All drawings contained in this Recommendation have been done in Autocad.
Recommendation Q.725
SIGNALLING PERFORMANCE IN THE TELEPHONE APPLICATION
1	Introduction
	This Recommendation gives the requirements of the telephone application of Signalling System No. 7.
	In Recommendation Q.706, the Message Transfer Part performance is described. The Message Transfer Part is the basis of the telephone application of Signalling System No. 7 and provision of a signalling network to serve the telephone service must take account of the performance of the Message Transfer Part and the requirements of the telephone application. For example, taking account of the message transfer times detailed in Recommendation Q.706 and the requirements for message transfer times between two telephone exchanges, a figure may be derived for the total permissible number of signalling links in signalling relations in tandem for a particular call.
2	Unsuccessful calls due to signalling malfunction
	The proportion of calls that are unsuccessful due to signalling malfunction should be less than 1 in 105.
	By means of error detection (see Recommendation Q.703) as well as transmission fault indication (see Recommendations G.732 [1] and G.733 [2]), it is ensured that, overall, not more than one error in 108 of all signal units transmitted is accepted and will cause false operation.
	Unsuccessful calls may be caused by undetected errors, loss of messages or messages delivered out of sequence (during emergency situations within the signalling network) and may result in:
–	incomplete call set–up,
–	misrouted calls (e.g. connection of wrong numbers),
–	calls routed correctly but mishandled (e.g. false clearing).
3	Unavailability of a signalling route set
	The overall unavailability of a signalling route set causing the unavailability of a signalling relation should not exceed a total of 10 minutes per year.
	Note – The availability of a signalling route set within a signalling network may be enhanced by replication of signalling links, signalling paths and signalling routes.
4	Labelling potential
	The label of the Telephone User Part of Signalling System No. 7 provides the potential to identify 16 384 signalling points and up to 4096 speech circuits for each signalling relation.
5	Cross–office transfer time
5.1	Functional reference points and transfer time components
Figure 1/Q.725 - CCITT 35600

5.2	Definitions
a)	cross–office transfer time, Tcu
	Tcu is the period which starts when the last bit of the signal unit leaves the incoming signalling data link and ends when the last bit of the signal unit enters the outgoing signalling data link for the first time. It also includes the queueing delay in the absence of disturbances but not the additional queueing delay caused by retransmission.
b)	user handling time, Thu
	Thu is the period which starts when the last bit of the message has entered the Telephone User Part and ends when the last bit of the derived message has left the Telephone User Part.
5.3	Queueing delay
	The formulae for the queueing delays are described in Recommendation Q.706, § 4.2.
	The telephone traffic model assumed is given in Table 1/Q.725, from which the proportion of signal messages may be obtained as shown in Table 2/Q.725. Using Table 2/Q.725, examples of queueing delays are calculated as shown in Figures 2/Q.725 to 5/Q.725, where one call attempt per second per 64 kbit/s signalling data link may yield 0.00577 Erlang of the traffic loading of each channel.
5.4	Estimates for message transfer time
	The figures in Table 3/Q.725 are related to a signalling bit rate of 64 kbit/s.
5.5	Effect of retransmission
	As a consequence of correction by retransmission, not more than one in 104 signals should be delayed more than 300 ms as a long–term average. This requirement refers to each signalling link.
	This requirement is laid down in order to ensure satisfactory answer delays.
TABLE 1/Q.725
Traffic model
Sending procedure			“En bloc”				Overlap				
Type of call			AW	SB	CC	AB	AW	SB	CC	AB	
Percent calls			30	10	5	5	30	10	5	5	
		Length (bits)									
	12–digit IAM	176	1	1	1	0					
	6–digit IAM	152					1	1	1	1	
	3–digit SAM	128					1	1	0	1	
Messages per call	1–digit SAM	112					3	3	0	0	
	Address complete	112	1	1	0	0	1	1	0	0	
	Others	112	3,5	2	3	0	3,5	2	3	2	
AW	Answered
SB	Subscriber busy and not answered
CC	Circuit congestion
AB	Abortive
Note – The assumptions used in this model are chosen for illustrative purposes, and should not be considered to be typical.
TABLE 2/Q.725
Proportion of messages
Length (bits)	176	152	128	112	104	Total	
Messages per call in both directions	0.45	0.5	0.45	2.0	2.9	6.3	
Percent	7.1 	7.9	7.1 	31.7	46.0	100	
Mean message length (Tm)	117.2 bits						
k1	1.032						
k2	1.107						
k3	1.239						

Figure 2/Q.725 - CCITT 41210

Figure 3/Q.725 - CCITT 41200

Figure 4/Q.725 - T1115651-88

Figure 5/Q.725 - T 1109931-88

TABLE 3/Q.725
Message type	Exchange call 	Cross–office transfer time 
Tcu (ms) a)		
	attempt loading	Mean	95%	

Simple (e.g. answer)	Normal 
+15% 
+30%	110
165
275	220
330
550	
Processing intensive 
(e.g. IAM)	Normal 
+15% 
+30%	180
270
450	360
540
900	
a) Provisional values.
References
[1]	CCITT Recommendation Characteristics of primary PCM multiplex equipment operating at 2048 kbit/s, Rec. G.732.
[2]	CCITT Recommendation Characteristics of primary PCM multiplex equipment operating at 1544 kbit/s, Rec. G.733.