com Business Transit switch Number of ports = Asw2/u = Call intensity (y) = Asw/s Asw is the traffic switched through GS is utilization of trunks (80%) is mean holding time Figure A4.3 Traffic flows through a transit switch ATM AAL1 is assumed, which means 64 kbit/s or 0.064 Mbits/s bandwidth per voice channel. As we have seen in the previous figures, the number of voice channels that we need in the circuit-switched network can be calculated by dividing the traffic (Erlang) with the utilization factor (u). Mapping (emulating) time slots into ATM cells means that we have to add a 5 + 1 octets header information into each 53 octet ATM cell, which corresponds to roughly 13 % overhead. Thus we get the conversion formula: Total traffic = Traffic/u X 0.064 Mbits/s X 1.13 traffic/u X 0.0722 Mbits/s. When using ATM AAL2 the actual voice compression and overhead ratio must be considered. The same type of formula is valid also for an IP backbone network. The main differ- ences are: The header in IP packets is much longer, especially in IPv6. When uncompressed, the overhead factor could be around 1.5 for IPv4 and 2 for IPv6. See Chapter 5, Section 5.7. If applied, header compression such as ROHC must also be considered. In an IP backbone network, voice compression will probably be used, and therefore the formula must include a voice compression factor. In an MSN we will have both voice and data traffic. Typically voice and data traf- fic have different busy hours. When dimensioning, it is necessary to find the peak for the total traffic. This is illustrated in Figure A4.4, which also shows that we can save network resources with an MSN compared with having separate networks for voice and data. Appendix 4: Dimensioning Media Gateways and Associated Telephony Servers Capacity with separate networks Capacity with MSN Required capacity Saved capacity Required capacity Voice Time of day Time of day Morning Afternoon Evening Morning Afternoon Evening Figure A4.4 Reducing required capacity with a multi-service network MSG Dimensio