Performance on Geostationary Satellite Links - - Lessons Learned Using Acts

The ACTS system has provided the opportunity to closely study the operation of various data communications protocols at speeds of up to 1.536 Mbps (Megabits per second). We performed performance measurements using the TCP/IP protocol suite, particularly the application layer implementations of FTP. In a separate experiment we studied the SNA/SDLC protocol in Normal Response Mode. Our observation of TCP/IP confirms the need for an expanded window size in the flow control mechanism; we also observe that the "Slow Start" algorithm defined in TCP for congestion control purposes plays a significant role in limiting the throughput achievable with TCP/IP. We further report on strategies designed to overcome these protocol limitations without modifications at or below the transport layer. Our study of SDLC suggests a response time degradation of at least two round-trip delay times when a terrestrial circuit is rerouted over the geosynchronous satellite link. Our results permits network designers to assess the applicability of satellites to disaster recovery situations (were some response time degradation is both expected as well as accepted). These results also point towards a series of steps that can be taken to improve the performance of predominantly terrestrial protocols over satellite links.