On the Web: TCP Over Satellite - The Final Frontier

Communication satellites have been in commercial use for more than three decades. Driving their early deployment were military communications , international telephony, and broadcast TV. Now these same satellites are being used to transport TCP/IP traffic between distant locations , and to offer Internet access. Satellites have thus become the celestial link of the global Internet, an " instant " infrastructure in the sky. Data communications over satellite has a clear appeal in areas where a terrestrial communications infrastructure is either not feasible or not present. Satellites also possess a natural broadcast capability that enables a single sender to direct a communications stream up to satellite, and then have it " reflected " down to a large downstream population. Many commercial Internet service providers now use satellite links in their networks as a less costly backbone link to land-based alternatives, especially when great distances or an ocean is involved, or as a means of bypassing a backbone or ISP network to deliver an advanced service to the edges of a network. Home Internet access via satellite is also now available: DirecPC, a satellite downlink, is capable of delivering Internet traffic at up to 400 kbps to the home PC. The rapid growth of satellite communications is evolving the TCP/IP protocol suite in positive ways. In particular, enhancements to the Transmission Control Protocol (TCP) to address the challenges of satellite transmission will benefit all high-bandwidth TCP communications. TCP, as you will remember, is the predominant unicast transport protocol used by Internet applications such as Telnet, FTP, and HTTP. 1 A TCP sender uses acknowledgments transmitted by the receiver to clock its sending rate and to ensure reliable data delivery. TCP also employs a window-based flow control mechanism to prevent buffer overruns in the receiver and network. The ability of TCP to maximize the link utilization of a satellite channel is being challenged by the inherent delays associated with space communications and some of TCP's own behaviors. In this column, we will help you understand the basics of using TCP for satellite transmission and describe the changes you can expect to see in the TCP protocol itself as a result of the increase in use of satellites for TCP/IP traffic. Satellite Basics A satellite acts as an overhead relay or repeater for communications between two geographically remote locations. As illustrated by the sample configuration in Figure 1, a router-1 is connected to …