Whatever Happened to Power Line Communications?

A while back, PLC was considered as an alternative to DSL and cable TV for high speed Internet connectivity. It not was widely implemented because it was never fast enough to really compete successfully.

Power line communications (PLC) is the technology that uses the ac power line as the medium for the transmission of digital data.  It has been around for decades and is now a highly developed wired communications alternative.  I have not heard much about it lately and wondered what was happening.

A while back, PLC was considered as an alternative to DSL and cable TV for high speed Internet connectivity.  It not was widely implemented because it was never fast enough to really compete successfully.  It was eventually abandoned because of the massive interference it caused to the ham radio and short wave listening bands (3-30 MHz).  Today, PLC is primarily a home networking technique.

There are several aspects to home networking.  The one we all know and use is Wi-Fi as it is what connects our smartphones, tablets and laptops to the home router connected to the Internet via cable TV or DSL.  It also connects our TV sets to Netflix by way of an Apple TV, Roku, or other over the top (OTT) TV box.  Wi-Fi is even increasingly used in connecting things to the Internet like home thermostats, video cams and some appliances.  Where PLC comes in is connecting multiple TVs, DVRs, DVDs, or other devices scattered around the home to the Internet.  Another technology that does the same thing is MoCA (Multimedia over Cable Alliance) that uses the existing home cable TV coax wiring.

The most widely used PLC technology is HomePlug (HP).  It is a standard of the HomePlug Alliance that was established in 2000.  The latest version of HP is AV2 that uses OFDM over the frequency range of 2 MHz to 86 MHz on the ac line.  Using modulation to 4096QAM it can achieve a raw data rate to 1 Gb/s or 500 Mb/s considering overhead.  This is useful for transporting HD video around the home.

I spoke with HomePlug President Rob Ranck who said sales of HP products were good.  They are not as visible as Wi-Fi products, but they are on the shelves at Best Buy and other retail outlets.  Usage is probably greater in Europe where Internet providers build in PLC to their modems and routers.  PLC gives better coverage than Wi-Fi in Europe where housing and building construction is harder for Wi-Fi to penetrate.  In the U.S. PLC helps whole home coverage by extending Wi-Fi coverage with re-broadcasters in a hybrid arrangement.  More and more, HP is being integrated into set top boxes, OTT routers, Internet TVs and other devices.  In any case, HP is alive and well and makes a great alternative for home connectivity.  Check out the resources at www.homeplug.org.

The other major PLC technology is G.hn.  This is a newer standard by the ITU designated G.9960 and G.9961.  It was designed so that it can use any available wired medium like existing telephone twisted pair, cable TV coax or the ac line.  It too uses OFDM to achieve data rates to 1 Gb/s as well.  There does not seem to be many commercial products available for the home networking market.  At least I could not locate any here in the U.S.  My inquiry to the Home Grid Forum was not answered in time for this article but you may want to check out their site at www.homegridforum.org.  My guess is that there is more activity in Asia than in the U.S.  It is an excellent technology that may have come too late for the U.S. market where HomePlug is the only PLC source.

Discuss this Blog Entry 5

on Jul 17, 2014

While you list Ham radio and Short Wave listening as the 'bad guys' helping in this system's demise, the horizon of interference is much wider, but not mentioned.
Low-Band commercial radio, operating from 30 to 50 MHz. also stood to be impacted. Fire departments, our local power company and many school bus companies operate there. I know, I maintain several school bus fleets.
They, in particular, would have been at risk as they would be the ones sitting right beneath the RF leaking power lines above their routes. Their ability to communicate, possibly in an emergency, severely diminished.
BTW, ham radio has become a proven emergency communications link. Katrina and the Twin Towers incidents, to name the most notable, bear ample witness.
Stan, amateur operator K2STN.

on Jul 21, 2014

The rampant spectrum pollution generated by HF PLC systems, described by Lou Frenzel, is certainly correct and is a simple fact; a physical reality. However, I would hesitate to cast blame on any individual spectrum user community for the failure of PLC. Culpability in this case lies with the inventors and marketers of HF PLC. HF PLC uses broad spectrum carriers in the 3 to 30 MHz range, representing wavelengths of 100 to 10 m. Consider deploying a communications system that uses copper conductors, with no shielding, with no attempt at balancing transverse impedance, on conductors whose physical length is on the same order as 1/2 wavelengths of the RF carriers, and said antennas are located in close proximity to radio receivers tuned to the same frequencies. This is an engineering disaster. The take away is: do not fight physics; you will usually lose.

on Jul 21, 2014

PLC was doomed before it ever got off the ground. Compare the carefully twisted pairs required for DSL or the carefully shielded, controlled impedance coax required for cable to the widely spaced, unshielded wires of power distribution, optimized not for MHz but for 60 Hz and laden with noise from other loads. It didn't stand a chance. The only people who didn't know it were the politicians, the FCC and the investors they duped. Two of those groups could be forgiven for their ignorance, and I guess, so could the FCC, since they are staffed with attorneys these days rather than engineers.

The model was further flawed, because it doesn't travel as well on medium voltage (4KV) distribution lines as it does on 120/240 neighborhood feeds. Hence it was generally injected after the transformer. Well, in urban areas where a number of households were served by a single transformer and transformers were close enough together to minimize backhaul, there was a lot of competition from DSL and cable. In rural areas where the politicians claimed this would be such a good solution, each house is served by a separate transformer, often miles apart. WISP has prove the only viable solution in those settings.

It is unfortunate that so much energy and money was thrown after that boondoggle. It just shows what happens when engineering decisions are made by people who don't understand the technology.

Power utilities and others have successfully use PLC for years, but generally low data rate circuits with a carrier frequency around 400 KHz. Even then, I have personally encountered interference to the AM broadcast band because the signal radiated from the wires. Generally it came into the receiver due to inadequate image rejection.

Certainly within a home its odds of success are much better. There is less distance to travel, less to interfere with, and the expected reliability is lower. However, even there, alternatives such as Wi-Fi are more likely to prove successful.

on Jul 22, 2014

We have more than enough interference on our Mains supply.
Is it now that we should consider outlawing all extra interference added by consumers?

Ban the import of this equipment.
Ban the selling of this equipment.
Ban the repair of this equipment.

on Jul 23, 2014

Having "fought the good fight" in the 2003-2009 timeframe against broadband over power-line (BPL) in Manassas, VA , I can attest to the flawed nature of this technology.

Amateur radio operators in the Manassas area during this time experienced noticeable interference from the BPL system, eventually requiring the system operator to "notch" frequencies causing the most pronounced interference. Interfering signals were detected from overhead medium voltage lines as well as underground installations. In the case of underground systems, energy was being radiated from the vertically-oriented wires leading to each street light.

While amateur radio operators complained of the egress (i.e., received interference) problem, a more worrisome problem with ingress interference was uncovered during testing. A mobile amateur HF transmitter was located underneath a BPL-equipped overhead power line. During transmission, the HF transmitter so completely overwhelmed the BPL system that the connection would drop at a nearby subscriber's home.

When one considers that 911 calls can be placed over Internet-based systems (i.e., VoIP traffic), this is cause for concern. In the previous scenario, the amateur radio operator would NOT be responsible for the dropped call because the BPL system, operating under Part 15 regulations, must accept any interference received from licensed radio services. Similar concerns would be warranted if the BPL system were to be used for municipal infrastructure such as traffic control cameras, etc.

Thankfully, the economics of this BPL system never panned out and the operator got out of the business. In the end, the operator had only secured about 1200 customers before Verizon came in with FiOS service.

To read more on this saga:

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Lou Frenzel

Lou Frenzel is the Communications Technology Editor for Electronic Design Magazine where he writes articles, columns, blogs, technology reports, and online material on the wireless, communications...
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