ITU-T E 421-1993 SERVICE QUALITY OBSERVATIONS ON A STATISTICAL BASIS《基于统计学的服务质量监视》.pdf

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1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 % TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G0.%47/2+G0G0!.$G0G0)3$.15!,)49G0G0/ for example, automaticobservations may be used to supplement observations taken by an operator. It was considered in 1968 that the need forautomatic observation

2、s would increase in view of the heavy cost associated with manual or semiautomatic observationson the rapidly expanding international network. It was also considered that automatic observations would not entirelysupersede observations taken by an observer within the foreseeable future.The relative m

3、erits of the three methods can be assessed as follows:2.2 Manual observationProvides 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 automatical

4、ly,e.g. very poor speech transmission (item 5.2 of Table 1/E.422), or difficulty with audible tones encountered in theinternational service (item 6.4 of Table 1/E.422).2.3 Semi-automatic observationProvides all the data required in Tables 1/E.422 and 1/E.423.There is a saving in staffing costs compa

5、red with manual observation.2 Fascicle II.3 - Rec. E.421Greater accuracy compared with manual observation is possible due to the fact that there is an automaticrecording of the number dialled, the time of the call, etc.It is possible for the observer to devote greater attention to the more critical

6、conditions being checked duringobservations 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

7、 day, to automatic operation.2.4 Automatic observationOperating 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 w

8、hich observations are made is facilitated.Some of the differences between internal and external automatic observations are given below:2.4.1 Internal automatic observations can be made in the switching centre itself, on the incoming side or the outgoingside or in between, according to the way the sw

9、itching centre is engineered:a) Only line signals, such as seizure, answer, etc. can be monitored, and also register signals as long as theydo 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

10、respect.c) Item b) applies also to outgoing signals. If there is a fault in the exchange it can happen that signals havenot 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 External a

11、utomatic observations are made by means of monitoring equipment which is supervising the trafficon incoming or outgoing lines:ll 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 re

12、quired 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 observationtechniques.3 Time of observationsThe 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

13、observations are not taken over the whole day the observation period is recorded under theheading “Time of observations“ and should include the three busiest hours of the day.4 Observation access points4.1 Observations for Table 1/E.422 or Table 2/E.422 should be carried out from access points as cl

14、ose as possibleto the outgoing international exchange.The following access points can be considered:i) outgoing relay 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 exch

15、ange.Observations will be made only while the call is being set up, and a few seconds after the called subscribersreply.Fascicle II.3 - Rec. E.421 3When the circuit access point1)is used for observation of international calls it is possible that the service qualityof the international exchange may n

16、ot 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 shouldbe carried out as close as possible to the international exchange on the national side. This would be more represent

17、ativeof service to the subscriber, and allows observation of call failure at the outgoing international exchange. Where it isnot possible to make the distinction between failures in the outgoing international exchange, and failures beyond thisexchange, or where there is a meaningful advantage in doi

18、ng 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 beenmade, as observations obtained at each one of the three access points mentioned above are not comparable.4.2 Observations for Table 1/E

19、.423 must be carried out from access points on the operators positions.5 Number of observations5.1 Service observing programmes should be established in such a manner that statistical results obtained be asreliable as practicable bearing in mind the cost of obtaining large samples.5.2 According to t

20、he studies carried out by the CCITT in 1964-1968, the quantities shown below are considered theminimum quantities to provide a general indication of the quality of service.5.2.1 Table 1/E.422The minimum number of observations per outgoing circuit group for Table 1/E.422 should be 200 per monthwhen m

21、ore than 20 circuits are included in a group, 200 per quarter when there are between 10 and 20 circuits in agroup and 200 per year if there are less than 10 circuits in a group.5.2.2 Table 1/E.423The minimum number of observations for Table 1/E.423 should be 200 per quarter when there are more than2

22、0 circuits in the group, 200 per semester when there are between 10 and 20 circuits and 200 per year when there areless than 10 circuits in the group.5.2.3 Transit trafficWhere an outgoing circuit group also carries transit traffic it is desirable to obtain data for each destinationcountry reached v

23、ia this circuit group. In principle, the number of observations for each destination should be obtainedas indicated above. To accomplish this, one should use for each destination country its corresponding number oferlangs and derive from these erlangs a theoretical number of circuits.However, where

24、only a very small amount of traffic is handled, e.g. less than 5 erlangs, each Administrationmay wish either to make a smaller number of observations or (e.g. in case of no complaints) no observations at all andrely on the information obtained at the transit exchange.5.3 The number of observations s

25、pecified above will provide a general indication of results on quality of service incertain broad categories. Administrations may desire more accurate results especially for the individual categories inTable 1/E.422.Attention is drawn to Table 1/E.421 which gives the number of observations required

26、to obtain a certain degreeof accuracy._1)For definitions of test access points see Recommendation M.700. See also Recommendation M.110.4 Fascicle II.3 - Rec. E.421TABLE 1/E.421Expected percentagerate of failureNumber of observations of a random sample required to predict with 95% confidence the true

27、 percentage offailure with an accuracy of: 25% 30% 35% 40% 45% 50%246810121416182030405031361536100373657646939333629225614996642178106769651140032627323320217810467441600784512376294239201171149131765033122560039228822518315413111410060382510305003302451951501281129585503020880440290215170132112988

28、070422416Annex to Table 1/E.421Examples of use of Table 1/E.421Example 1 It is estimated from previous results that a particular type of failure occurs on about 4% of calls. Ifit is required to confirm, with 95% confidence, that the existing failure rate is between 3% and 5% (i.e. 25% of 4%),then ob

29、servations 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 1200calls (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 th

30、at when 200 observations are taken over a period it is necessary to take the “rolling average“ ofconditions over six periods. The rate of failure for a number of categories important from the maintenance point ofview is expected to be about 2%.Example 3 After observations have been taken and the rat

31、e of failure in the sample has been calculated, thetable 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 dueto cause “Y“. The rates of failure in

32、the sample due to X and Y, respectively, are then 2.9% and 1.5%. From the table, itis apparent from this sample of 1000 calls that the true rate of failure due to X has an accuracy of about 35% (i.e. isbetween 1.9% and 3.9%), and that due to Y has an accuracy of about 50% (i.e. is between 0.8% and 2

33、.3%).6 Exchange and analysis of the results of observations6.1 Exchange of the results of observationsThe 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

34、 exchange is desirable.Nevertheless, in the case of small groups of circuits (less than 20 circuits) the information should be exchangedafter 200 observations have been made but never later than one year in any case; attention is drawn to the fact that lessthan 200 observations are of little value.F

35、ascicle II.3 - Rec. E.421 5Results 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 transitAdministrations and

36、 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 takento make the results available to those who can take action to bring about an improvement. The results of serviceobservations according to Table

37、s 1/E.422 and 1/E.423 should therefore be made available to the Administration in thecountries of destination as soon as possible after completion of the observation period and in any case within sixweeks.6.2 Analysis of observation resultsAn 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, serviceobservation statistics in the form of graphs.