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ANNEX B
(to Recommendation Q.543)
An example of a methodology for measuring exchange capacity
B.1 General
The capacity of an exchange used for call processing can be measured in a
laboratory or in the field and projections can be made to predict the maximum
processing capacity of the exchange design for the configuration and load
characteristics involved in the measurements. This Annex serves as an example of
a methodology that makes it possible to measure the processing capacity of an
exchange for the configuration and load characteristics involved in the
measurement.
B.2 Theory behind the measurement method
The call handling capacity of a processor can be expressed in terms of the
maximum number of calls (or call attempts) which can be processed in a fixed
interval of time while meeting all service criteria. In normal conditions, the
work functions performed by a switching system processor can be divided into
three categories (one fixed level and two variable) as shown in Figure B-1/Q.543.
Figure B-1/Q.543 - T1102850-86
At normal loads, a linear relationship is usually observed between offered
load and processor utilization. However, at heavy loads, some system components
may become overloaded and this can be reflected in non-linearity in the processor
utilization versus load characteristic.
In the case of a single processor controlled system, Figure B-1/Q.543
represents the processing capacity of the exchange. In a multi-processor system,
the capacity is distributed among processors and the exchange capacity is related
to the system configuration and the exchange processing capacity is a function of
the processors involved in call handling functions.
As shown in Figure B-1/Q.543, the processing capacity of a processor is
divided between three elements:
1) fixed overhead related to mandatory tasks (e.g. task scheduling and
scanning);
2) call processing work (including traffic-related overhead tasks);
3) deferrable (base-level) tasks (e.g. routine maintenance).
The tasks which a processor executes are assigned to three levels of
priorities, base, medium and high-level tasks (see Figure B-2/Q.543 a) and Figure
B-2/Q.543 b)).
Figure B-2/Q.543 - T1102860-86
As the traffic load (call attempts) increases call processing work expands
and the processing of deferrable tasks decreases.
Measurement of the percentage of time spent by the processor performing
base-level tasks gives an indication of the percent or processing capacity
required for a particular load on the processor.
As shown in Figure B-2/Q.543 a), at low traffic load, the percentage of
time used to perform base-level tasks is relatively high. In Figure B-2/Q.543 b),
at high traffic load, the percentage of time at base-level is relatively low.
Thus the measurement of percentage of time used to perform base-level tasks can
be used to determine call processing capacity.
B.3 Capacity measurement methodology for exchanges
Measurements can be performed on exchanges in laboratories or in the field
to measure capacity usage for various load levels and then to project the data to
estimate the call processing capacity of a processor.
The collection of data will depend on facilities available to perform the
required measurements. The exchange may be designed to provide indications of
time spent performing base-level tasks or it may be necessary to access the bus
system of a processor in order to measure this time. Equipment will be needed to
create loads, or loads in a working exchange must be measured in order to
establish load points. Various level loads for the various types of calls (or
services) should be observed in order to establish a basis for projecting the
load line to determine the maximum processing capacity for the mix of traffic
services assumed or measured. In projecting call capacity care must be taken not
to extrapolate beyond the linear region of the processor utilization versus
offered call attempts relationship.
Where multi-processors are involved, the exchange configuration, the
Fascicle VI.5 - Rec. Q.543 PAGE1
distribution of traffic types and processing capacity of each processor must be
examined to determine the limiting factors that controls the exchange capacity
(as discussed in Annex A. An example of methodology for computing the call
processing capacity of a digital exchange, taking into account ISDN services,
including packet data handling).
Figure B-3/Q.543 - T1102871-89
PAGE2 Fascicle VI.5 - Rec. Q.543