What is Power Line Technology?
What is PLT?
It has many names and acronyms, so let us bring you up to speed: PLT can mean both Power Line Telecommunications and Power Line Technology; and it is often referred to as Power Line Adapters (PLA). It is a form of Power Line Communication and it takes the shape of devices which you can plug into a standard mains outlet socket for the purposes of running an Ethernet computer network around your property. It is a "self-install" system designed to negate the need to install proper computer data networking (Cat 5E), which usually requires considerable effort and the skills of an electrician to route cables through walls and floor voids (although this is a one-off hassle). It is also sold as a replacement for Wi-Fi for those who experience over-crowded Wi-Fi channels in their locale; although this seems rather self-defeating?!
Here are some images of the typical PLA devices causing the trouble:
PLAs are manufactured against two competing industry "standards": The HomePlug Power Alliance (HPA) and the Universal Powerline Association (UPA). The two technologies are of course not compatible and they both use different techniques to transmit the signal around your home's mains wiring.
Do not be confused with the term Broadband over Power Line (BPL). BPL is a system whereby the local electricity company attempts to send broadband signals along the mains supply to your property for the purposes of providing Internet connectivity. To date, the vast majority of BPL deployments have been shutdown as commercial flops; they were too slow and generated too much interference.
PLT devices have been available from a number of outlets for several years, but the "technology" is poorly thought out and poorly implemented and throughput speeds are quite poor compared to Wi-Fi or hard-wired networking using Category 5 UTP or Category 6 UTP data cabling (both of which are capable of 1Gbps). The number of installed units suddenly increased when BT Vision launched its IPTV over Broadband package to home users. Instead of offering users a crash-course in networking, they chose to include a set of Comtrend PG902 PLT devices for end-user self-installation. This has greatly increased the number of PLT devices in use by an un-aware public!
Powerline networking (PLT) is an unethical technology which causes serious problems for radiocommunications. Despite the non-compliant nature of the technology with established internationally agreed limits for interference, the European Commission has permitted the industry to continue with impunity to the EMC Directive, leading to speculation of corruption within the Commission itself.
EMC experts, organisations, radio users and enthusiasts have tried hard to have their respective national administrations see sense. However in Europe, it has become clear that the Regulators are being directed by the EC's "no barrier to free-trade" mantra, which is used to ensure the non-compliant technology remains on the open market.
These devices have spent the last few years destroying the HF radio spectrum, much to the annoyance of Shortwave Radio listeners, Radio Amateurs and Citizens' Band radio users. Not content with that, a new breed of PLT is now available which wipes out VHF as well. You can kiss goodbye to your FM stereo, DAB, business radio, taxi radio, Air Traffic Control and more...
How does PLT "work"?
Power Line Adapters transmit their signal over the in-house mains wiring by injecting a very high level of radio frequency energy on to the phase (live) and neutral wires. They use a wide-band modulation method known as Orthogonal Frequency Division Multiplexing (OFDM) to encode the Ethernet frames into a radio signal. PLA manufacturers believe this signal happily travels along the mains wiring within the house and does not pass through the electricity meter or radiate from the cables. This sadly is a fundamental mis-understanding of how radio signals operate! The mains wiring in your property was designed for one thing and one thing only: mains electricity! It was not designed to handle radio energy. For that we have co-axial cable; used to connect TV/Satellite aerials (for example). And for networking, we have Category 5 UTP and Category 6 UTP, a balanced cable designed to handle high-frequency signals used by Ethernet networking. So, if you are operating a pair (or more) of PLT devices, your mains wiring is leaking radio energy like a sieve! Your PLT signals are also winding their way along the phase and neutral conductors leaving your property. UK domestic 3-phase wiring methods mean those neighbours who share the same power phase will see your PLT signals conducted on to their mains wiring. The same phase may also supply a lamp post, meaning your PLT noise will be radiated from there as well!
If you live in a village with over-head power cables and over-head telephone cables you will suffer a reduction in ADSL sync and throughput speeds. The PLT signals radiate from the mains and are coupled into the telephone lines via electromagnetic induction. This will not just affect your ADSL signal, but several properties in the local area. Your neighbours will be none too pleased to learn of this fact!
It has also been reported that the new breed of 1Gbps PLT (using frequencies up to 300MHz) cause considerable interference to the new VDSL system BT Openreach are installing to homes as part of their next-generation "broadband" offering. Yes that is right, BT Vision are shipping a product which wrecks what BT Openreach are rolling out!
The Orthogonal Frequency Division Multiplex modulation technique creates a swathe of noise across the radio spectrum running from 2 to 30MHz (in the case of the early HPA/UPA devices) and up to 300MHz in the case of the new 1Gbps devices now being sold. This radio energy leaking from the home of a PLT user can radiate in all directions up to 500 metres and further. When viewed at the level of a small town, this can ensure the entire town is blanketed with radio frequency interference (also known as Spectrum Abuse) by a small number of installations (approx. 25 PLT installations have been detected wiping out a small market town with a population of approx. 12,000). This level of interference renders the legal users of the radio spectrum unable to use their equipment.
Radio Hams, who we should really call Radio Amateurs as per their official title, have certainly been vocal about the issue - there can be no mistake. However in quantifiable terms, although some Amateurs have quite a hard time due to this technology, the greatest effect is being felt in the shortwave broadcast arena and with Citizens' Band radio users.
Knowing that the Amateurs would give them a hard time and in line with ITU recommendations, when the "Standard" was proposed for PLT, the Amateur bands were 'notched' out. This could be described as a kind of 'graphic equalizer' for radio in which certain ranges of frequencies are attenuated so as to reduce the likelihood of interference in that range. However, in the same way that different Hi-Fi speakers will respond differently to tweaks on a graphic equalizer, so too do the PLT devices react differently to the various wiring systems employed in peoples' homes.
Staying with the analogies for a moment, for those old enough to remember when most, if not all car radios had telescopic aerials, if your aerial snapped off or was left partially retracted, you knew the range of your radio would be much reduced unless you were lucky enough to live or work right next to your radio station - you still see workmen today using coat hangers for aerials on their portable radios! With PLT something similar is happening with the electrical wiring of your home or office. The more there is, the better "aerial" it becomes to the PLT signals which are constantly leaking out. Since the frequencies used by the current generation of PLT are 2-30 MHz, any length of wire more than a couple of metres has the capacity to radiate a significant signal. How much wiring does your house have in it? Think about your sockets and lights and light switches for a moment. I would bet you will have 100 - 200 metres if it is a house and plenty more if it is an office complex.
This problem is exacerbated by a phenomena known as intermodulation which is caused by semiconductor devices found in common household appliances. The "non-linear" devices as they are called, modify the PLT OFDM signals in such a way that the separate 'channels' become jumbled up creating new unwanted signals which cause even more interference than the "pure" PLT original. You could think of it like trying to listen to three songs simultaneously, all at a similar volume; not a pleasant prospect!
In the end, the effect is clear to see by anyone who wishes to look. Of course, you might say "Stuff'-em, they are only ham radio freaks. I need my broadband moved around the house - and my BT Vision (IPTV), why should I care? The Wi-Fi is all jammed packed round here and does not work properly any more."
So what is all the fuss about?
Page updated: 10th September 2012
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