Q.931 Call States

Q.931 Call Stats，在 Q.931 提到的 calls 有 circuit-switched、packet-mode、temporary signalling、和 global call reference。

null state 不存在

c(U0): No call exists.

c(N0): No call exists.

p(U0): No access connection exists.

p(N0): No access connection exists.

t(U0): No call exists.

t(N0): No call exists.

g(Rest 0): No transaction exists.

call initiated 撥出，user 送出請求或 network 收到請求

c(U1): This state exists for an outgoing call, when the user requests call establishment from the network.

c(N1): This state exists for an outgoing call when the network has received a call establishment request but has not yet responded.

p(U1): This state exists for an outgoing access connection when the user requests access connection establishment from the network.

p(N1): This state exists for an outgoing access connection when the network has received an access connection establishment request but has not yet responded.

t(U1): This state exists for an outgoing call when the user requests call establishment from the network.

t(N1): This state exists for an outgoing call when the network has received a call establishment request but has not yet responded.

overlap sending 撥出，user 收到認可後送 overlap 訊息或 network 認可請求可接收 overlap 訊息

c(U2): This state exists for an outgoing call when the user has received acknowledgement of the call establishment request which permits the user to send additional call information to the network in overlap mode.

c(N2): This state exists for an outgoing call when the network has acknowledged the call establishment request and is prepared to receive additional call information (if any) in overlap mode.

t(U2): This state exists for an outgoing call when the user has received acknowledgement of the call establishment request which permits the user to send additional call information to the network in overlap mode.

t(N2): This state exists for an outgoing call when the network has acknowledged the call establishment request and is prepared to receive additional call information (if any) in overlap mode.

outgoing call proceeding

c(U3): This state exists for an outgoing call when the user has received acknowledgement that the network has received all call information necessary to effect call establishment.

c(N3): This state exists for an outgoing call when the network has sent acknowledgement that the network has received all call information necessary to effect call establishment.

p(U3): This state exists for an outgoing access connection when the user has received acknowledgement that the network has received all access connection information necessary to effect access connection establishment.

p(N3): This state exists for an outgoing access connection when the network has sent acknowledgement that the network has received all access connection information necessary to effect access connection establishment.

t(U3): This state exists for an outgoing call when the user has received acknowledgement that the network has received all call information necessary to effect call establishment.

t(N3): This state exists for an outgoing call when the network has sent acknowledgement that the network has received all call information necessary to effect call establishment.

call delivered

c(U4):This state exists for an outgoing call when the calling user has received an indication that remote user alerting has been initiated.

c(N4): This state exists for an outgoing call when the network has indicated hat remote user alerting has been initiated.

t(U4): This state exists for an outgoing call when the calling user has received an indication that remote user alerting has been initiated.

t(N4): This state exists for an outgoing call when the network has indicated that remote user alerting has been initiated.

call present

c(U6)"This state exists for an incoming call when the user has received a call establishment request but has not yet responded.

c(N6): This state exists for an incoming call when the network has sent a call stablishment request but has not yet received a satisfactory response.

p(U6): This state exists for an incoming access connection when the user has received an access connection establishment request but has not yet responded.

p(N6): This state exists for an incoming access connection when the network has sent an access connection establishment request but has not yet received a satisfactory response.

t(U6): This state exists for an incoming call when the user has received a call establishment request but has not yet responded.

t(N6): This state exists for an incoming call when the network has sent a call establishment request but has not yet received a satisfactory response.

call received

c(U7) This state exists for an incoming call when the user has indicated alerting but has not yet answered.

c(N7): This state exists for an incoming call when the network has received an indication that the user is alerting but has not yet received an answer.

p(U7): This state exists for an incoming access connection when the user has indicated alerting but has not yet answered.

p(N7): This state exists for an incoming access connection when the network has received an indication that the user is alerting but has not yet received an answer.

t(U7): This state exists for an incoming call when the user has indicated alerting but has not yet answered.

t(N7): This state exists for an incoming call when the network has received an indication that the user is alerting but has not yet received an answer.

connect request

c(U8) This state exists for an incoming call when the user has answered the call and is waiting to be awarded the call.

c(N8): This state exists for an incoming call when the network has received an answer but the network has not yet awarded the call.

p(U8): This state exists for an incoming access connection when the user has accepted the access connection and is waiting to be awarded the access connection.

p(N8): This state exists for an incoming access connection when the network has received an answer but the network has not yet awarded the access connection.

t(U8): This state exists for an incoming call when the user has answered the call and is awaiting to be awarded the call.

t(N8): This state exists for an incoming call when the network has received an answer but the network has not yet awarded the call.

incoming call proceeding

c(U9) This state exists for an incoming call when the user has sent acknowledgement that the user has received all call information necessary to effect call establishment.

c(N9): This state exists for an incoming call when the network has received acknowledgement that the user has received all call information necessary to effect call establishment.

p(U9): This state exists for an incoming access connection when the user has sent acknowledgement that the user has received all access connection information necessary to effect access connection establishment.

p(N9): This state exists for an incoming access connection when the network has received acknowledgment that the user has received all access connection information necessary to effect access connection establishment.

t(U9): This state exists for an incoming call when the user has sent acknowledgement that the user has received all call information necessary to effect call establishment.

t(N9): This state exists for an incoming call when the network has received acknowledgement that the user has received all call information necessary to effect call establishment.

active

c(U10) This state exists for an incoming call when the user has received an acknowledgement from the network that the user has been awarded the call. This state exists for an outgoing call when the user has received an indication that the remote user has answered the call.

c(N10): This state exists for an incoming call when the network has awarded the call to the called user. This state exists for an outgoing call when the network has indicated that the remote user has answered the call.

p(U10): This state exists for an incoming access connection when the user has received an acknowledgement from the network that the user has been awarded the access connection. This state exists for an outgoing access connection when the user has received an indication that the local network has completed the access connection.

p(N10): This state exists for an incoming access connection when the network has awarded the access connection to the called user. This state exists for an outgoing access connection when the local network has indicated that the access connection has been completed.

t(U10): This state exists for an incoming call when the user has received an acknowledgement from the network that the user has been awarded the call. This state exists for an outgoing call when the user has received an indication that the remote user has answered the call.

t(N10): This state exists for an incoming call when the network has awarded the call to the called user. This state exists for an outgoing call when the network has indicated that the remote user has answered the call.

disconnect request

c(U11) This state exists when the user has requested the network to clear the end-to-end connection (if any) and is waiting for a response.

c(N11): This state exists when the network has received a request from he user to clear the end-to-end connection (if any).

p(U11): This state exists when the user has requested the local network to clear the access connection and is waiting for a response.

p(N11): This state exists when the network has received a request from the user to clear the access connection.

disconnect indication

c(U12) This state exists when the user has received an invitation to disconnect because the network has disconnected the end-to-end connection (if any).

c(N12): This state exists when the network has disconnected the end-to-end connection (if any) and has sent an invitation to disconnect the user-network connection.

p(U12): This state exists when the user has received an invitation to disconnect because the network has disconnected the access connection to end connection (if any).

p(N12): This state exists when the network has sent an invitation to disconnect the user-network access connection.

suspend request

c(U15) This state exists when the user has requested the network to suspend the call and is waiting for a response.

c(N15): This state exists when the network has received a request to suspend the call but has not yet responded.

resume request

c(U17) This state exists when the user has requested the network to resume a previously suspended call and is waiting for a response.

c(N17): This state exists when the network has received a request to resume a previously suspended call but has not yet responded.

release request

c(U19) This state exists when the user has requested the network to release and is waiting for a response.

c(N19): This state exists when the network has requested the user to release and is waiting for a response.

p(U19): This state exists when the user has requested the network to release the access connection and is waiting for a response.

p(N19): This state exists when the network has requested the user to release the access connection and is waiting for a response.

t(U19): This state exists when the user has requested the network to release and is waiting for a response.

t(N19): This state exists when the network has requested the user to release and is waiting for a response.

call abort

c(N22): This state exists for an incoming call for the point-to-multipoint configuration when the call is being cleared before any user has been awarded the call.

p(N22): This state exists for an incoming access connection for the point-to-multipoint configuration when the access connection is being cleared before any user has been awarded the access connection.

t(N22): This state exists for an incoming call for the point-to-multipoint configuration when the call is being cleared before any user has been awarded the call.

overlap receiving

c(U25) This state exists for an incoming call when the user has acknowledged the call establishment request from the network and is prepared to receive additional call information (if any) in overlap mode.

c(N25): This state exists for an incoming call when the network has received acknowledgement of the call establishment request which permits the network to send additional call information (if any) in the overlap mode.

t(U25): This state exists for an incoming call when the user has acknowledged the call establishment request from the network and is prepared to receive additional call information (if any) in overlap mode.

t(N25): This state exists for an incoming call when the network has received acknowledgement of the call establishment request which permits the network to send additional call information (if any) in the overlap mode.

restart request

(Rest 1): This state exists for a restart transaction when the user has sent a restart request but has not yet received an acknowledgement response from the network.

(Rest 1): This state exists for a restart transaction when the network has sent a restart request but has not yet received an acknowledgement response from the user.

restart

(Rest 2): This state exists when a request for a restart has been received from the network and responses have not yet been received from all locally active call references.

(Rest 2): This state exists when a request for a restart has been received from the user and a response has not yet been received from all locally active call references.

circuit		packet		temp		global
U0	N0	U0	N0	U0	N0	Rest0
U1	N1	U1	N1	U1	N1	-
U2	N2	-	-	U2	N2	-
U3	N3	U3	N3	U3	N3	-
U4	N4	-	-	U4	N4	-
U6	N6	U6	N6	U6	N6	-
U7	N7	U7	N7	U7	N7	-
U8	N8	U8	N8	U8	N8	-
U9	N9	U9	N9	U9	N9	-
U10	N10	U10	N10	U10	N10	-
U11	N11	U11	N11	-	-	-
U12	N12	U12	N12	-	-	-
U15	N15	-	-	-	-	-
U17	N17	-	-	-	-	-
U19	N19	U19	N19	U19	N19	-
-	N22	-	N22	-	N22	-
U25	N25	-	-	U25	N25	-
-	-	-	-	-	-	Rest1 (61)
-	-	-	-	-	-	Rest2 (62)

Q.931 STATUS IE Call state 3.1.16, 3.2.10, 3.3.11. 3.4.3. 4.5.7

Call state value (octet 3) Bits 6 5 4 3 2 1 User State Network state 0 0 0 0 0 0 U0 – Null N0 – Null 0 0 0 0 0 1 U1 – Call initiated N1 – Call initiated 0 0 0 0 1 0 U2 – Overlap sending N2 – Overlap sending 0 0 0 0 1 1 U3 – Outgoing call proceeding N3 – Outgoing call proceeding 0 0 0 1 0 0 U4 – Call delivered N4 – Call delivered 0 0 0 1 1 0 U6 – Call present N6 – Call present 0 0 0 1 1 1 U7 – Call received N7 – Call received 0 0 1 0 0 0 U8 – Connect request N8 – Connect request 0 0 1 0 0 1 U9 – Incoming call proceeding N9 – Incoming call proceeding 0 0 1 0 1 0 U10 – Active N10 – Active 0 0 1 0 1 1 U11 – Disconnect request N11 – Disconnect request 0 0 1 1 0 0 U12 – Disconnect indication N12 – Disconnect indication 0 0 1 1 1 1 U15 – Suspend request N15 – Suspend request 0 1 0 0 0 1 U17 – Resume request N17 – Resume request 0 1 0 0 1 1 U19 – Release request N19 – Release request 0 1 0 1 1 0 – - - - - - N22 – Call abort 0 1 1 0 0 1 U25 – Overlap receiving N25 – Overlap receiving Global interface state value (octet 3) Bits 6 5 4 3 2 1 State 0 0 0 0 0 0 REST0 – Null 1 1 1 1 0 1 REST1 – Restart request 1 1 1 1 1 0 REST2 – Restart All other values are reserved

Q.931 Called party number

Q.931 Channel Identification (ITU Q.931 section 4.5.8)

8	7	6	5	4	3	2	1	Octet
IE ID: 0x70								1
Length								2
ext. 1	Type of number			Number plan identification				3
0	Number digits (IA5 characters)							4 etc.

• Type of number
• Number plan identification
• Number digits：IA5 (International Alphabet No. 5) 字元也就是 ITU-T Recommendation T.50，也就是 ASCII。

參考

• E.164
• tel URI

Q.931 Cause

Q.931 Cause (ITU Q.931 section 4.5.12，定義在 Q.850)

8	7	6	5	4	3	2	1	Octet
IE ID: 0x08								1
Length								2
ext. 0/1	Coding Std.		0	Location				3
ext. 1	Recommendation							3a
ext. 1	Cause Value							4
Diagnostic(s)								5

• Coding Standard：編碼標準，無法度用 ITU-T 標準才使用其它標準。
• 0 = ITU-T 標準
• 1 = ISO/IEC 標準
• 2 = national 標準
• 3 = standard specific to identified location
• Location
• 0 = user (U)
• 1 = private network serving the local user (LPN)
• 2 = public network serving the local user (LN)
• 3 = transit network (TN)
• 4 = public network serving the remote user (RLN)
• 5 = private network serving the remote user (RPN)
• 7 = international network (INTL)
• 10 = network beyond interworking point (BI)
• 12~15：reserved for national use
• 其它：spare
• Cause Value：其中 bit 7~5 是 class 分類，表示 the general nature of the event。如下表：

Class	Hex	Dec	Cause
0, 1 = normal	01	1	Unallocated or unassigned number
	02	2	No route to specified transit network (Transit Network Identity)
	03	3	No route to destination
	04	4	Send special information tone
	05	5	Misdialed trunk prefix
	06	6	Channel unacceptable
	07	7	Call awarded and being delivered in an established channel
	08	8	Prefix 0 dialed but not allowed
	09	9	Prefix 1 dialed but not allowed
	0A	10	Prefix 1 not dialed but required
	0B	11	More digits received than allowed, call is proceeding
	10	16	Normal call clearing. 如果是呼叫失敗，可能是高層協定 (如 PPP)、認證、或 idle timeout 相關問題。Also, if you have requested a callback, the remote device disconnects the call, generates this code, and then calls you back.
	11	17	User busy
	12	18	No user responding
	13	19	T.301 expired: – User Alerted, No answer from user
	15	21	Call rejected
	16	22	Number changed to number in diagnostic field.
	17	23	Reverse charging rejected
	18	24	Call suspended
	19	25	Call resumed
	1A	26	Non-selected user clearing
	1B	27	Destination out of order
	1C	28	Invalid number format or incomplete address
	1D	29	EKTS facility rejected by network
	1E	30	Response to STATUS INQUIRY
	1F	31	Normal, unspecified
2 = resource unavailable	21	33	Circuit out of order
	22	34	No circuit/channel available
	23	35	Destination unattainable
	24	36	Out of order
	25	37	Degraded service
	26	38	Network out of order
	27	39	Transit delay range cannot be achieved
	28	40	Throughput range cannot be achieved
	29	41	Temporary failure
	2A	42	Switching equipment congestion
	2B	43	Access information discarded
	2C	44	Requested circuit channel not available
	2D	45	Preempted
	2E	46	Precedence call blocked
	2F	47	Resource unavailable, unspecified
3 = service or option not available	31	49	Quality of service unavailable
	32	50	Requested facility not subscribed
	33	51	Reverse charging not allowed
	34	52	Outgoing calls barred
	35	53	Outgoing calls barred within CUG
	36	54	Incoming calls barred
	37	55	Incoming calls barred within CUG
	38	56	Call waiting not subscribed
	39	57	Bearer capability not authorized
	3A	58	Bearer capability not presently available
	3F	63	Service or option not available, unspecified
4 = service or option not implemented	41	65	Bearer service not implemented
	42	66	Channel type not implemented
	43	67	Transit network selection not implemented
	44	68	Message not implemented
	45	69	Requested facility not implemented
	46	70	Only restricted digital information bearer capability is available
	4F	79	Service or option not implemented, unspecified
5 = invalid message	51	81	Invalid call reference value
	52	82	Identified channel does not exist
	53	83	A suspended call exists, but this call identity does not
	54	84	Call identity in use
	55	85	No call suspended
	56	86	Call having the requested call identity has been cleared
	57	87	Called user not member of CUG
	58	88	Incompatible destination
	59	89	Non-existent abbreviated address entry
	5A	90	Destination address missing, and direct call not subscribed
	5B	91	Invalid transit network selection (national use)
	5C	92	Invalid facility parameter 93 Mandatory information element is missing
	5D	93	Message type non-existent or not implemented
	5F	95	Invalid message, unspecified
6 = protocol error	60	96	Mandatory information element is missing
	61	97	Message type non-existent or not implemented
	62	98	Message not compatible with call state or message type non-existent or not implemented
	63	99	Information element nonexistent or not implemented
	64	100	Invalid information element contents
	65	101	Message not compatible with call state
	66	102	Recovery on timer expiry
	67	103	Parameter non-existent or not implemented – passed on
	6F	111	Protocol error, unspecified
7 = interworking	7F	127	Internetworking, unspecified
?	80 +	128 +	Proprietary diagnostic code (not necessarily bad). Typically used to pass proprietary control or maintenance messages between multiplexers.

範例

08 02 81 90：Class 0 Value 0

08 03 81 e2 0f ：Class 6 Value 2

參考

• https://www.viavisolutions.com/en-us/support/files/Q.931-Cause-Codes

Q.931 Channel Identification

Q.931 Channel Identification (ITU Q.931 section 4.5.13)

8	7	6	5	4	3	2	1	Octet
IE ID: 0x18								1
Length								2
ext. 1	Int. id. present	Int. type	0	Pref./Excl.	D-channel ind	Info. channel selection		3
ext. 0/1	Interface identifier							3.1
ext. 1	Coding Standard		Number/ Map	Channel type/Map element type				3.2
Channel number/Slot map								3.3

• interface identifier present 有設 => 有 octet 3.1 Interface identifier，可以有多個 byte，放界面的編號。
• interface type= 0 表示 BRI，此時沒有 3.2 及 3.3，用 information channel selection 表示用哪個通道：00 = No channel, 01 = B1 channel, 10 = B2 channel
• Preferred/Exclusive：0 = 偏好所選的通道 (也就是可以選別的)。1 = 排除眾議，就是要所選的通道。
• D-channel Indicator：1 = 指 D-channel，忽略 Information Channel Selection。0 = 不是 D-channel。
• Information Channel Selection
• Coding Standard：0 = ITU-T 標準
• Number/Map：0 = 用編號 (Channel number)，1 = 用 bit map (Slot map)。
• Element type：3 = B-channel
• Channel number：MSB = 1 表示最後 byte，剩下 7-bit 為 channel 編號。

範例
0x18 0x01 0x81：BRI 偏好 B1 channel
0x18 0x03 0xA1 0x83 0x83：PRI 偏好 B channel 3

LAPD

ISDN 的 datalink 層是 Q.921，也就是 LAPD (Link Access Procedure on the D-channel)，是 HDLC 的 close cousin。

在 frame 之間及閒置時送 Flag (0x7E, 01111110b)，而原本 0x7E 的資料為避免和 Flag 混淆，使用了 bit stuffing 機制，也就是資料在傳送端會每遇到 5 個連續 1 會插入一個 0，接收端再移除。如果出現連續 7 個以上的 1，則視為 abort 而忽略收到的資料。

LAPD 格式

Address	Control	Information	FCS
2	1~2	0~	2

LAPD 運作在 Asynchronous Balanced Mode (ABM)，也就是兩端是對等的、沒有主從之分，雙向各自獨立運作。2-byte FCS 檢查資料有沒有錯誤，通常用硬體計算，以下說明省略。重送時，每個 octet 的 bit 1 先送。

Address：2-byte

8	7	6	5	4	3	2	1	octet
SAPI						C/R	EA 0	1
TEI							EA 1	2

• EA：Address field extension bit，0 表示有下個 byte、1 則沒有。由於固定 2-byte，所以第一個 EA 為 0，第二個 EA 為 1。
• C/R：0 表示用戶端是發出的指令及網路端的回覆，1 則表示網路端發出的指令及用戶端的回覆。
• SAPI：Service Access Point Identifier
• 0：上層是 Q.931 呼叫控制 (GR303 TMC call proc)
• 1：I.451 封包模式的呼叫控制 (GR303 EOC, TMC switching)
• 12：teleaction
• 16：X.25 level 3 封包
• 63：自己本身 Layer 2 管理
• TEI：Terminal End Indentifier，在 SAPI = 0 時
• 0~63：用於非自動指定 TEI 的設備，PRI 使用 0
• 64~126：用於自動指定 TEI 的設備
• 127：廣播給所有 Terminal Endpoints

PRI 和點對點 BRI 使用 SAPI 0 TEI 0，點對多點 BRI 使用 SAPI 63 TEI 127。

Control 欄位包括 Q.921 的 frame type、序號等。

Control	8	7	6	5	4	3	2	1	octet
I Frame	N(S)							0	1	Information Frame：連線狀態正常及序號正確後往上層丟
	N(R)							P	2
S Frame	0000				SS		01		1	Supervisory Frame：datalink 本身 supervisory 用，看 SS 2-bit
	N(R)							P/F	2
U Frame	MMM			P/F	MM		11		1	Unnumbered Frame：Unnumbered 就是沒序號，看 MMM 及 MM 共 5-bit

P/F 在 command 是 poll，在 response 是 final。

I	N(S)							0	I-Frame，有 Information 欄位
S	0	0	0	0	0	0	0	1	RR (Receive Ready)：I Frame 的 acknowledgement，可以再收。
	0	0	0	0	0	1	0	1	RNR：Indicates a state of temporary occupation of station (e.g., window full)
	0	0	0	0	1	0	0	1	REJ：重傳給定的序號之後
U	0	0	0	x	0	0	1	1	UI (Unnumbered Information)，有 Information 欄位 => Q.931。P/F = 0 [Q.921 5.1.1]
	0	1	0	P	0	0	1	1	DISC (Disconnect command)
	0	1	1	F	0	0	1	1	UA (Unnumbered Acknowledgement response)
	1	0	0	Fx	0	1	1	1	FRMR (Framer reject response)
	0	0	0	F	1	1	1	1	DM (Disconnected Mode response)
	1	0	1	P/F	1	1	1	1	XID (eXchange IDentification)：不支援？
	0	1	1	P	1	1	1	1	SABME (Set Asynchronous Balanced Mode Extended)：command。在連線初始化時決定 window 大小，最大是 128，也就是可以有 128 個 frame 未完成 ACK。由 Address 的 C/R 可看出遠端是 Network 端還是 User 端

window size：modulo 8 待 acknowledge 的 outstanding frames
extended window size：modulo 128，使用不同的 link initialization frame type，frame type 名稱再加上 E，例如 SABM 變成 SABME。

範例：02 01 7f

• 02 01 (Address)：C/R = 1 = network command 或 user response，SAPI = 0 = Q.931，TEI = 0 = PRI or PTP
• 7F (Control)：U frame SABME，P/F = 1

範例：02 01 01 03：RR for Q.931

範例：FC FF 03 0F 5D 72 01 FF F7 6E：TEI Request

• FC FF (Address)：C/R = 0 = user command 或 network response，SAPI = 63 = Layer 2 Management，TEI = 127 = Broadcast connection for all TEs
• 03 (Control)：UI，P/F = 0
• 0F (Management entity identifier)：固定為 0x0F。
• 5D 72 (Ri)：Request number，一個亂數
• 01 (message type = REQUEST)：請求 64 ~ 126 的 TEI，成功的話回覆 2 = ASSIGNED，失敗的話回覆 3 = DENIED。另外還有 4 = CHECK_REQUEST、5 = CHECK_RESPONSE、6 = REMOVE、7 = VERIFY。
• FF (Ai 及 E-bit)：bit 1 為 E-bit，Ai 欄位的 externsion。剩下 7-bit 為 Ai = Action Indicator = 127 = 任何 TEI。
• F7 6E (FCS)

沒其他訊息時，每 10 秒送 RR 等回應。沒回應，每秒重送最多共四次，之後改送 SABME，也是每秒重送，直到收到 UA。

MDL-ERROR() 錯誤表

Error Code	Error Condition	Action	Error Type
A	Supervisory (F=1)	Error Log	Receipt of unsolicited response
B	DM (F=1)	Error Log	Receipt of unsolicited response
C	UA (F=1)	TEI removal procedure or  TEI check procedure, then if TEI: free: remove TEI single: no action multiple: TEI removal procedure	Receipt of unsolicited response
D	UA (F=0)	TEI removal procedure or  TEI check procedure, then if TEI: free: remove TEI single: no action multiple: TEI removal procedure	Receipt of unsolicited response
E	DM (F=0)	Error Log	Receipt of unsolicited response
F	SABME	Error Log	Peer initiated re-establish.
G	SABME	TEI check procedure, then if TEI: free: remove TEI single: no action multiple: TEI removal procedure	Unsuccessful retransmission (N200 times)
H	DISC	TEI check procedure, then if TEI: free: remove TEI single: no action multiple: TEI removal procedure	Unsuccessful retransmission (N200 times)
I	Status Enquiry	Error Log	Unsuccessful retransmission (N200 times)
J	N(R) error	Error Log	Other
K	Receipt of FRMR	Error Log	Other
L	Receipt of frame with undefined control field	Error Log	Other
M	Receipt of I field not permitted	Error Log	Other
N	Receipt of frame with wrong size	Error Log	Other
O	N201 error	Error Log	Other

參考

1. Q.921 ISDN user-network interface – Data link layer specification
2. http://www.freesoft.org/CIE/Topics/125.htm

• ISDN Q.921-User Adaptation (IUA)：透過 IP 傳送 ISDN Q.921 訊息

Q.931 訊息格式

Q.931 格式的基本欄位包括 Protocol Discriminator、Call Reference、及 Message Type，之後的欄位依據 Message Type 的需要而加，稱為 Information Element (IE)。

8	7	6	5	4	3	2	1	Octet
Protocol Discriminator								1
Call Reference								2~
0	Message Type
Information Elements as required

Protocol Discriminator 辨別是什麼協定，Q.931 用的值是 0x08，不過還有其它協定：

• 0x08：Q.931
• 0x4F：GR303
• 0x03 或 0x43：維護用

Call Reference 是用在 user-network 界面的通話參考編號，用戶 (user) 端和網路 (network) 端各自獨立編號，由通話建立端決定。

8	7	6	5	4	3	2	1	Octet
0000				Call Reference Length				1
Flag								2
Call Reference								etc.

• Call Reference Length：Call Reference 的長度，可能 0 ~ 3
• Flag：此 call 建立端設為 0，受話端使用時設為 1

Message Type

• 0000000
• 00-----：建立通話
• 0x01 ALERTING
• 0x02 CALL PROCEEDING
• 0x07 CONNECT
• 0x0F CONNECT ACKNOWLEDGE
• 0x03 PROGRESS
• 0x05 SETUP：建立通話
• 0x0D SETUP ACKNOWLEDGE
• 01-----：通話中
• 0x26 RESUME
• 0x2E RESUME ACKNOWLEDGE
• 0x22 RESUME REJECT
• 0x25 SUSPEND
• 0x2D SUSPEND ACKNOWLEDGE
• 0x21 SUSPEND REJECT
• 0x20 USER INFORMATION
• 10-----：掛斷通話
• 0x45 DISCONNECT
• 0x4E RELEASE
• 0x5A RELEASE COMPLETE
• 0x46 RESTART
• 0x4E RESTART ACKNOWLEDGE
• 11-----：其它
• 0x60 SEGMENT
• 0x79 CONGESTION CONTROL
• 0x7B INFORMATION：提供額外資訊，如 overlap sending。
• 0x6E NOTIFY
• 0x7D STATUS
• 0x75 STATUS ENQUIRY

Information Element (IE) 都會有 IE identifier 辨別哪種 IE，有三種格式，前兩種為單一 byte，bit 8 為 1。第三種可變長度，有長度欄位決定內容長度。

type	8	7	6	5	4	3	2	1	Octet
a	1	Information element identifier			Contents of Information element				1
b	1	Information element identifier							1
c	0	Information element identifier							1
	Length of contents of information element								2
	Contents of information element								3~

IE identifier

1	0	1	0	0	0	0	1	Sending complete
0	0	0	0	1	0	0	0	Cause
0	0	0	1	1	0	0	0	Channel Identification：可重複
0	1	1	1	0	0	0	0	Called party number

參考

1. Q.931
2. LAPD
3. Q.931 over IP
4. Q-Interface Signaling (QSIG)
• 同樣基於 Q.921/Q.931 的一種 D-channel voice signaling 變形，作為 PBX 間溝通用的標準協定，用來建立通話和管理 PBX 間的 QSIG supplementary service，如 Multiple Subscriber Number、Call Waiting、Calling-Line Identification Presentation (CLIP)、Calling-Line Identification Restriction (CLIR)、Connected-Line Identification Presentation (COLP)、Connected-Line Identification Restriction (COLR)、Malicious Call Identification、Call Hold、Advice of Charge、Three-Way Conference、Call Diversion、CFU Supplementary Service、Path Replacement (ANF-PR)、Call Transfer by Join (SS-CT)、Call Completion to Busy Subscriber (CCBS)、和 Explicit Call Transfer。
• 呼叫來自 private integrated services network exchange (PINX) 再繞到其它 PINX 設備。
• Tunneling over H.323：H.323 基於 Q.931，旗下包含基本的 H.225.0 和 H.245，和 supplementary services H.450.1。H.323 Annex-M1 有規範 Tunneling QSIG over H.323，但 Cisco 使用 ISDN Generic Transparency Descriptor (GTD) 在對應的 H.225 訊息不變地傳送 QSIG 訊息。沒對應的，則 tunneled 在 H.225.0 FACILITY 訊息。
• Tunneling over SIP：在 SIP 指示「Content-Type: application/QSIG」、「Content-Disposition: signal; handling=optional」放在 MIME 信體。非 QSIG 的 Q.931 訊息用「Content-Type: application/x-q931」。
• https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube_sip/configuration/15-mt/cube-sip-15-mt-book/voi-sip-trans-tun.pdf
• https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/sip/configuration/15-mt/sip-config-15-mt-book/voi-sip-tdm.pdf

• Calling-Line Identification Presentation (CLIP)：來電時提供主叫號碼給被叫。
• Calling-Line Identification Restriction (CLIR)：主叫限制提供主叫號碼給被叫。Calling-Line Identification Restriction Ignore：忽略主叫限制提供主叫號碼給被叫，例如撥緊急電話，即使主叫限制提供主叫號碼還是會提供。
• Connected-Line Identification Presentation (COLP)：接通時提供真正的被叫號碼給主叫。
• Connected-Line Identification Restriction (COLR)：被叫限制接通時提供真正的被叫號碼給主叫。
• http://shannon.cm.nctu.edu.tw/netexp/isdn03.pdf 整體數位服務網路簡介