Contents Telektronikk - Telenor

Observed traffic variations and their influence in choice of
intensity measurement routine
BY ASKO PARVIALA
Abstract
In this paper the traditional understanding
of stabilities and predictability
of the traffic profiles and models
have been weighted against the observations.
In Helsinki Telephone Company,
several wide scale traffic measurement
studies were made in the
1980s, in order to expose how regular
the traffic variations are. These
measurements indicated that the conventional
understanding about the stability
of the repeating profiles, together
with the popular traffic models during
the busy hour, is dubious. This has,
even in the international standardisation,
caused a need to rethink the traffic
intensity measurement routines, as a
basis for the dimensioning of networks.
1 Measurements of traffic
characteristics and
dimensioning
The quality of telecommunications services
depends on the network bottlenecks,
whereas the overdimensioning of other
parts in the network does not improve the
service. Existing or expected bottlenecks
are revealed by measuring the real traffic.
Directly measurable are the quality characteristics
observable by the subscriber;
like congestion, waiting time, or failure
rate. This information is used to control
the offered service quality, but it is also
necessary for immediate measures, such
as fault hunting and repair. The quality
characteristics are non-linear: they deteriorate
fast as the load increases,
approaching the network’s capacity limits.
So, these are not usable in forecasting.
But the load, measured in erlangs, bits per
second, etc., is linearily related to the use
of the service, and thus forecastable, e.g.
by time series. Therefore, the network is
dimensioned for a dimensioning hour
intensity, calculated according to certain
rules from the measured peak intensities.
In determining the required number of
circuits or other capacities, a mathematical
model is used, which combines the
intensity, the nominal quality norm and
assumptions of statistical properties of
traffic (distribution of call intervals and
holding times), concerning the network
structure in question (single circuitgroups,
alternative routing, waiting
opportunities). It has been popular to
assume a statistical equilibrium and the
Poisson-distribution for the offered traffic
during the dimensioning hour. Many academical
theories have been created and
presented on this or a slightly modified
basis. How to constitute the dimensioning
hour has met with less interest. The total
service quality over a year is a result of
the representativity of the dimensioning
hour among all hours of the year, together
with accuracy of the model, and the nominal
congestion during that hour. It could
be presented in number of minutes in the
year without service, caused by the network’s
bottlenecks.
2 Measurements of
intensity
The measurement costs concern the
reserved equipment, the work of preparing
and starting the measurements, and
the logical handling, pruning and reporting
of the measurement data. A balance
will be searched between the costs of the
carried measurements and the disadvantages
caused by the intensities which are
not measured. Among the disadvantages
are unknown or unexpected breakdowns
of the service quality, and unoptimal augmentations
in the network.
There are two different approaches with
the following measurement routines:
- Scheduled routines which minimise
the measurement periods and the data
to be handled by concentrating on the
expected high value periods only. The
nominal congestion is kept low in
order to compensate the presumably
lost peak intensities left outside the
measurement periods. This also results
in a high dependency on the mathematical
model, and a high investment
level due to the loose dimensioning.
These routines are the traditional way
of dimensioning, trying to minimise
the use of separate measuring equipment,
which is expensive in use due to
lots of manual handling of data.
- Continuous routines which activate the
measurements all the time. The abundance
of data is pruned in the earliest
phase by searching the relevant peak
intensities. The nominal congestion
can be higher while no compensation
for lost peak intensities outside the
measurement periods is needed. This
results in smaller dependency on the
mathematical model, and in a lower investment
level due to the possibility of
focusing augmentation of the network
equipment correctly. These routines
are in use today, the measurements
using just the basic data produced all
the time by the exchange control processor(s).
The worst choice is to create complex but
inaccurate measurements, needing a low
nominal congestion. Even this choice has
been made by some operators.
Every circuit-group between two points
has its traffic capacity, which depends on
the number of its circuits. If it is a potential
bottleneck, it must be measured distinctly.
But if, instead of one, several circuit-groups
in parallel, or an overflow
cluster, or a network between two subscriber
groups, is considered, then the
traffic capacity of the studied circuitgroup
is no more constant, but lower,
depending on the traffic caused by other
circuit-groups. This complicates the
dimensioning and the traffic management,
calling for special measures by
measurements. Complications of this
kind are less common in a radial network
structure, there including the cases where
circuit-groups for reliability reasons are
divided into two similar groups with
symmetrically offered traffic.
In comprehensive telecommunications
courses is commonly taught that the
offered teletraffic consists of rather regular
variations, obeying annual, weekly
and daily rhythms, and random Poissonian
distribution during the dimensioning
hour, the busy hour.
If these assumptions are valid, it is sufficient
to measure the busy hour intensities
during a few days annually only – the
rest of the year can be ignored – and to
dimension by the Erlang formulae and
their derivatives. The elementary conceptions
in question concern the existence of
the year’s high season, its annual consistency,
the peak-hour’s consistency between
seasons, representativity of the
average day, the absence of peak loads
outside the day’s peak-hour, the statistical
balance and Poissonian distribution
during the peak-hour.
Both the rule to select the dimensioning
hour and the mathematical model originated
early in this century, from manual
exchanges, and communication needs of
the past society. They have sometimes
been called into question, but they have
been persistently favoured by some influential
organisations. The new telecommunication
networks, the new services
and the nearly total renewal of users,
have only to a limited extent influenced
the common views.
While the word “busy hour” has been
used ambiguously, it is avoided here by
talking about the peak-hour as a postselected
hour having the highest inten-
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