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Build Your Own 10Mbps Microwave Data Link
Posted by
CmdrTaco
on Thu Jun 08, 2000 01:20 PM
from the now-thats-a-cool-hack dept.
from the now-thats-a-cool-hack dept.
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!"
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Build your own 10Mbps Microwave Data Link
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Home made microwave links... (Score:3)
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.
Re:I do it all the time - My cost (Score:4)
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.
Amateur Radio *NOT* for commercial use! (Score:4)
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. [arrl.org]
Chuck Milam, KF9FR
Already exists...with Linux drivers (Score:3)
Two I am familiar with are Breezecom [breezecom.com] and Aironet [aironet.com], just bought by Cisco. If you poke around, you'll find many of these products have Linux drivers [ucsc.edu].
Re:Drawbacks to Wireless (Score:3)
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.
I'm the author of the original 10-mbps project... (Score:5)
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 [gbonline.com]. 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
No. (Score:3)
I'm the author of the original 10-mbps project... (Score:3)
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 [gbonline.com]. 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
Re:Amateur Radio *NOT* for commercial use! (Score:3)
Re:How secure? (Score:3)
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 [findu.com]. You could build a home-made LoJack out of something like this.
Dave, WL7RO
RF propogation (Score:3)
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.
Re:I do it all the time - My cost (Score:4)
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
Very directional in this case. (Score:3)
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...)
Dear Slashdot (Score:5)
p.s. Do you have any information where I can get a free counter for my website?
--
Re:How directed is a microwave signal (Score:3)
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.
doesn't sound like he'll get real far (Score:3)
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?
Ham Radio use restrictions (Score:5)
- No profanity
- No music
- Nothing remotely commercial
- No encryption
After all, it's a privilege to generate an electromagnetic field...Drawbacks to Wireless (Score:3)
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