The table in this appendix summarizes Q.931 messages and the purpose of each. Q.931 is the layer 3 protocol of the subscriber signaling system used for ISDN and is known as Digital Subscriber Signaling System No. 1 (DSS 1). It employs a message set that is made for interworking with SS7's ISDN User Part (ISUP). As such, the message set maps to the ISUP message set.
Table E-1. Q.931 Messages
Binary Code | Message Name | Purpose |
|---|
| | Call Establishment Messages |
0 0 0 0 0 0 0 1 | Alerting | Direction:
Called User  Network and Network Calling User
The called user is being alerted; that is, "the 'B' party's phone is ringing." |
0 0 0 0 0 0 1 0 | Call Proceeding | Direction:
Called User Network or Network Calling User
Call establishment is taking place and, as such, no more call establishment signaling is accepted. |
0 0 0 0 0 1 1 1 | Connect | Direction:
Called User Network and Network Calling User
The 'B' party has accepted the call; that is, has "answered the phone." |
0 0 0 0 1 1 1 1 | Connect Acknowledge | Direction:
Network Called user or Calling User Network
Message sent to indicate that the called user has been awarded the call. If sent to the network by the calling user, the message allows symmetrical call-control procedures. |
0 0 0 0 0 0 1 1 | Progress | Direction:
User Network or Network-> User
Sent to indicate the progress of a call in the event of interworking or in relation to the provision of in-band information/patterns (for example, announcements). |
0 0 0 0 0 1 0 1 | Setup | Direction:
Calling user Network and Network Called User
Initial message sent to initiate a call. |
0 0 0 0 1 1 0 1 | Setup Acknowledge | Direction:
Called user Network or Network-> Calling User
Indicates that call establishment is underway, but additional information might be requested. |
| | Call Information Phase Messages |
0 0 1 0 0 1 1 0 | Resume | Direction:
User Network
Sent request to resume a previously suspended call. |
0 0 1 0 1 1 1 0 | Resume Acknowledge | Direction:
Network User
Indicates to the user that the request to resume a suspended call has been completed. |
0 0 1 0 0 0 1 0 | Resume Reject | Direction:
Network User
Indicates to the user that a failure occurred while trying to resume a suspended call. |
0 0 1 0 0 1 0 1 | Suspend | Direction:
User Network
Sent to request that a call be suspended. |
0 0 1 0 1 1 0 1 | Suspend Acknowledge | Direction:
Network User
Informs the user that a request to suspend a call has been completed. |
0 0 1 0 0 0 0 1 | Suspend Reject | Direction:
Network User
Informs the user that a request to suspend a call cannot be completed. |
0 0 1 0 0 0 0 0 | User Information | Direction:
User Network and Network User
Sent to transfer information to the remote user. |
| | Call Clearing Messages |
0 1 0 0 0 1 0 1 | Disconnect | Direction:
User Network or Network User
When sent by the network, indicates that the connection has been cleared end-to-end. When sent from user to network, it is used to request tear down of an end-to-end connection. |
0 1 0 0 1 1 0 1 | Release | Direction:
User Network or Network User
Indicates that the channel has been disconnected by the equipment sending the message, and that it intends to release the channel along with the call reference. As a result, the receiving equipment should release the channel and call references after sending a RELEASE COMPLETE. |
0 1 0 1 1 0 1 0 | Release Complete | Direction:
User Network or Network User
Sent to indicate that the equipment sending the message has released the channel and the call reference. The channel is ready for reuse and the receiving equipment shall release the call reference. |
0 1 0 0 0 1 1 0 | Restart | Direction:
User Network or Network User
Sent to request that the recipient restarts (returns to idle) the indicated channels or interfaces. |
0 1 0 0 1 1 1 0 | Restart Acknowledge | Direction:
User Network or Network User
Sent to acknowledge a RESTART message and to indicate that the requested restart has been completed. |
| | Miscellaneous Messages |
0 1 1 1 1 0 0 1 | Congestion Control | Direction:
Network User or User Network
Sent to indicate the beginning or ending of flow control on the transmission of USER INFORMATION messages. |
0 1 1 1 1 0 1 1 | Information | Direction:
User - > Network or Network User
Provides additional information in the case of overlap signaling for call establishment, for example, or for other miscellaneous call-related information. |
0 1 1 0 1 1 1 0 | Notify | Direction:
User Network or Network User
Indicates information relating to the call, such as when a user has suspended a call. |
0 1 1 1 1 1 0 1 | Status | Direction:
Network User or User Network
Indicates the current call state in terms of Q.931 state machine and is sent in response to a Status Enquiry message. Is also used to report certain error conditions at any time during a call. |
0 1 1 1 0 1 0 1 | Status Enquiry | Direction:
User Network or Network User
Requests a STATUS message, the sending of which is mandatory. |
0 1 1 0 0 0 0 0 | Segment | Used for segmented messages. |
