You live your life at 2.4GHz. Your router, your cordless phone, your Bluetooth earpiece, your baby monitor and your garage opener all love and live on this radio frequency and no others. Why? The answer is in your kitchen.
What We're Talking About
Before we charge too far ahead here, let's run over the basics. Your house or apartment, or the coffee shop you're sitting in now, is saturated with radio waves. Inconceivable numbers of them, in fact, vibrating forth from radio stations, TV stations, cellular towers and the universe itself into the space you inhabit. You're being bombarded constantly with electromagnetic waves of all kind of frequencies, many of which have been encoded with specific information, whether it be a voice, a tone or digital data. Hell, maybe even these very words.
On top of that, you're surrounded by waves of your own creation. Inside your home are a dozen tiny little radio stations: your router, your cordless phone, your garage door opener. Anything you own that's wireless, more or less. Friggin' radio waves: they're everywhere.
Really, it's odd that your cordless phone even has that 2.4GHz sticker. To your average, not-so-technically-inclined shopper, it's a number that means A) nothing or B) the wrong thing. ("2.4GHz? That's faster than my computer!")
What that number actually signifies is broadcast frequency, or the frequency of the waves that the phone's base station sends to its handset. That's it. In fact, the hertz itself is just a unit for frequency in any context: it's the number of times that something happens over the course of a second. In wireless communications, it refers to wave oscillation. In computers, it refers to processor clock rates. For TVs, the rate at which the screen refreshes; for me, clapping in front of my computer right now, it's the rate at which I'm doing so. One hertz, slow clap.
The question then is why so many of your gadgets operate at 2.4GHz, instead of the ~2,399,999,999 frequencies below it or the effectively infinite number above it. It seems almost controlled or guided. It seems maybe a bit arbitrary. It seems, well, regulated.
A glance at US Federal Communications Commission regulations confirms any suspicions. A band of frequencies clustered around 2.4GHz has been designated, along with a handful of others, as the industrial, scientific and medical radio bands. "A lot of the unlicensed stuff - for example Wi-Fi - is on the 2.4GHz or the 900Mhz frequencies - the ISM bands. You don't need a licence to operate on them." That's Ira Kelpz, Deputy Chief, Office of Engineering and Technology at the FCC, explaining precisely why these ISM bands are attractive to gadget makers: they're free to use. If routers and cordless phones and whatever else are relegated to a small band 2.4GHz, then their radio waves won't interfere with, say, mobile phones operating at 1.9GHz, or AM radio, which broadcasts between 535kHz and 1.7MHz. The ISM is a ghetto for unlicensed wireless transmission, recommended first by a quiet little agency in a Swiss office of the United Nations called the ITU, then formalised, modified and codified for practical use by the governments of the world, including the FCC.
The current ISM standards were established in 1985, and just in time. Our phones were on the cusp of losing their cords, and in the near future, broadband internet connections would come into existence and become magically wireless. All these gadgets needed frequencies that didn't require licenses, but which were nestled between the ones that did. Frequencies that weren't so high that they sacrificed broadcast penetration (through walls, for example), but weren't so low that they required foot-long antennae. In short, they needed the ISM bands. So they took them.
Now, there are many, many frequencies that qualify as "unlicensed", but only a handful get used in our phones, routers and walkie talkies.
In the case of something like phones, which are sold paired with a specific base station, choosing the right unlicensed frequency is a pretty straightforward calculation: a 900MHz system will be more easily able to broadcast through a multi-floor house, but a 2.4GHz system will have a longer range (if unobstructed) and generally requires a smaller antenna, which keeps the phone's size in check.
Wi-Fi routers started as proprietary paired systems operating on all manner of frequencies, only settling on a standard - 5GHz - with the codification of 802.11a. When 802.11b and then g came along, the Institute of Electrical and Electronics Engineers agreed that 2.4GHz, with its wide channel selection and range/penetration/cost potential, was a safer bet. Today, some wireless-n routers can operate on both 2.4GHz and 5GHz bands concurrently. Routers could function just as well at 2.3GHz or 2.5GHz, but they're not allowed. It's the rules. The 2.4GHz band, which runs from about 2400MHz to 2483.5Mhz, is where routers have to live.
For this, they can thank the microwave.
Microwave ovens heat food by blasting it with, literally, microwaves. At certain frequencies, such waves cause something called dialectric heating in water and fat, while passing straight through other materials, like plastic or glass, without exciting them much at all. (Metal, on the other hand, gets too excited.) For a full explanation of how dialectric heating works, click here, but for the purposes of this article, just know this: only certain materials are susceptible, and only when bombarded with waves of a certain frequency and power. One of those frequencies is 915MHz. Others fall at 5.8GHz and 24GHz. But the one that proved to be both effective and relatively cheap to achieve was 2.45GHz. That's the frequency emitted by your microwave, right there in the kitchen.
So when the FCC got around to establish just which frequencies unlicensed gadgets could broadcast on, they had a lot of things to think about. First, they had to consider which frequencies were already in use by stuff like radio and TV. Those would be off-limits. Then, of the remaining usable, unallocated frequencies, they sought out the ones that were already being used by existing equipment. One thing they noticed? Microwaves were popular! They'd been around commercially since 1947. And generally, they operated at a specific frequency: 2.45GHz. Despite heavy shielding, microwave ovens' powerful emissions could sometimes interfere with neighbouring frequencies, so it was decided that they should be given a few megahertz of space in both directions. And so the 2400 to 2483.5Mhz ISM band was born.
That these free-for-all frequencies could one day get overcrowded was always a possibility. But the FCC's primary concern is minding the frequencies it licenses; everyone working in ISM frequencies, then, must fend for themselves. And they do! Your microwave and your router might emit waves in the same frequency range, and this might screw with your router's connectivity a little bit. Generally though, the router companies have been able to minimise interference by boosting signal strength and writing more intelligent firmware. And outward emissions of microwaves are at least supposed to be minimised. (That perforated metal shield in the glass door of your microwave? It's a shield - the holes in it are smaller than the physical width of the 2.4GHz wave.) In the end, things work.
That's not to say that the 2.4GHz band isn't getting crowded. Many routers operate at least in part on the 5GHz band, and a quick survey of your local electonics store will find wireless phones at 900MHz, 1.9GHz and 5GHz. But the King of Frequency mountain, the band loved by billions of wireless connections around the world, be they Bluetooth, Wi-Fi or nonstandard RF remotes, is my band, your band, our band, 2.4. And all because we wanted to cook our food a little faster.
Original art by guest artist Chris McVeigh (AKA powerpig). You can catch all his work at flickr.com/powerpig, and follow him on Twitter. (@Actionfigured)
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