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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, S 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
Fascicle VI.8 - Rec. Q.725 PAGE1
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
55
Length (bits)
PAGE6 Fascicle VI.8 - Rec. Q.725
12-digit IAM 176 1 1 1 0
6-digit IAM 152 1 1 1 1
3-digit SAM 128
Fascicle VI.8 - Rec. Q.725 PAGE1
1 1 0 1
Messages per 1-digit SAM 112 3 3 0 0
call
Address 112 1 1 0 0 1 1 0 0
complete
Others 112 3,5 2 3 0
PAGE6 Fascicle VI.8 - Rec. Q.725
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 0.45 0.5 0.45 2.0 2.9 6.3
directions
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
Fascicle VI.8 - Rec. Q.725 PAGE1
k3 1.239
PAGE6 Fascicle VI.8 - Rec. Q.725
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%
Normal 110 220
Simple (e.g. answer) +15% 165 330
+30% 275 550
Processing intensive Normal 180 360
(e.g. IAM) +15% 270 540
+30% 450 900
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.
Fascicle VI.8 - Rec. Q.725 PAGE1
