Cost and Traffic Analysis of Demand Access Satellite Networks

A method using a Markov process model is described and used to analyze the traffic capacity and call blockage probability of an SCPC (single channel per carrier) satellite network. The model is shown to be suitable for telephone traffic. The traffic density for certain networks whose states can be ordered in a linear fashion (birth/death process) is derived and shown to exhibit an Erlang B distribution. The application of the Erlang B distribution to PA (preassigned) and limited cases of FVDA (fully-variable demand access) and SVDA (semi-variable demand access) is demonstrated. The Erlang B equation does not apply when an SVDA network exhibits partial sharing of channels by different earth stations. An example of this is given and the capacity and blockage probability solved exactly by the use of steady state Markov statistics. The traffic capacity and blockage for a satellite with channel switching capability in the repeater is shown to be equivalent to an SVDA network. SCPC should be used in place of preassigned frequency division multiplex (FDM) channels whenever the traffic load of the earth station is very light. In this case it is better to use a few expensive SCPC channels than many less expensive FDM channels. FDM is probably better if two stations have traffic between them requiring more than 15-25 channels. A simulation program was written to determine the blockage probability for those networks too large to do any other way. The goodness of the random number generator was tested and the results of the simulation were compared to known solutions. Traffic data obtained from Latin American and Caribbean nations was used to estimate the number of satellite channels and earth station modems required and the cost of such a network. We have also presented as an example the improvement of SVOA over PA in the case of Jamaica where l/4th as many modems would be required. A rough analysis of earth station costs was made based upon information obtained from several companies. It is shown that a large part of the network cost is in the modems. We estimate the cost of a regional network for Latin America and the Caribbean to be about $25M. The earth stations represent about 40% of this cost, and 65% of the earth station expense is in the modems.