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ISDN user-network interface layer 3 specification for basic call control
StatusIn force
Year started1988
Latest version(05/98)
May 1998
Related standardsH.323

ITU-T Recommendation Q.931 is the ITU standard ISDN connection control signalling protocol, forming part of Digital Subscriber Signalling System No. 1.[1] Unlike connectionless systems like UDP, ISDN is connection oriented and uses explicit signalling to manage call state: Q.931. Q.931 typically does not carry user data. Q.931 does not have a direct equivalent in the Internet Protocol stack, but can be compared to SIP. Q.931 does not 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. Amongst other things, Q.931 manages connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine.

Q.931 was designed for ISDN call establishment, maintenance, and release of network connections between two DTEs on the ISDN D channel. Q.931 has more recently been used as part of the VoIP H.323 protocol stack (see H.225.0) and in modified form in some mobile phone transmission systems[2] and in ATM.

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 setup, 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 the IE name (e.g. bearer capability), their length and a variable field of contents.

Message examples[edit]

Messages typically control or report the status of connections. For example:

  • 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).
  • RESTART (Reset D Channel to idle)

Disconnect causes[edit]

Hex Decimal Cause
0x1 1 Unallocated or unassigned number
0x2 2 No route to specified transit network (Transit Network Identity)
0x3 3 No route to destination
0x4 4 Send special information tone
0x5 5 Misdialled trunk prefix
0x6 6 Channel unacceptable
0x7 7 Call awarded and being delivered in an established channel
0x8 8 Prefix 0 dialed but not allowed
0x9 9 Prefix 1 dialed but not allowed
0xA 10 Prefix 1 not dialed but required
0xB 11 More digits received than allowed, call is proceeding
0x10 16 Normal call clearing
0x11 17 User busy
0x12 18 No user responding
0x13 19 T.301 expired: – User Alerted, No answer from user
0x15 21 Call rejected
0x16 22 Number changed to number in diagnostic field.
0x17 23 Reverse charging rejected
0x18 24 Call suspended
0x19 25 Call resumed
0x1A 26 Non-selected user clearing
0x1B 27 Destination out of order
0x1C 28 Invalid number format or incomplete address
0x1D 29 EKTS facility rejected by network
0x1E 30 Response to STATUS ENQUIRY
0x1F 31 Normal, unspecified
0x21 33 Circuit out of order
0x22 34 No circuit/channel available
0x23 35 Destination unattainable
0x24 36 Out of order
0x25 37 Degraded service
0x26 38 Network out of order
0x27 39 Transit delay range cannot be achieved
0x28 40 Throughput range cannot be achieved
0x29 41 Temporary failure
0x2A 42 Switching equipment congestion
0x2B 43 Access information discarded
0x2C 44 Requested circuit channel not available
0x2D 45 Preempted
0x2E 46 Precedence call blocked
0x2F 47 Resource unavailable, unspecified
0x31 49 Quality of service unavailable
0x32 50 Requested facility not subscribed
0x33 51 Reverse charging not allowed
0x34 52 Outgoing calls barred
0x35 53 Outgoing calls barred within CUG
0x36 54 Incoming calls barred
0x37 55 Incoming calls barred within CUG
0x38 56 Call waiting not subscribed
0x39 57 Bearer capability not authorized
0x3A 58 Bearer capability not presently available
0x3F 63 Service or option not available, unspecified
0x41 65 Bearer service not implemented
0x42 66 Channel type not implemented
0x43 67 Transit network selection not implemented
0x44 68 Message not implemented
0x45 69 Requested facility not implemented
0x46 70 Only restricted digital information bearer capability is available
0x4F 79 Service or option not implemented, unspecified
0x51 81 Invalid call reference value
0x52 82 Identified channel does not exist
0x53 83 A suspended call exists, but this call identity does not
0x54 84 Call identity in use
0x55 85 No call suspended
0x56 86 Call having the requested call identity has been cleared
0x57 87 Called user not member of CUG
0x58 88 Incompatible destination
0x59 89 Non-existent abbreviated address entry
0x5A 90 Destination address missing, and direct call not subscribed
0x5B 91 Invalid transit network selection (national use)
0x5C 92 Invalid facility parameter 93 Mandatory information element is missing
0x5D 93 Message type non-existent or not implemented
0x5F 95 Invalid message, unspecified
0x60 96 Mandatory information element is missing
0x61 97 Message type non-existent or not implemented
0x62 98 Message not compatible with call state or message type non-existent or not implemented
0x63 99 Information element nonexistent or not implemented
0x64 100 Invalid information element contents
0x65 101 Message not compatible with call state
0x66 102 Recovery on timer expiry
0x67 103 Parameter non-existent or not implemented – passed on
0x6F 111 Protocol error, unspecified
0x7F 127 Internetworking, unspecified
0x80+ 128 or


Proprietary diagnostic code (not necessarily bad). Typically used to pass proprietary control or maintenance messages between multiplexers.


Q.2931[3] is a modified and extended variant of Q.931 for use on "B-ISDN" or ATM networks. Q.2931 fulfils a purpose within BISDN similar to that of Q.931 in ISDN. Whilst ISDN allocates bandwidth in fixed 64k increments, B-ISDN/ATM incorporates an elaborate traffic management scheme, allowing precise specification of virtual circuit traffic parameters such as peak and mean bandwidth, jitter, cell loss ratio and so on. In order that ATM switches can manage bandwidth allocation in the network, encodings to express these parameters were added in Q.2931.[4]

Unlike Q.931, although Q.2931 was implemented by many switch manufacturers, it was never widely deployed.


  1. ^ "Q.931: ISDN user-network interface layer 3 specification for basic call control". www.itu.int. Retrieved 2021-04-27.
  2. ^ such as in GSM, where it is used for circuit switched call control between UE and MSC
  3. ^ ITU-T Recommendation Q.2931 Digital Subscriber Signalling System No. 2 – User-Network Interface (UNI) layer 3 specification for basic call/connection control
  4. ^ for example, see the Extended Quality of Service or eQoS Information Element, which can indicate many quality of service parameters'