Re: [MGSN-discussion] TNC?

[James C. <jw...@st...>]

> 1) What hardware does your TNC at Stanford consist of?
> 2) Which specific tasks does the TNC perform?

In the HAM world, the TNC (terminal node controller) traditionally
performs (de)modulation, bit synchronization, frame sync, and
packetization.   TNC's tend to be fairly intelligent with small
microprocessors embedded in them.  They often have a specialized
interface to allow you to configure their AX.25 packetization
capabilties.  Their is a group at the U of Wuerzburg that is also putting
FEC (forward error correction into a TNC).

We tend to operate our TNC's in KISS mode (KISS-Keep It Simple Stupid).
TNC's in KISS mode do not perform many of the standard packetization
functions.  They pass the AX.25 packets on to the CPU that is connected to
them so that the CPU can handle them. We connect our KISS TNC's to Linux
boxes that have kernel support for AX.25.

TNCs are dinosaurs and I expect them to be extinct soon.  General purpose
CPUs are replacing all of their functionality.  For instance, a CPU
running Linux with a sound card can perform all the functions of a
standard TNC.  We're combining these software radio/modem systems with
virtual machines.  Check out this paper:
http://swig.stanford.edu/pub/publications/space/RCSGSOJCutlerMS.pdf.
With this technology, we should be able to provide incredible multi
mission support in ground stations.

> 3) Is it sufficient to use the MX909 (GMSK Modem Data Pump), and a computer for
> the AX25 coding?

Yes it should.  We'll need to figure out how to export an interface to
your system through Mercury.  We haven't labeled all the pipeline outputs
yet.  In our AX.25 world, we've been using a TNC export and KISS export.
So, the default is that the GS exports KISS packets (with AX.25 packets
encoded into them).  Can we make your computer interface to the MX909 do
the same thing?  Other interfaces we'll be exporting include IP, IP over
AX.25, etc.  And actually, as we implement the virtual machines described
above, the GS and Mercury will no longer have to worry about explicitly
knowing about the interfaces. They will be handled by the VM.

I hope this helps.  Sorry for the late reply.  We just finished building a
ground station here and we're shipping it to Fairbanks, Alaska today!
That should give us and the MGSN some good northern coverage.  Can't wait
to get your station up and running too!  Also, I just received some CPU
hardware and we'll be setting up some of the initial MGSN central servers
here shortly.

Hope all is well!  Cheers!

--Jamie