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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:

Message examples[edit]

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

Disconnect causes[edit]

0x11Unallocated or unassigned number
0x22No route to specified transit network (Transit Network Identity)
0x33No route to destination
0x44Send special information tone
0x55Misdialled trunk prefix
0x66Channel unacceptable
0x77Call awarded and being delivered in an established channel
0x88Prefix 0 dialed but not allowed
0x99Prefix 1 dialed but not allowed
0xA10Prefix 1 not dialed but required
0xB11More digits received than allowed, call is proceeding
0x1016Normal call clearing
0x1117User busy
0x1218No user responding
0x1319T.301 expired: – User Alerted, No answer from user
0x1521Call rejected
0x1622Number changed to number in diagnostic field.
0x1723Reverse charging rejected
0x1824Call suspended
0x1925Call resumed
0x1A26Non-selected user clearing
0x1B27Destination out of order
0x1C28Invalid number format or incomplete address
0x1D29EKTS facility rejected by network
0x1E30Response to STATUS ENQUIRY
0x1F31Normal, unspecified
0x2133Circuit out of order
0x2234No circuit/channel available
0x2335Destination unattainable
0x2436Out of order
0x2537Degraded service
0x2638Network out of order
0x2739Transit delay range cannot be achieved
0x2840Throughput range cannot be achieved
0x2941Temporary failure
0x2A42Switching equipment congestion
0x2B43Access information discarded
0x2C44Requested circuit channel not available
0x2E46Precedence call blocked
0x2F47Resource unavailable, unspecified
0x3149Quality of service unavailable
0x3250Requested facility not subscribed
0x3351Reverse charging not allowed
0x3452Outgoing calls barred
0x3553Outgoing calls barred within CUG
0x3654Incoming calls barred
0x3755Incoming calls barred within CUG
0x3856Call waiting not subscribed
0x3957Bearer capability not authorized
0x3A58Bearer capability not presently available
0x3F63Service or option not available, unspecified
0x4165Bearer service not implemented
0x4266Channel type not implemented
0x4367Transit network selection not implemented
0x4468Message not implemented
0x4569Requested facility not implemented
0x4670Only restricted digital information bearer capability is available
0x4F79Service or option not implemented, unspecified
0x5181Invalid call reference value
0x5282Identified channel does not exist
0x5383A suspended call exists, but this call identity does not
0x5484Call identity in use
0x5585No call suspended
0x5686Call having the requested call identity has been cleared
0x5787Called user not member of CUG
0x5888Incompatible destination
0x5989Non-existent abbreviated address entry
0x5A90Destination address missing, and direct call not subscribed
0x5B91Invalid transit network selection (national use)
0x5C92Invalid facility parameter 93 Mandatory information element is missing
0x5D93Message type non-existent or not implemented
0x5F95Invalid message, unspecified
0x6096Mandatory information element is missing
0x6197Message type non-existent or not implemented
0x6298Message not compatible with call state or message type non-existent or not implemented
0x6399Information element nonexistent or not implemented
0x64100Invalid information element contents
0x65101Message not compatible with call state
0x66102Recovery on timer expiry
0x67103Parameter non-existent or not implemented – passed on
0x6F111Protocol error, unspecified
0x7F127Internetworking, 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. Whist 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. ^ ITU-T Recommendation Q.931 Digital subscriber Signalling System No. 1: ISDN user-network interface layer 3 specification for basic call control
  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