Broadband over power lines
Broadband over power lines (BPL) is a method of power line communication (PLC) that allows relatively high-speed digital data transmission over the public electric power distribution wiring. BPL uses higher frequencies, a wider frequency range and different technologies from other forms of power-line communications to provide high-rate communication over longer distances. BPL uses frequencies which are part of the radio spectrum allocated to over-the-air communication services therefore the prevention of interference to, and from, these services is a very important factor in designing BPL systems.
BPL is based on PLC technology developed as far back as 1914 by US telecommunications company AT&T. Electricity companies have been bundling radio frequency on the same line as electrical current to monitor the performance of their own power grids for years. More recently there have been attempts to implement access BPL, or the provision of internet services to customers via the grid. The prospect of BPL was predicted in 2004 to possibly motivate DSL and cable operators to more quickly serve rural communities.
The high level of attenuation (or data signal loss) from access BPL power cables had two critical effects: It limited bandwidth, and it attracted opposition from groups within the radio community.
Generally BPL is described as either In-House BPL to network machines within a building, or Access BPL which will carry broadband Internet using power lines and allow power companies to monitor power systems.
Because electric current and radio (data) signals vibrate at different frequencies they do not interfere with each other enough to significantly disrupt data transmission. This only works on low-voltage and medium-voltage cables. High-voltage cables do not vibrate at a consistent frequency, causing regular spikes which cancel the data signal and severely interrupt the transmission.
MV lines carry generally up to 100 kilovolts, over a few kilometres between the electricity distribution stations and pole-mounted transformers. Low voltage lines transmit a few hundred volts over a few hundreds of metres, usually from pole-mounted transformers into a home or business.
Typically modem couplers embed data signals on to MV lines at the substation, with extractors at the LV distribution transformer feeding power into a group of buildings.
BPL modems transmit in medium and high frequency (1.6 to 80 MHz electric carrier). The asymmetric speed in the modem is generally from 256 kbit/s to 2.7 Mbit/s. In the repeater situated in the meter room the speed is up to 45 Mbit/s and can be connected to 256 PLC modems. In the medium voltage stations, the speed from the head ends to the Internet is up to 135 Mbit/s. To connect to the Internet, utilities can use optical fiber backbone or wireless link.
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