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4.2.6 Load extremes
By specifying patterns where all cells are
assigned, a 100 % load is obtainable.
This is of interest as a debugging feature.
For traffic experiments, the maximum
load is defined by the user, and may
exceed 100 % and cause internal cell
losses. The superposition of the traffic
from the individual sources in a scenario,
Section 2.4, may be regarded as a multiplexer
with a limited buffer size. Cell
losses, internal to the generator, have no
effect on measurement accuracy, as
sequence numbering and time stamping
etc. is done in the subsequent test cell
module. The internal loss is logged and
reported at the MMI.
The lowest periodic traffic the user can
specify is about three cells per second.
This is attained by the use of maximum
long patterns and a one state source. Non
periodic traffic has no practical lower
bound – one cell per fortnight without
activation and deactivation.
4.2.7 Activation and deactivation
In the hardware an activation or a deactivation
of a source represents a workload
similar to a state change. The instantaneous
capacity is one activation – deactivation
per 12.5 µs. Requests exceeding
the instantaneous value will be buffered
and the generation of traffic will deviate
slightly from what the user has specified.
Due to the operating system, no hard real
time requirement can be met for the acti-
Set-up sequence
Steady state
Figure 4.5 Example of state model with
a set of initial transient states
vation – deactivation. An activation –
deactivation operation takes place within
0–250 ms. The delay is very close to the
lower bound when the system is lightly
loaded. During manual load control, this
inaccuracy is less than the timing precision
of the operator. A change from 0 to
2047 active sources can be undertaken
within a couple of seconds. This is
beyond the expected rate of load change
at the connection level.
4.3 Extended functionality
The sections above have described the
current functionality of the STG. This
was defined a number of years ago, and
advances within the ATM standardisation
and measurement technology invite
additional functionality. The rest of the
section describes such functionality.
Some of it is a rather simple task to
implement, other parts are quite demanding,
and advanced functionality is still at
the research stage.
4.3.1 Connection level
The STG was designed before any signalling
standard for ATM systems had
emerged. Hence, it lacks the possibility
to signal with the target system over the
ATM link. In the future, the ease-of-use
of the generator will increase with the
capability of signalling.
If such signalling is going to be implemented,
it invites a more advanced connection
level. A statistically defined
activity on this level, modelling the connects
and disconnects of the sources, may
be introduced. Furthermore, measurements
may be performed on the connect
– disconnect activity to determine
performance measures like set-up delay,
rejection probability, etc.
4.3.2 Basic functionality
A memory traffic generator, see Section
2.2.1, can be made as a sub-set of
the STG functionality. This will give the
user a possibility to configure some of
the STGs as a memory based generator
with a defined sequence, nearly without
any additional hardware. A simultaneous
STG and memory based generation is
also feasible.
The STG may be used to introduce delay
and cell loss impairments caused by a
finite or infinite buffer in a live cell
stream. Hence, an STG may be used to
simulate the effect of a connection, e.g. a
video transmission, caused by interfering
traffic in the network. The interfering
traffic is defined in the STG. A feasibility
study shows that this functionality is
less hardware demanding/costly than the
current generation functionality.
Since the STG originally was designed as
a module in the PARASOL measurement
system very little measurement functionality
is included. This may, however, be
done at a moderate cost. Such functionality
comprises:
- Sequence numbering of outgoing cells.
This may be done for all VCI/VPIs, or
for explicitly defined VCI/VPIs.
- Timestamping of outgoing cells.
- Cell payload integrity control by CRC
or other schemes.
4.3.3 Advanced functionality
Cell losses should be extremely rare
events in ATM systems. An end-to-end
loss rate of 10-9 is a typical objective. In
spite of this, cell losses represent severe
service degradations, and measurements
of cell losses are important to validate
system performance. However, even real
time measurements in this range take
from days to months. Hence, if QoS
requirement in this range shall be investigated,
speed up techniques are necessary.
Another paper in this issue is devoted to
this topic [1]. Ongoing research on introducing
such techniques in an STG based
measurement set-up is reported in [36].
There are, however, still stones to be
turned before such techniques are generally
applicable.
In the current version of the STG, all the
cells from a source are identical. The
user may, however, be interested in
measuring specific events related to the
source, e.g. start and/or stop of higher
level data units. For instance:
- Start and stop of an application packet
or a transport layer packet
- Start of frame in a video connection.
During the modelling process, such
events may be recognised while defining
a pattern, cf. Section 3.2. The pattern
may be extended to a larger alphabet than
{0,1} and these events may be given a
unique symbol in the pattern and the correspondingly
generated cell tagged
accordingly. This will enable delay and
loss measurements associated with these
higher level data units.
Currently all sources in a scenario have a
steady state behaviour. In the STG, the
sources may be initialised according to