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[ih] The origin of variable length packets



Although a bit off-topic, if anyone is interested in the history of the Interplanetary Internet there is a parallel. In the early days of the space program, data were transmitted to/from remote tracking stations and mission control facilities using a ground communications network called NASCOM: http://tmo.jpl.nasa.gov/progress_report2/42-36/36B.PDF.

NASCOM used fixed length blocks; I think that they started off at 836 bits and later migrated to 1200 and then to  4800 bit blocks. The system architecture was basically that the spacecraft telemetry and telecommand bit-streams were tunneled over NASCOM during their ground transit.

During the 1971 Mariner 9 mission to orbit Mars, we discovered that one of the scientific instruments was never getting a complete data set. After a lot of digging though data dumps we realized that if NASCOM operated at its specified block loss rate then it was guaranteed that there would be a hole in that user's data. After thinking about the problem, Ed Greenberg and I at JPL proposed the "Block Telemetry" concept (attached) in which a fixed length block of data from each instrument would be inserted into a fixed-length telemetry frame and transmitted to the ground. At the ground station the frames would be synchronized and placed 1:1 into the data field of a fixed length NASCOM block. That way, a missing NASCOM block would only hit one user. This system was flown on the SEASAT-A spacecraft in 1978.

Meanwhile, Greenberg and I discovered that Ed Greene at NASA Goddard was thinking along similar lines and in 1976 we started collaborating. About that time we also learned that a young whippersnapper named Cerf was messing around with packetized data transfer and it became clear that we would be much better served by moving from fixed length blocks of user data to variable length packets. The concepts of "Packet Telemetry" and "Packet Telecommand" soon followed; they are now the basis for space/ground communications on well over 500 spacecraft across the international community: http://public.ccsds.org/about/history.aspx

Now those same packet protocols will provide the space link underpinnings for the DTN protocol that a somewhat older whippersnapper called Cerf is promoting: http://en.wikipedia.org/wiki/Interplanetary_Internet

///adrian

Adrian J. Hooke
Manager: Space Disruption Tolerant Networking (DTN) Project
Space Communications and Navigation Office (SCaN)
Space Operations Mission Directorate
NASA Headquarters
Washington DC  20024-3210





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