Contents Telektronikk - Telenor

Number of type
2 connections
140
Feasible
state
for maximum cell delay is not so strict,
but the requirement for maximum cell
loss ratio is stronger than for telephony.
To cope with such problem QoS classes
(performance parameter classes) on the
cell level could be introduced. Otherwise
the network will have to be dimensioned
for the strongest requirements.
Some data services require large buffers.
A service like telephony which is delay
sensitive requires small buffers. Some
kind of buffer management scheme will
have to be implemented, possibly combined
with the use of separate physical
links in the network, to satisfy the needs
of both these services in the same network.
For a given buffer size the maximum link
allocation that the CAC can permit is
given by the cell loss requirements. This
maximum link allocation determines the
capacity of the physical link in terms of
the amount of bandwidth that can be allocated
to the virtual connections crossing
that physical interface. This then has
direct implication on the number of physical
interfaces needed towards a specific
destination (i.e. for the trunk group) to
satisfy the bandwidth needs determined
by the trunk group dimensioning.
3 The use of QoS classes
QoS classes may be introduced to cope
with different requirements for maximum
cell loss ratio and cell delay variation,
Legal transition
between states
Number of type
1 connections
Figure 1 Reversible transition diagram and coordinate
convex set of feasible states for an ATM link with 2 traffic
types
and may improve the utilisation of the
network depending on the traffic mix. A
way to achieve this is by use of the Cell
Loss Priority (CLP) bit in the ATM header
and some kind of buffer administration
that gives priorities to cells depending on
the value of the CLP bit. In this way we
will have two QoS classes, for example
with cell loss requirements of 10-9 for the
class with highest priority and 10-7 for
the class with the lowest priority.
The priority bit can be introduced at the
cell level so that cells belonging to the
same connection can have different priorities.
This has been suggested for video
services, but it seems not so much discussed
in standardisation bodies anymore.
The priority bit can also be used at the
connection level. This leads to a classification
of connections into two QoS classes
depending on their requirement for
maximum cell loss ratio. It is still unclear
how to use the CLP bit, and the first
switched ATM networks will probably
use other methods to distinguish between
QoS classes because it is possible to offer
more than two classes by using other
means than the CLP bit. One way is to
use for instance the VPI field combined
with some kind of buffer administration.
Another way is to use some signalling or
management message in the set-up phase
to be able to allocate the connections to
dedicated ATM links for the different
QoS classes or to enable buffer management
procedures to discriminate between
the different connections.
Different ways of dividing connections
into QoS classes have been proposed,
and it seems that at least three such
classes will be implemented in early
switched ATM networks:
- A QoS class with a requirement for
low cell loss (e.g. max 10-9 ) and high
CDV tolerance or no end-to-end requirement
on CDV (data applications).
- A QoS class with a more relaxed
requirement for cell loss (e.g. max
10-6 ) but with a low CDV tolerance
(voice and interactive video).
- A QoS class with unspecified values
(‘best effort’ and ‘low cost’ services).
The use of different links for the different
classes is only of interest if the offered
traffic is high enough to justify the reservation
of ATM links for the different
classes. Anyhow, QoS classes with different
requirements for maximum cell
loss will have impact on dimensioning
dependent on the way it is implemented.
If separate ATM links and buffers are
used with possible common overflow
links, we can dimension the amount of
resources needed for each QoS class separately.
In this case each connection
belongs to one QoS class, and the
requirement for maximum cell loss ratio
has implications for the maximum utilisation
of the ATM link, that is the maximum
number of simultaneous connections
on each link. The overflow links
will have to be dimensioned for the class
with the strongest requirements. If buffer
administration is used, this implies that
connections belonging to different QoS
classes may share the same ATM link but
experience different cell loss ratios. Calculation
of the admissible loading of the
links will then be more complicated and
is a matter for future research.
4 Dimensioning of trunk
groups in a complete
sharing environment
The task of this chapter is the dimensioning
of the transmission resources in terms
of the number of ATM links needed or
the total bandwidth needed between two
network nodes. The set of ATM links
between two nodes is termed a trunk
group.
In this chapter we will concentrate on the
general methodology for the dimensioning
of trunk groups. This methodology is
based on mathematical models that
approximate the statistical behaviour of
the traffic in the network. Inputs to this
problem are the services specified as traffic
types with certain traffic parameters,
the offered traffic and a connection
blocking objective for each traffic type.
We will be faced with the difficulty of
having services with different connection
blocking objectives or we have to provide
some service protection method (or
both). This problem will be treated in the
next chapter. We may also be faced with
the problem of having traffic components
with different QoS objectives even on the
ATM level (i.e. maximum cell loss ratio).
This problem was discussed in chapter 2
and will not be elaborated any further.
The connection blocking objective is the
connection blocking at the connection
level for a trunk group, defined as the
proportion of the offered connections that
are blocked. The connection is blocked if
the connection is not accepted by the
CAC function for any of the ATM links
in the trunk group. All blocked connec-