# LAPD primitives

The following table summarizes the supported LAPD primitives:

Message type | Description
--- | ---
DL_DA_IN | Indicates that the trunk from the remote party has received a packet of acknowledged data.
DL_DA_RQ | Requests that the stack transmit a packet of acknowledged data. Sent by the application to the NMS ISDN protocol stack.
DL_EST_CO | Indicates that equipment on the remote side of the ISDN trunk has acknowledged a SABME message sent by the ISDN protocol stack, and has sent back a UA message indicating that the data link is established.
DL_EST_IN | Indicates that the NMS ISDN protocol stack has received a SABME message. The stack automatically acknowledges the message by sending back a UA message, and establishes the data link.
DL_EST_RQ | Requests that the stack establish a data link. The stack transmits a SABME message over the trunk. Sent by the application to the NMS ISDN protocol stack.
DL_REL_IN | If the NMS ISDN software is in the IDLE state, this message indicates that an establishment request sent by the NMS ISDN protocol stack was not successfully answered. If the software is in the DATA_LINK_ESTABLISHED state, this message indicates that the data link has been broken.
DL_U_DA_IN | Indicates that the trunk from the remote party has received a packet of unacknowledged data.
DL_U_DA_RQ | Requests that the stack transmit a packet of unacknowledged data. Sent by the application to the NMS ISDN protocol stack