Q.931

ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly (not at the same layer) comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbit/s. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine. Q.931 has more recently been used as part of the VoIP H.323 protocol stack (see H.225.0) and in some mobile phone transmission systems.

Q.931 is a layer 3 protocol, mainly used for the ISDN call establishment, maintenance, and release of network connections between two DTEs on the D channel. A Q.931 frame contains the following elements:

* Protocol discriminator (PD) - Specifies which signaling protocol is used for the connection (e.g. PD=8 for DSS1)
* Call reference value (CR) - Addresses different connections which can exist simultaneously. The value is valid only during the actual time period of the connection
* Message type (MT) - Specifies the type of a layer 3 message out of the Q.931-defined Message type set for call control (e.g. SETUP). There are messages defined for the call set up, the call release and the control of call features.
* Information elements (IE) - Specify further information which is associated to the actual message. An IE contains of the IE name (e.g. bearer capability), their length and a variable field of contents.

Message examples

* SETUP (indicating the establishment of a connection)
* CALL PROCEEDING (indicating that the call is being processed by the destination terminal)
* ALERTING (tells the calling party that the destination terminal is ringing)
* CONNECT (sent back to the calling party indicating that the intended destination has answered the call)
* DISCONNECT (sent to indicate a request to terminate the connection, by the end that seeks to terminate)
* RELEASE (sent in response to the disconnect request indicating that the call is to be terminated).
* RELEASE COMPLETE (sent by the receiver of the release to complete the handshake).

References

* [http://www.itu.int/rec/T-REC-Q.931/en/ ITU-T Recommendation Q.931] .