David Rush wrote:
> Andrew:
>
> It's a cool idea and I've love to play with it (I am a licensed ham
> radio operator), and I don't want to rain on the parade, but there are a
> number of issues.
Not raining at all. I was afraid there wouldn't be any HAMs on the list
to tell me what's up :-)
> First and foremost are some of the rules preventing ham radio from
> carrying anything for profit or any "obscenity, profanity, or
> indecency".
Yeah, I really can't argue with that one. However, there are obviously
*some* licenses which allow such material to go over the airwaves -
802.11 equipment is unlicensed (or is licensed by purchase) and carries
such data regularly. So I'm guessing there's gotta be some way to make
it work. My only hope is that the proper license wouldn't cost out the
ass. Municipal wifi programs obviously license themselves somehow, but
like I said, hopefully it doesn't cost out the wazoo.
I'll be calling the FCC later to play phone tag, and see if I can get
an answer as to which type of licensing I should be investigating (huh
huh, wish me luck on getting an answer out of those dildos ^_^ )
> And the speeds with reasonably affordable equipment are pretty darn
> slow.... 9600 bps half duplex (with lethargic turn-around times between
> transmit and receive) is the fastest commonly available.
Common, yes. But it's not impossible to get much higher speeds if
you're willing to put some work into it.
From what I've been reading, the most common (and oldest) modulation
scheme for packet radio (X25/AX25) is FSK (frequency shift keying),
which doesn't do well above a few thousand kilobits/second, from what I
understand. PSK (phase shift keying) does much better, and there are
forms of PSK that can carry data at speeds up to megabit! Binary PSK
only has two phases and requires more bandwidth to carry a bit rate of
around 512kb (if what I've been reading is correct), while Quadrature
PSK can carry around twice the bitrate of regular PSK or BPSK on half
the bandwidth, I believe. And the kicker is that the linux soundmodem
module already has some basic PSK support in it, PSK1200 I believe - so
someone who was familiar with the math could easily hack BPSK or QPSK
into the kernel module and solve 1/2 of the bandwidth problem.
Ryan | Speed wrote:
> Not educated on the subject, but I imagine the problem would be
> interference.
And here we have the 2nd half of the problem. Interference isn't such a
problem when dealing with FSK - unless we're talking about intentional
interference, which is always a bone of contention - from what I
understand because of the fact that FSK is a variant of AM (and when
I've learned enough about the two modulations to explain *why* it
matters, yeah I'll do that :-P). But with PSK modulations - BPSK and
QPSK - interference is a much bigger problem because we're actually
changing the bandwidth of the signal. It actually consists of mixing 2
or 4 (respectively) different AM signals of slightly different frequency
together. Naturally, you can imagine how a receiver built for such a
signal would be more sensitive to interference than a regular AM receiver.
The problem of building transceivers for efficient PSK transmission at
high speeds has been solved many years ago. I'm sure there are others,
but Matjaz Vidmar was building 13cm transceivers for transmitting data
over packet radio at 1.2megabit/second as early as 1996. He also built
TNCs for such operation, so all of this stuff is documented and done
already; all that needs to happen is someone to implement the TNC design
in the soundmodem driver, and to build the transceivers.
Now, as for how much these 13cm transceivers cost to construct - I have
no idea. I'm looking into that now, as time permits. I also don't know
how easy it would be (or if it would even be possible) to take a 2m
radio and modify it to 13cm operation. My knowledge of electronics ...
probably isn't quite up to snuff for that just yet, but maybe I could
get with David and we could talk about it once I find out more about
Vidmar's transceivers.
While all this stuff is fascinating, the issue of licensing is a bit
haunting. It may very well come back to me from the FCC that the only
license suitable is the one 802.11 operates under, and the power limits
on that would put the kaibosh on the whole thing. We'll see though.
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