zation per speech-channel pV = MSG-internal, voice bV = Bandwidth needed for voice (Mbits/s) Channels to ATM = Nxatx (1-pV)/u bV = Channels X 0.064 x 1.13 (53/47) Voice Data: Data traffic = D (Mbits/s) pD = MSG-internal, data Total bandwidth on ATM network side: bD = Bandwidth needed for data (Mbits/s) bT = bV + bD (Mbit/s) STM-1's = bT/135 bD = Dx (1-pD) STM-1's/4 => number of ATM-boards (4 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 - 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 Te