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- Recommendation E.520
- NUMBER OF CIRCUITS TO BE PROVIDED IN AUTOMATIC
- AND/OR SEMIAUTOMATIC OPERATION, WITHOUT OVERFLOW FACILITIES
- This Recommendation refers to groups of circuits used:
- - in automatic operation;
- - in semiautomatic operation;
- - in both automatic and semiautomatic operations on the same group of
- circuits.
- 1 General method
- 1.1 The CCITT recommends that the number of circuits needed for a group should
- be read from tables or curves based on the classical Erlang B formula (see Supplements Nos. 1 and 2 at the end of this fascicle
- which refers to full availability groups). Recommended methods for
- traffic determination are indicated in Recommendation E.500.
- For semi-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 the same
- 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 be some smoothing 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 on exceptionally
- busy days, it is recommended that the proposed number of circuits
- should, if necessary, be increased to ensure that the loss
- probability shall not exceed 7% during the mean busy hour for the
- average traffic estimated for the five busiest days as specified in
- Recommendation E.500.
- 1.3 For small groups of long intercontinental circuits with
- automatic operation some relaxation could be made in respect to
- loss probability. It is envisaged that such circuits would be
- operated on a both-way basis and that a reasonable minimum for
- automatic service would be a group of six circuits. A table
- providing relaxation in Annex A is based on a loss probability of
- 3% for six circuits, with a smooth progression to 1% for 20
- circuits. The general provision for exceptional days remains
- unchanged.
- For exceptional circumstances in which very small groups (less
- than six intercontinental circuits) are used for automatic
- operation, dimensioning of the group should be based on the loss
- probability of 3%.
- 2 Time differences
- Time differences at the two terminations of intercontinental circuits are
- likely to be much more pronounced than those on continental circuits. In order to
- allow for differences on groups containing both-way circuits it will be desirable
- to acquire information in respect to traffic flow both during the mean busy hour
- for both directions and during 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 number of circuits, in spite of the fact that this
- overflow traffic is of a peaky nature. Such circumstances may arise if there is
- no traffic overflowing from high-usage groups during the mean busy hour of the
- final group.
- 3 Both-way circuits
- 3.1 With the use of both-way circuits there is a danger of simultaneous
- seizure at both ends; this is particularly the case on circuits with a long
- propagation time. It is advisable to arrange the sequence of selection at the two
- ends so that such double seizure can only occur when a single circuit remains
- free.
- When all the circuits of a group are operated on a both-way basis, time
- differences in the directional mean busy hours may result in a total mean
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- Fascicle II.3 - Rec. E.520 PAGE1
-
- busy-hour traffic flow for the group which is not the sum of the mean busy-hour
- traffic loads in each direction. Furthermore, such differences in directional
- mean busy hour may vary with seasons of the year. However, the available methods
- of traffic measurement can determine the traffic flow during mean busy hour for
- this total traffic.
- 3.2 Some intercontinental groups may include one-way as well as both-way
- operated circuits. It is recommended that in all cases the one-way circuits should be used, when free, in preference to the
- both-way circuits. The number of circuits 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
- corresponding 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 special procedure 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 two
- directions, some correction may be needed for the 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.520 may be applied to small groups of long intercontinental
- circuits. The values in column 2 are suitable 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 circuits
- (loss probabilities for these three values being based on the Erlang loss
- formula: see Supplement No. 1). The traffic flow values obtained from a smoothing
- curve coincide very nearly with those determined by equal marginal utility
- theory, i.e. an improvement factor of 0.05 Erlang for an additional circuit.
- For groups requiring more than 20 circuits the table for loss probability
- of 1%, mentioned in Supplement No. 1, should be used.
- TABLE A-1/E.520
- Traffic flow (in erlangs)
- Number of
- circuits Offered Carried Encountering
- congestion
- (1) (2) (3) (4)
- 6 2.54 2.47 0.08
- 7 3.13 3.05 0.09
- 8 3.73 3.65 0.09
- 9 4.35 4.26 0.09
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- PAGE2 Fascicle II.3 - Rec. E.520
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- 10 4.99 4.90 0.09
- 11 5.64 5.55 0.10
- 12 6.31 6.21 0.10
- 13 6.99 6.88 0.10
- 14 7.67 7.57 0.10
- 15 8.37 8.27 0.11
- 16 9.08 8.96 0.11
- 17 9.81 9.69 0.11
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- Fascicle II.3 - Rec. E.520 PAGE1
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- 10.54 10.42 0.11
- 19 11.28 11.16 0.12
- 20 12.03 11.91 0.12
- Reference
- [1] T┼NGE (I.): Optimal use of both-way circuits in cases of unlimited
- availability, TELE, English Edition, No. 1, 1956.
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- PAGE2 Fascicle II.3 - Rec. E.520
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