ISDN is a dial-up technology. It has to establish, maintain and terminate the circuit via a “Delta” channel. It transmits and receives data via “Bearer” channels. This can be done “in-band” or “out-of-band” depending on standards and equipment.

ISDN Technologies are broken down into BRI and PRI.

Bearer channels carry the data.
Delta channels are for call management.

BRI = 2 x 64kbps bearer channels + one 64kbps delta channel + 48kpbs of framing and synchronization. BRIs are generally used to place calls from point A to point B.
PRI (T1) = 23 x 64kbps bearer channels + one 64kbps delta channel + 8kbps of framing and synchronization.
PRI (E1) = 30 x 64kbps bearer channels + one 64kbps delta channel + 64kbps of framing and synchronization.
PRIs are generally used to connect to an ISPs access server for general Internet access.

Out-of-band signaling is done on a channel that is dedicated for the purpose and on a separate channel than the data communication. This is the standard for Signaling System 7 (SS7).

V.25bis is the serial protocol running between the NT1 and the modem.