Satellite Communications Problems Are Solvable
“Satellite Firms Dealt Blow on Internet Plans” (Oct. 3) made a significant error when it stated that “NASA scientists have discovered that a key transmission standard that is the foundation of communications over the Internet and corporate communications networks does not work well in space.”
The article was referring to the Internet’s Transmission Control Protocol (TCP), which has acquired a reputation of not working efficiently with satellites over the past 15 years. This is hardly news at all and certainly not a recent NASA discovery, as the article states.
Because of improvements in technology, notably in the area of error control, I believe that TCP can become satellite-friendly and that technical approaches to that end exist even today. The challenge is to convince people that any new version of TCP benefits everyone and does not diminish any of the characteristics that have made it so successful.
Technically speaking, TCP is inefficient with systems that have a high product of bandwidth and signal delay. Geostationary satellites fall into this class of systems ecause of their inherent signal delay. One motivation for wide acceptance of a new satellite-friendly TCP is that the massive growth of the Internet has led to the installation of high-bandwidth ground lines that also have a high product of bandwidth and delay.
Saying that TCP does not work efficiently with satellites is not the same as saying that Internet access cannot be provided over satellites. Satellite Internet systems are available to consumers today that work around the inefficiencies of TCP.
I was not the source for the unattributed material presented in the first four paragraphs of your article. I believe that the article has overstated the problem and that solutions are available.
DANIEL R. GLOVER
NASA Lewis Research Center
Cleveland, Ohio
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The notion that satellites cannot carry Internet traffic is erroneous. In fact, from the very earliest implementation of the Internet, when it was still the U.S. Department of Defense ARPAnet project, the first international traffic was carried by means of a regular geostationary Atlantic Ocean satellite. Satellites continue to carry Internet traffic around the world, including Intelsat’s global fleet of 24 satellites, which currently carry Internet traffic using the regular TCP protocol standard.
Experimental and theoretical work at Intelsat confirms that satellites are fully capable of supporting computer networking applications, including, among other applications, Internet and LAN-to-LAN communications at a level far superior to that actually realized in many optical fiber networks.
In practice, regular TCP via satellite, using the maximum receive window buffer size, can realize an information rate of approximately 860 kbps for a single session. There is absolutely no reason why multiple sessions cannot be bundled together into a larger satellite carrier in much the same way as multiple telephone calls are bundled together into trunk circuits. In addition, the unique broadcast ability of satellite technology makes it an ideal platform for multi-casting Internet applications.
M. HALSEY
Senior Communications
System Engineer
Intelsat
Washington
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The story contained an excess of conjecture. I know this because I spent last summer as a graduate student in the field of telecommunications, researching the Internet protocol, TCP/IP and its performance over geostationary satellites.
There is a propagation delay issue, but it is by no means “wreaking havoc” and is definitely a problem that is surmountable.
There are several modifications to TCP that address the delay issue that have already been adopted by the Internet Engineering Task Force. The real issue is convincing PC and software companies to implement these modifications. In that effort, the Internet and satellite industries are of the same mind because high-speed terrestrial networks can also stand to benefit from the modifications.
Apart from the protocol issues, the future role of satellite technology in the global infrastructure is very promising.
The delivery of Internet services via satellite has certain advantages over “wire-laden” technologies. The benefits of satellite usage include the flexibility to deliver high-speed Internet connections anywhere on a moment’s notice coupled with the high availability of dedicated bandwidth to carry that traffic, either as a backbone between Internet service providers or directly to the end user.
KAREN HANSEN
Athens, Ohio
