From Taridium

Contents 1 Q.931 Protocol Overview 1.1 ALERTING (1) 1.2 CALL PROCEEDING (2) 1.3 CONNECT (7) 1.4 CONNECT ACKNOWLEDGE (15) 1.5 SETUP (5) 1.6 SETUP ACKNOWLEDGE (13) 1.7 SUSPEND (37) 1.8 SUSPEND ACKNOWLEDGE (45) 1.9 SUSPEND REJECT (33) 1.10 RESUME (40) 1.11 RESUME ACKNOWLEDGE (48) 1.12 RESUME REJECT (34) 1.13 DISCONNECT (69) 1.14 RELEASE (77) 1.15 RELEASE COMPLETE (90) 1.16 STATUS ENQUIRY (117) 1.17 STATUS (125)

Q.931 Protocol Overview

Q.931 is ISDN's connection control protocol, roughly 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 kbps. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine.

Note: In accordance with the conventions of ITU standards, bits are numbered from LSB to MSB, 1 to 8 (Internet standards use MSB to LSB, 0 to 7).

The general format of a Q.931 message includes a single byte protocol discriminator (8 for Q.931 messages), a call reference value to distinguish between different calls being managed over the same D channel, a message type, and various information elements (IEs) as required by the message type in question:

The most important messages types are:

ALERTING (1)

IEs: Bearer capability, Channel identification, Progress indicator, Display, Signal, High layer compatibility

Direction: Called user → network → calling user The called user is being alerted, i.e "the phone is ringing".

CALL PROCEEDING (2)

IEs: Bearer capability, Channel identification, Progress indicator, Display, High layer compatibility

Direction: Called user → network → calling user Call establishment is proceeding.

CONNECT (7)

IEs: Bearer capability, Channel identification, Progress indicator, Display, Date/time, Signal, Low layer compatibility, High layer compatibility Direction: Called user → network → calling user

The call has gone through and been accepted.

CONNECT ACKNOWLEDGE (15)

IEs: Display, Signal

Direction: Calling user → network → called user

SETUP (5)

IEs: Sending complete, Repeat indicator, Bearer capability, Channel identification, Progress indicator, Network specific facilities, Display, Keypad facility, Signal, Calling party number, Calling party subaddress, Called party number, Called party subaddress, Transit network selection, Repeat indicator, Low layer compatibility, High layer compatibility

Direction: Calling user → network → called user

Initial message sent to initiate a call

SETUP ACKNOWLEDGE (13)

IEs: Channel identification, Progress indicator, Display, Signal

Direction: Called user → network → calling user

SUSPEND (37)

IEs: Call identity

Direction: User ? network

ISDN calls can be suspended (put on hold) to allow another call to use the B channel. SUSPEND/RESUME messages manage suspended calls.

SUSPEND ACKNOWLEDGE (45)

IEs: Display

Direction: Network → user

SUSPEND REJECT (33)

IEs: Cause, Display

Direction: Network → user

RESUME (40)

IEs: Call identity

Direction: User → network

RESUME ACKNOWLEDGE (48)

IEs: Channel identification, Display

Direction: Network → user

RESUME REJECT (34)

IEs: Cause, Display

   Direction: Network → user

DISCONNECT (69)

IEs: Cause, Progress indicator, Display, Signal

A message sent from the user to request call breakdown, or from the network to indicate the call has been cleared.

RELEASE (77)

IEs: Cause, Display, Signal

A message sent to indicate the channel is being released.

RELEASE COMPLETE (90)

IEs: Cause, Display, Signal

STATUS ENQUIRY (117)

IEs: Display

Direction: User → network

Requests a STATUS message from the network

STATUS (125)

IEs: Cause, Call State, Display

Direction: Network → user

Indicates current call state in terms of Q.931 state machine