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網際網路協議棧，但跟SIP相比,由於底層的協議被認為是可靠的協議並且電路交換網路的帶寬調整是固定的方式, 所以Q.931不提供流量控制和重傳機制.

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，在某種形式的行動電話傳輸系統。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），其長度和變量欄位的內容。

## 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路 (Transit Network Identity)

0x3｜3｜沒有到目的地的路由

0x4｜4｜發送特殊資訊

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隻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。

## 基本資料

- 車隊名稱：ISDN使用者網路接口第3層基本呼叫控制規範
- 狀態：已生效
- 開始年：1988
- version_date：May 1998
- 預覽版本：（12/02)
- 組織：ITU-T
- 有關標準：H.323
- 網站：https://www.itu.int/rec/T-REC-Q.931