Q.931

Q.931是ITU-T 制定的ISDN網路層介面協議（ISDN network layer interface protocol）。

Q.931一般不進行數據運送。Q.931並沒有直接相當於在互聯網協議棧，但跟SIP相比,由于底层的协议被认为是可靠的协议并且电路交换网络的带宽调整是固定的方式, 所以Q.931不提供流量控制和重传机制.

Q.931是ISDN相關的協議, Q.931最近被用來作為H.323協議（即H.225）的一部份，在某種形式的移動電話傳輸系統。H.323包括Q.931、H.225、H.245和ASN.1, 為了提供呼叫資訊，H.323部份融合了H.225和Q.931標準。Q.931流程包括呼叫建立和拆除。

一個 Q.931 frame包含以下元素：
協議鑑別器（Protocol discriminator, PD）-指定的信令協議是用於連接（如PD = 8為DSS1 ）
呼叫參考值（Call reference value, CR） - 針對不同的連接，可以同時存在。 只有在實際連接的一段時間值有效。
消息類型（message type, MT） - 指定的第3層消息類型的呼叫控制（如設置）設置在Q.931定義的消息類型。 有呼叫建立，呼叫釋放和通話功能的控制定義的消息。
資訊元素（information element, IE） - 指定關聯到實際的消息的進一步資料。Q.931 分組（packet）包含多個稱為資訊單元的參數。一個 IE瀏覽器包括IE瀏覽器的名稱（如承載能力），其長度和變量字段的內容。

訊息示例
 SETUP：指示建立一個連接
 CALL PROCEEDING
 ALERTING
 CONNECT
 RELEASE
 RELEASE COMPLETE
中斷原因
十六進制十進制原因 0x1 1 未分配或未指定的號碼 0x2 2 沒有路由到指定的運輸網絡 (Transit Network Identity) 0x3 3 沒有到目的地的路由 0x4 4 發送特殊信息 0x5 5 Misdialled trunk prefix 0x6 6 頻道不可接受 0x7 7 Call awarded and being delivered in an established channel 0x8 8 Prefix 0 dialed, 但不允許 0x9 9 Prefix 1 dialed, 但不允許 0xA 10 Prefix 1 not dialed, 但需要 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.

限制
Q.931是基於原語（primitive）的，但Q.931並沒有規定原語的字義，Q.931和H.225都定義了呼叫資訊流程，但定義不充分，Q.931的原語缺乏充分的文字說明。另一方面, Q.931和H.225融合不夠好，H.225將Q.931中的消息標記成“禁用”，但卻沒有規定出新流程。Q.931介面是不可移植的, 這是因為H.225只提到原語而沒有提供包含參數的資訊，使得H.323的發展大受影響, SIP有取代H.323的趨勢。

Q.2931
Q.2931是Q.931修改和擴展B-ISDN的變種。

参考资料

category:ITU-T标准
Category:ITU-T建议书
Category:整合服務數位網路