PRI Error Codes

Q.931 Protocol

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:

Messages

IEs (Information Elements) are defined below 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

Exchange

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)

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 according to the following chart: