1. In a communication system of the type wherein data processing machines in each of one or more user networks communicate through a TDMA common carrier including a satellite in earth orbit with at least one transponder and a plurality of geographically spaced earth station means, each for communicating with other of the earth station means in a TDMA format through the transponder, the improvement comprising: means for transferring TDMA frame timing signals from the common carrier to each of the user networks, said transferring means including, at each earth station, time base means responsive to TDMA frame timing signals for generating a pulse defining allocated capacity of a particular one of said user networks; and adapter means in each user network for synchronizing information transfer through the network in accordance with received TDMA frame timing signals.