Contents Telektronikk - Telenor

Testing ATM switches
BY SVEINUNG O GROVEN
Abstract
The telecommunications industry is
presently witnessing the broadband
integrated services digital network (B-
ISDN) evolution. B-ISDN, and specifically
asynchronous transfer mode
(ATM) technology as the chosen solution
by ITU-T, may be looked back
upon in a few years as being elementary
and simple. Today this is definitely
not the case. Experience in traffic profiles
in the various public B-ISDN network
topologies is limited. With various
equipment and private networks
connected to the public network the
complexity will increase many times.
The question asked by many is what
traffic profiles will the future telecommunications
networks experience.
Some of these foreseen traffical scenarios
and the influence these will have on
traffic profiles are presented.
B-ISDN pilot projects are being carried
out world-wide. Benchmark tests
and performance evaluations are constantly
being made. However likely or
unlikely, various traffic models are
suggested used in these tests. Thoughts
around the validity of these tests are
brought forward.
In order to test the Ericsson ATM
switch, traffical switch performance
tests were carried out. The theory
behind these tests are outlined. Finally,
a comparison of theory and reality
based on practical test experience is
made.
1 Introduction
One of the main problems associated
with the dimensioning and testing of
ATM networks is the fact that the standards
are not yet completed. Experience
gathered through tests will aid the completion
of these standards, and that is
why these performance measurements,
and many other quality of service (QoS)
measurements, are so important. Of
course, before performance measurements
can be made, conformance and
interoperability testing must be completed.
Even these tests reveal inconsistencies
and misunderstandings of the standards
and equipment being developed.
Figure 1 shows an example of a simple
B-ISDN network. Even with such a
simple network configuration the profiles
of many different traffic sources are difficult
to predict. Bearing in mind that this
is an early configuration, measurements
made today and the traffic profiles experienced
at the measurement points of
interest will be altered as the number of
users, the amount of traffic, and the types
of applications evolve.
1.1 A simple network scenario
Figure 1 gives a simplified view of a possible
network scenario and some of the
measurement points of interest.
Network topologies are one of the main
factors which will influence the traffic
profiles seen at the measurement points
of interest. For example, whether a user
network interface (UNI) is connected to a
local area network (LAN) or is used for a
video on demand (VoD) distribution service
is of a major consequence to the
traffic profiles.
Standardisation bodies estimate the performance
objectives which should be
provided by the network, but the end user
and applications being run will determine
the quality required.
The main objective of the tests outlined
in this article was to estimate the performance
of the switch under test. Some
tests needed to be repeated many times,
requiring quick and simple test routines,
while others were simplified due to limitations
in the test equipment.
Private
Network
Private
Network
Operator A
UNI
UNI
ICI - Inter Carrier Interface
NNI - Network Node Interface
UNI - User Network Interface
Public
Network
1.2 Broadband services
Based on a basic knowledge of the wide
range and variety of broadband services
and applications being defined today, we
realise that future applications may only
be limited by our imagination. These
may exhibit totally different traffic characteristics
from those foreseen today.
Bearing this in mind, we also realise the
need to define reference points and standard
test suites for performing QoS evaluations.
In due time, experience will supply
the information needed to dimension
these networks properly.
1.3 ATM traffic
Various different traffic types have been
specified to date. We have ATM traffic
in its most simple form, Constant Bit
Rate (CBR). The Cell Transfer Delay
(CTD) and Cell Delay Variation (CDV)
of CBR traffic is of course very low (typically
some tens or a few hundreds of
microseconds) and peak rate policing is
an adequate mechanism for controlling
this traffic. Variable Bit Rate (VBR) traffic
may exhibit greater CTD and CDV
(in the order of milliseconds) and will in
addition require the use of a sustainable
cell rate policing algorithm and a maximum
burst tolerance. Two types of VBR
traffics are defined; real-time and non
NNI
ICI
Operator B
Public
Network
ICI
Public
Network
Figure 1 ATM broadband network and relevant performance measurement points
ICI
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