QoS of a multimedia interactive satellite communications system, which uses a satellite circuit for downlinks and a terrestrial network for uplinks, has been measured and evaluated under inferior satellite circuit conditions. Our attention in this paper is focused on TCP throughput as the most significant QoS, and experiments and analysis on TCP throughput were performed. The measured results are in good agreement with theoretical values calculated by taking congestion avoidance algorithms of TCP into consideration. As a result, we were able to verify that the TCP throughput declines radically when the low signal quality of a satellite circuit causes packet retransmission to occur. In this case, congestion avoidance works to reduce network congestion. The congestion avoidance increases the value of a retransmission timer and decreases the window size even if the network is not congested. Assuming that throughput deterioration is caused by rain fade, the total time when the throughput declines is less than 4 hours a year in Tokyo, and the results show the system can be put to practical use.

Recently, computer communications, especially Internet services, have become popular and as a result, high-speed network access circuits are now desired. NTT has developed an economical and high-speed multimedia computer network, combining satellite and terrestrial circuits. The satellite circuit transmission rate is approximately 30-Mbit/s. To select IP packets from such high-speed satellite circuits, this system employs the asynchronous transfer mode (ATM) in the satellite section and we have developed a new economical satellite circuit receive adapter (SRA) for the satellite section. This paper describes the system configurations and the key network control technologies for multi-link routing, high speed processing and broadcasting.