Lab

Power plug networking

Anyone with more than one PC should have a network. With your computers connected together, you can save documents you're working on to a shared folder or hard drive, so they'll be available to you or anyone else in the home or office on one of the other PCs. Sharing a printer is also easy when you have a network; you can print from your laptop on the couch, for example, even if your printer is upstairs in the spare bedroom. Nor are networks just for sensible stuff. With network-capable games consoles, such as the Xbox 360, and media players now relatively cheap and widely available, you can use your network to pipe music, videos and even HD movies around your home.

All this sounds great in theory, but does it work in practice? The answer is 'Hmmm, sort of.' If you're prepared to go the whole hog, pull up the floorboards and lay Ethernet cables throughout your home or office, you're guaranteed a reliable network that can send and receive data at high speeds. For most people, however, that's too much hassle. We now have the excellent alternative of wireless networking, and for many people WiFi is great: just plug a router into your broadband connection and all your PCs can get onto the Internet, access each others' shared folders and so on. Rated speeds are decent (faster than your broadband) and there's no mess or hassle. But WiFi doesn't always work well. Signal strength tends to weaken over even short distances and is prone to interference from things like microwaves and mobile phones, mirrors and physical obstacles. The actual speed obtained with of WiFi equipment will be only a fraction of the rated speed, and we've seen WiFi networks suffer from unpredictable, intermittent signal loss. In old houses with thick walls, a wireless network's effective range is sometimes limited to little more than the area of a single room.

The answer is power plug networking, also referred to as home plug or power line networking. Whatever you call it, the idea is that networking adapters plug into your mains electricity sockets, which of course are already connected together. Having 240 volts of power running through the cables obviously presents some technical hurdles when it comes to transferring information cleanly, but that's what the network adapters are there for, and they seem to do a pretty good job. You don't have to install any new cables, transfer speeds are comparable to those that good WiFi equipment achieves in real life, and you have neither the reception issues nor the security worries associated with WiFi. This month we tested ten of the latest power plug devices to see which ones really are powerfully good value.

How it works

Power plug networking has been around in one form or another since around the turn of the millennium. However, many of the first products were difficult to set up, didn't achieve good transfer speeds and were subject to interference from power surges.

Electricity travels along the wiring in your home at a frequency of 50Hz. However, the wires themselves are capable of carrying more frequencies in addition to this. Older power line products only made use of two extra frequencies, one for binary ones and another for binary zeroes. Any interference to either frequency made the network slow down or temporarily stop working. Modern devices avoid this problem by transmitting data using frequencies between 2 and 28MHz. The adapters divide this frequency range into a number of sub-channels, and data packets are transmitted across several sub-channels simultaneously, enabling high transfer speeds. If a power surge, or crosstalk between frequencies, starts to interfere with transmission on a particular sub-channel, the adapters sense this and automatically transfer that data stream to another sub-channel. This allows for relatively consistent data transfer rates.

In other respects, power plug adapters work like any other network adapter. Each has a unique network identifier called a MAC address (nothing to do with Apple Macs, it stands for 'media access control), although most home plug adapters use the IP address of the Ethernet adapter they're plugged in to. Most require no special software: you just plug them into your PC, either into a USB port or more commonly an Ethernet networking port, which almost all modern PCs have.

The great majority of power plug adapters are built according to one of the standards created by the HomePlug Powerline Alliance, an industry group. However, as is so often the case, another group of companies has created a rival body, the Universal Powerline Association, which has released its own standard. Products built according to the two different sets of standards aren't compatible, so if you already have a power plug device you'll need to find out what standard it conforms to and make sure any other products you buy are also built to that standard.

If you have a WiFi network, you should be concerned about security. Anyone within range who has a WiFi adapter can join your network, use your Internet connection and perhaps read files on your PCs unless you encrypt and password-protected your network. That should be easy to do using the features built into the router, but you may well end up turning off security to get something awkward like a Nintendo DS connected, then forget to turn it back on. With powerline networking, the security risk is greatly reduced: only someone with physical access to your mains wiring could attempt to intercept the signal. However, some flats and offices may share a mains ring with their neighbours, so all powerline products should be able to encrypt the data they carry, and the higher the level of encryption the better.

There are a couple of ways you can use power plug networking. You could simply connect two PCs, but then the PC that's plugged into your broadband connection will have to be switched on for the other one to access the internet. This misses the point of having a network in the first place. Instead, you should plug a power line adapter into one of the Ethernet ports of your broadband router. Adding a power line adapter to any other PC in your house will then mean it can connect to the Internet, and other devices on your network, through the router. Power plug networking is also good for getting around the limitations of WiFi. If your signal is too weak to reach the back of the house, plug a power line adapter into your router and plug a wireless access point into another adapter. The access point can broadcast a second wireless network, which will still have access to the Internet through your router.