TAPPING out Morse code on your water pipes is one way to network information
around your home, but better ways are emerging. Some systems bathe people in
low-power microwaves, like the radio-based computer network known as Wi-Fi, or
the Bluetooth system, which aims to link up mobile gadgets.
If the Consumer Electronics Association in the US has its way, however, your
household devices鈥攕uch as cookers, washing machines, baby monitors,
burglar alarms or televisions鈥攚ill soon be able to talk to each through
their mains power plugs. For example, built-in modems will let you set up a link
between a baby alarm and your hi-fi speakers, or send images from a security
camera to your TV screen.
Networking via your power lines is not a new idea, but because of the
inadequacies of the technology it has never taken off. So the CEA is running a
鈥渂ake-off鈥濃�攁 contest between new power line networking technologies to see
which can crack the problems that have dogged their predecessors. Three firms
are in the running: Itran of Israel, Inari of Utah and nSine of Britain.
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Until now, sending signals around the ring main of houses has been hampered
by interference from the noise on the 50 to 60 hertz alternating current. You
can already buy adaptors that plug into a mains socket and send modem tones to
let a PC in one room connect to a phone line in another, but the data speeds are
limited to a few tens of kilobits per second.
To network digital devices usefully, the CEA wants to be able to inject
digital data into the mains at speeds of between 2 and 20 megabits per second on
a high-frequency radio 鈥渃arrier wave鈥�. The challenge is to keep the carrier
power low enough to stop the wires acting as an aerial and radiating
interference, while keeping the signal strong enough to overcome the mains
interference.
In the bake-off, CEA engineers will be dropping in on volunteer homes in
North Carolina, California, Utah and New England. The CEA insists that the mains
modem units must be cheap enough for manufacturers to embed in everyday consumer
equipment. And importantly, they must not need any control from a crash-prone
PC.
So what are the competing technologies? Itran of Tel Aviv uses a 鈥渟pread
spectrum鈥� system. Its transmitting modem puts data into labelled packets, adds
error-correcting codes and spreads the mix over a carrier signal that spans many
megahertz. Because the packets are labelled, the receiver can distinguish them
from mains noise.
Meanwhile, Inari has modified OFDM, Europe鈥檚 digital terrestrial TV and radio
broadcasting format. The data is split among 24 separate carrier signals. Each
carrier can be temporarily shut down when swamped with noise, allowing the data
to be shifted to other carriers and transmitted unsullied.
nSine鈥檚 transmitter sends the same, complete data stream on four carriers.
The receiver continually compares results and selects the best of the four,
supposedly dodging noise. The CEA鈥檚 results are expected in April.