X STM-1) ATM boards 4 X STM-1 per board, 16 boards per sub-rack 1 X STM-1 per board, 16 boards per sub-rack Figure A4.9 Calculation of the ATM interfaces on the network side To calculate the corresponding number of channels interfacing the backbone network, we divide by the utilization factor (u) : N X at (1 - pV) To convert the circuit-switched voice traffic, in Erlang, into packet-switched traf- fic (bV = bandwidth for voice), expressed in Mbits/s, we can use the formula: bV = Channels X 0.064 X 1.13. Data: if we assume that a certain portion of the data traffic (pD) will be switched internally in the MSG, the traffic that continues out in the backbone network can be expressed as bD = D X (1 - -pD). Note. As the voice and data traffic can be mixed in the backbone network, we add together the required bandwidth for voice and data (bT = bV + bD). Dividing the total bandwidth (bT) by 135 (payload) gives the number of STM-1s, and dividing by 4 gives the number of boards (4 X STM-1) needed on the network side. Even if the result is one board, we should use two boards for reliability reasons. CE/ATM Interfaces in an MSG - Summary To calculate the number of sub-racks we need, we simply add together all the different interface boards we have calculated above and divide by 16. See Figure A4.10. We have now calculated how many sub-racks we need, from an interface point of view. We also need to check the processor capacity in the MSG. Numerical Example In order to try to make things as clear as possible, we will illustrate the calculations with a numerical example. See Figure A4.11. Appendix 4: Dimensioning Media Gateways and Associated Telephony Servers CE/ATM-interfaces in an MSG Summary No. of boards CE voice ATM data - access side ATM data - network side ATM voice Total: Interface boards Voice 16 boards per sub-rack Figure A4.10 Calculation of total number of interface boards Voice 900 Mbit/s 4000 erl Assumptions Voice: 50 000 subs (200 E1) 0.10 erl./subs (tot.) 20% MSG-internal 80% utilization 120 S mht Data: 1500 Mb