Windows XP and 2000

Interference with Other Users

The sections of Part 15 that deal with interference are tricky enough to understand. The real problem, as I see it, lies in understanding how interference problems can be resolved within the Part 15 framework. There's a lot of misunderstanding about things like who has priority in the band and who doesn't, and even legal experts can be mistaken in their interpretation of FCC rules. I am neither a government bureaucrat nor a lawyer, and so my interpretation may be flawed as well. What I write here is for general orientation only. If you feel you have a true 'legal situation' regarding Wi-Fi or another service in conflict with a Wi-Fi installation, get legal counsel-ideally, counsel with some experience in FCC regulations and communications law generally. This might be a lawyer who represents local radio or broadcast TV stations, for example. A local legal referral service may be helpful, as this is a rather arcane legal specialty.

The big picture with Part 15 is fairly simple: Part 15 devices (including your Wi-Fi installation) must not cause harmful interference to others, and must gracefully accept interference received from others. This seemingly contradictory philosophy reflects (in legalese) the inevitable truth that interference is going to happen. We must try our best to avoid it, but when it happens, we have no legal basis to fight it.

This sounds grim, but it's not… at least, not yet. (More on which later.)

Keep in mind that 'harmful interference' as the FCC defines it does not mean deliberate jamming. That's a Federal crime, though it's difficult to prove. 'Harmful interference' means interference from devices that are legally certified to operate and are just doing their ordinary jobs correctly.

As I hope I made clear in Chapter 1, the 2.40 to 2.47 GHz stretch of the electromagnetic spectrum is not only 'the Wi-Fi band.' Wi-Fi technologies are a very recent addition to an already-substantial crowd of users in that region of radio space.

It is also sobering to note that virtually all of those earlier users have a better legal claim on the spectrum space than Wi-Fi does.

It's useful to take a look at some of the other users of the band and see what the problems might be.

• Part 18: Industrial, scientific, and medical RF devices. These are things like microwave ovens, induction heaters, medical diathermy machines, jewelry cleaners, and medical diagnostic equipment. They use RF (radio frequency) energy for things other than communication; mostly heating, sometimes imaging. The FCC has pretty tight standards on such gear, and unless you're right on top of a business using machinery of this type, it's unlikely to be an issue. That's good, because there's very little to be done about it. Part 18 ISM devices are primary in the band, and even if they interfere with you, that's in a legal sense their right. Users of Part 18 devices have no control over their frequency or power output. Many ISM device users may not even understand what their machines are doing nor that they have any 'radio' component at all. If you have an apartment over a business that uses Part 18 gear, well, consider a move to 802.11a gear operating at 5 GHz.

• Part 97: Amateur Radio. There is an Amateur Radio allocation from 2390 to

2450 MHz. There is some fine print about the amateur radio service being primary on some frequencies and secondary to others, but the kicker is that Part 15 devices (which include Wi-Fi networking gear) are secondary to all licensed services using the same frequencies. So if a ham radio station interferes with your network, you have no legal recourse and will have to accept that interference gracefully. On the other hand, ham radio ops have a lot more mobility on the bands, in that ham radio transmitters are allowed to move around within a band and are not tied to one frequency. So with a little diplomacy you may be able to persuade nearby ham operators to slide up or down away from the Wi-Fi channel on which you're operating. The very best diplomacy is to become a ham radio operator yourself. Apart from diplomacy, you'll have access to one of the brightest bunches of guys in the technology arena, and you'll learn something in the process. I've been a ham radio op for thirty years now, and the good news is that amateur radio use of 2.4 GHz is very thin, and almost no factory-made gear is sold for those frequencies. What gear is used on 2.4 GHz is hand-made or modified from some other service. Thus the guys who do use the band are extremely sharp technically, and may be able to work with you if approached gracefully. (They'll be much more inclined to do so if you're also a licensed amateur!)

• Part 80 and Part 87: Maritime and Aviation Radar. Aircraft and watercraft are allowed to use radar devices on broad stretches of frequencies from 470 MHz, past 2.4 GHz and all the way up to 10.5 GHz. There's not much to be done about this, but the interference is likely to be transient (as ships pass by or planes fly over) and your Wi-Fi signals are so weak as to not even be on their…radar.

• Part 90: Public Safety Radio. Part 90 radio communication devices operate from 2.45 to 2.835 GHz. Use of the band may be on a licensed or unlicensed basis, under parameters that I do not fully understand. Police and fire departments are the main users, along with rescue services and other odds and ends. Legally, they are considered licensed and have priority on the frequencies that they use. Unless you're next door to a police station or fire department Part 90 is unlikely to be a problem. Part 90 frequencies do not overlap the entire Wi-Fi band, so if you're asked by a public safety agency to move your network, you can take it to one of the channels (1 to 6) below 2.45 GHz.

• Part 74: Electronic News Gathering (ENG). Mobile TV news crews operate vans with their own high-gain parabolic antennas on retractable masts. You've probably seen these vans running around town or parked with their antennas cranked up in front of baseball games or major crime scenes. There is an ENG allocation from 2.45 to 2.4835 GHz. Because they're mobile and running at higher power levels you're unlikely to interfere with them. That's good, because they're a licensed service and have a claim on the band that you don't. If they interfere with you, well, you have to just wait it out-but by the nature of the beast, the interference is unlikely to last very long. As with Part 90, the overlap with Wi-Fi is not complete, and you can move to a lower channel (6 or under) if it becomes a problem.

That's the short list of people who have a better claim on the band that you and I and our Wi-Fi networks share. (There are some others but those are the ones you're most likely to encounter.) It sounds grimmer than it is. Most licensed sources of interference are mobile, and will come and go and won't be a problem for long. The exceptions are Part 18 machinery and public safety base stations. You can dodge Part 74 and Part 90 base station users by moving down to channels 1 to 6. Interference from Part 18 devices you may have to live with, or else move to 802.11a.

As for amateur radio operators, well, they have frequency mobility options that you don't, and their use of the band is still fairly thin. Join them, don't fight them- especially since a lot of ham radio people are also avid users of 802.11b gear. My sense is that many (perhaps most) computer enthusiasts who are also ham radio ops avoid using the 13 cm band (as they call it) to avoid interfering with their own networks. This is your saving grace. Take it gladly, because…

…your biggest single problem in terms of interference is likely to be other nearby Wi-Fi networks. Such interference is almost inevitable, and your legal recourse is close to nil. If you live in an apartment building or condo complex, your problems may well be intense, if not now then in coming years, as Wi-Fi use moves from sparse to ubiquitous. Your neighbors may have no idea how to change their access point channels, and they may not feel inclined to let you tinker with their networking machinery.

If you suspect interference from a neighbor's Wi-Fi network, download the NetStumbler utility (see Chapters 18 and 19) and determine what channels your neighbors are using. See what signals are strongest, and try to identify a channel as far removed in frequency from the strongest nearby access points as you can manage, and move there. (See Figure 1.3 for a frequency chart of 802.11b channels.) If you're on good terms with your neighbors, it's possible to work with them to limit the excursion of both your and their signals, possibly by placing a flat or curved mesh reflector (see Chapter 17) behind their access points or yours. (Try yours first to see if that helps. It's easier tweaking your own system.)

If things seem hopeless, you can switch to 802.11a equipment, which is shorterrange and more suited for use in an apartment or condo than in a largish house. (It is unaffected by interference from 802.11b equipment, and users of the 5 GHz spectrum are much fewer and farther between.)

Other Part 15 devices are occasionally a problem, including cordless phones and microwave ovens. The one I'm really worried about is still too new to weigh the interference threat: Bluetooth.This supposedly ultra-short-range 'personal area network' technology isn't quite as short-range as some claim, and it works on many of the same frequencies as Wi-Fi. I'm not convinced that Bluetooth is as useful as claimed, and I recommend against using both Bluetooth devices and Wi-Fi devices in the same computer installation, at least until we know more about how well the two technologies coexist. Bluetooth and Wi-Fi are both Part 15 devices and are on equal legal footing in terms of using the band, but that doesn't help when your connection locks up.

In summary: According to Part 15, you and I and others in Wi-Fi land have no legal standing against other users of the Wi-Fi frequencies. Finesse, technical flexibility (that is, learning how to change your access point's channel), and diplomacy are your only weapons. Use them. Complaints to the FCC and lawsuits will get you nowhere.