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Inspiring Adventures in SF Wireless Networking
JimDog writes "Here's a description I wrote of how I set up a
point-to-point 802.11b link over 3.5 miles for
Internet access at my house. The link runs at 3.5 Mbps, which I barely make a dent in, and I'd like to offer the rest of the bandwidth to anyone who's got line-of-sight to my location in San Francisco." The great thing about this story is both his terrific exposure to different parts of city and his willingness to share. It also makes it clear just how easy it is to set up a long distance link.
Not too hard to argue if you are a large bandwidth provider like a large ISP. Bandwidth is typically purchased in bulk, meaning at any given point in time, a provider is nowhere near maximum capacity. 3.5 megs doesn't even make a dent in a few OC3s.
In addition, it is a "perk of the job". No longer an employee, no longer free bandwidth. This is the problem with the community network designs relying on a few POP points to provide access...
For your own safety, and your employer's, you really need to install proper lightning arrestors on your outdoor antennas, or you could be in a world of hurt (and liability) if a fire starts in your house or your employer's building as a result of a strike.
(I also question the wisdom of allowing outsiders on your employer's network, since you never know what kind of illegal activities the random users might be up to. Your employer says everything's cool, though so he's probably assumed this risk.)
Just be careful.
-Isaac
I am not a lawyer, and this is not legal advice. For Entertainment Purposes Only.
The waveguide is horizontally polarized, his grid antenna is vertically polarized. He should turn his grid 90 degrees, and reaim... Prolly start seeing a full 6mb (the real-world thoughput of wifi) over that link.
The short answer is yes, but it would be incredibly hard to have every household. I'm sure there would be some places so dense that it couldn't be done too. You would have to have many cells of operation, using narrow beam antenna's both horizontal and verticle polarity and many many AP units. (For just one site) The big hangup on 802.11b is that there are 11 channels but only 3 of those channels do NOT overlap. So you have to be carefull how set it up.
By Tim Pozar - pozar@lns.com
for the Bay Area Wireless User Group
With the unlicensed use of 802.11b radio Ethernet devices in the Industrial, Scientific and Medical band that has been set aside for such use, there is confusion of what is allowed or limited by the Federal Communication Commissions Rules and Regulations. This paper is meant to help guide folks through the cryptic nature of these rules.
This paper does not cover other legal issues of using these devices such as FCC type-acceptance, Radio Frequency Radiation issues (ie. ANSI RFR levels) or Appropiate Use Policies (AUPs) of ISPs you may connect to.
The FCC is a regulation body whose purpose was defined in the Communications Act of 1934 as:
"For the purpose of regulating interstate and foreign commerce in communication by wire and radio so as to make available, so far as possible, to all the people of the United States a rapid, efficient, Nation-wide, and world-wide wire and radio communication service with adequate facilities at reasonable charges, for the purpose of the national defense, for the purpose of promoting safety of life and property through the use of wire and radio communications, and for the purpose of securing a more effective execution of this policy by centralizing authority heretofore granted by law to several agencies and by granting additional authority with respect to interstate and foreign commerce in wire and radio communication, there is created a commission to be known as the "Federal Communications Commission", which shall be constituted as hereinafter provided, and which shall execute and enforce the provisions of this chapter."
The FCC, with the Act of 1934, was empowered to regulate wire and wireless communications. Wired communications regulation was needed to monitor and regulate monopolies. Wireless regulation is needed as the spectrum is finite. The FCC is the "traffic cop" to ensure that communications is not interfered with.
Almost every bit of spectrum is regulated by the FCC with the exception of extreamly high or low frequency spectrum and bands managed by the Intergovernmental Radio Advisory Committee (IRAC) for the military and other goverment orginizations, by licensing operators of radio equipment. The part of the FCC's rules that cover the operation of equipment that does not need a license is (3) Except as shown in paragraphs (b)(3) (i), (ii) and (iii) of this section, if transmitting antennas of directional gain greater than 6 dBi are used the peak output power from the intentional radiator shall be reduced below the stated values in paragraphs (b)(1) or (b)(2) of this section, as appropriate, by the amount in dB that the directional gain of the antenna exceeds 6 dBi.
Do we need to turn down the transmitter?
15.247(b)(3) makes the assumption that you are running a point to multi-point network much like an Apple Airport or Cisco/Aironet AP box with a number of computers connecting to the network. They may be randomly surrounding the access point so you are not using a directional antenna.
But what does the FCC mean when they limit the "intentional radiator" to one watt?
This is a critical sticking point in understanding what the FCC is talking about. There is some question of what an "intentional radiator" consists of and what and where exactly is 1 watt measured. Unfortunatly if you just look at these poorly written rules you will not understand what the FCC means here. One has to look a bit deeper to the "Report and Order" and Notice of Proposed Rulemaking" that generated this section of the rules.
Things get a little clearer when we read this sentence in paragraph 4 of the Report and Order...
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"The current regulations limit spread spectrum systems to a maximum peak transmitter output power of one watt. When operating at that power level, the maximum directional gain of the associated antenna may not exceed 6 dBi, resulting in a maximum equivalent isotropically radiated power (EIRP) of four watts, i.e., 6 dBW."
With the old rules they are refering to the "intentional radiator" as a whole with a directional antenna can't exceed 6 dBw or 36 dBm and the antenna gain can't be more than 6 dBI. The transmitter can be up to one watt.In order to know if we are legal or if we need to turn down the transmitter we need to know the gain of your "intentional radiator". Let's say your access point actually puts out 1 watt of power and you want to put an omni-directiona antenna on it that has a gain of 5 dBi such as the ORiNOCO Range Extender Antenna".
We know the gain of the antenna, the transmitter but we also need to know the loss of the transmission line going to the antenna as this attenuats the transmitter output power going into the antenna. Looking up the attenuation of a common coax cable such as RG-8 on an coax attenuation table we find that at 2.4 GHz we have 16 dB of loss with 100 feet of cable. With a 10 foot cable your loss is about 1.6 dB. So your new "intentional radiator" will be radiating transmitter power output plus antenna gain minus coax loss or (30dBm + 5 dBi - 1.6 db) or 33.4 dBm or 2.2 watts EIRP.
Since this is a non-directional antenna you are limited to 1 watt EIRP or 30 dBm. The transmitter will need to be turned down 3.4 dB to 26.6 dBm or about 0.45 watts (450 mW) to get you back to 30 dBm or 1 watt EIRP.
If you think about this you may ask, "why add an omni-directional gain antenna it if I already was at 30 dBm?" You would be correct that it would be a waste of time. But if you had something like an Apple Airport that will only put out 15 dBm or 30 mW then you can add an omni-directional gain antenna and it will extend your "roaming" area. In fact you can add up to 15 dB of gain with an omni-directional antenna before you need to attenuate the output of the Wavelan card in the Airport.
Part 15.247(b)(3) actually gives you a free 6 dBi if you use a directional antenna your "intentional radiator". How do the do this? Only if the gain of the antenna is over 6 dBi will the Feds want you to roll back the EIRP of your "intentional radiator". You don't have to do it right at 1 watt EIRP. When would you do this? Say if you have an access point in the corner of a building and it needs to aim back into the work area. You don't want an omni-directional antenna as about 75% of the power would be going out the windows. Why not use a directional to keep the signal in the building and penetrate through the walls better? If we have antenna gain of about 12 dBi and in this case the antenna is a directional antenna. With the transmitter putting out 30 dBm and the coax has 1.6 dB of loss we have an "intentional radiator" that is putting out (30 dBm + 12 dBi - 1.6 dB) or 40.4 dBm or just over 10 watts EIRP. Since the antenna gain is 12 dBi and we have to reduce the power of this "intentional radiator" 1 db for every db we go over 6 dBi of the antenna we would have to roll the power back to 34.4 dBm or 2.2 watts EIRP (40.4 dBm - (12 dBi - 6 dBi)). Well, it is slightly better than 30 dBm or 1 watt EIRP.
There is another exception to this section of the FCC rules. Part 15.247(b)(3)(i) covers systems that are "fixed, point-to-point". That means this path only has two transmitters involved and they are bolted down by never moving their locations. Automobiles may not apply. An example would be if you have an access point and a user that is a couple blocks or even tens of miles away that you want to connect to.
This exception is more lenient as you only need to turn down the "intentional radiator" 1 dB for every 3 dB of signal over the 6 dBi of the antenna system. The FCC does this as it knows that these paths will not likely not be omni directional on each end and will have less of a chance to interfere with others as well as the need to span some long distances.
Lets look at an example using the same antenna, transmission line and transmitter as above. Without turning anything down we had an "intentional radiator" that was producing 40 dBm or 10 watts EIRP. Since the antenna gain is 12 dBi and we have to reduce the power of this "intentional radiator" 1 db for every 3 db we go over 6 dBi of antenna gain we would have to roll the power back to 38.4 dBm or 7 watts EIRP (40.4 - (12 dBi - 6 dBi) / 3).
Real world examples...
Recently I put up a short path between myself and a neighbor about 2 blocks away (.2 miles). I have an Apple Airport that uses the Lucent Wavelan Silver card that puts out 30 mW or about 15 dBm. The antennas have a gain of 24 dBi with a transmission line loss of about 6 db. This gives me an "intentional radiator" power of 48 dBi. Since the antenna gain is 18 dBi over the 6 dBi that the FCC gives you and since it is a fixed, point-to-point link I would have to limit my
[...]
Since the little Wavelan card only puts out 15 dBm, I am legal as far as part 15.247 goes.
Quicky Definitions...
dB, or one tenth of a Bel, is a unit of mesurment that looks at the ratio of one value to another. Gain or loss can be measured in dB. The dB scale is an exponential scale using the formula log(ratio)*10. This means that 3 dB is about twice the power, 10 dB is 10 times the power, 13 dB is about 20 times the power and 20 dB is 100 times the power.
dBm is deciBels referenced to a value of 1 miliWatt of power. Power over or under 1mW would be plus or minus dBm respectively.
If you have a transmitter that produces 1 watt of power that would be 1000 times more than 1 mW so that converts to 30 dBm.
dBW is deciBels referenced to a value of 1 Watt of power. Power over or under 1 Watt would be plus or minus dBW respectively.
Effective Isotropic Radiated Power defines the gain of an antenna over an "isotropic antenna" that would radiate equally in all directions.
An example would be a light bulb. A lightbulb is designed to radiate light equally well in all directions, except the direction that the base is in.
If you have an antenna that radiates better in one direction than another, it would likely have gain in this direction. The amount of gain would be shown as "dBi" or dB gain (or loss) over an "isotropic antenna".
To further our example above, if we have a light bulb and put it in front of a mirror, we would be taking the light radiation that would be heading in the direction of the mirror and reflecting it back in the same direction of the light not directed towards the mirror. Hence you would have twice the amount of light going in the direction of the refelction. As we are doubling the amount of light, we have a "gain" of 3dB or 3dBi.
Expect a response soon from the big ISPs. Wireless is currently in the phase that MP3 sharing was in its infancy, too small a blip to register much. But it will attract attention as more people do this. I wouldn't be surprised to see them even getting laws or FCC regulations that prevent wireless sharing.