# Sending messages across an established link

Once NMS ISDN is in the DATA_LINK_ESTABLISHED state, the application and the remote equipment can exchange messages. To send a packet of data, the application sends either of the following messages:

- If the packet is acknowledged (as defined in the Q.921 specification), send DL_DA_RQ to the stack with a pointer to the data.

- If the packet is unacknowledged, send DL_U_DA_RQ to the stack with a pointer to the data.

No confirmation is returned in response to either of these requests.

The following illustration shows the sequence of messages sent between the trunk, the stack, and the application when the application sends a message across an established data link:

When a packet is received from the remote party, the stack sends one of the following messages to the application:

- DL_DA_IN if the packet is acknowledged.

- DL_U_DA_IN if the packet is unacknowledged.

The application does not confirm receipt of a packet.

The following illustration shows the sequence of messages sent between the trunk, the stack, and the application when the application receives a message across an established data link:

If the data link is broken while NMS ISDN is in DATA_LINK_ESTABLISHED state, the stack sends DL_REL_IN to the application. NMS ISDN returns to the IDLE state.