[aprssig] Modems and FEC

John Langner WB2OSZ wb2osz at comcast.net
Sun Feb 13 19:31:29 EST 2022


The AX.25 and APRS protocol specifications say nothing

at all about modems.


The reason that we are using 1200 bps is because early 

packet radio developers, in Canada, in the late 1970s, happened 

to acquire a pile of old Bell 202 modems for 75 cents each.


The method that we use for 9600 bps was developed in

the late 1980s.  It has been widely implemented but won't

work with the microphone and speaker connections because

the audio processing distorts the signal and limits the bandwidth.


In the early 1990s, multiple TNC vendors produced products capable

of 2400 bps:  MFJ-2400, AEA PK232-2400, Kantronics KPC-2400.

Maybe if other vendors had offered the same we would all be using

2400 bps today.


With the shift from hardware modems to software modems,

the possibilities are endless.  Upgrades are easy.  



Two methods of Forward Error Correction are currently being used:

(1)    FX.25 which is completely interoperable with original AX.25.

(2)    IL2P which has less overhead but is not compatible.



Better alternatives are available today for those who want to use 

them.   The problem is trying to convince most of the installed base

to break free of a precedent set more than 40 years ago.





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