# Dialogic® NaturalAccess™ ISDN Messaging API Developer's Manual

The Dialogic® NaturalAccess™ ISDN Messaging API Developer's Manual provides:

- Background information about ISDN.

- A programming guide to NaturalAccess ISDN Messaging API applications.

- A summary of functions organized by task.

- An extensive reference section for functions, data structures, parameters, and codes.

The NaturalAccess ISDN Messaging product exposes ISDN protocols at three levels:

- The LAPD service access point (SAP) for layer 2 protocols, providing direct access to the D channel.

- The ACU SAP for access to a simplified abstraction of the Q.931 ISDN D channel layer 3 message protocols.

- An ISDN primary and basic rate trunk interface, which is channelized and integrated with the NaturalCallControl API.

This manual describes how to build applications that interface with the ISDN protocol stack at layers 2 and 3. For information on building applications that are integrated at higher layers, see the Dialogic® NaturalAccess™ ISDN Software Developer's Manual.

This document is for developers of ISDN applications in C who are programming at the messaging level. The developer should be experienced with ITU Q.931 ISDN call control messages and call states. The developer should also be familiar with Natural Access, basic telephony concepts, and the C programming language.

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