[aprssig] APRS Voice links?

Andrew P. andrewemt at hotmail.com
Sun May 19 17:47:56 CDT 2013

I wonder what ever happened to the FX.25 protocol? It is a backwards-compatible forward-error-correction extension to AX.25.

Andrew KA2DDO
------Original Message------
From: Stephen H. Smith
To: aprssig at tapr.org
Sent: May 19, 2013 5:44 PM
Subject: Re: [aprssig] APRS Voice links?

On 5/19/2013 4:34 PM, Kristoff Bonne wrote:
> If that is not the case, and require a 100 % correct packet every time -or else
> you drop the whole packet-; you completely lose the advantage of a DV protocol.
> Especially when dealing with mobile or portable stations as these streams (as
> received by the repeater) really contains lots of errors.
>  From my (althou limited) knowledge of the HAM packet networks, this is a
> completely different situation then what you have now.

This is exactly the reason this is not going to be practical shoe-horned into 
conventional AX.25 ham packet.

Ham AX.25 transmits packets of an arbitrary number of bytes tagged with a 
checksum at the end.  There is no FEC involved.

Either the entire AX.25 packet is received perfectly (passes checksum), or the 
packet is received defective (fails checksum) which triggers an NAK/ARQ 
transmission back to the sending station to resend the same entire packet. 
Even if the packet is received perfectly, the receiving station sends an ACK 
that essentially tells the sending station to go ahead with the next packet. 
(This is in the "connected" mode where two stations have a one-on-one two-way 
handshaking relationship.)

In the non-connected "UI" broadcast one-to-many mode, the sending station just 
"blasts out" sequential packets, again with no FEC.  If the receiving 
station(s) miss them, there is way to recover them.

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