# Responding to a data link establishment request

Before a data link is established, NMS ISDN is in the IDLE state. If the NMS ISDN protocol stack receives a SABME message, it automatically responds with a UA message, and sends a DL_EST_IN message to the application. The data link is established, and NMS ISDN enters the DATA_LINK_ESTABLISHED state. Messages and other information can now be passed across the data link.

The following illustration shows the sequence of messages sent between the trunk, the stack, and the application when the stack responds to a data link establishment request: