IP over HF packet, was Re: [HECnet] Area 19

[Peter Lothberg]

On Nov 2, 2012, at 6:14 PM, Peter Lothberg wrote:

If you guys actually try to do DECnet over radio I'd be interested...
=20
As long as you have a link-layer that takes a packet in and spit's it
out on the other side, taking care of retransmission, FEC coding etc,
it should work.

More precisely, you need either (a) a point to point reliable channel (anal=
ogous to DDCMP) or (b) a multicast datagram channel (analogous to Ethernet)=
.

IP over packet radio uses the latter, via the datagram mode of AX.25.   That=
's a reasonable option for DECnet as well if the packet loss rate is low.   =
If it's high, then the connection mode of AX.25 is a more efficient option,=
in spite of the design bugs that AX.25 inherited from X.25 and HDLC.

I was assuming using the "PTP" link mode.


The "error profile" of a HF chanel is such that the chances of
transmitting a packet without error with just HDLC decreeses with the
lenght of the packet. Ie, you might retransmitt the 576 byte packet
forever, having different bits being corupted every time. 

To do this succesfull on a HF chanel, you need some kind of forward
error correct scheme.

There is a "hard decition" FEC named CI-BCH-3, that does better than
10db. 

AX.25/HDLC likes VHF FM chanels with HIFI quality.

-P