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
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
PAGE2 Fascicle II.3 - Rec. E.520
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
18
Fascicle II.3 - Rec. E.520 PAGE1
10.54 10.42 0.11
19 11.28 11.16 0.12
20 12.03 11.91 0.12
Reference
[1] TÅNGE (I.): Optimal use of both-way circuits in cases of unlimited
availability, TELE, English Edition, No. 1, 1956.
PAGE2 Fascicle II.3 - Rec. E.520