# Overview of message primitives

This section describes the types and contents of the messages interchanged between the NMS ISDN protocol stack and the application.

To send a message to the stack, the application specifies the message primitive in the ISDN_MESSAGE structure passed to isdnSendMessage. For more information, refer to Sending ISDN messages to the stack.

Events received from the stack contain message primitives. For more information, refer to Network access identifiers (NAIs).

Messages with the prefix ACU_ are valid only when the NMS ISDN software is in its ACU configuration, and the protocol stack is in ACU stack mode. Messages with the prefix DL_ are valid only when the NMS ISDN software is in its LAPD configuration, and the protocol stack is in LAPD stack mode.