PRI Protocol Overview
From Taridium
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:
Message Types
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
Information Elements
The most important IEs are all multi-byte:
Bearer capability (4)
Specifies a requested service: packet or circuit mode, data rate, type of information content
Call identity (16)
Used to identify a suspended call
Call state (20)
Describes the current status of a call in terms of the standard Q.931 state machine
Called party number (112)
The phone number being dialed
Calling party number (108)
The origin phone number
Cause (16)
The reason a call was rejected or disconnected. A sample of possible cause codes: 1 Unassigned number 3 No route to destination 6 Channel unacceptable 16 Normal call clearing 17 User busy 18 User not responding 19 User alerting; no answer 22 Number changed 27 Destination out of order 28 Invalid number format 34 No circuit/channel available 42 Switching equipment congestion
Channel identification (24)
Identify a B channel
Date/time (41)
Poorly defined. Not year 2000 compliant!
Display (40)
Human-readable text. Can be specified with almost any message to provide text for an LCD display, for example.
Service Profile Identification (58)
Contains a Service Profile Identifier (SPID)
Signal (52)
Provide call status tones
