Operation

Once the network is installed and operating, the network operator has to "Keep 'em flying." This will include dealing with day-to-day challenges, the greatest of which is dealing with dynamic interference issues.

How to Deal with Interference Issues

According to the FCC, all Part 15 users must accept interference as part of doing business over public airwaves, but that does not mean we must accept harmful interference, which is an obstruction of such magnitude that signals carrying other telecommunication services are disrupted. What this means is that normal interference is inevitable; thus, it must be calculated as part of the plan and dealt with as issues come up. Simply cranking up the power causes harmful interference, so other peaceful ways of managing the interference must be found.

A variety of sources cause interference on the 2.4 GHz and 5 GHz bands; some of which are 2.4 GHz cordless phones, Bluetooth-enabled devices, microwave ovens, RF lighting, and other WLANs. Currently, the 5 GHz band is relatively free of interference, but as time goes on, this band too will be crowded.

2.4 GHz cordless phones marginally interfere with 802.11b. If it's necessary to buy cordless phones after an 802.11b system is installed, use a 900 MHz model instead.

Bluetooth enables users to link devices wirelessly. Interference from Bluetooth is limited to a short distance due to its limited range. Since Bluetooth devices are used all over the place, they really cannot be anticipated in a design or plan.

Microwave ovens that leak too much can be health hazards. Often the seals fail on older microwave ovens. Sometimes it's worth replacing a heavily used leaking microwave. If it's not heavily used, it only interferes once in a while. Distance from the unit works wonders. It's a good idea to plan APs away from microwave ovens. If an AP has to be close to an AP, plan its frequency on either channel 1 or 11 so that it's far away from the microwave oven's center frequency of 2.45 GHz (which is near channel 6).

RF lighting from Fusion Lighting, a company based in Maryland, uses a standard magnetron similar to the ones used in microwave ovens. The magnetron focuses energy on a quartz sphere the size of a golf ball filled with inert krypton or argon gas and sulfur. The microwave energy excites the gas and effectively lights up a plasma that releases extremely efficient light. The white light created is nearly identical to sunlight and costs much less than fluorescent lighting. Although this application has been approved by the FCC under Part 18, it has the potential of interfering with your Wi-Fi project. Used at North Carolina's Pope Air Force Base and at the Smithsonian Air and Space Museum in Washington, D.C., this type of lighting totally wipes out the use of 802.11b.^[17] Most likely, channels 1 and 11 are still usable. Proper planning and a site survey will uncover this.

As you can see, the planning step is very important when it is done methodically on paper because this forces you to think out every step and cover all the bases. Frequency coordination with other WISPs and WLAN users in your area is equally as important as good planning. It's important to start the relationships on a friendly and cooperative basis so that when new links or APs are planned and deployed, they are coordinated together, rather than creating a new crisis. It is important to settle on the same modulation method (FHSS and DSSS) if possible. Communication is probably the most important part of this process.

^[17]Ephraim Schwartz, "From Point B to A," Infoworld, www.infoworld.com/articles/op/xml/02/04/15/020415opwireless.xml.