In many cases it’s down to the strength and quality of your WiFi router’s signal.
OS X has a built in diagnostic that can help you to troubleshoot Mac connection problems. Firstly, Alt/Option click on the WiFi icon in your menubar, you’ll see a bit of extra information about the connection including the signal strength and the channel that Airport is connected to.
Choose “Open Wireless Diagnostics”. Login if prompted and run a diganostic on your set-up. After the diagnostic has completed, you can choose to monitor the connection over time or go straight to the summary screen. The summary screen contains some pointers to help you get the most from your connection and, if poor signal quality or strength is the culprit, you’ll see some tips to improve the issue.
Wireless Diagnostics, built into OS X from 10.7, can pinpoint problems with your WiFi and offer tips for improvement.
Usually, issues of signal strength are due to being too far away from your router or obstructions between your device and the source of the signal. The remedy, if you can, is to move your machine closer to the source of the WiFi.
That’s not always possible - but you can extend the range of your WiFi equipment with a “repeater”. Available for online purchase from as little as £15, WiFi repeaters plug into a domestic electricity socket and amplify the signal between your router and your Mac or other Apple device.
Sometimes it’s not distance or thick walls that affect the signal - but interference. It can come from a variety of electrical sources, including Christmas fairy lights! But the main cause of WiFi interference is other people’s WiFi...
If your neighbour has their WiFi signal set to the same channel as yours - or an adjacent channel - it can decrease the reliability of your connection significantly. How do find out? Hop onto your Mac again and Option/Alt click on the WiFi icon to open Wireless Diagnostics again. This time either you’ll need to perform a WiFi scan.
Sharing WiFi channels with neighbours can lead to interference. Check if you’re affected with Wireless Diagnostics.
On Lion you can hit Command (cmd) and N to open up the Network Utilities window. On Mountain Lion, you’ll need to go to Window > Utilities. In either case, you then choose WiFi Scan to get a list of signals in the vicinity - including your own. You can also find out what channel all signals are on. This is where the detective work begins. Find your own connection and note the channel number. Compare it with the channel numbers of any other WiFi sources in the area. Interference between channels can occur up to two channels either side.
For example, if your WiFi broadcasts over channel 6, you could experience interference from signals in channels 4 to 8 - though interference will be greater if both signals share the same channel.
The fastest fix for channel interference is to set your router to choose its broadcast channel automatically. The way you do this varies from router to router - but usually you will need to access your router’s admin panel via a web browser and find the WLAN or WiFi settings section. Check your broadband provider’s help documentation for complete instructions.