2 THE POWERLINE COMMUNICATION NETWORK 5
2 The Powerline Communication Network
Powerline Communications (PLC) technology uses the existing power system to communicate
without the need for additional or specific wiring. PLCnetTM devices use the existing electrical
wiring to transport information. The technology consists of overlaying a higher frequency (4.3
to 20.9 Mhz bandwidth), low-power signal on top of the 50 Hz signal. This signal travels over
the electrical wires and is then received and decoded by all the PLCnetTM devices on the power
system.
The power system is an environment subject to varying levels of interference. The distance bet-
ween outlets, the number of connections and the load are very hard to predict. To optimise this
media, Oxance has designed devices using PowerPacketTM technology, developed by IntellonTM.
This technology is the basis for the HomePlugTM specification.
PowerPacketTM operates with OFDM (Orthogonal Frequency Division Multiplexing) modulation
that is used in digital terrestrial television and the 802.11a standard. OFDM is a broadband wi-
reless transmission technology capable of overcoming obstacles on the transmit-receive system.
OFDM solves the problem of transmitter-receiver alignment. For a very fast broadband connec-
tion (several megabits per second), the transmitter and the receiver must be perfectly aligned
which is rarely the case in an urban environment where there are many buildings. With OFDM
devices, this is not a problem, since the system transmits a signal on several frequencies at once.
Even if only one weak signal passes, the receiver can reconstruct the message, regardless of any
obstacles encountered on the other frequencies.
The protocol used is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), similar
to 802.11, as well as priority classes and latency control. The frequency bandwidths range from
4.3 Mhz to 20.9 Mhz for a transfer rate of 14 Mb/s, or roughly 6 Mb/s effective data rate.
The communication distances are approximately 200 m, with the communication distance and
the transfer rate theoretically proportional to the SNR (Signal to Noise Ration) and dependant on
the quality of the network connections. Anything that reduces the SNR (switching power supplies,
halogen lamps, dimmer switches, etc.) or that modifies the impedance of the network (extension
cords, power strips, etc.) is likely to decrease the communication distances and/or the transfer
rates.
PowerPacketTM adjusts the transmission rates and times by forcing segmentation of frames that
exceed a certain duration to provide the best quality of service (QoS). The PowerPacketTM privacy
mechanism creates a logical network in which all stations share a common encryption key.
A 56-bit DES algorithm encrypts each frame. By using several different keys, several secure, in-
dependent logical networks can be created.
On the three-phase network, the high frequency coupling of the phases in the input cable may
be sufficient to allow PLCnetTM units plugged into different phases to communicate with each
other. For systems with long wiring or a band-stop filter, we recommend installing an Oxance
capacitive coupler (CPL3PH) for better performance.