A year or so ago, I rarely received email from TidBITS readers or those who have read my "Take Control of Your 802.11n AirPort Network" book detailing problems using Wi-Fi networking in the 5 GHz band. This higher-frequency radio band is available in the United States and many other countries worldwide, and was long seen as a less-crowded alternative to the 2.4 GHz band that was Wi-Fi's original home.
Now I get a few messages a week about mysteries involving channel selection in 5 GHz - normally an obscure option that most people don't need to deal with. I have a few tips, which I'll share after explaining what this is all about.
A Band, a Plan, a Channel: New Crowding -- The 5 GHz band was allotted more than a decade ago in the United States, but usage was sparse until it became cheap enough to put 5 GHz radios in cordless phones and until 802.11n appeared on the scene. Before 5 GHz was an affordable option, cordless phones, Wi-Fi, Bluetooth, and a bunch of other competing uses were crowded into a sliver of bandwidth in the 2.4 GHz range. Competing uses lead to interference and signal trouble.
These 5 GHz channels were heavily underused because only the 802.11a standard - introduced in 1999, at the same time as 802.11b - took advantage of 5 GHz. But 802.11a never took off, because 802.11a gear was initially expensive and not backwards compatible with 802.11b, which used the 2.4 GHz band.
It wasn't until the introduction of 802.11n, which could work over either the 2.4 or 5 GHz spectrum range, that 5 GHz saw any real employment outside of corporate networks that turned to 5 GHz often just for reliable voice-over-Wi-Fi communications.
Then simultaneous dual-band 802.11n base stations, like Apple's AirPort Extreme and Time Capsule revisions introduced in early 2009, started to become popular and affordable. These devices allowed users to retain legacy 2.4 GHz 802.11b and 802.11g hardware by broadcasting on both 2.4 and 5 GHz bands simultaneously.
This capability enabled the increasingly common dual-band 802.11n radios in computers and other devices to work at higher speeds over 5 GHz when close by and lower speeds over 2.4 GHz when further away, while still retaining compatibility with older Wi-Fi adapters. (An 802.11n adapter can be designed for only one band, but most such client hardware allows the choice of either band.)
With 5 GHz gaining popularity, the danger of overcrowding increased. While there are 23 possible distinct (or non-overlapping) channels for use in 5 GHz in the United States, Apple and most consumer equipment makers support only 8 of them in base stations. (Depending on your country, the 2.4 GHz band has just 3 or 4 channels that barely overlap, out of what is typically 11 or 14 channel divisions.)
Manufacturers ignore 15 of the 23 channels to avoid limitations that could be frustrating to users due to shared use with the military. While military radar can overlap with these 15 channels, for it to happen in the real world is rare, and even then it occurs only in limited parts of the country. However, the rules are in effect everywhere, and they can in theory lead to poor experiences, so consumer firms avoided including those channels as options in their base stations. (Apple and most other firms allow an adapter, such as is built into Macintosh computers, to access those channels if they are already in use by a base station, however.)
Those eight widely used channels, however, are split into two pieces: a lower band, called UNII-1 for the lowest band of the Unlicensed National Information Infrastructure range, and an upper band, UNII-3. The 15 other channels are in the UNII-2 range, split between what's generally called the lower UNII-2, which has four channels, and the upper UNII-2, which has 11 channels.
The lower four channels are numbered sequentially as 36, 40, 44, and 48. The upper four channels are likewise numbered as 149, 153, 157, and 161. (The reason for the four-unit gaps is that the channels are 20 MHz wide, but channel numbers are assigned in 5 MHz increments.)
I was reminded by Apple product managers a few months ago while talking about AirPort issues that the lower band uses 1/20th the power of the upper band. That's right: Choose channel 149, and your base station broadcasts signals at 20 times the power of channel 36! (The 5 GHz band, because of its shorter wavelength, requires more power to send the same quantity of data the same distance as a 2.4 GHz device.)
Power corrupts, however. The more power you use to broadcast a signal, the more potential there is for that signal to interfere with other networks, and to receive interference as well.
There's a second problem with the upper band: it's used for 5 GHz cordless DECT phones. These phones use an entirely different technology that can disrupt Wi-Fi performance without violating any FCC rules.
Finally, if you use so-called wide channels, which use 40 MHz of bandwidth to double 802.11n throughput, you're down to just four possible channels (36 or 44 in the lower band, or 149 or 157 in the upper band).
What to do? Read on.
Swap out your own 5.8 GHz DECT phones. The problem of so-called 5.8 GHz or even inaccurately named "6.0 GHz" DECT cordless phones can't be understated, as they tread all over the best 5 GHz Wi-Fi channels due to their signal pattern.
The solution is to switch to 1900 MHz DECT, a relatively recent flavor that has better in-home coverage because of the lower frequency employed. Lower frequencies typically penetrate objects better because of longer wavelengths, which are absorbed and reflected less well by seemingly solid objects. (That's why VHF TV runs from 40 to 200 MHz. If you recall, the higher-frequency UHF channels - from about 500 to 700 MHz - were always harder to pick up and required a separate and different antenna.)
I helped my in-laws get set up with the Panasonic DECT 6.0 Series, which builds in a digital answering system and supports up to five handsets, and we've all been generally happy with them. Amazon sells a base station and two handsets for under $60.
Switch to lower-numbered channels. Buying a new set of cordless phones is an expense, of course, so another option is to set your base station to use lower-band 5 GHz channels from 36 to 48 (or just 36 or 44, if you want to use wide channels). This can work in cases of interference from DECT cordless phones being used by nearby neighbors.
You change channels manually by launching Applications > Utilities > AirPort Utility, selecting your base station, and clicking the Manual Setup button. Click the Wireless tab. With a simultaneous dual-band base station, you see the options Manual and Automatic in the Radio Channel Selection pop-up menu. Choose Manual. Click the Edit menu. You can set the 2.4 GHz band to Automatic, and then select among the four lower-band (or four upper-band) channels for 5 GHz. Click Done, and then Update, which restarts the base station. (For a walk-through, watch my video.)
These lower channels, as noted above, are less likely to suffer interference because Apple and all other base stations put out lower power for that segment of the 5 GHz band, and because DECT and other cordless phones don't work in the lower band.
Because using the lower band substantially reduces the distance over which a signal can carry, you're limiting the higher speed uses of the 5 GHz band - it can top 100 Mbps in throughput with no other devices in use - to within probably the same room as the access point.
Try other upper-band channels, too. If you're having trouble making your network work, and examining the speed reported by holding down the Option key while selecting the AirPort Menu shows very low rates (below tens of megabits per second), you may need to experiment with changing channels manually on your base station. (For more on how to read the AirPort menu values in Snow Leopard, see "AirPort Menu Improves in Snow Leopard," 27 August 2009.)
In numerous conversations, Apple product managers have recommended letting the base station use automatic channel selection. But they acknowledge that the selection scans for interference only when the device is powered up, which won't help with DECT phones that are in sporadic use.
In Europe and some other regulatory domains, you will be unable to select channels numbered above 48, because all those higher-numbered channels have to use a process that avoids interfering with other nearby users. The base station may select one of those channels, however, when powering up or restarting.
Get a new base station. One trend in favor of improving 5 GHz interference is that Apple quietly updated its base stations for better range in October 2009. A new AirPort Extreme Base Station or Time Capsule base station now has two sets of three antennas instead of two sets of two antennas. These additional antennas produce better speed over greater range even using the lower band of 5 GHz channels, among other improvements.
Change the Channel -- There's no Wi-Fi spec in common usage yet that would allow all the adapters and the base station to agree to move to a different channel if interference were detected. (There is a standard from the IEEE for something like this, but it's not built into consumer gear, and would require near-universal adoption to make a difference.)