Contents Telektronikk - Telenor

Aspects of dimensioning transmission resources in B-ISDN
networks
BY INGE SVINNSET
1 Introduction
The complexity of current telecommunication
networks makes it very important
to have good network planning routines
to meet the demands of the customers
with the necessary investments. One
important part of this is the dimensioning
of the needed transmission resources, and
a subtask is the dimensioning of the number
of channels needed between each pair
of network switching nodes. This is what
we understand by dimensioning of trunk
groups. Input to this task is the forecast
traffic demands and the mapping of these
demands onto the network routes where
the various calls can be placed. This is
then again mapped onto the trunk groups
taking part in the route.
In telephony a channel is a dedicated
physical resource that must be reserved
for a single call for the whole lifetime of
the call. In today’s digital networks these
physical resources are divided in timeslots
(Time Division Multiplexing,
TDM). Every Nth slot is reserved for a
single call, i.e. every Nth slot belongs to
one channel where N depends on the rate
of the physical medium, e.g. 2 Mb/s or
34 Mb/s.
In ATM a channel has a more complex
definition and the amount of resources
needed for one channel is not static but
depends on the needed data rate and the
needed quality of service. The data rate
of a channel may also vary in time. This
complexity gives an increased flexibility
and a possibility for saving capacity (statistical
multiplexing gain) and these are
also two of the reasons for the attractiveness
of ATM. But it also creates new
problems and new challenges for the network
planning task.
In this paper we will present some methods
for dimensioning of the required
capacity of trunk groups in an ATM network.
The task is to provide enough
capacity for the forecast traffic needs and
at the same time satisfy the given quality
of service constraints. A subtask of this is
to look for ways of dividing capacity between
different traffic types to obtain a
more effective utilisation of the network
resources. The intention of such a division
is to give a lower dimensioned
capacity than we would have obtained
without using such methods. This division
will have implications for the connection
admission procedure.
An ATM link (= transmission path in
[5]) is in this context a physical transmission
channel, characterised by a constant
bit rate (for example 150 Mb/s or 600
Mb/s) and extending between a pair of
transmission path connection end-points.
A virtual channel connection (VCC) is an
end-to-end virtual connection, set up between
two users and characterised by
traffic parameters (for example peak bit
rate and mean bit rate). A virtual path
connection (VPC) is set up between two
ATM switching nodes, between a user
and a switching node or between two
users. It can be characterised in the same
manner as a VCC. An ATM link can
contain one or many VPCs and a VPC
can contain one or many VCCs. For
more exact definitions and more information
about these concepts we refer to [5].
This paper considers only VPCs or
VCCs, i.e. we do not treat the problems
related to bundling VCCs into VPCs.
Except for the first part of this paper
where we treat the multi-dimensional
Erlang loss formula, we only treat linear
Connection Admission Control (CAC)
functions. The connection admission is
then based on the calculation of an equivalent
bandwidth for each connection.
Ignoring the Cell Delay Variation (CDV)
tolerance this equivalent bandwidth may
equal the peak bit rate or may be less
than the peak bit rate but higher than the
mean bit rate. Taking CDV tolerance into
account the equivalent bandwidth may
become higher than the declared peak bit
rate if the CDV tolerance is allowed to
take a high value.
2 Performance parameters
that impact
dimensioning
Performance parameters are divided into
direct and derived performance parameters.
Derived parameters such as
availability will not be considered here.
Direct performance parameters can be
divided into
- call/connection processing performance
parameters
- information transfer performance
parameters.
The connection processing performance
parameters concern the establishment,
release and parameter change of connections.
These are
- connection rejection probability
- connection set-up delay
- connection release delay
- in-call connection parameter change
delay.
The information transfer performance
parameters to be considered are
- cell error ratio
- cell loss ratio
- cell misinsertion rate
- severely errored cell block ratio
- cell transfer delay
- cell delay variation.
Of these performance parameters the two
parameters that have most impact on
trunk dimensioning are:
- connection rejection (blocking) probability,
and
- maximum cell loss ratio.
The connection blocking probability is
the probability that the network is not
able to deliver a connection between two
users due to lack of network resources or
due to errors in the network. A network
dimensioned for every possible circumstance
will be a very expensive network,
if possible to realise at all. For this reason
we need to set a positive value for this
parameter, a connection blocking objective,
taking both cost and user satisfaction
into account.
The connection blocking objective may
differ between the services. If no service
protection is used the high capacity services
will certainly experience a higher
probability of connection blocking than
the low capacity services. This can be
very unsatisfactory. Although it is not
necessary to offer the same maximum
connection blocking probability for all
services, it is necessary to use some kind
of service protection method to control
the quality of service. Service protection
methods should be used to dimension the
network with specified connection blocking
objectives for the different services.
Dimensioning should be made for busy
hour in the same manner as for traditional
circuit switched networks, but the busy
hour may be different for the different
services.
When it comes to the information transfer
performance, different services will
have different requirements with regard
to cell delay, cell delay variation and cell
loss ratio. For the exact definitions of the
information transfer performance parameters,
see [6]. Some services like telephony
are not so sensitive to cell losses,
but cell delay and delay variation must be
small. For data services the requirement
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