or data traffic through the backbone network, which means that the data traffic will put very little load on the processors. However, when using data services that are switched using SVCs, the data traffic must also be considered when checking the processor capacity. The call handling capacity of the processors is expressed in number of calls per second. The call intensity in the network can be calculated with the formula A = y S, where A is the traffic (in Erlang), y is the call intensity (calls/s) and S is the mean holding time. The processor has to handle all the originating and incoming calls. The overall possible load on a processor is normally expressed as a percentage of the processor total capacity. To estimate the loadability for traffic handling we have to take away the 'idle load' and the 'overload margin'. The 'idle load' is the part of the processor capacity that is used for running the operating system. The 'overload margin' is introduced to avoid instability problems that can occur if the processor is loaded close to 100 %. A small part of the processor capacity is needed for administrative tasks like O&M. This is called usage load. Many operators are also introducing an extra security margin called 'dimensioning factor', in order to avoid problems in case of extreme traffic situations. Typically, a traffic load of 90 % of the processor capacity is used for dimensioning purposes. Figure A4.12 illustrates the loads and margins. Impact of Voice Traffic Mix If we know how many milliseconds it takes for a processor to set up different types of voice calls, we can calculate the call handling capacity for a certain voice traffic mix. As an example, we assume a traffic loadability of 90 % for the CP in the telephony server. Let's also assume that we have a traffic mix with 10 % IN calls and 90 % ordinary ISUP calls. To set up an IN call takes more processor capacity than to set up an ordinary ISUP call. Let's assume that an IN call takes 7 ms in the processor and an ordinary ISUP call takes 2 ms. With the