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Network Working Group                           Jon PostelRequest for Comments: 346                       Computer Science                                                UCLA-NMCNIC : 10425                                     30 May 72Categories :  Echo Plex, SatelliteReferences :  RFC's 1, 5, 51                        Satellite ConsiderationsThe consideration of using space satellite transmission links in theARPANET should be cause for some thought by the parties making use ofthe network.  The satellite transmission path will not necessarily affectthe transmission rate but it will affect the delay.  The change in thedelay characteristics can be approximated by the change in path length.Thus if the satellite is in synchronous orbit about 22,000 miles abovethe earth, the path length is about 44,000 miles compared to (worstcase) 3,000 miles or about a 15 to 1 increase in path length and delay.(The time for light to travel 3,000 miles is .016 seconds, to travel44,000 miles is .236 seconds.)In the current (surface) ARPANET delays are such that interactiveservers with character-at-a-time remote echo are only marginally useful.While I believe that this delay (unmeasured) is largely due to the hostsystems, adding a half second transmission delay will cause thesemarginal systems to become unusable.Thought should also be given to buffer allocations.  If a receivingsystem allows only one line of text to be buffered at a time andrefreshes the allocation as each line is output to a human user therewill be at least a half second delay between the arrival of each line atthe receiving system.  This need not be a problem until the speed of theoutput device is above about 150 characters/second.  This "small buffer"problem can be expected to occur even with lower speed devices sincehost delays are estimated to be in the range 0.1 second to 1.0 second.I suggest that it is appropriate to resume a discussion of measures tocircumvent the difficulties brought about by these large delaycharacteristics. Some areas of discussion could be: buffer sizes inservers and users, echo plex techniques, moving part of the inputprocessing to the user system.  If it is decided to move the echo plexfunctions to the user system, it would be wise to try for a "standard"package, thus reducing a M times N problem to a M plus N problem.Please dig out and read RFC's #1 Crocker, #5 Rulifson, #51 Elie to seesome previous thinking about this type of problem.       [ 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]