Q.931

ITU T Recommendation Q.931 is the ITU standard ISDN connection control signalling protocol, forming part of Digital Subscriber Signalling System No. 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 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.

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

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