sioning a single-service network. However, we will show in the following that the traditional traffic engineering principles will apply to a large extent also for MSN dimensioning. TASK 1: CALCULATING NUMBER OF INTERFACE BOARDS Theory Figure A4.1 also shows what type of input data we need in order to perform the dimen- sioning. For the voice traffic we need traditional input data such as number of subscribers, average traffic per subscriber during busy hour, and mean holding time. Appendix 4: Dimensioning Media Gateways and Associated Telephony Servers We also need to know how the traffic is distributed, i.e. how much traffic is internal in an MSG, how much traffic stays within a domain (controlled by one TeS), how much traffic is going between two domains etc. Local Switch Traffic Distribution The traffic flow through a local switch is defined as originating and terminating traffic on the access side, and incoming and outgoing traffic on the network side. If it is a combined local and transit switch it can also handle transit traffic. Traffic between subscribers connected to the same local switch is called internal traffic. A typical traffic distribution (in percentage) is shown in Figure A4.2. The traffic is expressed in Erlang, which is the product of call intensity and mean holding time (A = y x s). Transit Switch Traffic Distribution The required number of ports (input/output) in the group switch is related to the switched traffic and the utilization of the connected trunks. A typical utilization (u) of the trunks is 80 % (0.8 Erl/ch). As each call occupies two ports in the group switch, the number of ports in the group switch can be expressed as A X 2/0.8. The call intensity (y) can be calculated using the ordinary formula A =yxs. See Figure A4.3. Converting TDM Voice Traffic into Packets To calculate the resources needed in the backbone packet-switched network, we have to be able to convert circuit-mode Erlang traffic in the access network into packet-mode Mbits/s in the backbone network. The preferred bac