[Sis-ipo] New Draft IP over CCSDS Links Document Available

[Keith Hogie]

Greg,

   In the D-1 and D-2 diagrams the stacks on either side of the RF link
only operate at the physical layer.

   The stacks just outside those have HDLC frames over some physical
layer with a gated clock as  indicated.  But there is no data link
layer framing that goes on between those stacks.

   The stacks in the middle do not need to do any data link framing.
Their function is to add/remove any coding and modulation needed
for the RF link.

   If there is no coding or convolutional coding it's pretty clear
that there is no data link layer.  When you use Reed-Solomon coding
there is a frame sync function that is performed but that is a
physical link function just like convolutional coding.  It does
a frame sync function but that framing is only to support the
operation of the R/S coding.  The R/S sync is not a data link
operation in this case since the HDLC framing covers that in the
next stack.

   In the other CCSDS formats the R/S sync is a data link function
since it locates the frames.  It actually serves a dual purpose of
providing the framing to locate the R/S blocks and it also locates
the CCSDS frames.

   As indicated the stacks on either side of the RF link are
basically media converters that pass bits between the wire and RF
media.  They do not provide a data link framing operation.  They
are similar to satellite modems that are used to send network
traffic over commercial communication satellites.

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   Keith Hogie                   e-mail: Keith.Hogie at gsfc.nasa.gov
   Computer Sciences Corp.       office: 301-794-2999  fax: 301-794-9480
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