3 km wi-fi link, how?


I'm wondering if it's possible to set up a decent wi-fi link between two points ca. 3 km (2 miles) distant from each other (I have line of sight). Most important, I'd like to know what gear I'd need for a project like this one. Which kind of (directional) antennae should I choose? how much should their gain be? do I need special APs to drive them, or external amplifiers, if anything like this is needed at all? I hope that you can give me some suggestions, because I have no clue. I tried to search the net, but these questions must be either too stupid or too difficult to answer, as I found nothing (that is, nothing that I'd be able to understand anyway).

Thanks in advance.

Reply to
Demetrio Girardi
Loading thread data ...

ok, here's something I don't get.

according to that calculator, using two 16dbI antennas with AP transmission power of 10mW (don't know how feasible the last one is) would lead to a 'Theoretical System Operating Margin' of 14.8 db. If I got it right, this means that as long as real-world noise doesn't cause more than 14 db power loss, it should work.

however, this other tool:

formatting link
says that power output (for the same antenna gain and tx power) is 26 db. Power loss over 2 miles is 110 db (both tools agree on this). How can it work then? what am I missing?

Reply to
Demetrio Girardi

I would like to pursue projects like this where I live also, maybe we can brainstorm together. I also know a very knowledgeable electronics wiz who seems willing to give me free pointers.

Reply to
Joseph Stewart

You need to do a link analysis. There are any number of web sites that offer online calculators. Here's just one:

formatting link

Reply to
Clark W. Griswold, Jr.

You need MORE than line of sight. You need Fresnel zone clearance. At

2 miles, that's about 33ft radius from the center line at midpoint. Do you have Fresnel zone clearance?
formatting link

Well, first you need to disclose what you plan to do with this link. The details will determine the type of equipment. For example:

  1. Do you need to pass only one MAC address or do you need a transparent bridge (where you glue to networks together via wireless)?
  2. How fast do you need to go? Mbits/sec thruput. If you're sharing an internet connection, there's little need to go faster than the broadband connection.
  3. Need bandwidth management or QoS features? VoIP? Games?
  4. How fanatical do you want to be about security? VPN? PPTP? WPA2-AES?
  5. Is there a cost limit?

Never mind the type for now. It's the gain that's important. Once the gain is established, then you can consider the type. For example, if you need more than about 15dBi of antenna gain, you won't find a yagi with that high gain. Similarly, panel antennas become huge above

14dBi. That leaves a dish which goes to 24dBi gain.

That needs to be calculated. See below. Incidentally, I've only done this about a dozen times in this newsgroup but I guess it's difficult to find with Google Groups or something. I guess I should add it to the FAQ.

Nothing special. Depending on the answers to the aforementioned 5 questions, the hardware is fairly generic. For simplicity, I'll assume a transparent bridge, where each end is identical, and passes more than one MAC address.

Hmmm... suggestion: Do the numbers first.

Yeah, that's the problem. If you don't have a vocabulary with the necessary buzzwords, you can't find anything useful with Google. Welcome to the wonderful world of wireless calculations.

Let's start with some assumptions.

Linksys WRT54G 802.11g with DD-WRT alternative firmware setup as a bridge. The transmit output is about +17dBm but can be cranked up (illegally) higher. I'll assume that you'll mount the WRT54G somewhere near the antenna with a single 3ft cable (pigtail) using LMR-240 coax. Loss will be about 4dB including connectors. Longer cables will require larger LMR-400 coax plus a pigtail.

Now, I gotta do some guesswork. I'll assume that you can live with about 6Mbits/sec thruput. That means your connection speed must be about twice that or 12Mbits/sec. The receiver sensitivity varies with the speed and modulation type. The following was lifted from the DI-624 datasheet but is close enough for most similar 802.11g radios: * 54Mbps OFDM, 10% PER, -68dBm * 48Mbps OFDM, 10% PER, -68dBm * 36Mbps OFDM, 10% PER, -75dBm * 24Mbps OFDM, 10% PER, -79dBm * 18Mbps OFDM, 10% PER, -82dBm * 12Mbps OFDM, 10% PER, -84dBm * 11Mbps CCK, 8% PER, -82dBm * 9Mbps OFDM, 10% PER, -87dBm * 6Mbps OFDM, 10% PER, -88dBm * 5.5Mbps CCK, 8% PER, -85dBm * 2Mbps QPSK, 8% PER, -86dBm * 1Mbps BPSK, 8% PER, -89dBm

12Mbits/sec sets the receiver sensitivity at -84dBm.

PER means Packet Error Rate which is similar to BER (Bit Error Rate) but is easier to measure. You would not want to operate at a 10% error rate. 1 in 10 packets trashed is useable, but no fun. This is

*ROUGHLY* the same as a BER of 1E10^5 which is one of the common reference levels for measuring receiver sensitivity. Again, these are measurement reference levels, not operating points.

Next I gotta guess the fade margin. That's how much stronger the operating level is above the receiver reference level. 20dB is considered to be a good minimum. In other words, your operating signal level must be 100 times stronger than the receiver sensitivity or the link is going to be flaky and unreliable. The relationship between fade margin and reliability is: Reliability Fade Margin 90% 8 dB 99% 18 dB 99.9% 28 dB 99.99% 38 dB 99.999% 48 dB

99% reliability might sound great, but that means your link will be useless for 1% of the year, or dead 3.6 days per year. Don't go below 20dB fade margin.

Plugging the above guesswork into

formatting link
playing with the antenna gains until I get 20dB fade margin:

TX power = +15dBm TX coax loss = 4dB (3ft LMR-240 plus a mess of connectors) TX ant gain = unknown Distance = 2 miles RX ant gain = unknown RX coax loss = 4dB (same at other end) RX sens = -84dBm (at 12Mbits/sec) Fade margin = 20dB yields a minimum antenna gain of +21dBi. That makes the antenna required to be a dish. You can get away with a lower gain antenna by running at a slower speed, shorter coax, or more tx power (at both ends).

In general the numbers and assumptions above can only get worse, not better. These calculations are for ideal environmental conditions, without interference, Fresnel zone obstructions, reflections, multipath, atmospherics, and terrain problems. If you have another

802.11b/g user along the line of sight, the high gain of the antenna will create a substantial interference problem. I suggest you do a site survey to see if there are any other users around.

I have some suggestions as to specific hardware and sources, but I would like to see the answers to my questions, and corrections to my assumptions first.

Reply to
Jeff Liebermann

Wow, thanks for your extensive answers.

The short answer is: I think so. However my setup would be significantly different from the scheme terabeam proposes. One end of the link would be approximately 1000ft higher (in altitude) than the other. If this doesn't change the shape of the Fresnel zone too much, it should be clear.

I'm not sure about what you mean by 'transparent' (no hops in between the networks?), however I don't need anything special. I don't mind if the two ends are on different subnets. So I'd settle for the cheapest / easiest solution.

here the broadest-band internet connection avaliable goes 4Mbps, and it's likely to remain such for a long time, so that would be enough.

No need for QoS, but what has a router/AP to do with VoIP or games in particular? One end needs to connect to an ADSL line and do NAT, but an additional router for that is no problem, they are fairly cheap.

I don't know. As long as one cannot simply power up a laptop to steal my bandwidth, I think I'm OK; but if slightly better security does not mean a lot more money, I'd probably give it a thought.

Approximately =80500 ($600). Above that it would be probably better to pay to get a phone line. Anyway, I'm hoping to spend much less.

I think you guessed right enough what I want to do. However I don't get what fade margin is all about. Is 20db considered to be a good minimum because of real-world interferences, or is it what I need in ideal conditions? Considering 9Mps throughput, two 18dbI antennas (the highest gain I can get without having to buy stuff abroad) would mean a SOM of 19.8 db. Is this too risky?

How much can it be boosted? I don't worry much about legality, since here (in Italy) the government is vaguely aware of the existence of wireless technologies, and I don't think anything is regulated. I'll check, though.

That would be wonderful, thanks.

Reply to
Demetrio Girardi

I'm not done yet. I threw together a collection of spreadsheets stolen from various manufacturers, students, experts, and myself on calculating the path. Some are a bit old and may have been updated.

formatting link

The height at each end become signifigant to not hit the ground due to the curvature of the earth. At 2 miles, that's not very important, especially with your altitude. The drawing on the Terrabeam page is clear enough. At midpoint (1 mile) do you have sufficient clearance? You should be able to visually guess. If you cannot see the 33ft radius at the midpoint, then some other objects are in the Fresnel zone between your endpoint and the midpoint clearance area. The sloppy way to do this is to take a photograph of path from some location that is perpendicular to the path. Draw the line of sight and Fresnel zone on the photograph using a straight line approximation. That will show any problems.

802.11 wireless is ALL bridging. There are no subnets in a bridge because there is nothing on Layer 3 (TCP/IP) involved. Everything is done at the MAC layer (layer 2). The only thing the access ponit IP address is used for is managing and configuring the box.

Transparent means that it looks to each end exactly as if it were connected with a CAT5 cable. This implies that all broadcasts are passed through the bridge, and that the wireless bridge can pass all MAC addresses that it sees.

However, if you only have one computah at the remote end, and don't need the bridge to be transparent, then a simple client radio at one end, and a simple access point at the other, is sufficient and cheaper. You decide.

To insure some headroom, 6Mbits/sec thruput is sufficient. Therefore, you will probably operate at a 12Mbit/sec connection rate (as I guessed). You will probably want to fix your connection speed in the bridge setup instead of using "auto".

Lots. If you're bridging two busy networks, the wireless bridge needs to decide how much junk will go across the bridge. It tends to also become slowest thing on the network which constitutes a bottleneck. Wireless is also a shared resource, where other users on the channel fight for airtime or cause interference. Therefore, latency tends to be erratic and longer than expected. If you have time critical traffic (VoIP, video conferencing, streaming content server), then you will need to prioritize the traffic through the bridge.

Minimum feature is therefore WPA encryption. No need for a RADIUS server based 802.1x authentication as you will be the only user.

You pay for a phone line every month while the wireless solution is only one payment. $600 is insufficient for a plug and play solution such as: |

formatting link
you'll probably end up doing it yourself. I still cannot determine if you want a bridge-bridge solution, or an AP-client solution. I'll go with the bridge-bridge because it's easier for me to do as both ends are identical.

WRT54G or WAP54G radio $60 Coax pigtail $15

24dBi dish antenna $70 waterproof box $20 Misc wire, mounts, etc $20 ======= Total (one end) $185

$370 is the price if you build it yourself. Less if you scrounge the parts or have existing hardware.

Unfortunately, 20dB under ideal conditions. Incidentally, fade margin and SOM (system operating margin) are the same thing. It's how much stronger the signal needs to be than the reference value used to measure the receiver sensitivity. Very roughly, it's the amount that you can REDUCE the signal and still have a functional (but error prone) link. When installing a point to point link, I always install attenuators in the coax line to measure my fade margin. If I calculate for 20dB of fade margin, and the link dies when I add 20dB of attenuation, then I did something wrong, somewhere.

Real world interference is difficult to quantify and calculate.

802.11g can handle considerable intereference and still function. What you'll see is an increase in packet loss. Ping the other end of the link continuously. Ideally, the latency should be less than 5msec for a 2 mile wireles bridge. If you see > The transmit output is about +17dBm but can be cranked up

The Linksys WRT54G box is based on Linux and therefore is open source firmware. Alternative firmware is available. One of the features is the ability to adjust the transmit power. I don't approve much of turning up the power from the usual +15dBm to +23dBm, but some value in between is probably acceptable. For firmware, I use and recommend DD-WRT: :

formatting link
you purchase a WRT54G or WRT54GS, please avoid the WRT54G v5 as this hardware mutation does not run Linux. See: :
formatting link
is chart is a bit old but gives a fair idea of the features.

Not yet. You decide if it's going to be bridge-bridge or AP-client type of system.

Reply to
Jeff Liebermann

Dead easy if its in the clear thus might help .

formatting link

Reply to

I have used the terabeam figures with success many many times. assuming its in the clear

Reply to

I'm not associated with this company in any way, but a couple of their devices (as Jeff suggested, an access point and a client device) would make that distance easily. They can be found on eBay much cheaper than suggested retail. I don't know anything about your country's laws, but apparently they do make models for each world region. Mount the radio/antenna device outside at each location, point it at the other location, run a single cat5 cable inside and do some simple hookup and networking.

formatting link
Remember, the suggested retail prices are exaggerated. They can be had cheaper.

Reply to

The tx limit in Italy (and in most European Countries) is 20db at the antenna side. So, antenna + Access Point (or equivalent) do NOT have to exceed 20db. Obviously you have to include cable/connector loss. So if you have 30db antenna well you will have some problems in tx. No problem with rx ;)

By the way, you have also ask for an authorization to operate in outdoor environments. So it's not exactly "free for use" in outdoor environments, it's regulated.

Regarding the Italian regulation, it's very clear who can do what, and it's easy to be issued of specific legal actions. So I would suggest to double check before starting the transmission.

Have fun :)

Reply to
Paolo Di Francesco

I'm not always right. However, I come close most of the time.

Smartbridges AirHaul boxes transmit up to +24dBm (adjustable). The best a WRT54GS can do is about +17dBm. 7dB of additional fade margin is worth about an 8 times improvement link reliability.

formatting link

I bought a pair of TR-6000F boxes on Craigslist for $430. Add two

19dbi dish antennas and installed it on a 1.5mile link across downtown Santa Cruz. Except for erratic interference problems and the roofers, I haven't had any difficulties. I hate to base my opinions on just one installation, but that's all I have to offer.

Everyone lies on their specs, but that's ok because nobody every reads or understands them.

Smartbridges numbers look too good to be true to me. They don't specify the BER or PER reference. Airhaul's -73dBm at 54Mbits/sec is quite a bit better than the DI-624 claim of -68dBm at 54Mbits/sec 10% PER reference. However, I would be interested in knowing their method of testing, reference level, and just how "typical" a result. Their

-97dBm at 1Mbit/sec looks like it's right at the noise floor and is IMHO rather suspicious. I'll do the numbers later and check. Also, better sensitivity can have a detrimental effect on receive dynamic range in that it might reduce the receiver overload point. That will result in more intermod, blocking, and interference susceptibility in a crowded RF environment.

Reply to
Jeff Liebermann

As always Jeff gives great info. I agree that the WRT54GS setup is the best for cheaper costs but have found it not to be as reliable as using Smartbridges equipment in setups more than a mile long. Smartbridges equipment obviously costs a little more if gotten off of ebay maybe twice as much if gotten brand new compared to Jeff's estimates, but I think it is worth the quality/reliability of the connections. I used Tranzeo equipment 2-3 years ago and had the worst headache ever. Maybe the stuff has come around by now but had a bad experience with it and they did not do anything to help me solve the problem or take the equipment back and was stuck with $750 in equipment I ended up unloading on ebay for nothing so can't justify using them ever again myself. I have personally setup numerous smartbridge equipment going up to 8 miles + and still running using there airbridge outdoor product with a

24 db grid. Th> >

formatting link
Therefore, you'll probably end up doing it yourself. I still cannot

Reply to

Jeff Liebermann wrote: Snip.....................................

For firmware, I use and recommend

formatting link
you think they had some ver4's left?


Reply to

WRT54GL CL7A = WRT54GL / BCM5352E Chipset / CPU 200Mhz

Translation: After the WRT54G v5 was totally trashed in the blogs, mailing lists, and newsgroups, and where pundits (like me) were advising everyone not to buy a V5, Linksys finally realized that they had screwed up. Linksys apparently didn't know why their customers were buying WRT54G routers, were trying to save literally pennies on the RAM, and are now offering a resurrected V4 router for Linux users at $70-$80 where the V4 formerly sold at about $55 (street price). Well, it's commendable that they figured out the obvious after only 2 months. I don't have sales figures but my guess is V5 sales fell through the floor, or there were LOTS of V5 returns. The GL version is nothing better than pre-xmas damage control. Also, the official pitch line might be baloney. There's very little price difference between 8MB and 16MB ram in 100K and up piece prices. There may be other differences that affect cost, but it's not the chip prices.

The good news is that they probably won't make the same mistake again.

Reply to
Jeff Liebermann
[POSTED TO alt.internet.wireless - REPLY ON USENET PLEASE]

And modest too. :)

Reply to
John Navas

WRT54GL < note "L" Avoid WRT54G V5 and up (without "L")

fundamentalism, fundamentally wrong.

Reply to

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.