Appendix H. ISUP Timers for ANSI/ETSI/ITU-T Applications

This appendix lists all ISUP timers. The timer values are specified in ITU-T Q.764 [78]. ETSI ISUP timers [18] are identical to ITU-T timers.

Timers applicable to the US network are specified in ANSI T1.113-2000 [3].

Table H-1. ISUP Timers Specified in ANSI, ETSI and ITU-T

Timer	Duration Before Time-Out	Starts	Normal Termination	Action at Time-Out
T1	15–60 sec (ITU) 4–15 sec (ANSI)	When Release Message is sent	Upon receipt of Release Complete Message	Retransmit Release Message and start timer T1
T2 (ITU-T ONLY)	3 min	When controlling exchange receives Suspend (User) Message	Upon receipt of Resume (User) Message, at controlling exchange	Initiate release procedure
T3 (ITU-T ONLY)	2 min	Upon receipt of Overload Message	Upon expiry	Initiate release procedure
T4 (ITU-T ONLY)	5–15 min	Upon receipt of MTP-STATUS primitive with the cause "inaccessible remote user" or at receipt of MTP-RESUME primitive[1]	Upon expiry, or at receipt of User Part Available Message (or any other)	Send User Part Test Message and start T4
T5	5–15 min (ITU) 1 min (ANSI)	When initial Release Message is sent	Upon receipt of Release Complete Message	Send Reset Circuit Message, alert maintenance personnel, and remove the Circuit from service, stop T1, and start T17
T6	10–32 sec, with preference for 30 sec (specified in Rec. Q.118 [113])	When controlling exchange receives suspend (network)	Upon receipt of Resume (Network) Message or Release Message	Initiate release procedure
T7	20–30 sec	When the latest Address Message is sent	When the condition for normal release of address and routing information is met (receipt of ACM and CON Messages)	Release all equipment and connection (send Release Message)
T8	10–15 sec	Upon receipt of Initial Address Message requiring continuity check on this circuit, or indicating that continuity check has been performed on a previous circuit	Upon receipt of Continuity Message	Release all equipment and connection into the network (send Release Message)
T9	1.5–3 min (Specified in Q.118 [113])	When national controlling (ITU ONLY) or outgoing international exchange receives ACM (ANSI and ITU)	Upon receipt of Answer Message	Release connection and send Release Message
T10 (ITU-T ONLY)	4–6 sec	Upon receipt of last digit in interworking situations	Upon receipt of fresh information	Send Address Complete Message
T11	15–20 sec	Upon receipt of the latest address message (i.e., IAM) in interworking situations	When Address Complete Message is sent	Send Address Complete Message
T12	15–60 sec (ITU) 4–15 sec (ANSI)	When Blocking Message is sent	Upon receipt of Blocking Acknowledgement Message	Retransmit Blocking Message and restart T12
T13	5–15 min (ITU) 1 min (ANSI)	When initial Blocking Message is sent	Upon receipt of Blocking Acknowledgment	Retransmit Blocking Message and alert maintenance personnel, start T13, and stop T12
T14	15–60 sec (ITU) 4-15 sec (ANSI)	When Unblocking Message is sent	Upon receipt of Unblocking Acknowledgment	Retransmit Unblocking Message and start T14
T15	5–15 min (ITU) 1 min (ANSI)	When initial Unblocking Message is sent	Upon receipt of Unblocking Acknowledgment Message	Retransmit Unblocking Message, alert maintenance personnel, start T15, and stop T14
T16	15–60 sec (ITU) 4-15 sec (ANSI)	When Reset Circuit Message is sent not due to expiry of T5	Upon receipt of the Acknowledgment (RLC Message)	Retransmit Reset Circuit Message and start T16
T17	5–15 min (ITU) 1 min (ANSI)	When initial Reset Circuit Message is sent	At the receipt of the Acknowledgment (RLC Message)	Alert maintenance personnel, retransmit Reset Circuit Message, start T17, and stop T16
T18	15–60 sec (ITU) 4–15 sec (ANSI)	When Circuit Group Blocking Message is sent	At receipt of Circuit Group Blocking Acknowledgment	Retransmit Circuit Group Blocking Message and start T18
T19	5–15 min (ITU) 1 min (ANSI)	When initial Circuit Group Blocking Message is sent	Upon receipt of Circuit Group Blocking Acknowledgment	Retransmit Circuit Group Blocking Message, alert maintenance personnel, start T19, and stop T18
T20	15–60 sec (ITU) 4–15 sec (ANSI)	When Circuit Group Unblocking Message is sent	Upon receipt of Circuit Group Unblocking Acknowledgment	Retransmit Circuit Group Unblocking Message and start T20
T21	5–15 min (ITU) 1 min (ANSI)	When initial Circuit Group Unblocking Message is sent	Upon receipt of Circuit Group Unblocking Acknowledgment	Retransmit Circuit Group Unblocking Message, alert maintenance personnel, start T21, and stop T20
T22	15–60 sec (ITU) 4–15 (ANSI)	When Circuit group Reset Message is sent	Upon receipt of the Acknowledgment	Retransmit Circuit Group Reset Message and start T22
T23	5–15 min (ITU) 1 min (ANSI)	When initial Circuit Group Reset Message is sent	Upon receipt of the Acknowledgment	Alert maintenance personnel and start T23; retransmit Circuit Group Reset Message, and stop T22
T24	< 2 sec	When check tone is sent	Upon receipt of the backward check tone	Send Continuity Message indicating failure, and (ITU-T ONLY) a) Start T25 if continuity check was asked in IAM and make automatic repeat attempt, or b) Start T24 if continuity check was asked in CCR (ANSI ONLY) c) Start T25 and make automatic repeat attempt (if applicable)
T25	1–10 sec	When initial continuity check failure is detected	Upon expiry	Send Continuity Check Request Message and repeat continuity check
T26	1–3 min	When second or subsequent continuity check failure is detected	When continuity is detected	Send Continuity Check Request Message and repeat continuity check (starting T26)
T27	4 min (ITU-T) >3 min (ANSI)	Upon receipt of continuity check failure	Upon receipt of Continuity Check Request Message	Send Reset Circuit Message, start T16 and T17
T28	10 sec	When a Circuit Query Message is sent	Upon receipt of Circuit Query Response Message	Alert maintenance
T29 (ITU-T ONLY)	300–600 ms	Congestion indication received when T29 not running	–	New congestion indication will be taken into account
T30 (ITU-T ONLY)	5–10 sec	Congestion indication received when T29 not running	–	Restore traffic by one step if not yet at full load, and start T30
T31	> 6 min	Release of ISDN user part end-to-end signaling connection, based on connection oriented SCCP	Upon expiry	Call reference reusable
T32	3–5 sec	When response to request of end-to-end connection establishment is sent	Upon receipt of first End-to-end Message from the remote end	End-to-end Message allowed to be sent
T33	12–15 sec	When Information Request Message is sent	Upon receipt of an Information Message	Release call and alert maintenance personnel
T34 (ITU-T)	2–4 sec	When indication of a Segmented Message is received on an IAM, ACM, CPG, ANM or CON Message	At receipt of a Segmentation Message	Proceed with call
T34 (ANSI)	10–15 sec	When Loop-Back Acknowledgment Message is sent in response to receipt of Continuity Check Request Message	Upon receipt of Continuity or Release Message	Release all equipment, send Reset Circuit Message, and start T16 and T17
T35 (ITU-T)	15–20 sec	Upon receipt of the latest digit (< or >ST) and before the minimum or fixed number of digits have been received	Upon receipt of ST, or when the minimum or fixed number of digits have been received	Send Release Message (cause 28)
T36 (ITU-T)	10–15 sec	When transit or incoming international exchange receives Continuity Check Request Message	Upon receipt of Continuity or Release Message	Release all equipment, send Reset Circuit Message, start T16 and T17
T36 (ANSI)	2–4 sec	When a message is received indicating that another segment follows	Upon receipt of a Segmentation Message	Proceed with call processing
T37 (ITU-T) —reserved for ISUP '92	2–4 sec
T37 (ANSI)	30 sec	When ISUP availability test is started	Upon receipt of a message from the affected ISUP	Proceed with call processing
T38 (ITU-T)	Interval Specified in Rec. Q.118 [113]	When the incoming international exchange sends a Suspend (network) Message to the preceding exchange	Upon receipt of Resume (Network) or Release Message	Send Release Message (cause 102)
T39 (ITU-T ONLY)	4–15 sec interval specified in Rec. Q. 731-7 §7.9	When a MCID request is sent	Upon receipt of a MCID response	Call continues
TACC,r (ANSI ONLY)	5 sec	Upon receipt of ACC indicator	Upon expiry	Remove ACC controls in the exchange
TCCR (ANSI ONLY)	2 sec	When Continuity Check Request Message is sent	Upon receipt of Loop-Back Acknowledgment Message	Disconnect transceiver, send Reset Circuit Message, and start T16 and T17
TCCR,r (ANSI ONLY)	20 sec	Upon receipt of initial Continuity Message, indicating failure	Upon receipt of Continuity Check Request Message	Send Reset Circuit Message and start T16 and T17
TCGB (ANSI ONLY)	5 sec	Upon receipt of Circuit Group Blocking Message	Upon receipt of Circuit Group Blocking or Circuit Group Unblocking Message	Accept subsequent Circuit Group Blocking Message as a new message
TCRA (ANSI ONLY)	20 sec	When Circuit Reservation Acknowledgment Message is sent	Upon receipt of Initial Address Message or Release Message	Initiate release procedure
TCRM (ANSI ONLY)	3–4 sec	When Circuit Reservation Message is sent	Upon receipt of Circuit Reservation Acknowledgment Message	Initiate release procedure
TCVT (ANSI ONLY)	10 sec	When Circuit Validation Test Message is sent	Upon receipt of Circuit Validation Response Message	Retransmit Circuit Validation Test Message and restart TCVT; alert maintenance personnel at second expiry
TEXM,d (ANSI ONLY)	Network Dependent	When Initial Address Message is sent to succeeding network	Upon expiry	Send Exit Message to proceeding exchange
TGRS (ANSI ONLY)	5 sec	Upon receipt of Circuit Group Reset Message	Upon receipt of Circuit Group Reset Message	Accept subsequent Circuit Group Reset Message as new message
THGA (ANSI ONLY)	0–5 min	Carrier loss	Carrier restoral	Alert maintenance personnel
TSCGA (ANSI ONLY)	0–2 min	Upon failure of initial Demand Continuity Check in SCGA group	Upon success of initial Demand Continuity Check in SCGA group	Alert maintenance personnel
TSCGA,d (ANSI ONLY)	5–120 sec	Upon failure of initial Demand Continuity Check in SCGA group	Upon expiry	Initiate Demand Continuity Check on another circuit in failed group
min = minute(s) sec = seconds(s) ms = millisecond(s)

^[1] Extra condition for ETSI only [18]