I don't think there needs to be any TCP or server-side involvement if you
want to catch-up with GPS fixes, as long as XCSoar can read the state of
the GSM/3G/4G network.
All you would need to do is read the state of the network when you are
going to send a skylines packet.
If the network is unreachable/out of range then save the timestamp.
Continue recording fixes, and each time you go to send to skylines, check
for network availability before sending.
When the network is available again, send the fixes since the network went
down.
That way, you should not get straight lines between mobile coverage areas.
On Apr 17, 2013 9:03 PM, "Ian" <ian-xcs...@zsd.co.za> wrote:
> On 17/04/2013 14:17, Max Kellermann wrote:
>
> > However, we don't use TCP for a good reason - TCP requires a three-way
> > handshake, and that's often not possible with flaky networks.
>
> I was thinking about the "catch up" transaction. One TCP session could
> be used to report the missing data to XCSoar and then for XCSoar to
> re-transmit the data block to the logging server. At least at this stage
> one would have the benefit of error detection.
>
> >> PS: Is there any compression of the data before it is sent?
>
> > No. It's hard to imagine how 48 bytes can be compressed. However it
> > would be possible to implement a smaller "GPS fix" datagram by
> > omitting many of the rarely used fields (like "airspeed" or "ENL").
> > Maybe we could go slightly below 40 bytes, but I'm not sure if that's
> > worth the trouble.
>
> Once again, probably not worth it for logging live traffic, but may be
> worth considering for "catch up" traffic, which could entail quite a lot
> of data in a single upload.
>
> If you want to compress the live traffic, you could look at a protocol
> like "Van Jacobson" compression as described in rfc1144
>
> http://tools.ietf.org/html/rfc1144#page-4
>
> This is for compression of TCP/IP headers, but you could use a similar
> principal for compressing, say, the most significant characters in GPS
> co-ordinates, which wont change from fix to fix.
>
> Regards
>
> Ian
>
>
>
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