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Technology : Digital data comes down the mains

作者:沙引    发布时间:2019-02-27 09:11:01    

By Mark Ward INTERNET access may come with your mains electricity supply if trials planned by the power company Norweb in the northwest of England this week prove successful. In at least 20 households interactive monitors will check electricity consumption, instantaneously reveal how much money Norweb is owed and automatically report faults. Net access and video-on-demand could follow, Norweb says. The company is experimenting with power-line communications to cut the cost of connecting customers to the phone network operated by its subsidiary, Norweb Communications. Last year, 20 other Norweb customers were the first people in the world to make their domestic phone calls over power cables. Calls made this way are connected to conventional phone lines at the electrical substation that serves the home, usually no more than 300 metres away. The power lines between an electricity substation and a typical home are coaxial cables, similar to those that connect TVs to aerials. They can carry enough compressed data to provide several TV channels. The normal 100-amp, 240-volt AC supply has a frequency of 50 hertz. Norweb has found that the cables can carry additional signals at over 1 megahertz without interfering with the power supply. Digital data can be superimposed on this carrier wave. The researchers have used a modified version of a mobile phone standard known as CT2. The phones typically operate at frequencies of 100 megahertz, but Norweb has converted the system to work with frequencies between 4 and 20 megahertz. However, CT2 was originally developed for mobile phones, so it has some drawbacks, says Paul Brown, research and development manager with Norweb Communications. “Every phone call gets allocated one of 40 channels [by the CT2 base station that sits in the electricity substation] but only 12 can be used simultaneously.” Brown says this could be a problem because substations typically serve around 150 customers, and during peak demand more than 12 people may want to make phone calls, watch films or surf the Internet. He also says that the way CT2 encodes digital data is not very efficient, and the company is working with the Communications Research Centre at the University of Lancaster to increase the amount of data it can carry per second and the number of channels available. Each terrestrial TV channel occupies about 8 megahertz of frequency and Brown estimates there are 10 megahertz of frequency available in power cables, so squashing multiple channels down the wire will require some compression. The group has so far managed to fit one bit of data on each wave in the carrier signal. Currently, the trial households receive 32 kilobits per second, but Brown believes this could be increased to several megabits once the CT2 standard is streamlined and compression techniques are used to cram more data into the carrier signal. To ensure the data and power supplies do not get mixed up, each home is fitted with a conditioning unit. This splits and filters the two signals as they enter each house, sending electrical power into the ring main and data to the phone,

 

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