BRI lines

ISDN Basic Rate Interface (BRI) consists of 2B + D channels.

Which stands for 2 Bearer channels of 64 kbps each for data and one D channel of 16 kbps for handshaking and control. Having a separate channel for handshaking and control is called "out of band" signalling.

The 2B channels can be bonded together for a single data channel with a 128 kbps transfer rate. Channels 1 and 2 are bearer channels, channel 3 is the D channel.