# NMS ISDN software overview

NMS ISDN protocol software enables you to write Natural Access applications that communicate on T1, E1, or BRI trunks to perform voice processing functions and call control using ISDN common channel signaling (CCS
) protocols.

Use NMS ISDN with one or more AG or CG boards, which provide the physical interface to trunk lines. Most of these boards also feature on-board digital signal processing (DSP) resources that can perform call control and voice processing functions.

You can configure NMS ISDN software to access ISDN services in any of the following ways:

Configuration | Description
Channelized | The application performs call control and other operations using Natural Call Control (NCC). For more information about this configuration, see the NMS ISDN for Natural Call Control Developer's Manual.
ACU | The application accesses ISDN services at the ACU SAP using the NMS ISDN Messaging service. This configuration enables the application to perform a wide range of Q.931 ISDN D channel functions. For more information about this configuration, see the NMS ISDN Messaging API Developer's Reference Manual.
LAPD | The application accesses ISDN services at the data link layer (layer 2) using the NMS ISDN Messaging service. This setup enables the application to send and receive I-frame data in LAPD messages. This data typically consists of Q.931 messages. For more information about this configuration, see the NMS ISDN Messaging API Developer's Reference Manual.

Choose a method for accessing ISDN services when you initialize the NMS ISDN protocol stack. The access methods are described in both the NMS ISDN Messaging API Developer's Reference Manual and the NMS ISDN for Natural Call Control Developer's Manual.