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