Traffic in 2.4Ghz
July 6, 2010 8:51 AM   Subscribe

why does almost every wireless device use the same 2.4Ghz spectrum?

A lot of portable phones, Baby monitors, wireless mice, 802.11b/g/n wifi, wireless speakers, security cameras, etc. all use the 2.4Ghz spectrum and often collide with each other. Why isn't there a more sane breakdown of frequencies so folks can use multiple devices in the home without having to deal with all this interference?

Yes, i know cordless phones can use 6.0Ghz now. And I think 802.11n can run on 5.8Ghz. But it seems there are still many, many products using that same 2.4Ghz frequency.
posted by escher to Technology (11 answers total) 1 user marked this as a favorite
 
Because that's the part of the spectrum that they're allowed to use. Start reading Wikipedia and it becomes clear that it's pretty arbitrary. But you can't just set up and broadcast on a random frequency.
posted by Lemurrhea at 8:57 AM on July 6, 2010


I read that the only reason why the Telcom industry was willing to let the radio spectrum used by WiFi go cheaply was that they believed that baby alarms, microwave ovens, etc made it so noisy as to be effectively useless. However the development of better equipment for dealing with such noise has made WiFi both popular and a threat to their revenue. Cannot find source just now but others might.
posted by rongorongo at 9:23 AM on July 6, 2010


The simple answer is that that's what they're legally allowed to do. Consumer devices are unlicensed -- which is why you are able to have them without any sort of government registration, fees, etc. To protect licensed services from interference, unlicensed devices are restricted to certain free-for-all bands. In the USA this is covered by Part 15 of the FCC regulations, which essentially says (among other things) "you may transmit up to X power in Y band, and if you step on each other, don't look for any help from us".

RF spectrum allocation is actually a pretty important economic issue these days, because even though demand for neat wireless stuff is constantly rising, the spectrum isn't getting any bigger. For a chart of the way the RF spectrum is carved up, check out this chart (note: big PDF, suitable for poster size printing if you want a wall decoration). Anywhere you see "ISM" that's one of the free-for-all bands -- often they overlap with amateur radio or similar things. While it would be nice if they would carve out a few more chunks for unlicensed stuff, one look at that chart should show just how big a problem it is to find something that is practical to reallocate. The best candidates right now are TV channels at a luxurious 6 MHz each (the recent digital TV transition was done to enable that to happen) and the various bits that are allocated to general government use. But it's a long process, and it will still be a while before new stuff arrives to use the now-available frequencies.
posted by robt at 9:24 AM on July 6, 2010 [1 favorite]


Spectrum is precious. Part of the spectrum freed by the digital TV transition was auctioned by the FCC for billions of dollars. If the FCC can get companies to pay them money for spectrum, they're not about to give any more up for free.
posted by zsazsa at 9:29 AM on July 6, 2010


Indeed. If the ISM bands hadn't been allocated long before anyone thought of auctioning off huge licensed chunks of spectrum, we probably wouldn't have them. Unfortunately the post-transition auction of TV channels didn't result in any new bands for unlicensed consumer devices, but there is work being done on "white space" devices which may be allowed to work in whatever slices of that spectrum are not in use in the immediate area. Time will tell if that actually becomes a legal reality though.
posted by robt at 9:40 AM on July 6, 2010


There are other unlicensed bands, just by the way -- first 27 MHz, then 49 MHz, then 900 MHz, all of which are still used -- but 2.4 GHz had the benefit of being in the gigahertz range. The data carrying capacity of a signal is related directly to its frequency, so 2.4 GHz was chosen for 802.11b. Once that happened, 802.11b radios became commodity items, and they became a popular choice for newer equipment.
posted by kindall at 10:45 AM on July 6, 2010


So, to see if I have this right:


consumer devices mostly use 900Mhz, 2.4Ghz, 5.8Ghz, etc because:

- These are unlicensed frequencies and can be used without paying a fees or government registration (as long as you stay within X power in Y band)
- And the 2.4Ghz one in particular is used because the higher frequency translates into higher data bandwidth

The FCC has allocated pretty much every frequency out for various items already, and there isn't any easy space for another "free" spectrum. You'd have to free some space up, which isn't easy or quick. Plus, now that the FCC can get huge amounts of money for selling spectrum space, so there isn't a real big incentive to just give it out for free.

So it sounds like we're kinda screwed in the 2.4 problem for a while at least. bummer.

Thanks for the links so far! I'd read most of the wikipedia pages so far but the story of how we got into this situation is what I was looking for.
posted by escher at 11:36 AM on July 6, 2010


I think another reason why 2.4GHz is crowded is marketing. Things like baby monitors and cordless phones really don't need the bandwidth available at 2.4GHz or 5.8GHz; in fact, range can decrease as the frequency goes up. However, a giant 2.4GHz on the box sure looks better than 900MHz. The latest standard for cordless phones, DECT, is marketed as "DECT 6.0" when it's actually in the uncluttered 1.9GHz band.
posted by zsazsa at 12:05 PM on July 6, 2010


This article on "the history of wifi" in the Economist was the one I was thinking of when I talked about "an auction of garbage spectrum".
posted by rongorongo at 3:24 PM on July 6, 2010


- These are unlicensed frequencies and can be used without paying a fees or government registration (as long as you stay within X power in Y band)

Not just fees and registration, but a site permit for every person that owns that device. In essence, a radio station permit. I worked somewhere that used some oddball spectrum onsite and there was an FCC radio station permit that had to be displayed.

If you are feelin' fancy, you can set yourself with 802.11a.

Minor television trivia- I forget the numbers, but not only did each analog channel use up 6mhz, because of the unclean signals they put out, whenever channel X was in service in an area, the adjacent channels couldn't be used, obviously. But the bad harmonics would also prevent something like 3 other channels from being used. (Completely wrong example: if 24 was in use, 48 and 72 were also right out.)
posted by gjc at 4:43 PM on July 6, 2010


all use the 2.4Ghz spectrum and often collide with each other

Note that these things all use spread-spectrum techniques which are designed to make sharing common radio channels possible. If you picture a graph with power as the Y axis and frequency on the X axis, the traditional method is to divvy it up into tall, narrow bins, and assign each device its own bin. With spread spectrum you make the bins short and wide (that is, low power) and spread over many different frequencies. Because the power is less you can 'stack' multiple transmissions on top of each other and still have each receiver recover the signal. In principle all these devices can share the same channels, so it's not like a walkie-talkie where every person has to be on a different channel or else nobody can talk.
posted by Rhomboid at 2:03 AM on July 7, 2010


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