ation of the number of calls/s that the processor has to handle, we will use the originating traffic per subscriber. For simplicity reasons we assume that the originat- ing and the terminating traffic are equal. Thus, if we have the total average traffic per subscriber available, we simply take half of this value as originating traffic. We also assume that the outgoing and incoming traffic (on the network side) are equal. Then the incoming traffic can be expressed as: N X ao (1 - pV). See Figure A4.15. If we add the originating (N X ao) and incoming traffic together, we get the formula: N X ao (2 - pV). If we then divide this traffic by the mean holding time we get the number of calls/s. The result of this calculation can then be compared with the value 55 calls/s for one processor pair, and we can conclude how many sub-racks we need, from the processor point of view. Figure A4.15 includes an example where some assumed input data values have been inserted into the formula. The example shows that, with the assumed input values, the processor is not any limitation if the MSG is equipped with E1 interfaces. If STM-1 interfaces are used, the processor puts a limit at approximately four STM-1 interfaces per sub-rack for a MSG with one CP pair. Furthermore, the maximum number of E1s will depend on the ratio between intra- and inter-MSG traffic. Note: with higher mean holding time values, more STM-1 CE (or Els) boards can be supported. Access Signalling and Call Distribution Impact on MSG Workload What has been shown SO far is of course a simplified example. In reality the calculations are influenced to a large extent by factors like different access methods (ISUP, DSS1, V5.2 etc.) and the distribution of the calls (intra-domain, inter-domain, intra-MSG etc.). N X ao(1-pV) N = No. of subscribers Network side N x ao(1-pV) ao = orig. traffic/sub (orig = term) u = utilization per speech channel pV = MSG-internal voice (s) sV = mht for voice (s) Data traffic = D (Mbit/s) pD = MSG-internal data (s) Access side D (Mbits/s) Vo