[aprssig] Modems and FEC

[Randy Hall]

Hey guys, I love the blue sky nature of this. Seriously. I use Direwolf for
a couple of big footprint digi/igate sites, and I'm hugely excited about
M17 as a data and digital voice mode.

But there is a ridiculous amount of installed hardware that only speaks
1200 baud AX.25 AFSK packet, and it needs to be comprehended in whatever
vision we might create.

That isn't to say the physical layer definition shouldn't be expansive to
include and accommodate many different OTA modes. But physical layer is
just part of it.

As for OpenTrac, I tried visiting http://opentrac.net/ and was visited with
a "site down" notification. Is the site meant to be offline?

--R
*--*
*Randy AA6RH*
*aa6rh at socorad.io <aa6rh at socorad.io>*
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On Sun, Feb 13, 2022 at 9:28 PM Mobilinkd LLC <mobilinkd at gmail.com> wrote:

> I helped define the M17 data modes, which includes a raw packet mode.  M17
> uses 4-FSK modulation with FEC.  There is experimental firmware available
> for the Mobilinkd TNC3 and NucleoTNC that will do M17 packet mode. I've had
> an M17 APRS repeater up at times in Chicago.
>
> Data modes on M17 approaches 5kbps throughput.  It uses 4800 symbols/sec,
> 9600bps, with a slightly punctured convolutional code.
>
> https://m17project.org/
>
> Kind Regards,
>
> Rob Riggs WX9O
> Mobilinkd LLC
>
>
> On Sun, Feb 13, 2022 at 10:54 PM Gregg Wonderly <gregg at wonderly.org>
> wrote:
>
>> OpenTrac is an even better idea and more extensible and more easily
>> processed than APRS packets!  Let’s really take a moment to reflect on what
>> it could mean to start with a better solution end to end!
>>
>> Gregg
>>
>> Sent from my iPhone
>>
>> On Feb 13, 2022, at 6:32 PM, John Langner WB2OSZ <wb2osz at comcast.net>
>> wrote:
>>
>> 
>>
>>
>>
>> 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.
>>
>>
>>
>> 73,
>>
>> John WB2OSZ
>>
>>
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