After pulling the old reference materials out, I was hoping to be able to
provide some quantitative data here, but I can't find even one reference
that talks definitively about potential throughput using varying
bandwidths/signal separations on HF frequencies. It's a fun theoretical
question but putting together a system to test any potential answers would
be expensive to build and illegal to use.
As a side note, I've learned one interesting thing that may seem obvious
when you take a moment to think about it: one of the main factors that
limits data transfer rate over long distances using HF frequencies is
propagation delay. Propagation delay increases when the physical distance
between nodes increases and imposes a hard limit on what your potential
transfer rate can be. The longer the distance, the longer the propagation
delay, the smaller your potential transfer rate can be.
Because of that (and the fact that radio doesn't provide a clean enough
signal to work with at HF frequencies) I don't think it's possible to "go as
fast as you want" using HF.
The total available signal bandwidth below 30MHz is... well... 30MHz. That
includes everyone's frequencies, not just amateur radio frequencies. When
you consider that the bandwidth of just one 802.11 wifi channel is 22Mhz and
there are 11 of them (in most countries), you can see that it takes a
tremendous amount of bandwidth even when you have an excellent signal to
transfer a significant amount of data at high speed. There just isn't
enough bandwidth available in the HF bands to do that.
Here's a good summary of digital modes that are available on HF. According
to this and most of the other resources I've looked at today, 300 baud is
about the best you can expect reliably at HF frequencies over long distances
although 1200 can be workable.
http://www.qsl.net/yo5ofh/doc/digital%20hf%20radio%20operating%20modes.htm
Llew Roberts
> -----Original Message-----
> From: p2p-hackers-boun...@lists.zooko.com [mailto:p2p-hackers-
> boun...@lists.zooko.com] On Behalf Of Serguei Osokine
> Sent: Tuesday, February 08, 2011 9:19 AM
> To: theory and practice of decentralized computer networks
> Subject: Re: [p2p-hackers] What we should build for the Egyptian (and
other)
> protesters
>
> On Tuesday, February 08, 2011 David Barrett wrote:
> > How fast is it actually possible to go?
>
> You can go as fast as you want - megabits, technically. But as you
increase the
> modulation frequency (raise the baud rate), you increase the band width of
> the channel that you're using. This is shared media, remember. CB, for
> example, has 40 channels allocated to it, with freqs from 26.965 MHz
> (Channel 1) to 27.405 MHz (Cannel 40). These channels are typically
> separated by 10 kHz intervals (Channel 2 is 26.975 MHz,
> etc) with a few 20-KHz intervals thrown in just for fun.
>
> So as you raise the baud rate, your signal starts to take wider and wider
> channel band width, eventually getting into the next channel's frequency
> interval allocation. And since channel allocations were originally
designed just
> wide enough to carry the low-quality voice
> - just as the phone network was - their digital carrying capacity is also
close to
> what an old-school modem could do over the telephone network. I think you
> can actually go to 9600 baud with relative ease, but anything higher would
be
> a challenge.
>
> Best wishes -
> S.Osokine.
> 8 Feb 2011.
>
>
> -----Original Message-----
> From: p2p-hackers-boun...@lists.zooko.com [mailto:p2p-hackers-
> boun...@lists.zooko.com] On Behalf Of David Barrett
> Sent: Tuesday, February 08, 2011 8:43 AM
> To: p2p-hackers@lists.zooko.com
> Subject: Re: [p2p-hackers] What we should build for the Egyptian (and
other)
> protesters
>
> On 02/08/2011 08:03 AM, Eugen Leitl wrote:
> >> HF radio frequencies that enable reliable communication over long
> >> terrestrial distances don't have enough bandwidth to pass large
> >> amounts of data. 1200 baud is the maximum legal rate that Ham Radio
> >> operators can send using the 10 meter band, which ~28-30 MHz. ("CB"
> >> is ~26.xx-27.xx MHz). And the 10 meter band is not always reliable
over
> long distances...
>
> Interesting: why is there a "maximum legal rate"? I assumed 1200 baud was
> just the fastest you could reliably get over such a low frequency
connection.
> How fast is it actually possible to go?
>
> -david
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