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Recommendation E.421
SERVICE QUALITY OBSERVATIONS ON A STATISTICAL BASIS
1 Definitions
1.1 service observation
F: observation de la qualité de service
S: observación de la calidad del servicio
Monitoring to obtain a complete or partial assessment of the quality of
telephone calls, excluding test calls.
1.2 manual observation
F: observation manuelle
S: observación manual
Monitoring of telephone calls by an observer without using any automatic
data-recording machine.
1.3 automatic observation
F: observation automatique
S: observación automática
Monitoring of telephone calls without an observer.
1.4 semi-automatic observation
F: observation semi-automatique
S: observación semiautomática
Monitoring of telephone calls using equipment which records some data
automatically. For example, equipment in which information, such as exchange
being observed, number dialled by the subscriber, metering pulses and time of
call, is recorded automatically on some means suitable for data processing. The
observer merely has to key in a code indicating the condition observed.
2 Relative merits of manual, automatic and semi-automatic observations
2.1 The three methods mentioned above in SS 1.2, 1.3 and 1.4 are not
exclusive; for example, automatic observations may be used to supplement
observations taken by an operator. It was considered in 1968 that the need for
automatic observations would increase in view of the heavy cost associated with
manual or semiautomatic observations on the rapidly expanding international
network. It was also considered that automatic observations would not entirely
supersede observations taken by an observer within the foreseeable future.
The relative merits of the three methods can be assessed as follows:
2.2 Manual observation
Provides all the data required in Tables 1/E.422 and 1/E.423.
Observations can be carried out with a minimum of equipment.
Observations can permit the detection of a number of abnormalities which
cannot be detected automatically, e.g. very poor speech transmission (item 5.2 of
Table 1/E.422), or difficulty with audible tones encountered in the international
service (item 6.4 of Table 1/E.422).
2.3 Semi-automatic observation
Provides all the data required in Tables 1/E.422 and 1/E.423.
There is a saving in staffing costs compared with manual observation.
Greater accuracy compared with manual observation is possible due to the
fact that there is an automatic recording of the number dialled, the time of the
call, etc.
It is possible for the observer to devote greater attention to the more
critical conditions being checked during observations of calls.
The results are produced in a form suitable for subsequent mechanized
analysis.
Owing to the reduction of costs it is possible to obtain a larger sample
for the same expenditure.
Semi-automatic equipment may be converted, during certain hours of the
day, to automatic operation.
2.4 Automatic observation
Operating cost is minimum (staff reduction).
Continuous observation is possible.
It is possible to have a larger sample.
Human error is eliminated.
Automatic processing of data is facilitated.
Conversational privacy is ensured.
Control of the time at which observations are made is facilitated.
Some of the differences between internal and external automatic
observations are given below:
Fascicle II.3 - Rec. E.421 PAGE1
2.4.1 I automatic observations can be
made in the switching centre itself, on the incoming side or the
outgoing side or in between, according to the way the switching
centre is engineered:
a) Only line signals, such as seizure, answer, etc. can be monitored, and
also register signals as long as they do not pass through the exchange
in an end-to-end signalling procedure.
b) Signals received are only monitored if the exchange itself operates
correctly in that respect.
c) Item b) applies also to outgoing signals. If there is a fault in the
exchange it can happen that signals have not been sent in the
appropriate way without the exchange being aware of it.
More information on this type of observation technique is given in
Recommendation E.425.
2.4.2 E automatic observations are made
by means of monitoring equipment which is supervising the traffic
on incoming or outgoing lines:
- All signalling signals can be monitored.
- The detection of tones, speech and data is possible if advanced
equipment is used.
- This observation technique provides all the data required in Table
2/E.422 and Table 2/E.423.
- The application is very flexible and can be used instead of manual or
semi-automatic observation techniques.
3 Time of observations
The results of all observations taken over the whole day should be
recorded in Table 1/E.422 or Table 2/E.422.
In the case where observations are not taken over the whole day the
observation period is recorded under the heading "Time of observations" and
should include the three busiest hours of the day.
4 Observation access points
4.1 Observations for Table 1/E.422 or Table 2/E.422 should be carried out from
access points as close as possible to the outgoing international exchange.
The following access points can be considered:
For definitions of test access points see Recommendation M.700. See also
Recommendation M.110.
i) outgoing set of an international circuit
("exchange" side), i.e. international circuit access
point1);
ii) incoming relay set of a national circuit;
iii) link circuits of the international exchange.
Observations will be made only while the call is being set up, and a few
seconds after the called subscriber's reply.
When the circuit access point is used for observation of international
calls it is possible that the service quality of the international exchange may
not be checked by either international or national observation programmes.
Preferably, and where technically feasible for the most complete results,
observations for Table 1/E.422 should be carried out as close as possible to the
international exchange on the national side. This would be more representative of
service to the subscriber, and allows observation of call failure at the outgoing
international exchange. Where it is not possible to make the distinction between
failures in the outgoing international exchange, and failures beyond this
exchange, or where there is a meaningful advantage in doing so, observations
should be taken on the outgoing side.
It is necessary to state in Table 1/E.422 or Table 2/E.422 the access
point where the observations have been made, as observations obtained at each one
of the three access points mentioned above are not comparable.
4.2 Observations for Table 1/E.423 must be carried out from access points on
the operators' positions.
5 Number of observations
5.1 Service observing programmes should be established in such a manner that
statistical results obtained be as reliable as practicable bearing in mind the
cost of obtaining large samples.
1) For definitions of test access points see Recommendation M.700. See also
Recommendation M.110.
PAGE4 Fascicle II.3 - Rec. E.421
5.2 According to the studies carried out by the CCITT in 1964-1968, the
quantities shown below are considered the minimum quantities to provide a general
indication of the quality of service.
5.2.1 Table 1/E.422
The minimum number of observations per outgoing circuit group for Table
1/E.422 should be 200 per month when more than 20 circuits are included in a
group, 200 per quarter when there are between 10 and 20 circuits in a group and
200 per year if there are less than 10 circuits in a group.
5.2.2 Table 1/E.423
The minimum number of observations for Table 1/E.423 should be 200 per
quarter when there are more than 20 circuits in the group, 200 per semester when
there are between 10 and 20 circuits and 200 per year when there are less than 10
circuits in the group.
5.2.3 Transit traffic
Where an outgoin t group also carries transit
traffic it is desirable to obtain data for each destination country
reached via this circuit group. In principle, the number of
observations for each destination should be obtained as indicated
above. To accomplish this, one should use for each destination
country its corresponding number of erlangs and derive from these
erlangs a theoretical number of circuits.
However, where only a very small amount of traffic is handled,
e.g. less than 5 erlangs, each Administration may wish either to
make a smaller number of observations or (e.g. in case of no
complaints) no observations at all and rely on the information
obtained at the transit exchange.
5.3 The number of observations specified above will provide a
general indication of results on quality of service in certain
broad categories. Administrations may desire more accurate results
especially for the individual categories in Table 1/E.422.
Attention is drawn to Table 1/E.421 which gives the number of
observations required to obtain a certain degree of accuracy.
TABLE 1/E.421
Expected percentage Number of observations of a random sample required to predict
rate with 95% confidence the true percentage of failure with an
accuracy of:
of failure ▒ 25% ▒ 30% ▒ 35% ▒ 40% ▒ 45% ▒ 50%
2 3136 2178 1600 1225 1030 880
4 1536 1067 784 600 500 440
6 1003 696 512 392
Fascicle II.3 - Rec. E.421 PAGE1
330 290
8 736 511 376 288 245 215
10 576 400 294 225 195 170
12 469 326 239 183 150 132
14 393 273 201 154 128 112
16 336 233
PAGE4 Fascicle II.3 - Rec. E.421
171 131 112 98
18 292 202 149 114 95 80
20 256 178 131 100 85 70
30 149 104 76 60 50 42
40 96 67 50 38 30 24
50
Fascicle II.3 - Rec. E.421 PAGE1
64 44 33 25 20 16
Annex to Table 1/E.421
Examples of use of Table 1/E.421
Example 1 - It is estimated from previous results that a particular type
of failure occurs on about 4% of calls. If it is required to confirm, with 95%
confidence, that the existing failure rate is between 3% and 5% (i.e. ▒ 25% of
4%), then observations must be made on a random sample of 1536 calls.
Example 2 - For an expected failure rate of 2%, observations must be made
on a random sample of about 1200 calls (1225 in the table) to predict, with 95%
confidence, that the true percentage is between 1.2% and 2.8% (i.e. ▒ 40% of 2%).
This means that when 200 observations are taken over a period it is necessary to
take the "rolling average" of conditions over six periods. The rate of failure
for a number of categories important from the maintenance point of view is
expected to be about 2%.
Example 3 - After observations have been taken and the rate of failure in
the sample has been calculated, the table may be used in a "backward" direction
to give a rough indication of the accuracy of the result.
Suppose that out of a sample of 1000 observations, there were 29 failures
due to cause "X" and 15 failures due to cause "Y". The rates of failure in the
sample due to X and Y, respectively, are then 2.9% and 1.5%. From the table, it
is apparent from this sample of 1000 calls that the true rate of failure due to X
has an accuracy of about ▒ 35% (i.e. is between 1.9% and 3.9%), and that due to Y
has an accuracy of about ▒ 50% (i.e. is between 0.8% and 2.3%).
6 Exchange and analysis of the results of observations
6.1 Exchange of the results of observations
The following periodicities are proposed for the exchange of results
between Administrations:
Table 1/E.422 or Table 2/E.422 - a monthly exchange is desirable;
Table 1/E.423 or Table 2/E.423 - a quarterly exchange is desirable.
Nevertheless, in the case of small groups of circuits (less than 20
circuits) the information should be exchanged after 200 observations have been
made but never later than one year in any case; attention is drawn to the fact
that less than 200 observations are of little value.
Results of observations will be reported without delay:
- to the Administrations and the network analysis point of the country
where observations are carried out;
- to the Administrations and the network analysis point of the other
country (including transit Administrations and their network analysis
point when involved).
The benefits to be derived from service observations tend to decrease if
there is any increase in the time taken to make the results available to those
who can take action to bring about an improvement. The results of service
observations according to Tables 1/E.422 and 1/E.423 should therefore be made
available to the Administration in the countries of destination as soon as
possible after completion of the observation period and in any case within six
weeks.
6.2 Analysis of observation results
An analysis of the results should be carried out in the country of origin
as well as in the country of destination.
Some Administrations have found it useful to distribute to other
Administrations concerned, service observation statistics in the form of graphs.
PAGE4 Fascicle II.3 - Rec. E.421
