Contents Telektronikk - Telenor

76
cost increase which must be covered by
diminishing the radial (e.g. the lastchoice)
circuit-group. The calculation
methods to discover the optimum size of
the circuit-groups are mainly developed
under the assumption of the ElCos above,
e.g. the coincident peak-hours of different
traffic flows in a cluster, even assuming
corresponding regularities not only in
the individual traffic flows, but synchronously
in all flows belonging to the
cluster.
If the peak-hours are non-coincident, the
advantage gained from diminishing the
last-choice circuit is missed more or less.
In other words, the optimal size of the
first-choice circuit-group diminishes with
growing non-coincidence of peak-hours.
In order to control the dimensioning by
real traffic relations, the measurement
routines need different qualifications
from those of the single circuit-groups.
Dimensioning of the last-choice circuitgroup
defines the service quality of the
cluster. Due to overflowing, the traffic
distribution during the peak-hour is more
peaky in the last-choice circuit-group
than traffic offered to the first-choice circuit-group.
This should be made clear by
means of measurement routines and
modelling.
Dimensioning of the first-choice circuitgroups
defines the overflow and thus, the
economy of the cluster. Starting from
balanced cost, caused by carrying one
erlang alternatively by last-choice or
first-choice circuits, a Moe-minded
thumb-rule for dimensioning the firstchoice
circuit-group is created:
F(A) n = ε H,
where F(A) n = carried traffic caused by
the n’th first-choice circuit, ε = cost of
one first-choice circuit in units of the cost
per one last-choice circuit, and H = traffic
per last-choice circuit. Example:
ε = .5, H = .6, and F(A) n = .3 gives the
needed relations between A and n. – The
formula is valid as long as the augmented
first-choice circuit causes gain in the lastchoice
circuitry in question. This is not
realised in cases where, in spite of a
noticeable overflow, some last-choice
circuits have no load due to minor traffic
offered to them from other sources. This
relation should be clarified by means of
measuring and modelling.
Peak intensities in last-choice circuitgroups
can be clarified in two alternative
ways, namely by calculating from peak-
hour average by model or by measuring a
short peak period to be calculated further.
Calculating the peaks by some equivalence
rules and thereby the congestion, is
based on offered traffic having Poissonian
distribution. Such methods have been
developed by Wilkinson, Bretschneider,
Rapp and others. The method is mathematically
clear. In practice, it is ambivalent
when the offered traffic is more
peaky (see above) and the real distribution
cannot be studied case by case. The
method has been used in pre-selected
measurement hours, too. When omitting
the peaks outside the peak-hour and measurement
period, it gives an indefinite
picture of the service quality, and is not
usable in network structure optimisation.
Measuring the peaks during a period
shorter than one hour reveals higher
peaks, during which the distribution
approaches the Poissonian. Calculating
congestion leads to a better result. The
method is mathematically less defined,
but yields results with better applicability
through its diminished sensibility to the
model of offered traffic. Consequently,
the peaks outside peak-hours and the
daily variations are included thereby.
In dimensioning the first-choice circuitgroup,
the load during peak-hour of the
last-choice circuit-group is of most
importance to optimising; the first-choice
circuit-groups’ own peak-hours do not
influence the optimum.
Measurement routine of first-choice circuit-groups
can be created in two ways in
order to minimise the collected data,
namely by a synchronised or a softened
routine. Both of them yield the intensity
for the above mentioned thumb-rule
equation.
In a synchronised routine, the measurement
of the last-choice traffic is activated
continuously, but the first-choice is on
stand-by. The last-choice meter discovers
and reports the days’ peak hour intensities,
but the first-choice meters are activated
to report the coincident hour’s
intensity, not their own peak-hours. Such
a routine works in cases where the successive
last-choice circuit-groups’ peakhours
can be treated as one, alternatively
due to their peak-hours actually being
coincident, or while only one of the circuit-groups
is relevant (the others being
e.g. overdimensioned). The measurement
becomes complicated when the cluster
consists of several first-choice circuits
with non-coincident peak-hours, and several
last-choice circuit-groups are eco-
nomically relevant. Such a many-conditioned
synchronising is hardly realisable,
not even by post-processing the complete
profile data. – The synchronised measurement
can be regarded as theoretically
sound, but inadequate in practice for the
complicated clusters.
In a softened routine the measurements
of first-choice traffic aims at an intensity
value which at least will be reached during
the last-choice peak-hour with considerable
certainty. Instead of the firstchoice
peak-hour, it is aimed at a lower
value fitting with the last-choice peak
timing. One approximation of the
searched intensity might be an average
integrated over a period of a few hours.
Its length depends on the variations of
the day’s profiles and the seasonal variations,
and it should be fixed after wide
studies of real cluster traffic, non-existent
so far. A good guess of two to four hour
periods could be a starting point for the
studies.
10 Comparison of
measurement routines
in overflow clusters
The usability of some measurement routines
were studied in some existing overflow
clusters. The study object in
Helsinki local network was the 115 circuit-groups
in the Sörnäinen AKE-transit
exchange in May 1985, as described
above. The complete study is published
in references [7, 8].
Fifty-eight circuit-groups among the 115
were combined as overflow clusters. Half
of them were ignored in this study,
because the first-choice circuit-groups
were too loosely dimensioned to have
overflow of high congestion, but only of
a low congestion. The planners had
apparently overestimated the first-choice
traffic during the last-choice peak-hour.
Thus, a marked overflow was available
in three clusters only. Among them there
was a small cluster with simultaneous
peaks in different flows, and a big one
with non-coincident peak-hours. All the
17 circuit-groups in question were measured
in parallel using the three continuous
routines, namely TCBH, ADPH
and ADSI. ADSI (Average of Daily Synchronised
measured Intensities) routine
measures the first-choice circuit-groups
continuously, but is automatically activated
to report only during the peak-hour
in the common last-choice circuit-group,
revealed by its continuous measurement.
It was observed that