| Broad
Band Over Power Line
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Definition
Power line communications
(PLC) or Broadband over Power Lines (BPL) allows transmission of data over power
lines.Power line communications uses the RF signal sent over medium and low voltage
AC power lines to allow end users to connect to the Internet. The RF signal is
modulated with digital information that is converted by an interface in the home
or small business into Ethernet compatible data.
To gain a good understanding
of how PLC Works an excellent understanding of the Power Grid is required. Unlike
telephony and it's associated technologies there is no set standard for providing
power. An example of such a difference in standards can be seen in the difference
in voltage provided in the U.S. and the EU. Ireland uses a 220v AC power source
whereas in the US a 110v power source is used. These differences lead to differences
in basic equipment such as plugs e.g. The 3 pin plug used in Ireland compared
to the 2 pin plug used in the US made possible due to the lower voltage used in
the U.S. Despite these differences in basic equipment the basic network is similar
in nearly all countries.
Power is generated at Power stations and distributed
around a medium to large geographical area via HV lines or High Voltage lines.
In areas where power needs to be distributed to consumers transformers
will be used to convert this high voltage into a lower voltage for transport over
MV or Medium Voltage lines. These transformers are generally located at electrical
substations operated by the utility or power supplier. Such medium voltage lines
will be used to transport electricity around smaller geographical areas such as
towns and small counties etc.
At the customer's house or premises a transformer
is used to drop the voltage down to safer more manageable voltages for use in
the home or business. This power is usually transported over LV or Low Voltage
lines. These Low Voltage Lines include the lines that traverse a customer's home
or business.
PLC encoding
Though there are no set standards
in PLC all implementations act in the same manner. PLC is based on the idea that
any copper medium will transport any electrical signal for a certain distance.
Basically a radio signal is modulated with the data we wish to send. This radio
signal is then sent down the copper medium (our power lines) in a band of frequencies
not used by for the purposes of supplying electricity and managing electricity.
The frequencies and encoding schemes used greatly influence both the efficiency
and the speed of the PLC service. Most PLC radio traffic generally occurs in the
same bandwidth roughly 1.6 MHz to 80 MHz. These frequencies are in the MF Medium
Frequency (300KHz-3 MHz), HF High Frequency (3MHz - 30 MHz) and some of the VHF
Very High Frequency (30MHz - 300 MHz) spectrum. Various encoding schemes have
been used for sending the data along the Power Lines these include:
GMSK
Used with the Single Carrier Version of PLC providing low bandwidths <1 MB
CDMA
Used with the Single Carrier Version of PLC providing low bandwidths
<1 MB
OFDM
Used with the Multi Carrier version of PLC providing a
bandwidth of 45 MB
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