# B channel assignment overview

When a TCP instance is launched, a B channel is assigned to it. This channel is called the default channel. When placing a call, the TCP requests the use of the default channel from the network. It directs the network to respond in one of two ways, or modes:

Mode | Description
--- | ---
Exclusive | If the TCP's default B channel is not available on the network, the network clears the call.
Non-exclusive (also called preferred mode) | If a TCP's default B channel is not available on the network, the network directs the TCP to use another channel. In this case, the application must perform switching to connect the new B channel with the DSP resource associated with the TCP.

In the US, ISDN systems almost always place and receive calls in exclusive mode. In Europe, both modes are used: many central office (CO) switches require the TE equipment side to use non-exclusive mode.

For details about the switch model for a board, see the installation and developer's manual for the board.

## Default channel assignment

A default channel is assigned to each TCP instance in either of two ways.

Ordinarily, the network stream and timeslot are inferred from the DSP stream and timeslot that were specified when the context was opened. This is the trunk slot that is connected to the DSP slot when the Clocking.HBus.ClockMode board keyword is set to STANDALONE. See Making switch connections for ISDN Software for more information.

The default channel for the NCC API can be specified by setting the following board keywords for that TCP instance. These parameters are documented in ISDN Software parameter files.

- NCC.ADI_ISDN.START_EXT.networkstream

- NCC.ADI_ISDN.START_EXT.networkslot