The paper presents a study and a performance analysis of a modified version of the TCP (Transmission Control Protocol) over a GEO (geostationary orbit) satellite link. The round trip time (RTT) is above 500 ms. TCP has not been designed for these network characteristics: the high delay to receive acknowledgements decreases the performance and makes the quality perceived by the users really poor. The performance of TCP may be improved by properly tuning some parameters and modifying algorithms. The behaviour of the protocol with a modified slow start algorithm is investigated on the basis of previous work concerning variations of the buffer length and of the initial congestion window. Configurations that drastically improve the performance (measured by the throughput in bytes/s and by the overall transmission time) are proposed. A real test-bed, composed of two remote hosts connected through a satellite channel is adopted to obtain the results. The analysis has included both the single application case and the multiple application case, where several connections share the satellite link at the same time.
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