If you made it through the italicized technical protocol descriptions of the prior page, you encountered numerous references to bit errors and their mitigation; forward error correction approaches, retransmit schemes, hundreds or thousands of discrete transmission channels with independently configured modulation densities, etc. Such workarounds exist because the power grid was never intended for networking purposes and, as such, is a quite unfriendly environment for reliable high-speed data transfers.

Consider, first and foremost, that every AC-fed device creates a momentary dip or surge (however slight) when it first powers up or off. Such situations are usually occasional, and as such can be dealt with packet retransmission (in the case of TCP) or brief loss (with UDP). More egregious, on the other hand, are devices that inject a constant stream of high frequency noise onto the power grid, such as:

  • Switching power supplies (including AC-to-DC converters used in cellphone chargers and the like)
  • Motors in devices such as fans, hair dryers, vacuum cleaners, washers and dryers, furnaces and air conditioners, and refrigerator compressors
  • Illuminated CFLs (compact fluorescent lamps)

Such devices' noise patterns can destructively interfere with one or multiple channels' worth of powerline networking data. And at this point, I should also point out that the active powerline network can itself be a destructive interference source, specifically for shortwave radios, by virtue of the fact that current passing through a wire creates a magnetic field surrounding that wire, thereby turning it into an antenna. Powerline technologies are a longstanding sworn enemy to many 'ham' radio operators, although LAN-based powerline approaches are far less egregious in this regard than are WAN BPL (broadband over powerline) approaches spanning a large region. And powerline adapters are also intentionally designed with notch filters that, when activated, create channels (at the tradeoff of reduced peak bandwidth) that might interfere with other transmitters and receivers in a particular geography.

Next is the issue of networking signal attenuation, which is first and foremost caused by old or otherwise low-quality electrical wiring. Other potential problems include narrow-gauge wiring, with excessively high impedance; poor intra-span connections and variable gauge wiring across the span both result in unwanted reflections. Powerline packet 'jumps' across circuit breakers are performance-problematic; even more so are source-to-destination paths that involve a transition from one 110V (U.S.) phase of the incoming 220V source to the other phase. Even within a particular circuit breaker wiring spur, the presence of GFCI (Ground Fault Circuit Interrupter) outlets can cause problems, even if a powerline adapter isn't directly connected to them.

Don't try to connect a powerline adapter to a surge protector, which will filter out the high frequency data modulated on the 50 or 60 Hz carrier, unless the adapter is three-prong and implements Sigma Designs' ClearPath approach. ClearPath, according to Sigma Designs, alternately routes packets over the earth ground connection, which is normally not filtered. (Atheros also eventually plans to implement a similar approach, called Smart Link.) Keep in mind that surge protection circuitry is increasingly not just included in standalone power strips but also embedded within wall outlets. And a UPS (uninterruptable power supply) also acts as an effective deterrent to powerline packet propagation.

Speaking of circuit breakers, now's as good a time as any to discuss security. Don't worry about your next-door neighbor accessing your LAN if a transformer is in-between your respective street-side power connections. On the other hand, there's a tangible possibility that multiple powerline networking users sharing a common transformer feed (such as, for example, in the same multi-apartment building) could tap into each others' equipment. That's where encryption comes in. HomePlug 1.0 and 1.0 Turbo harness 56-bit DES encryption, while HomePlug AV leverages even more robust 128-bit AES. And altering an adapter's password requires access to a 16-digit unique password stamped on the unit. Just change your equipments' passwords from the 'HomePlug' or 'HomePlugAV' default, and other folks on the same transformer feed won't subsequently have access to them.

Technology Fundamentals Testing Setup
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  • metageek - Thursday, September 1, 2011 - link

    So, what happens if a power surge comes in through the outlet? Do these things include surge suppression, or is that surge going to travel out over the Ethernet cable and fry your computer? Reply
  • kmmatney - Thursday, September 1, 2011 - link

    They have built-in power surge features. These are mainly used to keep the network signal clean, but they also protect the device. Reply
  • metageek - Thursday, September 1, 2011 - link

    Thanks. Reply
  • ranster - Thursday, September 1, 2011 - link

    I've been using Linksys PLE200s in my home for a long time now. I got the first pair (sold as a PLK200) for like $140, then later Sears had a sale online for $90, a good deal at the time, so I got another pair. I picked up a third pair off ebay for $14 since only one worked, but not an issue to add on just one. Lastly, I got a new PLK when Circuit City was going out of business, for a mighty good price. I have five of the seven in use, and the Linksys utility shows throughput from ~60Mbit to ~130 Mbit, depending on which PLE and its location in the house.

    I might check into the newest standard since they are said to interoperate with the HomePlug AV standard that PLE200s use.
    Reply
  • durinbug - Thursday, September 1, 2011 - link

    I've been interested in powerline networking for the last couple of years, and I appreciate the attempt to do a thorough investigation of the technology, but the results just don't turn out to be useful.

    I've been considering connecting my PS3 in the living room with the router (and desktop) in my office. Right now I use 802.11g (since the PS3 doesn't support n), which nets me a connection speed that ranges from ~10-15 mbps (a laptop also on G in the same location gets 30-40 mbps - the PS3 wireless adapter really seems to be craptastic). This is sufficient for playing games, but can be a major headache for streaming video.

    While the throughput data you present here suggests that powerline networking might be an improvement, it doesn't really help make that decision. At an apparent average of about 30-40 mbps that you got, it wouldn't take a whole lot of interference to bring it down to the speeds I'm seeing already.

    That said, I understand why you wanted to show the "best case" - I just wish you had also shown the "normal use" case so we might have some idea of the potential speed reduction we might see. I have numerous compact fluorescent lights, refrigerator, etc. (plus living room and office are on separate breakers, don't know about phases) - so even after reading your article I am no closer to knowing whether powerline networking will work for me. So after all the work of writing the article, the take-away seems to be "this is what I got; I have no idea what performance you will see."
    Reply
  • bdipert - Thursday, September 1, 2011 - link

    Dear durinbug, I appreciate the feedback. The fundamental issue that a reviewer such as myself always struggles with in a situation iike this (or Wi-Fi or phoneline or coax, for that matter...any non-networking-optimized interconnect media) is what is 'normal'. Note that my testing (for unfortunate reasons that I discuss in the article) was TCP-only, whereas your streaming situation would likely use UDP instead. From my past experience with powerline products, you would likely experience a 50%-to-100% performance improvement in average UDP transfer rate versus the TCP numbers that I published. Every technology subsequent to first-generation HomePlug 1.0 has focused the bulk of its development and implementation attention on UDP (for likely obvious reasons) Reply
  • demonbug - Thursday, September 1, 2011 - link

    I appreciate the response, and wanted to reiterate that I really do appreciate the in-depth information and test results you provided. It would just be nice to get some sense of the impact on performance things like running the refrigerator or CFLs might have, even if your case doesn't represent the "normal". There is enough variability in throughput as it is, even with the semi-idealized setup (lights off, etc), that I'm a little concerned about maintaining throughput with such sources of interference present.

    Your mentioning that you generally see 50%-100% faster UDP transfer rates makes me hopeful that it actually would be a significant improvement over what I have now (and save me from trying to run cat5 through an exterior wall, which looks to be no fun at all), so I just might have to go and pick a couple up.
    Reply
  • dennishodge - Thursday, September 1, 2011 - link

    Now that the Sonos Bridge is only $50, went Sonos and bought an extra Bridge just to plug my laser printer into. I used to have powerline ethernet but the devices were flaky and eventually the last pair died. Each Sonos component has 1-2 10/100 switches. I haven't done a speed test, but the latency is superb. I even have an old wifi router plugged into a Sonos amp in one end of the house to extend my wifi coverage for weak devices like cell phones.

    It would be cool to trial two $50 bridges vs. powerline ethernet :-)

    - Dennis
    Reply
  • EarthwormJim - Thursday, September 1, 2011 - link

    It seems to me for about the same cost as some of these high speed powerline networking setups, you could just hire an electrician to wire some ethernet jacks throughout your house. Reply
  • bdipert - Thursday, September 1, 2011 - link

    Dear EarthwormJim, the hourly labor rate for electricians must be much lower where you live than where I do ;-) Your comment also prompts me to make a related point...retrofitting an existing structure for Cat5e by "burrowing through dirty, spider- and snake-infested crawlspaces" underneath floors (of which I have repeated past personal experience, back where I used to live in Sacramento CA) isn't even an option for a basement- and crawlspace-less residence built directly on a concrete slab... Reply

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