# Initiating a data link

Before a data link is established, NMS ISDN is in the IDLE state. To establish a data link, the application sends a DL_EST_RQ message to the NMS ISDN protocol stack. This message requests the ISDN stack to transmit a SABME (set asynchronous balanced mode extended) message to the trunk. NMS ISDN remains in the IDLE state until the request is acknowledged by the remote side of the ISDN trunk.

If the equipment on the remote side of the ISDN trunk is prepared to complete the data link, it acknowledges the SABME message with a UA (unnumbered acknowledgement) message. When the stack receives the UA message, it sends a DL_EST_CO 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 application requests a data link:

If the SABME message is not acknowledged (the physical link is down, or the remote equipment is not prepared to complete the link), the stack sends the SABME message four more times, at one second intervals. If the message is not acknowledged within five tries, the stack sends a DL_REL_IN message to the application. NMS ISDN remains in the IDLE state.

The following illustration shows the sequence of messages sent between the trunk, the stack, and the application when the remote party does not respond to a data link request: