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Monday, March 31, 2008, 05:00 am PT (08:00 am ET)

Exploring Time Capsule: WiFi 802.11n and the 5GHz band


The Pros and Cons of 5 GHz

As noted by Glenn Fleishman in the article Wi-Fi Networking News: 5 GHz or Bust, the 5 GHz band has much greater radio spectrum available; there are 12 non-overlapping channels, each with 20 MHz of bandwidth. The entire 2.4 GHz band is only 80 MHz wide, which only allows for three non-overlapping 20 MHz channels; while you can select any channel between 1 and 11 in the standard 2.4 GHz band, there is so much overlap between channels that only 1, 5, and 11 can really coexist in the same area without interference. When using 5 GHz, the base station selects the channel for you automatically.

However, 5 GHz is also a higher, shorter radio frequency, which means that at the same amount of radio transmission power, its radio waves propagate shorter distances than those of 2.4 GHz base stations. The 5 GHz band is also worse at penetrating solid objects such as wooden walls in a home (neither band can penetrate metal walls, such as lath and plaster walls using a metal mesh found in some older buildings). There are also power transmission restrictions that affect the use of the 5 GHz band. These factors combine to result in users likely seeing a significant drop in their signal range when using 5 GHz.

In our initial tests, switching to 5 GHz initially had no positive impact on wireless transmission speeds. In fact, it actually seemed to slow things down. However, Apple's 802.11n AirPort devices, including Time Capsule, support a wide channel mode when using 5 GHz that does make a big difference in network speed.

Use Wide Channels For a Big Boost

The greatest advantage to using 5 GHz is the ability to bond two channels together, which Apple calls "Use wide channels." (below) This allows the base station to grab twice as much radio bandwidth (40 MHz) and should be turned on by default when using the 5 GHz band. This will also boost the reported connection speed from 130 to 300 Mbit/sec, at the expense of possibly interfering with other nearby 5 GHz networks, giving up some signal range, and dropping compatibility with 802.11b/g devices.

Time Capsule


However, once configured to use wide channels in the 5 GHz band (the setting hides behind the "Wireless Options..." button on the Wireless tab of AirPort Utility, below), our gigabyte of test files copied to the Time Capsule in 2:11, a dramatic improvement that put 802.11n in the running next to Time Capsule's Gigabit Ethernet performance (which took 1:38) and was well below the 3 to 8 minutes required by wireless configurations only using a single 20 MHz channel.

Time Capsule


Of course, connecting the base station directly via an Ethernet cable makes the biggest difference in performance, but also negates the convenience of wireless networking. In any case, the fact that 802.11n wireless networking can approach the ballpark of base station Gigabit Ethernet speeds is pretty impressive. The dark side of the same coin is that the AirPort Extreme and Time Capsule have relatively poor Gigabit Ethernet performance (as noted in the next segment).

It also needs to be pointed out that WiFi performance will degrade rapidly as the user loses signal strength; the poor performance of the first tests were exaggerated by a drop in signal strength related to unknown environmental factors. During the first tests we ran, the AirPort software reported a signal strength that fell from 130 to 117 or below. With a 300 Mbit/sec, 40 MHz wide channel network, the loss in signal reception coverage might offset the faster data rate, or require more base stations to extend the same signal coverage.

If you're thinking that configuring wireless networking sounds a lot more like voodoo than engineering, you might be right. Experimentation to suit your own needs related to signal reception area versus data speed, and accounting for the type of barriers or sources of interference in your specific setup is essential to gain the best possible performance.

The next segment in this series will look at our actual test results comparing wireless performance between modern 802.11n devices, older computers that only support 802.11g, and systems directly connected over Ethernet. We'll also compare performance of 10/100 Fast Ethernet offered by last year's AirPort Extreme, and the Gigabit Ethernet performance of currently shipping Time Capsule and AirPort Extreme base stations, and compare how the base stations stack up against a dedicated file sharing server.

Previous articles related to Time Capsule and its AirPort Extreme cousin:

Exploring Time Capsule: theoretical speed vs practical throughput
Exploring Time Capsule: how it fits into Apple's AirPort family
An in-depth review of Apple's 802.11n AirPort Extreme Base Station
Apple Time Capsule unboxing and preview
A Look Inside Apple's New Time Capsule
Answers to Time Capsule reader questions