ITU-T E 845-1993 CONNECTION ACCESSIBILITY OBJECTIVE FOR THE INTERNATIONAL TELEPHONE SERVICE《国际电话业务的连接接入能力目标》.pdf

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1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 % TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G0.%47/2+G0G0!.$G0G0)3$.15!,)49G0G0/(b) that customers rank connection inaccessibility as one of the most annoying of call set-up impairments;(c) that an objective for connection accessibility whic

2、h takes into account customer opinion about the call set-up phase is consistent with other Recommendations which have recommended an objective for service retainabilitybased, in part, on customer opinion;(d) that connection accessibility will not be constant over time, even for a particular calling

3、and called linepair. One suitable measure is a long-term average network connection failure probability. (Other suitable measures mayalso be required.);(e) that the overall objective for connection accessibility should be allocable to the national systems and theinternational chain of the internatio

4、nal connection;(f) that the objective should take into account the concerns of network planners and system designers, provideuseful guidance to both and may be used by Administrations in providing a method for verifying whether or notnetwork performance is acceptable;(g) that the overall connection

5、accessibility should be controlled by the accessibility performances ofindividual exchanges and circuits, and that to obtain this control, the overall connection accessibility must bemathematically linked to the equipment availability and reliability,recommends1 Measure of connection accessibilityCo

6、nnection accessibility shall be measured using the long-term average network connection failure probability,which is the complement of the connection access probability as defined in Recommendation E.800.The network connection failure probability PNCFcan be estimated by using the following formula:P

7、QNNCFN=_1)Formerly G.180, in Red Book, Fascicle III.1.2)Some of the terms in this Recommendation, for example, the noun “measure”, are used in the sense of their definition given inRecommendation E.800.2 Fascicle II.3 - Rec. E.845where QNis the number of unsuccessful connection access attempts and N

8、 is the total number of connectionaccess attempts in some time period (to be determined).A method for estimating the required call sample size is contained in Annex A.For purposes of network design, the network connection failure probability, PNCF, can also be calculated usingthe method outlined in

9、Annex B. Annex C describes how the busy and non-busy hours affect the network connectionfailure (NCF).Note 1 Those unsuccessful connection access attempts reflecting failure of the network to work properly,from the users perspective, are called network connection failure. They are call failures an a

10、stute caller can determineand are caused by network faults and congestion. A network connection failure is any valid bid for service whichreceives one of the following network responses:1) dial tone returned after dialling is completed;2) no ring and no answer;3) all circuits busy signal or announce

11、ment;4) connection to the wrong number (misrouting);5) double connection.This list may not be exhaustive.Note 2 This definition of network connection failure is based on the response the caller can hear.Note 3 There are two generic causes of network connection failures: equipment faults and traffic

12、congestion.Note 4 The averaging interval (to be determined) used for estimating the connection failure probability shallinclude normal and peak hour traffic periods. In the event of exceptionally high traffic demand (public holiday, naturaldisaster, etc.) failure rates higher than the objective may

13、be tolerated.Note 5 The network connection failure probability should be estimated by Administrations in a mannerconsistent with obtaining, from the Administrations point of view, reasonably accurate estimates.2 Objective for connection accessibilityConnection accessibility is acceptable if the long

14、-term average connection failure probability, expressed as apercentage, does not exceed a value (overall average for all international calls) of A% to B% (values to be determined).Additionally, the long-term average failure probability at any single international homing exchange should neverexceed C

15、% (value to be determined).Note Possible values for A, B and C are in the range of 10% to 20%.3 Allocation of the overall objective to the national systems and international chainThe network connection failure probability objective shall be apportioned as follows:X % to the originating national syst

16、em,Y % to the international chain,Z % to the terminating national system,where X + Y + Z = P, and P is the overall objective stated in 2.Note 1 The connection access attempt may fail in the national systems or the international chain of theconnection.Note 2 The objective takes into account all means

17、 of “defense“ of the network against failure to complete theconnection, including alternate routing, if used.Note 3 The network connection failure probability of the national systems or international chain is defined asthe probability that the call access attempt will fail because of some problem (e

18、quipment fault or congestion) in thesystems or chain.Note 4 Values for X, Y and Z are in the range of 3% to 7%.Fascicle II.3 - Rec. E.845 3ANNEX A(to Recommendation E.845)Method for selecting the required call sample size, NThe network connection failure probability shall be estimated by Administrat

19、ions in a manner consistent withobtaining reasonably accurate estimates.The number of call access attempts sampled shall be sufficiently large to obtain a good estimate of theprobability.A method of picking a sample size N could be used which could produce a maximum error of measurement, e,(to be de

20、termined) with confidence level, (to be determined).Recommendation E.850 contains a method for estimating the sample size required to estimate cutoff callprobability. This method should be studied for application here.ANNEX B(to Recommendation E.845)Method for relating overall networkconnection fail

21、ure probability to the reliability and availabilityperformance of exchanges and circuitsThe following equation gives the relationship between the overall network connection failure probability,PNCF, and the probabilities of connection failure in the national systems and international chain of the co

22、nnection:()()()PPPPNCF OE I TE= 11 1 1where POEis the probability that the access attempt fails in the originating national system, PIis the probabilityof failure in the international chain and PTEis the probability of failure in the terminating national system.Hypothetical reference connections for

23、 the three parts of an international connection are shown inFigure B 1/E.845. The proportion of calls (Fn) which are routed over the parts are also given in the figure. The valuesare taken from Table 1/G.101.The probability that a connection access attempt fails in either of the parts is given by th

24、e following equations:()()PFPPOE nncnn=1151- 1-s()()PFPPInncnsn= =+111121 -()()PFPPTE n cnsnn=1151- “ 1- “where n is the number of circuits in a selected part. Fnis the call frequency for an n-circuit system or chain(from Figure B-1/E.845).4 Fascicle II.3 - Rec. E.845Pc, Pcand P“care the probabiliti

25、es that the connection access fails in the originating system, internationalchain or terminating system circuits, respectively. (It is assumed here for simplicity that all circuits in a system or chainhave the same probability of failure. However, this is not a requirement.)Ps, Psand P“sare the prob

26、abilities that the connection access attempt fails in the originating system,international chain (note that ISC is assumed part of the international chain) or terminating system exchanges,respectively. (For simplicity, all exchanges are assumed to have the same failure probability, but this is not a

27、requirement.)A circuit or exchange can cause a network connection failure for one of three reasons:1) The call is blocked because of congestion. The probability of blockage is PCBand PSBfor circuits andexchanges, respectively.2) The circuit or exchange fails during the call set-up time. The probabil

28、ity of such a failure is PCFand PSFfor circuits and exchanges, respectively.3) The circuit or exchange is unavailable to arriving calls, so all calls arriving during the downtime fail to becompleted. These probabilities are PCD and PSDfor circuits and exchanges, respectively.The probability that a c

29、ircuit or exchange causes a network connection failure is given by the followingequations, respectively:()()()PPPPCCBCFCD= 11 1 1 - -()()()PPPPSSBSFSD= 11 1 1 - -Fascicle II.3 - Rec. E.845 5The failure probabilities PCF, and PSFcan be expressed in terms of the long-term mean failure intensifies Zcan

30、d Zs of circuits and exchanges, respectively, by the following equations:PCF =ZcTs,PSF =ZsTs,where Tsis the long-term average call set-up time.Similarly, the failure probabilities PCDand PSDcan be expressed in terms of the long-term mean accumulateddowntime (MADT)cand (MADT)s, of circuits and exchan

31、ges, respectively, by the following equations:()PMADTNCDcc=K()PMADTNSDs s=Kcand sare the long-term average call arrival rates for circuits and exchanges, respectively, and N is the long-term average number of call attempts (in some interval, such as one year).K is a constant equal to the number of u

32、nits of time (minutes or seconds) used to express the downtime in thelong-term averaging interval selected (such as a year).For example, if the downtime is expressed in minutes and the averaging interval is one year, thenK = 525 600 min./year.ANNEX C(to Recommendation E.845)Effects of busy hours and

33、 non-busy hours on the network connection failureThe two major components of network connection failure (NCF) are the blocking rate due to congestion andconnection access attempt failures due to equipment faults. Equipment faults are further divided into major and minorfaults. These components affec

34、t NCF differently.C.1 Influences of faultsFaults of subsystems in a telephone network may be divided into two categories, according to their influence onnetwork performance. Table C-1/E.845 shows two fault categories: major and minor.TABLE C-1/E.845Failure category Definition Network componentsMajor

35、 (considerable)influence faultFault wherein a connection access attempt encounters asituation such that service degradation of networkcomponent(s) lasts for some period of time, owing to largescale failure of equipment, and a subscriber cannot beassured of normal serviceSubscribers line, subscriber

36、terminalb)exchange, transmission line, service centerMinor (less important)influence faulta)Small scale fault wherein a connection access attempt ishandled incorrectly and encounters no signal (e.g. dial tone,ring-back tone), no connection, low level speech signal, etc.,i.e. less important service d

37、egradation is experienceda)Intermittent fault is excluded and its treatment is an unresolved problem.b)In some Administrations the subscriber terminal is not considered a network component.6 Fascicle II.3 - Rec. E.845C.2 Relationship between NCF, congestion and faultCongestion-related NCF depends on

38、 the traffic offered to a system being considered (a switching system, anetwork, etc.).The effects of a minor fault will be considered as so-called white noise where the absolute value is small andfluctuates at random.The effects of a major (complete) fault depend on the offered traffic volume at th

39、e time of fault. If a major faultoccurred during busy hours, there would be an extremely high value for NCF. Conversely, a major fault during non-busy hours will merely yield a small NCF, no matter how large the affected system is. This is because the traffic loaditself is small. Since it is usually

40、 expected that major faults will be very rare, NCF characteristics under major faultconditions are different from those under minor fault conditions which may be daily occurrences.C.3 Long-term NCF (averaged throughout a year)The long-term NCF concerned with traffic congestion during non-busy hours

41、will be much smaller than thatduring busy hours. Since both cumulative call failures Nf and total calls offered Noduring non-busy hours are muchsmaller than those during busy hours, the averaged 24-hour NCF including non-busy and busy hours effects will not bemuch different from the busy hour NCF.A

42、major fault can be identified but a minor fault cannot be specified correctly when network operators maintainnetwork equipment. By measuring long-term NCF during non-busy hours, the effect of minor faults can be estimatedbecause NCF during non-busy hours is attributed not to traffic congestion but t

43、o minor faults.C.4 NCF and busy hour patternIn a country (international region) with several standard time zones, there will be several busy hours. In suchcases, a connection in the network may include busy and non-busy network components. Thus, an averaged24-hour NCF would be helpful to administer

44、a network with different time zones.However, the averaged 24-hour NCF does not seem to be appropriate to administer a network having only onestandard time zone because its fault-related term is too small to affect the total NCF, and it might be too late by the timean extraordinary NCF value has been detected. The NCF averaged during non-busy hours would be one measure formonitoring the effect of equipment faults (minor faults) on subscribers, since this will become a major factor duringnon-busy hours.