lexed Els, we get the number of STM-1 interface boards by further dividing the number of E1s by 63. If we do not have any data accesses, we can have maximum 14 circuit emulation boards connected to each sub-rack. ATM Interfaces in an MSG, Access Side Estimating the data traffic on the access side is quite a tricky task. We will here make some assumptions in order to show some basic principles. See Figure A4.8. The interfaces for data traffic are usually STM-1, both on the access side and the network side. The data traffic is expressed in Mbits/s. The payload in STM-1, for data traffic, is roughly 150 Mbits/s. 5 Mbits/s consists of overhead information (out of 155 Mbits/s). The utilization of STM-1 is approximately 90 %, SO the dimensioning value of payload is = 150 X 0.9 = 135 Mb/s. Access side: we assume that the total data traffic is D Mbits/s. Dividing the total data traffic by the payload (135 Mbits/s) gives the number of STM-1s needed, and dividing the result by 4 (4 X STM-1 per board) gives the number of boards needed. ATM Interfaces in an MSG, Network Side On the network side the voice and data traffic are mixed together. Therefore the ATM interfaces on the network side are calculated by adding the traffic for voice (expressed in Mbits/s) and data. See Figure A4.9. Network side: Voice: if we assume that a certain portion of the voice traffic (pV) is switched internally in the MSG, the traffic that will continue out in the backbone network is N X at (1 - pV). AT-interfaces in an MSG Network side ATM for data, access side Data traffic = D (Mbits/s) STM-1s = D/135 Pay-load STM-1 = 155 X (261/270) = 149.8 Voice STM-1s/4 => number of ATM boards Access side ATM boards 4 STM-1 per board, 16 boards per sub-rack 1 X STM-1 per board, 16 boards per sub-rack Figure A4.8 Calculation of the data traffic on the access side Understanding Changing Telecommunications - Building a Successful Telecom Business ATM for voice and data, nework side Voice: N = No. of subscribers at = total traffic per subs. (orig + term) u = utili