# Setting up and using NFAS

NFAS groups are specified in the board keyword file. In the file, you specify which trunks on which boards belong to which groups. You also specify which trunk in the group will carry the D channel. All other trunks are set to NetworkInterface.T1E1[x].SignalingType=RAW so all 24 channels on these trunks can be used as D channels. For more information, see the NMS ISDN Installation Manual.

Your Messaging API application initializes just as described in Initialization tasks. The only difference is that when the application calls isdnStartProtocol, the NAI specified in the call can be greater than 3.

The fact that a given trunk is a part of an NFAS group and the D channel resides elsewhere is largely transparent to the application. There are two slight behavioral differences:

- A maximum of four groups on one board can be defined in the configuration file.

- The range of connection IDs available for the D channel is higher. In an NFAS group containing multiple trunks, there will be ACU_MX_CALLS connection IDs for each NAI.

## NFAS groups and Hot Swap configurations

If an NFAS group spans multiple boards in a Hot Swap system, and you remove or insert a board while the system is running, the NMS ISDN protocol stacks on the other boards are not affected. If you remove a board containing only B channels, the stack on the D channel board does not detect that the board is missing. It is the application's responsibility to detect this change and take appropriate action (for example, not accessing B channels on that board).