Q.931

Q.931是ITU-T 制定的ISDN網路層介面協議（ISDN network layer interface protocol）。^[1]

Q.931

ISDN用户网络接口第3层基本呼叫控制规范
状态	已生效
开始年	1988
最新版本	May 1998
预览版本	（12/02)
组织	ITU-T
有关标准	H.323
网站	https://www.itu.int/rec/T-REC-Q.931

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

1. Q.931 : ISDN user-network interface layer 3 specification for basic call control. www.itu.int. [2021-04-28]. （原始内容存档于2021-04-27）.