AirPort Extreme 802.11n Throughput Limits
The configuration manuals for the new 802.11n-based AirPort Extreme Base Station were posted last week on Apple’s support Web site, and they offer some insight into whether you should immediately purchase the new equipment or not. (See “AirPort Extreme Updated,” 2007-01-15.) Most tellingly, one of the manuals shows that 802.11n’s highest bandwidth modes may not be available in most people’s preferred network configuration.
The new AirPort Extreme Base Station can use either of two frequency ranges for wireless networking – 2.4 gigahertz (GHz), which is the range used for the original AirPort (802.11b), and for the original AirPort Extreme (802.11g); and 5 GHz, used for 802.11a, a standard Apple never previously supported.
The wireless data protocols that use 2.4 GHz and 5 GHz bands divvy up spectrum into channels, each of which is about 20 megahertz (MHz) wide; no spec previously allowed more than 54 megabits per second (Mbps) of raw data in a channel. The 802.11n spec ups the bandwidth ante to 65 Mbps of raw data per channel, and uses two radios and other techniques to more than double that to 150 Mbps of raw throughput.
In ideal circumstances, 802.11n can reach up to 300 Mbps of throughput by using a special wide channel mode in which 40 MHz of spectrum (two channels) are used simultaneously. The latest draft of the protocol, still in progress, forces 802.11n equipment to drop from wide to normal channels if the device detects any other network using the same channels. However, even the occasional capability to use wide channels can boost overall throughput to 100 Mbps.
Now Apple’s “Designing AirPort Extreme 802.11n Networks” manual notes that wide channels can be used only in 5 GHz, not in the 2.4 GHz range. This could be limiting, because your real-world throughput might not exceed 50 Mbps, only about twice the real-world throughput of the original 802.11g-based AirPort Extreme and not nearly as much of a speed improvement as promised. According to chipmakers I’ve spoken with, Apple has chosen to not offer wide channels at this point in 2.4 GHz, but could upgrade firmware later; the 802.11n specification doesn’t require equipment makers to offer wide channels in either band. In some countries, wide channels won’t be legal in 5 GHz.
Apple suggests in “Designing AirPort Extreme 802.11n Networks” that you can easily set up a combination network by using older and newer AirPort equipment. If you already own an AirPort Extreme or other base station, you could connect its WAN (wide area network or broadband) port directly to a new 802.11n AirPort Extreme Base Station using one of the three ports on its built-in Ethernet switch. You could set the new 802.11n Extreme to use 5 GHz, and leave the old unit available for older Wi-Fi clients. This would give you the best of both worlds: the highest possible speeds for any new, 802.11n equipment, and better performance – through reduced congestion – for older hardware.
By the way, it’s possible that Apple TV, which uses 802.11n, will work only in the 5 GHz band. It’s not clear in the AirPort Extreme manual, which illustrates an Apple TV being connected but fails to note which band is in use.