration

NFAS configurations are supported only for T1 trunks.