Q.931

Q.931是ITU-T 制定的ISDN网络层界面协议（ISDN network layer interface protocol）。

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）的一部分，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部分融合了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

- SETUP：指示建立一个连接
- CALL PROCEEDING
- ALERTING
- CONNECT
- RELEASE
- RELEASE COMPLETE

## 中断原因

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

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界面是不可移植的, 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

Q.2931是Q.931修改和扩展B-ISDN的变种。

## 基本资料

- 车队名称：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
- version_date：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