Why is the speed of ISDN sometimes referred to as 56 kbps, 64 kbps, 112 kbps, 128 kbps or even 144 kbps?
While most of Southwestern Bell’s central office switches are capable of “clear channel coding” which supports 64 kbps per B channel, a few older switches only support 56 kbps per B-channel. Many ISDN devices allow you to aggregate two B-channels for a total of 128 kbps, or if you are on an older switch 115 kbps. The ISDN protocol actually supports a third 16 kbps channel (D-channel) for network signalling, so the total line speed is 144 kbps. The type of ISDN device you use will also impact speed since some devices only support a single B-channel at 64 kbps or even 56 kbps.
