Contents Telektronikk - Telenor

56
Architectures for the modelling of QoS functionality*
BY FINN ARVE AAGESEN
A service is the behaviour of some functional
capability. Quality of service
(QoS) is a measure of the relative frequency
of specific events within a service.
These specific events are used as
quality criteria for proper service functioning.
The functional architecture of a telecommunication
service providing system is
defined as the total set of functional elements
and the dynamic relationship between
these functional elements. This
architecture has both an operational and
a management part. The operational
architecture defines the primary functionality
related to the real-time handling of a
call, while the management architecture
represents the additional functionality
needed for the administration of the operational
functionality. A QoS architecture
is a view of the functional architecture,
focusing on traffic-resource-related
aspects. A QoS service is defined as the
QoS-related aspects of a service relationship.
An optimum traffic solution is related
to the existence of an optimum QoS
architecture.
This paper discusses state-of-the art for
QoS architectures for telecommunication
service providing systems. The ISO/OSI
and the ITU-F/ISDN QoS frameworks
are presented. Some reflections on the
ideal QoS architecture are also given.
Keywords: Computer-Communication
Networks, Network Architecture and
Design, Network Protocols, Telecommunication
Networks, B-ISDN, Quality of
Service.
1 Introduction
1.1 QoS in perspective
During the last few years, an increased
attention has been given to the concept of
Quality of Service (QoS). For telephone
networks and circuit-switched data communication
networks, QoS has always
been taken seriously. During several
decades, there has been a significant traffic
research related to the QoS aspects of
these systems, and the system design,
standardisation and planning cultures
have been QoS-oriented. The distances
* Part of the work reported in this paper
was done under contract with Telenor
Research as a part of the project “QoS
in Layered Architecture”.
between the traffic research, system
design, standardisation and traffic planning
cultures have been relatively short.
As the packet switching technology was
introduced in the 1970s, this co-operative
QoS-oriented culture was not inherited.
Considering the standardisation, QoS
concepts have been defined and discussed,
but many QoS-related items are
either “for further study”, “left to the network
service provider”, or not mentioned
at all. The driving force behind packetswitching
has been service- and protocol-oriented
rather than QoS-oriented.
There has been a significant traffic research
related to packet switching. However,
there are gaps between the traffic
research, the system design and standardisation
and the traffic planning cultures.
The B-ISDN specification and standardisation
process has shown similar symptoms.
It is well-known that ATM is much
more vulnerable to bad dimensioning and
non-optimal functionality than packetswitching.
There has been a considerable
traffic research related to the traffic
aspects of ATM. However, there has
been a gap between the traffic research
and the system specification and standardisation
cultures.
So, how come that QoS has not been
appropriately handled for packet-switching
and ATM? One obvious explanation
is that QoS is difficult to handle. The
appropriate handling requires a total view
of the user behaviour parameters as well
as an understanding of the various
aspects of the system functionality. One
other explanation is that the objective of
the protocol-related standards is to focus
on the interaction aspects between systems
and on the information that is
exchanged. QoS-related aspects can
therefore fall out of the standardisation
context. Protocol data units (PDUs) related
to traffic-resource allocation are of
interest, but locally applied functionality
for the allocation of traffic resources are
not. In a way QoS has not been important
enough. It seems now, however, that the
time is mature for the handling of QoS in
a more systematic way and from the view
of totality.
One reason for this increased focusing on
QoS is that the explosion in the use of
Internet has demonstrated the insufficiency
of QoS-less protocols in high load situations.
Internet is periodically so overloaded
that the network is practically
useless for planned professional work. A
second reason is the awareness of the
QoS-related problems faced in the ATM
standardisation. ATM is by politicians
and industry allotted the role of the backbone
of the future Information Super
Highway. QoS concepts are given much
attention in the ATM standards. However,
there is a lack of conclusions. If the
QoS problems of ATM are not solved,
ATM can adopt the nature of the connection-less
Internet or the circuit-switched
X.21, with upgraded bandwidths. A third
reason for the increased focus on QoS is
the growing number of applications with
well-defined QoS requirements. The
obvious inadequacy of the OSI transport
layer to handle the well-defined QoS
responsibility has stimulated the QoS discussion.
The new evolving set of applications
are represented by the “multimedia
culture”. This culture, seeing the communication
system from above, has also
contributed with their own solutions to
many of the non-solved problems of
packet-switching and B-ISDN.
So it seems that time wants QoS solutions
rather than “for-further-study” classifications.
The QoS-oriented culture that
has been typical for the circuit-switched
communication era may get its renaissance
in the high capacity and multimedia
communication era.
1.2 Outline of this paper
A summary of state-of-the art for QoS
architectures for telecommunication service
providing systems is given. It will
be focused on the QoS aspects of the primary
operational functionality of a system
in a proper functioning state.
Section 2 gives a presentation of generic
concepts. Concepts such as service, QoS,
QoS service, QoS architecture and traffic
handling functionality are defined.
Section 3 presents the ISO/OSI view of
QoS, which is defined within the framework
of the hierarchical OSI model. The
OSI QoS Framework that is now under
work is a valuable contribution to the
introduction of QoS in packet-switched
and ATM systems. However, so far, the
framework is preliminary, and the intention
must be followed up in the specific
standards. Section 4 presents the narrowband
and broadband ISDN views of QoS,
defined within the framework of a usernetwork
relationship.
In Section 5, the characteristics of an
ideal QoS architecture is discussed. An
application-oriented QoS service is an
important element within an ideal QoS
architecture. Section 6 gives summary
and conclusions.