Monday, March 28, 2005

Turning Junk Frequencies Into Gold

We take the 2.4 GHz WiFi band for granted these days. Hotspots, wireless routers and access points are so prolific that it seems reasonable that this was all carefully planned and allocated. WiFi does have it's own special frequency allocations, right? Wrong!

WiFi Hotspots are actually using a chunk of radio spectrum that was originally carved out as a toxic dump for radiation that no one wanted in their backyard. It's called the ISM or Industrial, Scientific and Medical band. There are all sorts of ISM frequencies assigned by the FCC. They range from 6.78 MHz in the shortwave band through 245 GHz in the EHF or Extremely High Frequency range. The ones we're mostly interested in today are the 900 MHz band from 902-928 MHz, the 2.4 GHz band from 2.4 to 2.5 GHz, and the 5.8 GHz band from 5.725 to 5.875 GHz. Does that middle one sound familiar? It's the frequencies used by 802.11b & 802.11g WiFi.

The ISM bands were originally carved out to allow unintentional radiators some place to radiate. Unintentional means that no one is trying to communicate. It's radiated energy that is a byproduct of some industrial, scientific or medical process. What sort of equipment? Turn your microwave oven around and look at the information on the back. Or if it weighs too much, look at the specifications in the manual that came with it. What frequency does it operate on? My late model Sharp Carousel Microwave says 2450 MHz which is also 2.45 GHz. Good grief. That's right in there with channels 8 and 9 of the IEEE 802.11b WiFi specification.

Yes, the ISM bands really were meant for things like commercial and home microwave ovens, industrial microwave heating like curing glued wooden parts, medical diathermy which is therapeutic heating of body tissues, and scientific instruments. For these applications to work they need to use radio frequencies and can't help but radiate some energy even when they are well shielded. These ISM assignments weren't set up as licensed bands because the emissions were a byproduct of other processes.

That changed in 1985 when the FCC opened up these bands for intentional radiators to give new technologies a place to go in the crowded radio spectrum. The requirements to put ISM frequencies to work include a fairly low radiated power of 4 watts or less and a no complaints policy. If your radio system suffers interference from a microwave oven or other equipment, too bad. You didn't buy a license to have exclusive use the channels so nobody is coming to your rescue.

At first blush, who would want these unprotected frequencies? Isn't operating on them just begging for trouble? Turns out that the interference situation isn't all that dire. ISM's attraction is license free operation with enough power for wireless phones, wireless networking and even video transmission. Cordless phones operate successfully on the 900 MHz, 2.4 GHz and 5.8 GHz bands. WiFi and Bluetooth have claimed 2.4 GHz. The 5.8 GHz band is being put to good use for such things as transmitting security camera video and is earmarked for WiMAX operation.

So why isn't the interference problem a show stopper? Partly because the range of both the intentional and unintentional signals is very limited. It's not like AM or Short Wave Radio where stations interfere with each other over hundreds and thousands of miles. The other big factor is the use of modulation techniques that are tolerant of interference. The two main forms of spread spectrum modulation, Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) are used by telephones, 802.11b WiFi and Bluetooth. Another resilient modulation technique, OFDM or Orthogonal Frequency Division Multiplexing is used for 802.11a, 802.11g, WiMAX and Broadband over Power Line, to name a few.

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