# NMS ISDN state machine (LAPD configuration)

An application can perform various operations at the data link layer with NMS ISDN software in an LAPD configuration. The following illustration shows the NMS ISDN software LAPD configuration state machine:

lapddemo, the LAPD demonstration program, demonstrates LAPD operations using NMS ISDN software.

NMS ISDN events are generated by the NMS ISDN protocol stack running on the board. When an NMS ISDN event occurs, the event ID ISDNEVN_RCV_MESSAGE is returned in the CTA_EVENT structure. When decoding the message, the following information is extracted:

Item | Description
--- | ---
LAPD message | The message is a single byte code.
Sender | The sender is the ISDN entity that generated the message. For layer 2 ISDN call control messages (LAPD), the sender is ENT_DL_D (the data link layer).
Recipient | The recipient is ENT_APPLI (the application).