Contents Telektronikk - Telenor

The situation is familiar: Some
nice, new communication system
with fancy facilities is installed,
and everybody is happy. Until
some day the system response
gets slow, or blocking occurs
more and more frequently. Something
must be done, but what? In
a simple network it may be a
straightforward matter of adding
capacity, even though on the way
costly time is wasted. In the more
complicated systems diagnosis is
also more difficult. One may have
to do systematic observations and
carry out sophisticated analyses.
The problem is no longer that of
correct operation in accordance
with the functional design of the
system. It is rather a matter of
how to give service to many
uncoordinated users simultaneously
by means of a system
with limited capacity.
With the extremely large and
complicated telecommunications
networks of today two main considerations
may be pointed out: functionality and quality. An
important subset of quality characteristics is that of traffic performance.
A functionally good solution may at times be rather
useless if the traffic dimensioning and control are inadequate. In
this issue of “Telektronikk” teletraffic is chosen as the theme in
focus.
In the early days of telephony – around the last turn of the century
– users encountered blocking and waiting situations because
of shared subscriber lines, inadequate switchboard or
operator capacity, or busy or unavailable called users. Later,
trunk lines between switchboards became a concern, and the
introduction of automatic switches – for all their advantages –
stripped the network of intelligent information and control functions.
Many of today’s teletraffic issues were in fact present in
those early systems: shared media, limited transmission and
switching capacities, control system limitations and called side
accessibility. Like in the early days, blocking and delays result.
The first systematic studies of teletraffic were carried out about
ninety years ago. Several people initiated studies of telephone
traffic, using probability theory. However, it was the Danish
scientist A.K. Erlang who pioneered a methodical study that is
still fully valid. His main publications appeared in the period
1909 – 1926, with the most important contribution in 1917.
The state of the development of teletraffic theory today can be
illustrated in several ways. The main forum of contributions is
the International Teletraffic Congress (ITC). Since 1955 fourteen
congresses have taken place with increasing world-wide
participation. Only at the last congress in 1994 more than 1500
publication pages were presented. In addition, an impressive
number of regional and national conferences on the subject take
place. Many other telecommunications conferences include teletraffic
as part of their program, and standards organisations have
teletraffic on their agenda. Teletraffic theory is taught in many
Guest editorial
BY ARNE MYSKJA
universities, journal articles
abound, and numerous textbooks
have appeared. Queuing theory
and operations analysis are concepts
closely related to teletraffic
theory, but distinctions will not
be discussed here.
Traffic definition in itself is
extremely simple. The instant
traffic value at a certain point in a
system is simply A(t) = i(t),
0 ≤ i ≤ n, where i is the number of
occupied servers among n accessible
servers at that point. The
mean traffic value A over a given
interval T is the time integral of
A(t) divided by T. Thus, a traffic
value is simply given by a number
with no denomination. Traffic
is created by calls (arrivals) and
service times, and the most basic
traffic formula is Little’s formula:
A = λ ⋅ s, where λ is the mean
arrival rate in calls per time unit
and s is the mean holding time.
This formula applies to any part
of a system or to the whole system,
and it is independent of distributions, so that the single
parameter A may often replace the two independent λ and s.
Given the simplicity of concept, why then the virtually endless
number of different cases and the complexity of problems? The
answer is best given by first assuming the simplest conditions:
time-invariant basic process, independence between single
events and fully accessible service system. This is one single
case, where only a small set of parameters is a matter of choice.
However, as soon as one or more of these conditions are
dropped, variations are virtually endless. Not only are the cases
numerous, also the analyses grow much more complex.
A question sometimes posed is: When electronics and software
tend to produce functions of control, switching and transmission
at a much lower cost now than earlier, would it be sensible to
avoid sophisticated dimensioning and simply add capacity to be
on the safe side? I readily admit that I find the question worth
considering. Still, the proposition sounds like an echo. At each
new major step in the development of telecom networks the
focus of performance analysis has shifted. Up till now these
shifts have not led to loss of interest in the performance issue.
The increasing frequency of and attendance at teletraffic conferences
bear witness to the opposite. But there is not only that evidence,
there is also good reason behind. Simply trying to guess
the needs would in many cases lead to underprovisioning of
capacity with initial troubles and costly additions, or otherwise
to overdimensioning with unknown amount of unnecessary capital
invested. An interesting observation is that overdimensioning
very often went undetected since nobody ever complained!
My presumption is that one will always need to understand the
mechanisms and to carry out analyses of traffic performance,
whatever are the traffic types, the system solutions and the cost
of establishment and operation. There are no indications that the
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