1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 % TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G0.%47/2+G0G0!.$G0G0)3$.15!,)49G0G0/ in semiautomatic operation; in both automatic and semiautomatic operations on the same group of circuits.1 General method1.1 The CCITT recommends that the numbe
2、r of circuits needed for a group should be read from tables or curvesbased on the classical Erlang B formula (see Supplements Nos. 1 and 2 at the end of this fascicle which refers to fullavailability groups). Recommended methods for traffic determination are indicated in Recommendation E.500.For sem
3、i-automatic operation the loss probability p should be based on 3% during the mean busy hour.For automatic operation the loss probability p should be based on 1% during the mean busy hour.Semiautomatic traffic using the same circuits as automatic traffic is to be added to the automatic traffic and t
4、hesame parameter value of p = 1% should be used for the total traffic.The values of 3% and 1% quoted above refer to the Erlang B formula and derived tables and curves. The 3%value should not be considered as determining a grade of service because with semiautomatic operation there will besome smooth
5、ing of the traffic peaks; it is quoted here only to determine the value of the parameter p (loss probability)to use in the Erlang B tables and curves.1.2 In order to provide a satisfactory grade of service both for the mean busy-hour traffic and for the traffic onexceptionally busy days, it is recom
6、mended that the proposed number of circuits should, if necessary, be increased toensure that the loss probability shall not exceed 7% during the mean busy hour for the average traffic estimated for thefive busiest days as specified in Recommendation E.500.1.3 For small groups of long intercontinenta
7、l circuits with automatic operation some relaxation could be made inrespect to loss probability. It is envisaged that such circuits would be operated on a both-way basis and that areasonable minimum for automatic service would be a group of six circuits. A table providing relaxation in Annex A isbas
8、ed on a loss probability of 3% for six circuits, with a smooth progression to 1% for 20 circuits. The generalprovision for exceptional days remains unchanged.For exceptional circumstances in which very small groups (less than six intercontinental circuits) are used forautomatic operation, dimensioni
9、ng of the group should be based on the loss probability of 3%.2 Time differencesTime differences at the two terminations of intercontinental circuits are likely to be much more pronouncedthan those on continental circuits. In order to allow for differences on groups containing both-way circuits it w
10、ill bedesirable to acquire information in respect to traffic flow both during the mean busy hour for both directions andduring the mean busy hour for each direction.It is possible that in some cases overflow traffic can be accepted without any necessity to increase the numberof circuits, in spite of
11、 the fact that this overflow traffic is of a peaky nature. Such circumstances may arise if there is notraffic overflowing from high-usage groups during the mean busy hour of the final group.3 Both-way circuits3.1 With the use of both-way circuits there is a danger of simultaneous seizure at both end
12、s; this is particularly thecase on circuits with a long propagation time. It is advisable to arrange the sequence of selection at the two ends so thatsuch double seizure can only occur when a single circuit remains free.2 Fascicle II.3 - Rec. E.520When all the circuits of a group are operated on a b
13、oth-way basis, time differences in the directional mean busyhours may result in a total mean busy-hour traffic flow for the group which is not the sum of the mean busy-hour trafficloads in each direction. Furthermore, such differences in directional mean busy hour may vary with seasons of the year.H
14、owever, the available methods of traffic measurement can determine the traffic flow during mean busy hour for thistotal traffic.3.2 Some intercontinental groups may include one-way as well as both-way operated circuits. It is recommendedthat in all cases the one-way circuits should be used, when fre
15、e, in preference to the both-way circuits. The number ofcircuits to be provided will depend upon the one-way and total traffic.The total traffic will need to be determined for:a) each direction of traffic;b) both-way traffic.This determination is to be made for the busy hour or the busy hours corres
16、ponding to the two cases a) and b)above.In the cases where the number of one-way circuits is approximately equal for each direction, no specialprocedure is necessary, and the calculation can be treated as for a simple two-group grading 1.If the number of one-way circuits is quite different for the t
17、wo directions, some correction may be needed forthe difference in randomness of the flow of calls from the two one-way circuit groups to the both-way circuit group.The general techniques for handling cases of this type are quoted in Recommendation E.521.ANNEX A(to Recommendation E.520)Table A-1/E.52
18、0 may be applied to small groups of long intercontinental circuits. The values in column 2 aresuitable for a random offered traffic with full availability access.The table is based on 1% loss probability for 20 circuits and increases progressively to a loss probability of 2%at 9 circuits and 3% at 6
19、 circuits (loss probabilities for these three values being based on the Erlang loss formula: seeSupplement No. 1). The traffic flow values obtained from a smoothing curve coincide very nearly with thosedetermined by equal marginal utility theory, i.e. an improvement factor of 0.05 Erlang for an addi
20、tional circuit.For groups requiring more than 20 circuits the table for loss probability of 1%, mentioned in Supplement No. 1,should be used.Fascicle II.3 - Rec. E.520 3TABLE A-1/E.520NumberTraffic flow (in erlangs)of circuitsOffered CarriedEncounteringcongestion(1) (2) (3) (4)6789101112131415161718
21、19202.543.133.734.354.995.646.316.997.678.379.089.8110.5411.2812.032.473.053.654.264.905.556.216.887.578.278.969.6910.4211.1611.910.080.090.090.090.090.100.100.100.100.110.110.110.110.120.12Reference1 TNGE (I.): Optimal use of both-way circuits in cases of unlimited availability, TELE, English Edition. No. 1, 1956.
