# NMS ISDN software

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

NMS ISDN software is designed to use one or more NMS digital boards (such as the CG 6000C, AG 4000, or AG 2000-BRI board) as the physical interface to trunk lines. In addition to line interfaces, these boards also use powerful on-board digital signal processing (DSP) resources that can handle much of the call control and voice processing overhead.

This topic describes the:

- NMS ISDN product configurations

- NMS ISDN ACU configuration

- NMS ISDN LAPD configuration

- Reference points supported by NMS ISDN

## NMS ISDN product configurations

You can use NMS ISDN software to access ISDN services in three ways:

- Configure the NMS ISDN software so an application can perform call control and other operations, using the Natural Call Control service.
  This NMS ISDN configuration is called the channelized configuration. For more information about this access method, see the NMS ISDN for Natural Call Control Developer's Manual.

- Access ISDN services at the ACU SAPI using the NMS ISDN Messaging API. An application can use the API to perform a wide range of Q.931 ISDN D channel functions.
  This NMS ISDN configuration is called the ACU configuration. This access method is discussed in this manual.

- Access ISDN services at the data link layer (layer 2) using the NMS ISDN Messaging API. An application can send and receive I-frame data in LAPD messages. This data typically consists of Q.931 messages.
  This NMS ISDN configuration is called the LAPD configuration. This access method is discussed in this manual.

Specify the configuration to use when initializing the ISDN protocol stack, as described in Initializing ISDN protocol stack instances.

## NMS ISDN ACU configuration

The NMS ISDN ACU configuration allows access to Q.931 (layer 3) call control, using the NMS ISDN Messaging API. The application can send and receive switch- and country-invariant D channel messages using this interface. Access at this level allows you direct control over D channel messages and greater control over the contents of these messages.

In the ACU configuration, one or more instances of the NMS ISDN protocol stack runs on the board, one for each D channel. The stack runs in ACU stack mode. In this mode, the protocol stack implements all ISDN layer 2 and layer 3 functionalities. The application uses the NMS ISDN Messaging API to command an entity in the stack called the ACU, which in turn commands the D channel through the lower ISDN layers. Events received by the stack from the D channel are placed in the same event queue as other Natural Access events, allowing the application to access ISDN events in the same way that other events are accessed.

B channel information is routed to the DSP resources through the board's H.100, H.110, or MVIP switch. The switch has certain default behavior, described in Making switch connections for NMS ISDN. The switch can also be controlled using the Natural Access Switching service.

The following illustration shows the NMS ISDN application architecture (ACU configuration):

## NMS ISDN LAPD configuration

Access at the data link layer is useful if an application must support a private data link protocol, or if the user wants to create a complete Q.931 protocol at the application level. At this level, the messages sent and received by the application constitute LAPD frames.

The NMS ISDN protocol stack runs in LAPD stack mode, as shown in the following illustration:

In this mode, the protocol stack implements ISDN layer 2 functionality. No ACU is present. Instead, the application uses the NMS ISDN Messaging API to send LAPD frames directly to the data link layer (layer 2).

Events coming from the data link layer are placed in the same event queue as other Natural Access events, allowing the user to access ISDN events in the same way that other events are accessed.

As in other configurations, B channel information is routed to the DSP resources through the board's H.100, H.110, or MVIP switch. The switch has certain default behavior, described in Making switch connections for NMS ISDN. Alternatively, the switch can be controlled using the Natural Access Switching service.

## Reference points supported by NMS ISDN

NMS ISDN supports access across the S and the T reference points (as shown by the solid arrows on the upper left of the following illustration). At the ACU SAP, access to S/T is transparent.

You can configure an ISDN protocol stack to emulate the network by using the partner_equip field passed to isdnStartProtocol. See Function summary for more details on the parameters available to configure the ISDN protocol stack.

The following illustration shows the reference points supported by NMS ISDN: