Contents Telektronikk - Telenor

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QoS user
Service
catagories user
QoS
QoS characteristic
Service
provider
Figure 3.1 QoS user categories, characteristics and parameters
tecture focuses on how the QoS functionality
is modelled. In the QoS architecture,
QoS is the objective and the QoSservice
is the solution. The service in the
functional architecture is replaced by the
QoS-service in the QoS architecture. The
service parameters are replaced by the
QoS parameters. An optimum traffic
solution is related to the existence of an
optimum QoS architecture, i.e. an architecture
that gives the necessary possibilities
without being too costly. In the B-
ISDN standardisation, simplicity has
been a strong requirement. Focus on simplicity,
however, can lead to non-optimal
solutions.
2.3 Traffic-handling
functionality
The functionality for the allocation,
administration and reallocation of trafficcarrying
resources within the operational
QoS architecture, is denoted as traffichandling
functionality. The design objective
for the telecommunication services
providing system is the offering of
the defined set of services with a defined
QoS at a minimum cost. The QoS architecture
and the traffic handling functionality
are tools for reaching this objective.
The B-ISDN classification [8] of traffichandling
functionality is as follows:
- traffic control: the set of actions taken
by the network to avoid congested
conditions
- congestion control: the set of actions
taken by the network to minimise the
intensity, spread and duration of congestion.
In networks “up to LAN”, the traffic control
functionality was: switching, multiplexing,
routing, access, priority control
and ack-based flow-control. Traffic handling
functionality for high capacity networks
has been a topic for discussion for
several years. The traditional ack-based
stop-and-wait and sliding window flow
QoS parameters
QoS requirement parameters
QoS data parameters
control mechanisms are in the general
case insufficient. With high-capacity networks
came transport protocols with
rate-based flow-control. In this case the
number of information units over a predefined
time period is controlled, rather
than the number of outstanding acknowledgements.
With B-ISDN came concepts such as:
connection admission control (CAC),
usage/network parameter control
(UPC/NPC) and traffic shaping. CAC is
the set of actions taking place during call
set up in order to establish a traffic contract
and a connection, UPC/NPC is the
set of actions taken by the network to
monitor and control traffic, and traffic
shaping comprises modification of the
traffic characteristics. Network resource
management, priority control and feedback
control are also defined as part of
the B-ISDN traffic handling functionality.
Network resource management is
the allocation of resources in order to
separate flows according to service characteristics
and feedback controls are the
set of actions taken by the network and
by the users to regulate the traffic submitted
on ATM connections according to
the state of network elements.
3 The ISO/OSI QoS framework
The Basic Reference Model for Open
Systems Interconnection (OSI) defined in
ITU recommendations X.200 and
ISO/IEC 7498-1 provides a description
of a model and the activities necessary
for systems to interwork using communication
media. The ISO/IEC “Quality of
Service Framework” [27] will be a supplement
to the description of QoS contained
in this Basic Reference Model. In
addition to the definition of QoS-related
concepts, it contains the definition of a
model of QoS and the definition of QoS
parameter semantics.
The present version of the Quality of Service
Framework still has holes. The document
is formally and compactly written.
The following presentation in the Sections
3.1–3.4 will therefore focus on the
basic contents of this document rather
than details. Section 3.1 presents QoS
user categories, characteristics and
parameters, Section 3.2 QoS parameter
semantics. Section 3.3 presents the model
of QoS for OSI. Section 3.4 is not
directly related to the Quality of Service
Framework document, but discusses QoS
in existing layer-specific OSI recommendations.
3.1 QoS user categories, characteristics
and parameters
The concepts QoS user categories, QoS
characteristics and QoS parameters are
essential in the OSI QoS framework. The
relationships between these concepts are
illustrated in Figure 3.1.
A QoS user category is defined as “a policy
objective that leads to the identification
of a set of QoS characteristics”. The
basic idea is to identify various classes of
users and to define the QoS requirements
based on these classes. A QoS characteristic
is defined as “a quantifiable aspect
of QoS, which is defined independently
of the means by which it is represented
or controlled”. QoS characteristics are
intended to be used to describe the actual
behaviour of systems. A QoS parameter
is defined “as a variable relating to one
or more QoS characteristics, values of
which are conveyed between objects as a
part of a QoS mechanism”. QoS parameters
are classified as QoS requirement
parameters and QoS data parameters.
QoS requirements are a statement of
requirements for one or more QoS characteristics.
The defined fundamental QoS user categories
are: the secure, the safe, the time
critical, the highly reliable, the easy to
use, the extensible/flexible, the low cost
and the QoS monitorable/testable/auditable
user categories. Most of these categories
are self-explanatory. The QoS
monitorable/testable/auditable user category
is related to the requirements for
monitoring, testing and auditing of the
QoS for the provided service. QoS auditability
means that the service user must
be able to obtain verifiable evidence of
the QoS as actually provided and of the
cost incurred as related to the service
provided and to the quality of that service.
The requirement for audit of the
service provided may be met by the pro-