If your WiFi network isn’t delivering the speeds you expect (you shouldn’t be getting less than 20Mbps over 802.11n), the leading suspect is interference – when competing wireless signals disrupt the transmission of your data; that data then has to be resent or sent at a slower speed. Diagnosing and eliminating interference is one of the best fixes for slow WiFi.

Measure the problem
The first thing to do is launch AirPort Utility (/Applications/Utilities) and select your base station. (For the purposes of this story, we’ll assume you’re using an AirPort base station; many of the tips apply to other wireless routers as well, but the details will obviously change.) Click on Manual Setup and then on the Advanced view icon. Select Logs and Statistics at the bottom of the screen and then the Wireless Clients tab. There, you’ll see which devices are connected to your base station and measurements of those connections.

Look at the numbers for signal and noise. Both are measured on a negative scale; the further a number is below zero, the weaker the signal or noise. You want a strong signal and a weak noise reading. Signal should be closer to zero than noise, with -25 being far better than -50. You’d rather have noise down at -90 (very weak) than at -50. (The measurement is logarithmic; every 10 units represents a tenfold change.)

Poor signal or noise numbers (or a lower- than-expected rate reading) could be a sign of interference. If that’s what you find, the next step is to identify the culprit. Download and install iStumbler (payment requested; www.istumbler.net). This utility provides a list of all the networks that your WiFi adaptor can sense, and the channel each one is using. If one of them is using the same channel you are, it could be the cause of your problem. 

If neighbouring networks interfere with yours, switching to another channel can help

Change channels and bands
If that’s the case, try changing the channel you’re using. The easiest way to do that is to restart your router. When it starts back up, it will automatically pick the channel that’s least used.

Unfortunately, the channel it picks isn’t always the best choice; also, the longer your router stays on, what was once the least used channel may become the most crowded. To make sure you’re using the best channel, switch manually.

In the 2.4GHz band, you want to pick among 1, 6, and 11, whichever is being used by the fewest neighbouring networks; if performance stays poor, try another. In the 5GHz band, you can choose one of four channels available in the EU – 36, 40, 44, and 48). These channels, sadly, have less reach than the higher-numbered ones – 149, 153, 157, and 161 – available in the US (which are illegal to use in the EU) but they can help to reduce interference.

To change the channel, select your base station in AirPort Utility, click on Manual Setup, then open the Wireless tab in AirPort view. On a 2007 or 2008 AirPort Extreme or Time Capsule, or on any 802.11n AirPort Express, select a channel from the Channel pop-up menu. On AirPort Extreme and Time Capsules from 2009 and later, select Manual from the Radio Channel Selection menu, click on Edit, and then select a channel from the 2.4GHz or 5GHz Channel menu. 

If your base station can use only the 2.4GHz band, you could upgrade to a simultaneous dual-band router. Then you could move devices that support 5GHz to that band without giving up on older 2.4GHz hardware. Most Macs released since 2006 can use 5GHz, as can the iPad and the Apple TV. The iPhone and the iPod touch use 2.4GHz only.