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Build Your Own 10Mbps Microwave Data Link
Rob writes: "I found this site called The Microwave Data Link Project where a guy is attempting to link 2 LANs (2 miles apart) by a 10Mbps Microwave Data Link. I've seen the plans for these links, but never wanted to learn all that amateur radio stuff, but this guy is going to lead people step by step how to build it, including how to get your FCC License, and more. Should be interesting to follow the progress!"
10Mbps wireless ethernet you can build:
http://www.technocrat.net/937014982/index_html
--Kevin
=-=-=
Not over ham radio...
You are NOT allowed to encrypt any of your data, unless you are controlling a space based transmitter (that is, a satellite).
If you are needing to encrypt, try something like the Apple Airport thingy.
I looked into setting up some WAN links such as this with commercial equipment a couple of years ago. I think California Microwave and TRW were a couple of companies that make LOS (line of sight) microwave equipment designed exactly (or close enough) for this purpose.
At the time, the equipment was expensive, but still financially viable over a year or two compared to dedicated T1 lines. I believe in 1997 I was getting quotes of around $10,000 for a single 2 mbps link (anyone, which is it, Mbps or mbps?). The price of the link did not increase significantly as the speed went up. For instance, I belive it was only a couple of grand more for the 10 mbps link. The large initial cost was for the antenna systems and the radios. For instance, 10 mbps data and 2 mbps data traveled over the same frequency, but the 10 mbps equipment had tighter tolerences and was slightly more expensive.
Anyhow, the biggest problem with a system like this is security. To really secure a link like this, one will need a spread spectrum modem. Those aren't cheap. Spread spectrum "spreads" a data signal out over a wide band of frequency. If done correctly, the signal is difficult to detect apart from normal background noise. Also, to eavesdrop on the signal, one would need the right codes that the modem uses to spread the signal out and then put it back together.
For instance, with spread spectrum, one could transmit two different signals on (nearly) the same frequency, but because 2 different codes are used to split up the signals, they could be decoded at the distant end without interfering with each other.
Other advantages to spread spectrum include resistance to jamming and low succeptibility to background noise. One would be less likely to lose a signal due to rain or snow or some jerk with a 300 watt CB radio in his car.
I believe this microwave project will meet with success, but it won't be anywhere near what I would call reliable enough to use in a business environment.
The cost of me doing it is quite a bit more than his project since I have to relay to get around trees. Microwave has to be line of sight. However if I had line of sight between 2 networks up to 30 miles away I could use:
2 lucent wavelan cards: $150 ea
2 24db grid dish antennas: $150ea
2 40' cables $50ea.
Total $700
I could save money here on the antenna's, get a couple 10dbi antenna's for $70ea, and still get 10 miles.
Figure the time going into making the stuff, and licensing (this is unlicensed stuff I use), the excellent software, stability, etc, not to mention the range and that $200 difference goes away really fast. Not to mention we'll have to see if he make it at $500.
Before everyone gets excited about doing this project, please take the time to understand that Amateur Radio is specifically prohibited from being used to carry commercial communications. Be mindful that what constitutes "commercial" communications is subject to interpretation. Note also, that encryption *may* be prohibited under this regulation (No SSL for you!)
Here's an excerpt of the relevant federal regulation:
(a) No amateur station shall transmit:
(1) Communications specifically prohibited elsewhere in this Part;
(2) Communications for hire or for material compensation, direct or indirect, paid or promised, except as otherwise provided in these rules;
(3) Communications in which the station licensee or control operator has a pecuniary interest, including communications on behalf of an employer. Amateur operators may, however, notify other amateur operators of the availability for sale or trade of apparatus normally used in an amateur station, provided that such activity is not conducted on a regular basis;
(4) Music using a phone emission except as specifically provided elsewhere in this Section; communications intended to facilitate a criminal act; messages in codes or ciphers intended to obscure the meaning thereof, except as otherwise provided herein; obscene or indecent words or language; or false or deceptive messages, signals or identification;
(5) Communications, on a regular basis, which could reasonably be furnished alternatively through other radio services.
The full regulation can be found here.
Chuck Milam, KF9FR
Two I am familiar with are Breezecom and Aironet , just bought by Cisco. If you poke around, you'll find many of these products have Linux drivers .
"one treats others with courtesy not because they are gentlemen or gentlewomen, but because you are" --G. Henrichs
Yes, exactly. The low-cost Gunn-diode units being used couldn't be modulated much faster than about 2 Mbps.
Second, the statement about "not much operating at 2Mbps" as a reason for trying for 10Mbps makes it sound like he really doesn't understand the way asynchronous networks function. At least with tcp...
It has nothing to do with protocols. The problem is finding something that pumps data in a useful manner at 2 Mbps. A 10-Mbps Ethernet card can be had for next to nothing. But what card do you use if you need to slow down to 2 Mbps? Thus the desire to increase the project's speed to 10 Mbps.
Of course, as other people have pointed out, the legality of a 2Mbps HAM link, let alone 10Mbps, is quite suspect. Someone with access to recent FCC rulings should comment on it. A licensed amateur may design and build their own device to transmit legally in the HAM bands, and it seems from the rules most people know about that the 10ghz HAM band is limited to 56kbps.
Not so. On any frequency above 450 MHz, US hams can use any digital bandwidth that will fit inside the band. See the FCC Rules at 97.305(c) (and 97.307(f)).
Does anyone know what the equivalent license in Canada would be?
BlackNova Traders
What kind of relay? I've got six miles through a hill.
73
KK5WA
sine puella vita suget
I don't know how correct your information is there. Just about all microwave transmission takes place in the Ghz range. Satellite uplink/downlink are frequently in the 8 - 25 Ghz range and line of site microwave is only slightly less.
Shorter wavelengths are better than longer for travelling through objects and over long distances (line of sight). Remember your 900Mhz cordless phone, now pushing up to the 2.4 Ghz range? The reason behind that is farther transmission with less interference.
As to the ISP you mentioned, did they use directional (parabolic) antennas? All microwave transmission is line of sight. You need really low frequencys, like AM to be able to go over the horizon. Unless of course, you've got a couple of thousand watts of transmitting power, and then you can do something called troposcatter. Ranges of over 100 miles are available in that situation.
Without anything blocking your view, you can easily get 30 miles by line of sight. Also consider all of the microwave links you will see the phone companies have connecting small villages and cities out in the boonies. They are much cheaper than stringing enough copper that far.
If you live in a downtown area, if you look to the tops of buildings, you'll see many antenna horns and dishes (look like dinner plates almost) littering the top of the skyline. All of those are high freq microwave links.
... referenced in the article, at http://www.qsl.net/ke5fx.
/.-worthy. The response came back in about 5 minutes:
:-)
:-)
Amusingly, I submitted the project page to Slashdot myself about a year ago, when I first got the link working. When the submission was rejected, I wrote to one of the well-known admins at Slashdot asking what I needed to do to make it
I just didn't think it was interesting enough.
Sorry.
--
| (admin name omitted) | Just want to be misunderstood,
| (admin name omitted) | I wanna be feared in my neighborhood.
| http://slashdot.org/ | Just want to be a moody man,
| codejockey*gangstero | say things that nobody can understand.
| flove*lovemachine*wr | --Pete Townshend, Misunderstood
Dejected, I was forced to admit that my news-for-nerds quotient just couldn't compete with the guy who built a PC case out of Zebrano wood, or whatever the story-of-the-day was at the time. Consequently, Rob gets major props from me for doing a better job writing up the submission than I did.
Now for the bad news: out of many thousands of hits my site's received over the past year, only one guy, an EE professor at a local university, has successfully duplicated my efforts. In my mind, that somewhat validates the unnamed Slashdot admin's decision not to publicize the project in the first place. People, this is not a project for beginners or casual electronics buffs! In the year since I finished the project, a couple of excellent alternative solutions for RF data links that make good use of off-the-shelf hardware and homemade antennas have come to light. For instance, there's a good page on the topic of modifying Proxim Symphony cards for long-range service at Low-Cost Wireless Network How-To. I would have to say that this is a better approach for 99% of the people who have looked at my microwave link page and thought about building it. Heck, in retrospect this is what I should have done, instead of putting in all that engineering time on a 100%-custom solution.
My own 10-megabit link has been running great since last June with only a couple of hours' downtime for maintenance and tweaks, and it's easily the most 'educational' project I've ever tackled. But I think it's important to temper peoples' expectations. If you are an experienced electronics hacker with access to a very well-equipped RF test bench or a university EE lab, and you just want to spend a lot of time and money building something something cool, then I highly recommend my microwave link project. If not, do yourself a BIG favor and check out the various wireless-Ethernet card tweaks that have been springing up on the Web.
-- John Miles, KE5FX
Dahlmann tightly grips the knife, which he may have no idea how to use, and steps out into the plain.
Cabs do not use HAM (Amateur) radios. They use radios and frequencies specifically leased and licensed to them by the FCC for this commercial purpose. Amateur radio is totally different.
... referenced in the article, at http://www.qsl.net/ke5fx. I'm going to be sort of redundant here, and repost this message as a reply to the "first post", just to make sure all interested parties see it. I got to work late today, so my initial reply is lost somewhere in the noise at the bottom of the article. :-) Sorry for the duplicate post -- if someone can down-moderate the copy at the bottom I'd appreciate it.
/.-worthy. The response came back in about 5 minutes:
:-)
:-)
Amusingly, I submitted the project page to Slashdot myself about a year ago, when I first got the link working. When the submission was rejected, I wrote to one of the well-known admins at Slashdot asking what I needed to do to make it
I just didn't think it was interesting enough.
Sorry.
--
| (admin name omitted) | Just want to be misunderstood,
| (admin name omitted) | I wanna be feared in my neighborhood.
| http://slashdot.org/ | Just want to be a moody man,
| codejockey*gangstero | say things that nobody can understand.
| flove*lovemachine*wr | --Pete Townshend, Misunderstood
Dejected, I was forced to admit that my news-for-nerds quotient just couldn't compete with the guy who built a PC case out of Zebrano wood, or whatever the story-of-the-day was at the time. Consequently, Rob gets major props from me for doing a better job writing up the submission than I did.
Now for the bad news: out of many thousands of hits my site's received over the past year, only one guy, an EE professor at a local university, has successfully duplicated my efforts. In my mind, that somewhat validates the unnamed Slashdot admin's decision not to publicize the project in the first place. People, this is not a project for beginners or casual electronics buffs! In the year since I finished the project, a couple of excellent alternative solutions for RF data links that make good use of off-the-shelf hardware and homemade antennas have come to light. For instance, there's a good page on the topic of modifying Proxim Symphony cards for long-range service at Low-Cost Wireless Network How-To. I would have to say that this is a better approach for 99% of the people who have looked at my microwave link page and thought about building it. Heck, in retrospect this is what I should have done, instead of putting in all that engineering time on a 100%-custom solution.
My own 10-megabit link has been running great since last June with only a couple of hours' downtime for maintenance and tweaks, and it's easily the most 'educational' project I've ever tackled. But I think it's important to temper peoples' expectations. If you are an experienced electronics hacker with access to a very well-equipped RF test bench or a university EE lab, and you just want to spend a lot of time and money building something something cool, then I highly recommend my microwave link project. If not, do yourself a BIG favor and check out the various wireless-Ethernet card tweaks that have been springing up on the Web.
-- John Miles, KE5FX
Dahlmann tightly grips the knife, which he may have no idea how to use, and steps out into the plain.
Why would you want to use a regulated communications medium which makes it impossible to do anything useful without paying a fortune when you can use a laser?
(Mainly for the benefit of others reading your comment.) The idea behind amateur radio is that it's a hobby, and meant for experimentation and for trying new things, learning, etc. It's not meant to do the job of something you can find commercially.
So generally speaking every signal you send over amateur frequencies must be things that a) you're perfectly comfortable sharing with the rest of the amateur radio community (thus the rest of the world); and b) aren't things that should probably be sent via some existing, equivalent (probably commercialized) route.
In this case, the guy's having fun trying to learn how microwave transmissions work, and what better way to learn how to do these things than to get an amateur license and actually build the darn thing? But the regulations say if he wants to use this link for anything but completely open, completely personal use, he needs to pursue a commercial license (or use a non-amateur product).
For this reason you will not find amateur digital links directly connected to the Internet for any Joe Schmoe to (perhaps inadvertantly) send anything violating FCC regulations over amateur radio frequencies. Things like porn, advertisements and commercial activities do not belong on amateur radio frequencies, and nowadays the web is full of it. In a way, this also hinders a lot of amateur radio digital activities, since we're effectively limited to e-mails and other communications between HAMs.
But this is just one area of digital amateur radio. We've got several satellites in orbit, some capable of doing digital work, and we have new cool things like APRS for instant world-wide messaging (even via satellite if you're in the boonies) and GPS positioning. You could build a home-made LoJack out of something like this.
Dave, WL7RO
All microwave transmission bit error rates can be improved using standard diversity techniques. Time diversity, sending the same data twice. Accomplished through forward error correction and literally using two transmitters/ receiver pairs with lag placed on the second transmission. Space diversity, using two transmitter/receiver pairs separated laterally relative to the transmission path (a Flock of birds flies across one transmission path but does not affect the other) Frequency diversity, again two transmitters or a single frequency hopping transmitter (ala CDMA cell phones).
Using two transmitters does not mean having to use more frequencies. Using the same frequencies but changing the polarization of your antenna provides 83 dB of attenuation between transmission paths. Ideally you would use several of these diversity methods to give you the most paths possible. your modem also must be smart enough not only to handle multiple redundant paths it must also decide what the "best" data bit is if two bits in the same frame from different paths are interpreted differently by the receivers.
My eyes gloss over when hackers discuss the linux kernel in minutiae. I'm sure you are all there now.
If voting were effective, it would be illegal by now.
Most microwave transmissions use very little power. Line of sight transmissions often use 10 watts or less. Even satellite transmission often use 10-20 watts of power. Do not put microwave communications in the same league with radars that are pumping out 10,000 watts to reach over 100 miles to bounce a signal off of something. Your microwave oven is more likely to kill a bird that a microwave comm link.
Keep in mind that an exceptionally large proportion (compared to general web users) of Slashdot users are using a text-based browser and/or blocking counters (counters are, in my experience, even more annoying than banner ads). So I wouldn't put any faith in any of the numbers you're getting.
Frequencies work differently in different media. For instance, high frequency is very good through the atmosphere. It'll blow right through almost anything. That's why FM radio only has a range of 50 miles or so (depends on the power of the transmitter). FM signals will generally pass right through the upper atmosphere, while the lower frequency AM signals will bounce off and extend the range a couple of hundred miles.
Low freqs work well in a more densely packed media, such as water. Sonar and underwater communications devices use low frequencys. Those signals carry over long distances because of the media they are travelling through, rather than because the signals themselves.
First off, sound frequency is the frequency of vibration in a medium. Examples are voice in air and sonar in water.
Electromagnetic Frequency is the frequency of the EM wave. Examples are UV (suntan), Infrared (remote control), regular light, AM, and FM.
they are totally different!
Nearly OT but, on the topic of wireless transmission.... I've been searching around for a way to add ethernet connectivity to the computer that's going in my car (mp3s, gps, etc...). This microwave thing peaked my interest briefly, but I don't think it's what I need. I just need a range of about 10 feet and would settle for 1.5 Mbps. The only commercial solutions I've come across are >$200. Does anyone know of a cheap package for these slow, short range applications. Right now, I'm planning on going with my back-up plan: RJ45 in the grill.
Extremely directional. Depending on the antenna.. could be 30 degrees or less.. probably less.
So it's not a 2 mile radius.. it's a 2 mile point-point link.
I don't remember reading that in Part 97, could you post the relevant citations?
TAPR is in the process of building a Spread Spectrum radio that will be faster than 56k.
http://www.tapr.org/tapr/html/Ftaprfhss.html for details on this project
I think the 56K symbol/second limit only applies to the 70 cm band. See Part 97 of the FCC regulations.
Also, as the freq goes up the signals start acting more like light than fields, and things start wanting to work more point-to-point. That's one of the reasons I doubt you'll ever see cell phones at 26GHz: the directionality of the signals would be too great to allow you to just whip out your phone: you'd have to point it at the cell tower.
www.eFax.com are spammers
www.eFax.com are spammers
Now, why did I keep italicizing baud? Because baud ain't bits per second! It one of my pet peeves when I hear somebody talking about his 33.6kbaud modem, or 56kbaud modem. There ain't such a thing! You have either a 3 kBaud modem or an 8 kBaud modem. A baud is a symbol transition per second, not a bit per second. In the old 300 baud days each symbol was worth 1 bit, so 300baud == 300 bits/second. However, in a 33.6kbit/sescond modem, the baud rate is 3 kbaud, with each symbol worth 11 bits. A 56kbps modem uses 8 kBaud, with 8 bits/symbol (with 1 bit robbed for line signaling).
On 2 meters, I could modulate a signal with C4FM (FM signal, but with 4 levels rather than 2) and get 2 bits/symbol, or 19200bps. If I went to 256QAM, I could get 8 bits per symbol, and still be perfectly legal. Why don't hams do this? Because you cannot hook an external TNC (terminal node controller, the radio equivelent of a modem+network interface) to a standard narrowband FM tranceiver and have it make those kinds of signals - you need a special data radio. Also, 256QAM has far less signal to noise ratio than does BFSK. Hence, the amateur community doesn't avail themselves of the bandwidth they have.
One final point: signal bandwidth does not directly control data bandwidth: as Shannon proved, it's all about signal to noise ratio. The only thing signal bandwidth does is give you a better S/N ratio.
www.eFax.com are spammers
The driver support under Linux for the WaveLAN card is fantastic, and the driver is even included with Linux-Mandrake 7.1 -- no special configuration required other than to make sure pcmcia inits before network.
The downside? It's not a 10-Mbps connection, it's only 2-Mbps. However, I routinely run X connections over SSH from a dedicated ssh gateway at my business. The link has fantastic uptime, the signal-noise ratio is almost as good as it gets with a 24dB antenna, and I'm completely undisturbed by power outages (UPS here), snow, rain, etc. High winds (60 m.p.h. + gusts outside right now) do take a toll, though. A 1 watt amplifier allows my 24dB antenna to punch right through the walls of my attic and lets me forget about the many trees, houses, and other obstructions between me and my ISP, 3 miles away.
The WaveLAN card with a pair of antennas and amps would be a much easier, more convenient option for most home users trying to set up this type of connection, IMHO. The amps are pretty expensive (a couple hundred dollars) but do wonders to ensure connection integrity.
Here's what my S/N ratio looks like, catting
Matthew P. Barnson
I learn what I think when I read what I write
The first issue, which a few people have mentioned, is that microwaves are LOS. On a lake with two mile range, that isn't really a problem.
The second is that developing high power in the microwave regions is very difficult. Note the quote of a 100 mW diode costing $45 in addition to the $90 cost of the rest of the unit. 100 mW isn't much power.
Fortunately, for link builders, the antenna size to achieve a given gain is proportional to the wavelength of the signal. In the case of this link, extremely high frequency means a very short wavelength and very high gain with not much of an antenna. A 1-2 foot dish will give incredible gain, but reduce your beamwidth to 5 degrees. (I don't have my ARRL Handbook with the exact quotes handy...)
retrorocket.o not found, launch anyway?
p.s. Do you have any information where I can get a free counter for my website?
--
Have fun: Join D.N.A. (National Dyslexics Association)
These may be microwaves, but they're very low power. (Max 100 mW)
Compare this to the 500-1000 watts used by microwave ovens. Also, microwave ovens are specifically tuned to be absorbed at maximum efficiency by water.
retrorocket.o not found, launch anyway?
Expectations were that this could, quite quickly, result in a hot and tasty bird, even in the cool temperatures of Ottawa autumn.
I suspect the station manager nixed the idea; if it interrupted Max Keeping doing the news, there would doubtless be thousands of complaints...
If you're not part of the solution, you're part of the precipitate.
Only if the water was very calm, the ship wasn't vibrating much, and the antennas were being kept in line with each other by tracking systems.
It's really amazing just how directional radio waves become as you get into the microwave range. The best analogy would be light: imagine, x miles apart, trying to point two searchlights at each other and keep them properly aimed.
Now, if you mount one of them on a moving target, both of them have to be able to track each other. Very expensive to buy, very time-consuming to build or troubleshoot.
If you want comms to a boat, use lower-frequency RF. The problems you'll have there - ie. availability of frequencies to use, lower bandwidth, etc. will be easier to overcome than the directional properties of microwaves.
Fire and Meat. Yummy.
One of the advantages of microwave frequencies, esp. the 10 GHz X-band used by the Gunnplexer, is the ability to build very small and very high gain antennas. You can easily get 20 - 30 dB of gain over an isotropic source with a small antenna.
First, i imagine the original design stopped at 2Mbps because it wasn't really feasable to go over that speed using the parts on hand.
Second, the statement about "not much operating at 2Mbps" as a reason for trying for 10Mbps makes it sound like he really doesn't understand the way asynchronous networks function.
At least with tcp, you simply transmit more and more (use larger and larger windows) until you start to experience transmission errors, or it just isn't getting you any better throughput, and then step one back from the last increase (well, you're supposed to) and call that your transmission rate.
Basically, the device on the other end ACK's every once in a while to let you know your packet got through. If the device on the other end didn't understand the data, or wasn't ready to recieve it, it can't ACK it. If you don't get your ACK in time, you retransmit. If you're having to retransmit too often, your transmission rate is obviously too high, so you back off your window size and slow down.
It's a cheezy form of bandwidth controll, but it works, sortof. It's the electronic equivalent of not reading a signifigant portion of your email in hopes that people will stop sending so much of it. Except that the protocol is designed such that a correctly written application *will* stop sending so much.
Yes, speed is nice. but relatively low-speed long haul links are the way the world goes round at this point. "T1" is only a 1.4Mbps link and it's more than enough for most corporate internet connections, even if the internal lan is 100Mbps.
The trick is, you have to remember that long haul links, no matter how fast, make really crappy bridges between homogenous networks. The segment length limitations are there for a reason. The speed of light is not just a plot complication in a larry niven novel.
The best way to handle a long haul link is to put a router on each end and let the routers deal with the general bullshit inherent in a really distant bridge.
Better yet, employ some form of traffic shaping (The buzzword is "QOS" but most people don't actually need or want guaranteed quality of service) in order to make sure that the pipe gets used to it's fullest capability rather than let applications choose their own window size and thus often cause the network to behave in a "bursty" manner, where a rude application or server occasionally kicks all other connections off the link for a few miliseconds so it can send it's data in first-class, and then let all the polite apps try and figure out if they can use a large window again.
The traffic controller code in recent 2.2 kernels is great for that kind of thing, btw.
Of course, as other people have pointed out, the legality of a 2Mbps HAM link, let alone 10Mbps, is quite suspect. Someone with access to recent FCC rulings should comment on it.
A licensed amateur may design and build their own device to transmit legally in the HAM bands, and it seems from the rules most people know about that the 10ghz HAM band is limited to 56kbps.
Consumer devices can use unlicensed frequencies such as the 900mhz range used by cordless phones and cellular devices, and the 2.4ghz range that's starting to be used by cordless and cellular devices, but this requires that the design of the device be FCC approved, not an easy thing for joe solderingiron to acheive.
It's posible to use a licensed frequency for high speed data transmission of just about anything you like, but this also requires an FCC approved design.
Is there anyone here who's recently researched FCC rules regarding packet data rates?
This is just like television, only you can see much further.
http://www.qsl.net/ke5fx/uwave.html
Definately something to watch, though. Maybe I can get my neighbor to get DSL and then beam it over microwave to me (I'm a little too far...)
dc
--
Wooden armaments to battle your imaginary foes!
I believe the original specs from N6GN indicated that even the 100 mW transmitters could probably reach 40 miles with a good LOS. (Of course, getting that good an LOS is TOUGH. You'd need VERY tall towers and no hills in between.)
retrorocket.o not found, launch anyway?
This project is prety non-informative at this point. C'mon, how about linking to his page after he reads his Microwave Data Link for Dummies(TM) handbook.
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Have fun: Join D.N.A. (National Dyslexics Association)
When was this announced?
I do recall that 56k was the limit in the 440 band, but I haven't head of 56k limits in higher bands. (I think most of them have some limit, but higher than 56k once you get above 1 GHz.)
retrorocket.o not found, launch anyway?
It links to the same design this project links to.
retrorocket.o not found, launch anyway?
Sure. But you really won't need to get into things like IP encryption or scrambling the signal at an analog level.
The limitation of using microwaves turns into a terrific advantage: line of sight is required.
I mean, this thing is almost as much of a line-of-sight system as pointing a laser pointer at the side of a building. The path has to be clear, and the two transceivers will have to be pointed at each other.
It depends on the antennas, but to get any range with the power levels discussed, you're going to need something with a fairly good reflector, preferably parabolic. (Hacking a DirecTV antenna comes immediately to mind.) These are incredibly directional; being off by a degree could lose 3dB of signal strength (half your signal) or more.
This means that someone who wishes to intercept your communications has to be directly in line of sight with your existing equipment. If he's behind one antenna or the other, he'll only be able to receive packets from the more distant antenna, since the closer antenna will be transmitting away from him.
Placing anything in the line of sight - let's say a reflector going to a receive-only rig - will attentuate the signal available for communications to a point where your users will notice it.
In fact, I used to work, on a freelance basis, for a TV station in Ottawa. CJOH-TV is located at Clyde and Merivale, and has a microwave uplink to their transmitter at Camp Fortune, Quebec.
One year, the technicians went out on strike, and it was starting to get pretty nasty.
You know those little plastic-foil balloons? Someone from the NABET union went out and bought thousands of these, along with enough helium to fill them. Somehow, the station management found out and immediately gave in to the demands of the union.
It is speculated, and is the subject of great water cooler and craft services table lore in Ottawa's TV community, that the plan was to inflate the balloons and tether them with strings from some location where they'd be blocking the microwave link. Foil balloons would very nicely have scattered CJOH's uplink and taken the station off the air.
That's to illustrate how much line-of-sight issues will be relevant. Line-of-sight usually implies height (to get above trees and buildings and things), which usually makes things pretty difficult to intercept. In CJOH's case, their uplink was only about 30-50 feet above ground level, and would have been fairly easy to stop with balloons but not practical to eavesdrop (not that you'd want to eavesdrop the microwave when you could just have tuned your TV set to channel 13). Eavesdropping this would have required being in the line of sight and actually placing a receiver high enough to be in the beam - which would have to be a fairly permanent installation, you couldn't do it with a balloon or a kite because it would sway too much to keep the antenna aimed.
Now, that's not to say that there isn't some risk of reflection from buildings and things that will be scattering the beam a bit. But the scattered signal reflected off objects will be very tough to receive with any degree of reliability.
I think the point is, if someone really wants to intercept your communications, they will. Take reasonable precautions to prevent it. How reasonable the precautions are will therefore be dictated by how sensitive the information is. I'd suggest that the difficulty in piecing together such a system, as well as the line-of-sight issues involved, will probably be sufficient security for even fairly sensitive data.
Fire and Meat. Yummy.
If you want a reliable link, you have to do link margin calculations and take weather and wind into account. If the path has 20 dB fades, you need an ERP 20 dB higher than the best case.
When trying to determine if a license is necessary for transmitting on any given frequency, you have to look at the FCC regulations for that part of the radio spectrum. In general, there is nowhere that you have free reign to transmit anything you want. This is a very old problem whose solution is well known. They tried the free-for-all thing 100 years ago when radio was new and found that everyone just interferes with each other and can't do anything, unless they group similar users in certain bands (frequency ranges) of the radio spectrum. So there's a reason why they do that.
On Amateur Radio frequencies, you need an Amateur Radio license. They're not hard to get. Dave lists a good start on his web page on how to study to get one.
On 802.11 Wireless Ethernet, you have to stay on low power to avoid interfering with the licensed users of those frequencies. So even though you don't have a license, that's the condition under which you may transmit. Other commercial uses require commercial licenses that you probably can't afford for a hobby project.
Don't just flaunt the rules. You'll undoubtedly end up interfering with someone who belongs there, which just isn't nice. It may take them a lot of work to find you and they won't be happy when they do because of all the work you caused them. And the FCC started doing enforcement again last year after neglecting it since 1993. Some people are still of the attitude that there's no enforcement. They won't accept ignorance as a defense.
Microwaves are line of sight. This is mentioned in the document that provides the actual plans to build this device, which is linked to the website that this article is about. For boat cruising, I would recommend RF. You might also try a more powerful transmitter, so that you can go farther out. You will, however, need to get several ham radio licenses before you are allowed to legally construct such a device.
Eh...
L0pht's already working on it. guerrilla.net
-Spazimodo
Fsck the millennium, we want it now.
Fsck the millennium, we want it now.
Millennium Crisis Line: 0890 900 2000 [calls cost 50p/min]
why can't you crypt it? Say you use this for email, do you want the entire world "packet sniffing" on your converastions?
Given the insecure nature of shooting you data out into waves avaiable for anyone to listen to, there should be some legal way to secure this...
Maybe "incode" all your senastive information into "quake3 packets"?
"`Ford, you're turning into a penguin. Stop it.'" -THHGTTG
OK, fine i'm wrong. I admit it. :)
-Craig
We're here to give you an OS, not a religion.
That may have actually been his goal, though, so your impression may not have been mistaken at all.
In order to reach that goal, though, he heard about and began investigating amateur radio as a means to that goal, which isn't quite appropriate, but isn't a bad thing at all if he doesn't mind using it strictly in a hobby capacity (not using it with live, potentially regulation-breaking content).
Seriously, unless you are very careful, microwaves are extremely dangerous.
I don't know how many of you have been shocked by 110v, but I can attest to the fact that I would rather spend an hour with 110 going through my body than even spending a single second getting a burn from RF.
So, please know what you are doing before you attempt to build something like this. Even standing in the wrong place with relation to the feedhorn is enough to make you sterile in nothing flat, if not cook you from the inside out.
the infrared transmitters could go through buildings
Infrared can't penetrate several pieces of paper, let alone buildings. Infrared is just below visible light in the frequency spectrum, at about 70 terahertz.
The higher you go with frequencies, the more bandwidth you can easily get. (Exceptions are TV broadcasters, who somehow managed to swindle the FCC out of 6mhz per channel in the 55 through 800mhz range). The more bandwidth, the more clarity (perhaps what you meant by "less interference") you can encode in whatever digital encoding scheme you're using. However, the signal also gets more directional and starts to not penetrate things (like raindrops and trees). HF (signals below about 30mhz, this includes shortwave) can be reflected by the atmosphere and travel around the world (depending on conditions and the specific band you're using). VHF/UHF (30mhz to about 1ghz) will go through hills/concrete structures/etc usually, but there's not much bandwidth left in the VHF/UHF range for people wanting to set up 2mbps links to their friends. Microwave (1GHz on up) has plenty of room, but the signals can't penetrate much.
Robert Woodcock, KC7WOZ
...is his domain name. What a great way to get back all those nice, short domain names! Too bad underscores aren't legal.
- No profanity
- No music
- Nothing remotely commercial
- No encryption
After all, it's a privilege to generate an electromagnetic field...--Just the place for a snark!
You can't use an encrypted connection. It's against FCC rules for amateur operation.
One of the unfortunate drawbacks to high-speed wireless data transmission, especially in the 10mbps range, is the tradeoff of frequency vs. transmission range. (I'm pulling this all from memory of a conference I attended on wireless data transmission, please correct me if I'm wrong). The problem is that high bandwidth wireless pipes (microwave) require very high transmission frequencies, typically in the 2.x GHz range IIRC. Waveforms with high frequencies have short wavelength, and therefore do not travel far, or through objects (in the same way that you can hear the bass from your neighbor's stereo, but not the treble).
Unless this technology has advanced substantially in recent years, most microwave transmitters require line-of-sight, and rarely transmit more than a few miles. An ISP I used to work for purchased a set of microwave transmitters to allow them to expand their business into a neighboring state without paying taxes and tarrifs on a leased line crossing state borders. The band these microwave transmitters use isn't retulated by the FCC, so transmission at those frequencies is open game. We had to set up the transmitters on opposite sides of the border (a few miles apart), and align them for line of sight. Even at a short range, we still only got E1 speeds (which I understand to be ~2mpbs).
alt3r3go
badges? we don't need no steenking badges!