Contents Telektronikk - Telenor

which typically lasts for 5 ms. This,
however, requires a co-operative flow
control between various protocol layers.
- No spacing. When a PDU (Protocol
Data Unit) arrives at the AAL, the
entire PDU is sent continuously, i.e.
cells are transmitted back-to-back.
This will cause compact peaks
(minibursts) in the cell pattern, shown
in Figure 3.6.
The latter obviously results in a much
worse traffic for the network to handle.
During the establishment of a connection
a maximum allowable peak rate is negotiated
between the network and the user.
This will usually correspond to something
in-between the two extremes
above. This negotiated rate rp will determine
the intercell distance
IC = r -1
P
Note that this must be the case for all the
patterns related to a source, irrespective
of the corresponding average load of the
pattern, as long as there is no mechanism
to inform the AAL whether the source is
in a “peak state” or not.
Depending on the “regularity” of the
equipment there may be slight variations
of the value of IC during a miniburst and
pattern.
3.2.2 Miniburst volume
The number of filled cells in a miniburst
is obtained by considering the following
items:
- The minimum Protocol Data Unit
(PDU) size the traffic is split into,
VPDU . For instance, keeping Figure 3.1
in mind, the FTP traffic over TCP/IP,
CSMA/CD (Ethernet) and Frame
Relay have a minimum PDU size of
1500 information bytes + 26 header
bytes given by the CSMA/CD frames.
- The segmentation and reassembling
strategy at each level in the hierarchy.
For instance, are small PDUs reassembled
to larger PDUs at any level?
Still keeping Figure 3.1 in mind,
Frame Relay has a maximum segmentation
size of 1600 bytes. Whether two
packets (PDUs) of e.g. 600 bytes are
reassembled to one Frame Relay PDU
or not, is a question of implementation
and may therefore not be generally
stated.
The V B may vary within a cell pattern.
How to use the Composite Source Model
to describe source types
- Fitting to measured user behaviour State models
Section 3.1.1 and Section 2.4.3
- Varying information transfer rate or activity level State models
⋅ distinct states Section 3.1.1
⋅ continuous Section 3.1.2
- Modelling measured data transfer volumes and anticipated State models
end-to-end protocol behaviour
Section 3.1.1, third example
- Packet formation, segmentation and reassembly Patterns
⋅ protocol stack
⋅ intermediate LAN
Section 3.2.2
- End-system and CPN performance and contention Patterns
Section 3.2.3
- AAL protocol behaviour – cell spacing Patterns
Section 3.2.1
3.2.3 Minibursts interdistance
The minibursts interdistance, IB , i.e. the
number of cell periods or duration between
start of minibursts depends on a
number of factors, as well. While the
minibursts volume is primarily dependent
on the behaviour of the protocols
involved, the miniburst interdistance is
primarily dependent on the rate of information
transfer and the speed of the protocol
execution.
- The rate of information which shall be
transferred. If, in a certain state, ai bit
per second is going to be transferred, it
is seen that this must correspond to the
ratio between miniburst volume and
inter-miniburst distance for the pattern
of this state, i.e.
ai = E(VB)
E(IB)
- The service rates of levels in the protocol
hierarchy which the PDU passes on
its way from the application to the end
level, may influence the inter-miniburst
distance. The service rate will be determined
by equipment speed, protocol
implementation, transmission speed in
a CPN (Customer Premises Network)
and similar. The minimum/ slowest of
these will be the dominate factor in
determining I B . Note that this factor
may determine the information transfer
rate, ai , as well. For instance, in a configuration
like the one in Figure 3.1,
the raw transmission rate of an Ethernet
is 10 Mbit/s which may be the limiting
factor for some services.
- In addition, contention in a CPN, e.g.
collisions in a CSMA/CD based LAN,
may cause variations in the interminiburst
distance and reduce the maximum
effective service rate. For
instance, the back-off retransmission
strategy in Ethernet may introduce
variation between Protocol Data Units.
In this context, however, this is significant
only for a heavily loaded network
with a high probability of collision. In
general, contention in a CPN may be
modelled by a variable inter-miniburst
distance IB within a cell pattern where
the degree of contention is reflected in
Var(IB ) and Max(IB ) - Min(IB ).
4 The synthesised traffic
generator
In the above sections, the more theoretical
and principal aspects of generation of
load for ATM measurement have been
dealt with. In this section a generator system
based on this principle will be presented.
In Section 4.1 its design is outlined
and in Section 4.2 the capabilities
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