Q.931
Encyclopedia
ITU-T Recommendation Q.931 is ISDN
's connection control protocol
, roughly (not at the same layer) 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 kbit/s. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine
. Q.931 has more recently been used as part of the VoIP H.323 protocol stack (see H.225.0
) and in some mobile phone
transmission systems.
Q.931 is a layer 3 protocol, mainly used for the ISDN call establishment, maintenance, and release of network connections between two DTEs on the D channel. A Q.931 frame contains the following elements:
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
's connection control protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
, roughly (not at the same layer) comparable to TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
in the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
Protocol stack. Q.931 doesn't provide flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
or perform retransmission, since the underlying layers
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
are assumed to be reliable and the circuit-oriented
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
nature of ISDN allocates bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
in fixed increments of 64 kbit/s. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine
Finite state machine
A finite-state machine or finite-state automaton , or simply a state machine, is a mathematical model used to design computer programs and digital logic circuits. It is conceived as an abstract machine that can be in one of a finite number of states...
. Q.931 has more recently been used as part of the VoIP H.323 protocol stack (see H.225.0
H.225.0
H.225.0 is part of the H.323 family of telecommunication protocols.The main objective of H.225.0 is the definition of messages and procedures for:* Call Signaling: establish, control and end an H.323 call...
) and in some mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
transmission systems.
Q.931 is a layer 3 protocol, mainly used for the ISDN call establishment, maintenance, and release of network connections between two DTEs on the D channel. 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
- 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).
Disconnect causes
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 higher |
Proprietary diagnostic code (not necessarily bad). Typically used to pass proprietary control or maintenance messages between multiplexers. |