Q.931
Q.931是ITU-T 制定的ISDN网路层介面协议（ISDN network layer interface protocol）。[1]
| 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.931一般不进行数据运送。Q.931并没有直接相当于在互联网协议栈，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相关的协议, Q.931最近被用来作为H.323协议（即H.225）的一部份，在某种形式的移动电话传输系统。H.323包括Q.931、H.225、H.245和ASN.1, 为了提供呼叫资讯，H.323部份融合了H.225和Q.931标准。Q.931流程包括呼叫建立和拆除。
一个 Q.931 frame包含以下元素：
- 协议鉴别器（Protocol discriminator, PD）-指定的信令协议是用于连接（如PD = 8为DSS1 ）
- 呼叫参考值（Call reference value, CR） - 针对不同的连接，可以同时存在。 只有在实际连接的一段时间值有效。
- 消息类型（message type, MT） - 指定的第3层消息类型的呼叫控制（如设置）设置在Q.931定义的消息类型。 有呼叫建立，呼叫释放和通话功能的控制定义的消息。
- 资讯元素（information element, IE） - 指定关联到实际的消息的进一步资料。Q.931 分组（packet）包含多个称为资讯单元的参数。一个 IE浏览器包括IE浏览器的名称（如承载能力），其长度和变量字段的内容。
讯息示例
编辑- SETUP：指示建立一个连接
- CALL PROCEEDING
- ALERTING
- CONNECT
- RELEASE
- RELEASE COMPLETE
中断原因
编辑| 十六进制 | 十进制 | 原因 |
|---|---|---|
| 0x1 | 1 | 未分配或未指定的号码 |
| 0x2 | 2 | 没有路由到指定的运输网络 (Transit Network Identity) |
| 0x3 | 3 | 没有到目的地的路由 |
| 0x4 | 4 | 发送特殊信息 |
| 0x5 | 5 | Misdialled trunk prefix |
| 0x6 | 6 | 频道不可接受 |
| 0x7 | 7 | Call awarded and being delivered in an established channel |
| 0x8 | 8 | Prefix 0 dialed, 但不允许 |
| 0x9 | 9 | Prefix 1 dialed, 但不允许 |
| 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
编辑Q.931是基于原语（primitive）的，但Q.931并没有规定原语的字义，Q.931和H.225都定义了呼叫资讯流程，但定义不充分，Q.931的原语缺乏充分的文字说明。另一方面, Q.931和H.225融合不够好，H.225将Q.931中的消息标记成“禁用”，但却没有规定出新流程。Q.931介面是不可移植的, 这是因为H.225只提到原语而没有提供包含参数的资讯，使得H.323的发展大受影响, SIP有取代H.323的趋势。
Q.2931
编辑Q.2931是Q.931修改和扩展B-ISDN的变种。
参考资料
编辑- ^ 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）.