1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/.G0G0/6%2G0G04(%G0G04%,%0(/.%.%47/2+,)-)43G0G0 when they are exceeded the maintenance services shouldconsider the transmission channel defective. The period of measurement is about 15 minut
2、es (more precisely, theperiod corresponding to the transmission of the total number of sequences which is closest to 15 minutes).TABLE 1/V.53Modulation rate(bauds)Connection Maximum bit error rate12001200600600200200switched(when possible)leasedswitchedleasedswitchedleased10-35 10-510-35 10-510-45 1
3、0-5Note - These values are not intended for use in planning circuits, but for the information ofmaintenance services.2 Fascicle VIII.1 - Rec. V.532.2 Block error rateInformation on the error rate for sequences of 511 bits would be given in a form similar to that for the bit errorrate, the two measur
4、ements being made simultaneously. However, no limit for the sequence error rate can berecommended for the time being.Note - To enable Administrations to appreciate the value of sequence error-rate measurement, Table 2/V.53shows the maximum and minimum theoretic values of error rates for sequences of
5、 511 bits corresponding to differentvalues of bit error rate.These theoretic values do not depend on the modulation rate. For the purposes of this table, a modulation rateof 1200 bauds has been taken as an example.Modulation rate: 1200 baudsPeriod of measurement: 15 minutes = 900 secondsNumber of bi
6、ts transmitted: 1 080 000Length of sequence: 511 bitsNumber of sequences transmitted: 21133 Limit of uniform-spectrum random noiseSee Recommendation G.153 l.TABLE 2/V.53Bit error rate Number oferroneous bitsErroneous sequencesMaximumnumbera)Maximumrate in %Minimumnumber b)Minimumrate in %2 10-310-3
7、c)5 10-410-45 10-52160108054010854211310805401085410051.125.55.12.5532110.240.150.100.050.05a)The maximum number of erroneous sequences corresponds to a uniform distribution of erroneous bits (one bit per sequence).b) The minimum number of erroneous sequences corresponds to a grouped distribution of
8、 erroneous bits (sets of 511 bits affectingthe sequences).c)It will be seen that for a bit error rate of 10-3, the sequence error rate can vary between 0.15% and 51.1%. (This shows the valueof sequence error-rate measurement, not only for users, but also for Administrations, which can thus obtain us
9、eful information onthe causes of bit and sequence error).4 Limits for impulsive noise4.1 Bearing in mind the following two points:- that Recommendation V.2 demands a maximum data signal level of -10 dBm0 for a simplex transmissionand -13 dBm0 for duplex transmission,- that there has been considerabl
10、e experience of using the threshold -18 dBm0 and -22 dBm0,the threshold settings should be -18 dBm0 for telephone-type circuits and -21 dBm0 for the special quality circuitsmentioned in Recommendation M.1020 2, the standard measuring instrument (see Recommendation 0.71, 3) beingadjustable to thresho
11、lds 3 dB apart (see Note 1).Fascicle VIII.1 - Rec. V.53 34.2 For counting the number of impulses, the instrument shall be used in the “flat“ bandwidth condition (see Note2).On a leased circuit, the admissible limit should be 70 impulse counts per hour; but in realizing that error ratemeasurements ar
12、e conducted for periods of 15 minutes each, the recommended maintenance limit should be 18 countsin 15 minutes for leased circuits (see Note 3). The measurements should be made during peak hours.At the time of measurement the line should be terminated at both ends by impedances of 600 ohms. Themodem
13、 may be used for this purpose if it complies with this impedance.4.3 For the general switched telephone network, there should be no recommended maintenance limits forimpulse counts, but the instrument might be useful as a diagnostic aid at the discretion of the Administrations. This isbecause the im
14、pulse count results taken on any one connection vary considerably with time and even greaterdifferences appear between various connections.4.4 The correlation between the bit error rate and the number of impulse counts thus determined has not yet beenestablished.Note 1 - Levels should be expressed i
15、n dBm0, becausea) the difference between the various national transmission plans is taken into account, andb) the level value is related to the value of the data signal level to a close degree.Note 2 - Owing to lack of experience, no external filter should be used for present maintenance purposes.Ho
16、wever, the study of the use of external filters should continue. By means of additional filters the instrument mayprovide other optional bandwidths (see the Recommendation cited in 4).Note 3 - These values are given as an indication. The question of the duration of the measurement andpermissible max
17、imum standards for impulsive noise forms the subject of future studies.References1 CCITT Recommendation Characteristics appropriate to international circuits more than 2500 km in length,Vol. III, Rec. G.153.2 CCITT Recommendation Characteristics of special quality international leased circuits with special band-width conditioning, Vol. IV, Rec. M.1020.3 CCITT Recommendation Specification for an impulsive noise measuring instrument for telephone-typecircuits, Vol. IV, Rec. O.71.4 Ibid., 3.5.2.
