# ISDN Software channelized configuration

ISDN Software allows you to access ISDN call control services using the NaturalCallControl API (NCC API). ISDN Software provides access through standard function calls. The same function calls are used with any other trunk protocols (such as CAS or DPNSS). The ISDN Software NaturalCallControl interface is described in this manual.

The following illustration shows the ISDN Software application architecture in a channelized configuration:

In a channelized configuration, the ISDN protocol stack and a trunk control program (TCP) run on the board.

Component | Description
ISDN protocol stack | One or more instances of this stack run on the board, one for each D channel. In the ISDN Software channelized configuration, the protocol stack runs in channelized stack mode. In this mode, the stack implements all ISDN layer 2 and layer 3 functionality. When signaling messages for a particular B channel arrive on the D channel, the channelizer within the stack routes them to the TCP associated with that channel. This description also applies to DPNSS.
Trunk control program (TCP) | A TCP serves as the call control interface between NaturalAccess and a B channel on a trunk. At initialization, the application creates a context/TCP instance pair for each B channel (voice channel). The application communicates with the TCP through the context. The TCP for a B channel serves as the interface between the application and the ISDN protocol stack for all signaling for that B channel. It translates NaturalAccess commands involving signaling for the B channel into ISDN messages, and sends them to the ISDN protocol stack, which controls the D channel. It interprets signaling messages coming from the ISDN protocol stack, translates them into appropriate events, and sends them to the application through the context. During call setup, the TCP has exclusive use of the DSP resources allocated to the B channel. Once the call is in the connected state, the application also has direct control of the DSP resources for the channel. For DPNSS, the mechanism described for ISDN also applies.

Note: DSP resources are temporarily available to the application during call setup if the application elects to play user audio while accepting or rejecting a call.

In channelized configuration, ISDN protocol stack instances are started and stopped using two functions from the ISDN Software library. No other functions from this library are used in this configuration. A daemon program included with the ISDN Software can start the stack before ISDN applications are launched, and stop the stack after they are shut down.

B channel information is routed to the DSP resources through the board's CT bus switch. The switch has default behavior, described in Making switch connections for ISDN Software. Alternatively, the switch can be controlled using the Natural Access Switching API.