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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation E.541
OVERALL GRADE OF SERVICE FOR INTERNATIONAL CONNECTIONS
(SUBSCRIBER-TO-SUBSCRIBER)
1 Introduction
1.1 The overall grade of service (subscriber-to-subscriber) on international
connections - relating only to the phenome n the
entire network as a result of the traffic flow - depends on a
number of different factors, such as the routing arrangements in
the national and international parts of the connection, congestion
allowed per switching stage, the methods used to measure traffic
and compute the traffic base, and the time differences between the
busy hours of the various links
involved in the connection.
1.2 The most satisfactory way in which this grade of service could
be described would be to give its distribution. The design average
grade of service during the busy hour of the complete connection
would be the most useful single parameter. However, until such time
as continuous traffic measurements are carried out during the busy
season in all parts of the network on a routine basis, it is not
possible to compute this average grade of service. Therefore, at
this stage it cannot be used as a criterion for the dimensioning of
the network.
1.3 The only practical way of ensuring an acceptable overall grade
of service on international calls is to specify an upper limit on
the design loss probability per connecting link in
the national network as is done for the links in the international
network (see Recommendation E.540).
2 General considerations
2.1 Since the success of the international automatic service is highly dependent on the grade of service of all
links involved in the connection from subscriber-to-subscriber, it
is desirable that the originating and terminating national network
involved in the connection has grade of service standards comparable with those of the international network.
2.2 It is especially important that the links in the country of
destination should have a good grade of service for handling the
traffic, since high congestion in the terminating national network
could have serious effects on the international network. High congestion in the network of the country of
destination causes added retrials with consequent increased loading
on common switching devices as well as increased occupation of the
routes with ineffective calls.
3 Design objectives
3.1 It is recommended that the links in the national network should be
designed for a loss probability1) not exceeding 1 per cent per link in the final
choice route during its applicable busy hour. It is recognized, however, that in
some countries additional congestion is permitted for the internal switching
1) The loss probability mentioned refers to busy hour traffic values as defined in
Recommendation E.500.
Fascicle II.3 - Rec. E.541 PAGE1
stages of the transit exchanges . It is also recognized
that, where this recommended grade of service is not provided for
the national service, it may not be economically feasible to
provide it for international relations.
3.2 The maximum number of links in tandem used by an international
connection is defined by Recommendation E.171 [1].
3.3 Although the worst overall grade of service would be
approximated by the sum of loss probabilities for individual links
connected in tandem, on most calls the overall grade of service
will be significantly better.
PAGE4 Fascicle II.3 - Rec. E.541
4 Maximum traffic loading
4.1 An acceptable automatic service on a final circuit group is difficult to
maintain traffic loading on the group exceeds a
level corresponding to a calculated Erlang grade of service of 10
per cent. Beyond this traffic loading, service on the route may
rapidly deteriorate. This condition will be accentuated under the
cumulative effect of repeat attempt calls if these should occur.
4.2 The curves of Figure 1/E.541 indicate the proportionate
reduction in circuits that may be tolerated for a short period, 15
minutes for example, under normal busy-hour conditions, on a
full-availability circuit group dimensioned for 1 per cent Erlang
loss, in accordance with the above traffic overload criterion.
Table 1/E.541 gives the figures used to plot the curves.
Figure 1/E.541 - CCITT 48110
TABLE 1/E.541
Percentage reduction in the number of circuits if the calculated Erlang grade of service
is not to exceed 10 %
If originally operating at 1 % congestion, %
reduction
in circuits allowed to yield 10 % congestion
Number of circuits Random traffic Peakedness factor = 2.5
(peakedness factor =
1.0)
5 37.7 -
10 32.3 40.2
20 27.2 33.3
30 24.8 30.1
50 21.7 26.5
100 18.3 22.4
150 16.7 19.7
4.3 The curves of Figure 1/E.541 are intended merely as a guide. If the
breakdown occurs during an exceptionally busy hour, the permissible proportionate
reduction will be less. Conversely, if the breakdown occurs during an hour of
light traffic, a higher proportionate reduction in circuits could be tolerated. A
higher reduction might also be acceptable after an appropriate oral announcement
has been introduced. In the general case, a knowledge of the circuit occupancy
will enable an estimate to be made of the prevailing Erlang loss figure with the
reduced number of circuits.
The permissible reduction in the case of large groups should
not be exceeded; otherwise very serious congestion can result from
repeated attempts.
5 General notes
Note 1 - Teletraffic implications for international switching and
operational procedures under failure of a transmission facility are discussed in
Supplement No. 5 of this fascicle.
Note 2 - Al e routing in the national and
in the international networks provides on average a grade of
service that is better than that provided in the theoretical final
route.
Note 3 - Non-coincidence of traffic peaks in the national and
international networks will provide reduction in the overall grade
of service compared with the sum of the design grade of service
values per link.
Note 4 - Time differences will also improve the resulting
grade of service.
Note 5 - The methods of measuring and calculating the traffic
base for provisioning purposes in the national networks may be
different in various countries and differ from the methods for the
international network given in Recommendation E.500. This means
that the national traffic values are not always comparable among
themselves or with the values of the international network. Each
Administration must estimate how its design traffic level compares
with that recommended for the international network.
Note 6 - The design grade of service value of each link will
Fascicle II.3 - Rec. E.541 PAGE1
only apply if the traffic at each switching stage is equal to the
forecast. In practice, such a situation will seldom occur.
Furthermore, the planning procedure normally is such that the
specified grade of service should not be exceeded until the end of
the planning period. In a growing network, this means that the
circuit groups during almost the whole planning period give a
better service than the specified critical standard.
In conclusion, the overall grade of service depends on the
accuracy of forecasts made and the planning procedure used, i.e. it
depends on the interval between plant additions and on the specific
traffic value in future to which the grade of service is related.
Reference
[1] CCITT Recommendation International routing plan, Rec. E.171.
PAGE4 Fascicle II.3 - Rec. E.541