Network Working Group Jon Postel
Request for Comments: 346 Computer Science
UCLA-NMC
- NIC : 10425 30 May 72
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- Categories : Echo Plex, Satellite
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- References : RFC's 1, 5, 51
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Satellite Considerations
- The consideration of using space satellite transmission links in the
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- ARPANET should be cause for some thought by the parties making use of
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- the network. The satellite transmission path will not necessarily affect
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- the transmission rate but it will affect the delay. The change in the
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- delay characteristics can be approximated by the change in path length.
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- Thus if the satellite is in synchronous orbit about 22,000 miles above
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- the earth, the path length is about 44,000 miles compared to (worst
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- case) 3,000 miles or about a 15 to 1 increase in path length and delay.
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- (The time for light to travel 3,000 miles is .016 seconds, to travel
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- 44,000 miles is .236 seconds.)
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- In the current (surface) ARPANET delays are such that interactive
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- servers with character-at-a-time remote echo are only marginally useful.
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- While I believe that this delay (unmeasured) is largely due to the host
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- systems, adding a half second transmission delay will cause these
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- marginal systems to become unusable.
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- Thought should also be given to buffer allocations. If a receiving
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- system allows only one line of text to be buffered at a time and
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- refreshes the allocation as each line is output to a human user there
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- will be at least a half second delay between the arrival of each line at
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- the receiving system. This need not be a problem until the speed of the
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- output device is above about 150 characters/second. This "small buffer"
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- problem can be expected to occur even with lower speed devices since
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- host delays are estimated to be in the range 0.1 second to 1.0 second.
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- I suggest that it is appropriate to resume a discussion of measures to
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- circumvent the difficulties brought about by these large delay
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- characteristics. Some areas of discussion could be: buffer sizes in
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- servers and users, echo plex techniques, moving part of the input
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- processing to the user system. If it is decided to move the echo plex
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- functions to the user system, it would be wise to try for a "standard"
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- package, thus reducing a M times N problem to a M plus N problem.
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- Please dig out and read RFC's #1 Crocker, #5 Rulifson, #51 Elie to see
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- some previous thinking about this type of problem.
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[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by BBN Corp. under the ]
[ direction of Alex McKenzie. 12/96 ]
[Page 1]
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