# Overview of API data structures

NMS ISDN uses data structures to configure the ISDN protocol stack, send messages to the protocol stack, and receive messages from the protocol stack. This topic describes the NMS ISDN data structures in detail. These data structures are defined in the isdnparm.h header file.

## Messaging structures

The message structures used to convey messages and message data between the ISDN protocol stack and the application are:

- ISDN_MESSAGE

- ISDN_PACKET

For more information on sending and receiving NMS ISDN messages, refer to Sending ISDN messages to the stack and Receiving events and ISDN protocol stack messages.

## Protocol parameter structures

Data structures are passed to isdnStartProtocol. The parms argument points to one of these parameter structures, depending upon the stack mode in which the ISDN protocol stack is running:

For... | Specify...
--- | ---
ACU stack mode | ISDN_PROTOCOL_PARMS_Q931CC
LAPD stack mode | ISDN_PROTOCOL_PARMS_LAPD
Channelized stack mode | ISDN_PROTOCOL_PARMS_CHANNELIZED

Refer to the Message primitives section for more information on the default values of these parameters.