This page is brought to you by the engineering department of Synaptech Solutions Inc.
The power line medium is widely viewed as the most ideal infrastructure to allow
connectivity between everyday devices. Unfortunately, it is also one of the most
difficult environments to communicate in. Unpredictable noise levels and sources,
along with dynamic attenuation characteristics make most conventional
communication techniques ineffective.
These obstacles involved in communicating over the power line are well discussed
in various literature such as: "Power Line Communications: Channel Properties and Communication Strategies"
, from which some excerpts have been posted later on in this section.
Many of the devices best suited for PLC, cause some of the worst obstacles evident
on the power line. Devices such as dimmer switches on lights, and switching power
supplies used by many computers, use the 60Hz AC power characteristics to operate
properly. As the direction of the current on the power line changes, components
inside these devices 'switch' to compensate.
This causes noise, or a 'spike', on the power line, and as these devices 'switch' along with the AC power cycles,
repetitive noise in the form of 'spikes' is created. In many cases these noise
spikes can be many times greater in amplitude than the average ambient noise,
thereby making the effect of repetitive noise a greater obstacle to overcome,
in many case, than ambient noise on the power line.
A major benefit of the power line as a connectivity infrastructure is the fact that
almost all devices in the home are connected to power in some form or another.
However, that very fact creates another great obstacle for PLC: Unpredictable,
constantly changing attenuation characteristics.
In most electronic communication situations, calculations can be made to determine to what extent the signals will
be attenuated during their travel between source and destination. These calculations
usually involve simple factors such as distance between the devices and the
communication medium used to connect the devices.
Generally speaking, the inherent impedance of the communication medium and the distance that the signal has to travel
along that medium will determine how much of the signal will be lost during the trip.
However, on the power line, these calculations become unattainable. The impedance
of the power line is not a constant, as it is in most communication mediums. The
impedance on the power line is affected by the type and number of devices attached
to it at one time.
As devices turn on and off, and even during the regular use of many devices, their effect on the impedance of the power line changes. This creates
a purely dynamic impedance environment. In general, the physical distance between
two devices can usually be used to determine just how far the signals have to
travel. This is not the case on the power line.
The power wiring in any house or building is a complex network and in many cases two devices, only meters apart
physically, may be hundreds of meters apart when traveling by the power line.
Both these factors further increase the complexity of determining the amount of
attenuation a signal will realize between two devices.
The following excerpt is taken directly from the thesis by L. Selander refered to above.
It provides a more detailed, technical look into using the power line as a communication
medium and some of the problems that are faced in doing so.
"Chapter 4 - Measurements of the Characteristics
of the Low-Voltage Grid"
There is a growing interest in the use of PowerLine Carrier (PLC) for data
communication using the intrabuilding electric power distribution circuits. Power lines
were not designed for data communications and exhibit highly variable levels of impedance,
signal attenuation and noise. Many studies are available that describe in some detail the
impedance, signal attenuation and noise characteristics of powerline networks. In their
study of residential powerline noise sources, Vines et al. identified 4 types:
1- Sources that generate impulse noise in synchronism with the 60 Hz power frequency.
2- Smooth spectrum noise generated by loads not synchronous with the power frequency
(Ex : Universal motor in an electric drill)
3- Non synchronous, single-event impulse noise, (Ex : thermostat or lighting switching)
4- Non synchronous periodic noise.
In general, intrabuilding powerline noise can be considered to consist of continuous, relatively low level background noise punctuated by high level noise impulses.
Background noise is typically Gaussian, and its effects on communication performance is well
understood. In the case of impulse noise, its time-domain characteristics (amplitude, width
and interarrival time) is very important to determine the influence on data communication systems.
Chan and Donaldson characterized noise impulses on PLC networks. They conclude:
1- Impulse strength is typically more than 10 dB above the background noise level and can exceed 40 dB.
2- Impulse frequency for the dominant impulse train is usually 120 Hz.
3- Impulse width can vary up to a few percent of the impulse period for 120 Hz impulse noise.
4- Because noise as well as wanted signal are subjected to attenuation, noise sources close to the receiver will have the greatest effect on the received noise structure, particularly when the network attenuation is large.
5- Harmful effects of impulse noise on data communications systems can be expected.
Item 4 above bears significant consequences. It says that when a noise source is located
close to a receiver, and when the signal is attenuated (across-phase communication, or
attenuation caused by the combination of line impedance and the presence of low impedance
loads along the communication link) a local noise source could make a receiver exceed its
noise tolerance (signal to noise ratio), yielding erroneous received data.
Single-event noise type can be easily overcome, by simply repeating the data packet.
However, the situation is more complex with line-synchronized noise type, because this
type of noise can introduce one (or more) error 120 times per second.
If the data transmission process lasts more than 1/120 second (or 8.33 mSec), errors will likely
occur, and packets will be lost. In many circumstances, it is unlikely that repeating the
packet will circumvent the problem. Breaking the data packet into smaller segments
lasting less than 8.33 mSec, and transmitting these segments between noise bursts is also
impossible, because the transmitter could have a local noise source, different from the
noise source at the receiver.
In other words (and remembering item 4 above), in any PLC system, one should assume that noise at the receiver is unknown to the transmitter.
Consequently, it could be difficult (if not impractical) for an intelligent transmitter
to analyze the powerline noise characteristics and adapt its transmission strategy,
because noise at the receiver end is very likely to be different from noise measured at
the transmitter. Moreover, such an intelligent transmitter could not broadcast messages
to N receivers, each receiver having its own and different local noise characteristics.
Local noise is not the sole source of communication errors.
Sudden impedance variation can also induce similar effects. Different loads (some compact fluorescent lights,
personal computer power supplies, switching power supplies) are known to make the
impedance of the communication link change abruptly with time, generally synchronously
with the powerline frequency. (These loads generally offer a higher impedance level
around the zero crossing of the power wave.) For the same reasons as above, impedance
variations at the receiver end is almost undetectable at the transmitter location, making
adaptive algorithms difficult to implement. The same principles apply to impedance
variation: more severe effects are generally observed when the impairment source is close
to the receiver, and the impedance between the receiver and the transmitter is high.
Once again, repeating a greater than 8.33 mSec packet will likely not overcome line
synchronous, impedance variation related error bursts.