Radio Network


10 posts
by The_Pixie » Wed May 23, 2012 12:27 pm
While still waiting for my R-Pi, I began chatting to one of my school mates about the potential of such a small cheap device. We both have started A-Level computing, and class ourselves as "adapt" at programming, all be it in Visual Basic. Since our college is a boarding college, there is a network in place already. Yet this network is managed by Dazza T, an arch-nemisis so to speak.with a policy is that only college issued laptops can interface with the college system, and so all chances of getting an R-Pi onto it is shot, never mind anyother laptop, tablet or PC.

So, after a daily chat of the possibilty, an idea was sown. Creating our own private network, that would allow us to connect R-Pi's and other equipment together. A few basic ideas have been thrashed about, from using lasers to transmit the data. It is forturnate that our rooms have line of sight, at the moment. After a bit of googling, that idea was thrown out due to speed and cost. The next idea, an idea I think others may be doing, is using radio waves. Since the boarding houses are not much more than 80m apart, and that a WiFi connection for this size would be expensive, the general concensus is that radio is the best option. We thought that since possible parts for radio communications are found in walkie talkies, and these can be found cheaply on the net, they could be used.

So, I ask, is there a way through the GIO Pins for a small walkie talky be used to cycle bits across this distance?
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by rurwin » Wed May 23, 2012 1:55 pm
You could use PPP or SLIP if you had a modem, but converting a telephone modem to work over a walkie-talkie would be a significantly challenging task.

AX.25 modems are fairly cheap, but I'm not sure they are cheaper than WiFi, and most walkie-talkies do not come with microphone sockets.

The way I would do it is to get a standard WiFi Access Point. Then build a directional aerial for it. (Or even buy one.) It might even work with the standard aerial if there is clear line of sight and no walls in the way. For even more chance use a WiFi dongle with an external aerial.
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by bredman » Wed May 23, 2012 2:09 pm
I agree that WiFi is the best solution. It can be great fun to hack WiFi to create a directional antenna. Search the internet for WiFi cantenna (that was not a spelling error).

Also, if you college-issued laptop includes two interfaces (such as Ethernet and WiFi) you could use the laptop as a network bridge to join your unofficial network to the official network. Assuming your laptop uses Windows, search the internet for Windows network bridge.

If you get expelled from college, don't blame me.
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by error404 » Thu May 24, 2012 12:10 am
WiFi is the obvious choice here. Anything you roll your own is going to be pathetically slow (think maybe 9600bps at best, if you're good at DSP and modulation theory), or very complicated and expensive (see RONJA). 80m isn't really a big deal, even for consumer grade WiFi equipment. In fact I'd be quite surprised if your machines have line of sight and can't see each other already (performance likely won't be good, but I think it would work). A simple parabolic reflector made from card and tinfoil combined with a USB dongle on one of the two machines should get you the range you need. Here's a template for one: http://www.freeantennas.com/projects/template/ . Use whatever WiFi dongle you have and it should work pretty well. One reflector on each side would be even better.

If you want to step it up a notch, you could use cheap WiFi routers on each end that you set up semi-permanently with the reflectors and configure one as an AP and one as a repeater. That would get you a long-range link, plus the 'leaking' signal around the reflectors would give you a wireless network across a room or two in proximity to where the routers are. No wires!

If you really wanted to do AX.25 (1200bps!), the Linux soundmodem should work for this with a cheap eBay USB audio dongle.
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by The_Pixie » Fri May 25, 2012 10:17 am
So, I've just finished a computing exam, and read this. And all the long words like, PPP, SLIP, RONJA and dongle are making me confused. If I'm getting this, using a cantenna, or a wokfi, I can extend the range of a USB Wireless dongle; but will this work both ways?
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by rurwin » Fri May 25, 2012 11:50 am
Yes.

It is basic radio theory that what works one way, also works the other way. An aerial that boosts transmit performance will boost receive performance by exactly the same amount and in exactly the same way.

Although I think error404's idea was that you put a dongle and a reflector at both ends of the link. You can set the computers up to talk to each other without using an Access Point.
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by The_Pixie » Fri May 25, 2012 4:11 pm
This is good! I like that. Heck, if I build things right, it'll be amazingly adaptable.
So, while my Pi, slowly makes its way to me, I'll start plotting my world domination...
Thanks for all your help guys!
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by philwebman » Mon May 28, 2012 8:23 pm
Two 5GHz capable APs, 2 directional antennas and NAT from your permitted college device. You can transmit at up to 1W EIGRP using UNI2 (Band B) 5GHz channels.
Sounds easy enough to me!
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by bredman » Tue May 29, 2012 8:18 am
Another suggestion - 2 cheap WiFi adapters and 2 cans of Pringles crisps should give you a 54Mbit/second connection and something to chew on at the same time.

If you need examples of cheap equipment, dx.com/p/24688 works with the RPi and you may need a USB extension cable such as dx.com/p/33717.
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by CCitizenTO » Mon Jun 25, 2012 11:46 am
Interesting dillema.

You could do the obvious method of handling things and have your RPi spoof your laptops MAC address. If they are using a MAC address whitelist then this will solve your problem. You then have your laptop spoof the RPi MAC address (because having two MACs the same on the same subnet causes things to screw up) and use your RPi as a router between you and the college system.

Alternatively as others have suggested you can use WiFi and a directional antenna. For a span of 80m it shouldnt be too hard to do. You could probably hack it with some tinfoil reflectors to get enough signal in one direction. You could always try something like a Wok-Fi setup ( https://en.wikipedia.org/wiki/WokFi ) which will get you like 15db gain in a 60 degree arc typically and could reach ranges of 5km easily.
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