It’s time for another look at Powerline telecommunications (PLT). This is a generic term for methods of transmitting data over power cables both in buildings and in the power distribution network.

PLT was touted as a technology to compete with ADSL for broadband access but it never got beyond a few trials. It couldn’t provide the high speeds that are common with ADSL2 and there was always a question mark about radio interference caused by emissions from unbalanced overhead cables. The PLT signals are in the high-frequency (3MHz to 30MHz) radio band, also known as the short-wave band.

In-building PLT is becoming a serious technology for many applications. The shorter distances make good performance possible and the radio emissions are not as bad as feared.

The PLT standards created by the HomePlug Powerline Alliance now dominate the market. There are two standards: HomePlug 1.0 and HomePlug AV. The first provides connections at speeds up to 14Mbit/s, resulting in user rates of about 6Mbit/s, roughly the same as 10BaseT Ethernet. HomePlug AV can run at up to 200Mbit/s, delivering speeds of more than 100Mbit/s. This is comparable to 100BaseT. There is also a very common proprietary extension to HomePlug 1.0, which runs at up to 85Mbit/s, called HomePlug 1.0 Plus Turbo.

These performance numbers are available only under good conditions; HomePlug devices will reduce their speed to cope with noise and interference on the cables. Throughputs are lower on long runs or where there is electrically noisy equipment.

All HomePlugs use Orthogonal Frequency-Division Multiplexing, which distributes the data over more than 900 separate carriers. The modulation system on each carrier is optimised to suit the capacity of the piece of spectrum it sees. There can be up to 10bits per symbol. In addition, the modulation uses a Turbo code, which allows the performance to be very close to the Shannon limit. Turbo codes were devised in the early 1990s and have only recently been used in mass-market products.

HomePlug devices use encrypted transmission to avoid eavesdropping problems, but remember to change the key from the default before you use them. Electricity meters and mains filters usually block transmission and HomePlugs will work only if the connection points are on the same electrical phase.

Nevertheless, I expect we will see more HomePlugs in commercial premises where they can provide temporary connections very cheaply and reach places that have no Ethernet cabling.