This is, I think, the first time that AnandTech has published in-depth hands-on testing of powerline networking equipment, although staffers such as Ganesh T S have covered the technology both conceptually and via hands-on overviews in the past. As such, I thought a short upfront tutorial might be in order. Powerline networking transceivers employ the 50- or 60 Hz sine wave as a carrier, superimposing the higher-frequency data packets on it. Sounds simple, right? While it may be elementary in concept, the implementation is quite complex. Take this excerpt from the technical white paper for HomePlug 1.0 (PDF), which touted up-to-14 Mbps PHY rates and dates from mid-2001:


Orthogonal Frequency Division Multiplexing (OFDM) is the basic transmission technique used by the HomePlug. OFDM is well known in the literature and in industry. It is currently used in DSL technology, terrestrial wireless distribution of television signals, and has also been adapted for IEEE’s high rate wireless LAN Standards (802.11a and 802.11g). The basic idea of OFDM is to divide the available spectrum into several narrowband, low data rate subcarriers. To obtain high spectral efficiency the frequency response of the subcarriers are overlapping and orthogonal, hence the name OFDM. Each narrowband subcarrier can be modulated using various modulation formats. By choosing the subcarrier spacing to be small the channel transfer function reduces to a simple constant within the bandwidth of each subcarrier. In this way, a frequency selective channel is divided into many flat fading subchannels, which eliminates the need for sophisticated equalizers.

The OFDM used by HomePlug is specially tailored for powerline environments. It uses 84 equally spaced subcarriers in the frequency band between 4.5MHz and 21MHz. Cyclic prefix and differential modulation techniques (DBPSK, DQPSK) are used to completely eliminate the need for any equalization. Impulsive noise events are overcome by means of forward error correction and data interleaving. HomePlug payload uses a concatenation of Viterbi and Reed-Solomon FEC. Sensitive frame control data is encoded using turbo product codes.

The powerline channel between any two links has a different amplitude and phase response. Furthermore, noise on the powerline is local to the receiver. HomePlug technology optimizes the data rate on each link by means of an adaptive approach. Channel adaptation is achieved by Tone Allocation, modulation and FEC choice. Tone allocation is the process by which certain heavily impaired carriers are turned off. This significantly reduces the bit error rates and helps in targeting the power of FEC and Modulation choices on the good carriers. HomePlug allows for choosing from DBPSK 1/2, DQPSK 1/2 and DQPSK 3/4 on all the carriers. The end result of this adaptation is a highly optimized link throughput.

Certain types of information, such as broadcast packets, cannot make use of channel adaptation techniques. HomePlug uses an innovative modulation called ROBO, so that information is reliably transmitted. ROBO modulation uses a DBPSK with heavy error correction with bit repetition in time and frequency to enable highly reliable communication. ROBO frames are also used for channel adaptation.


HomePlug 1.0 was an industry standard, at least in concept, although Intellon (now a part of Atheros, which was subsequently acquired by Qualcomm) supplied the bulk of the transceivers used in HomePlug 1.0 transceivers. Follow-on HomePlug 1.0 Turbo, first unveiled in product form at the January 2005 Consumer Electronics Show, was a more overt Intellon-proprietary offering, backwards compatible with HomePlug 1.0 (at HomePlug 1.0 speeds) but delivering up to 85 Mbps PHY rates in Turbo-only adapter topologies. Marketing claims aside, however, Home Plug 1.0 Turbo products delivered little to no performance improvement over their HomePlug 1.0 predecessors in most real-life configurations.

Next up was HomePlug AV, which represented a return to the consortium-inclusive (albeit still Intellon-led) approach and was spec-wise first unveiled in August 2005. Like HomePlug 1.0 Turbo, it focused the bulk of its performance improvement attention on UDP (User Datagram Protocol) typically employed by high bitrate streaming multimedia applications (therefore the AV acronym within its name), versus TCP (Transmission Control Protocol). And how did it accomplish its 200 Mbps peak PHY rate claims? Here's a quote from its corresponding technical white paper (PDF):


The Physical Layer (PHY) operates in the frequency range of 2 - 28 MHz and provides a 200 Mbps PHY channel rate and a 150 Mbps information rate. It uses windowed OFDM and a powerful Turbo Convolutional Code (TCC), which provides robust performance within 0.5 dB of Shannon Capacity. Windowed OFDM provides flexible spectrum notching capability where the notches can exceed 30 dB in depth without losing significant useful spectrum outside of the notch. Long OFDM symbols with 917 usable carriers (tones) are used in conjunction with a flexible guard interval. Modulation densities from BPSK (which carries 1 bit of information per carrier per symbol) to 1024 QAM (which carries 10 bits of information per carrier per symbol) are independently applied to each carrier based on the channel characteristics between the transmitter and the receiver

On the transmitter side, the PHY layer receives its inputs from the Medium Access Control (MAC) layer. There are separate inputs for HPAV data, HPAV control information, and HomePlug 1.0 control information (the latter in order to support HomePlug 1.0 compatibility). HPAV control information is processed by the Frame Control Encoder block, which has an embedded Frame Control FEC block and Diversity Interleaver. The HPAV data stream passes through a Scrambler, a Turbo FEC Encoder and an Interleaver. The outputs of the three streams lead into a common OFDM Modulation structure, consisting of a Mapper, an IFFT processor, Preamble and Cyclic prefix insertion and a Peak Limiter. This output eventually feeds the Analog Front End (AFE) module which couples the signal to the Powerline medium.

At the receiver, an AFE operates in conjunction with an Automatic Gain Controller (AGC) and a time synchronization module to feed separate data information and data recovery circuits. The HPAV Frame Control is recovered by processing the received stream through a 3072-point FFT, a Frame Control Demodulator and a Frame Control Decoder. The HomePlug 1.0 Frame Control, if present, is recovered by a 384-point FFT. In parallel, the data stream is retrieved after processing through a 3072-point FFT for HPAV, a demodulator with SNR estimation, a De-mapper, De-interleaver, Turbo FEC decoder, and a De-scrambler for HPAV data.

The HPAV PHY provides for the implementation of flexible spectrum policy mechanisms to allow for adaptation in varying geographic, network and regulatory environments. Frequency notches can be applied easily and dynamically, even in deployed devices. Region-specific keep-out regions can be set under software control. The ability to make soft changes to alter the device’s tone mask (enabled tones) allows for implementations that can dynamically adapt their keep-out regions.


HomePlug AV can coexist with HomePlug 1.0 and HomePlug 1.0 Turbo, although it doesn't interoperate with either predecessor technology. It also (at least initially) competed against two other '200 Mbps' powerline networking approaches. UPA (the Universal Powerline Association) was largely controlled by DS2 (Design of Systems on Silicon), much as the HomePlug Powerline Alliance was Intellon-dominated in its early days. UPA was, like HomePlug AV, OFDM-based, but the two technologies' implementation specifics were incompatible (UPA, for example, used 1,536 carriers across a 3-to-34 MHz frequency range). Nor did they even coexist, in fact; they'd notably degrade each other if you tried to simultaneously run both approaches on the same power grid. Spain-based DS2 achieved some success, especially in Europe, but declared bankruptcy in 2010 and was subsequently acquired by Marvell.

The same non-coexistence situation occurred with Panasonic-championed HD-PLC approach, which was also somewhat popular in its day, especially in Asia. Here, the versus-HomePlug AV outcome was somewhat different, although HD-PLC has also largely faded from the market. The IEEE 1901 standard supports HomePlug AV, HD-PLC, or both technologies; the latter implementation resolves technical issues that had previously precluded coexistence, but it requires a costly dual-MAC design, since HomePlug AV is a FFT-based approach whereas HD-PLC harnesses wavelet transforms. IEEE 1901 also optionally expands the employed spectrum swath beyond 28 MHz all the way up to 50 Mhz, with a corresponding peak PHY rate increase from 200 to 500 Mbps. Note, however, that just as with 5 Ghz versus 2.4 Ghz Wi-Fi, higher frequency powerline channels travel shorter distances (before attenuation leads to insufficient signal strength) than do their lower frequency, longer-distance peers.

Then there's G.hn, an ITU-sanctioned standard whose participants include many past representatives of DS2, now with various new employers. Chano Gómez, DS2's former VP of Marketing, is now Director of Business Development at Lantiq (the former networking division of Infineon), for example. Sigma Designs, specifically the corporate division formed by the late-2009 acquisition of CopperGate Communications (which had itself obtained powerline networking expertise via purchase from Conexant), is developing G.hn chipsets, too, although in this particular case the company is hedging its bets by also (and, in fact, initially) designing HomePlug AV transceivers. G.h n is an attempt to unify powerline, phone line and coaxial cable-based networking with a single protocol stack that can run on multiple physical media backbones. As such, it competes not only against IEEE 1901 but also with technologies such as HomePNA and MoCA. And the IEEE is also developing a unified approach, the 1905 standard.

Introduction Implementation Issues
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  • quiksilvr - Thursday, September 01, 2011 - link

    Have you tried getting a better router and/or perhaps a better wireless card for your laptop? Reply
  • akedia - Thursday, September 01, 2011 - link

    I have a current generation Airport Extreme, which is generally regarded as one of the best wireless routers available, and the built-in WiFi antenna in my Mac mini is not upgradable, as far as I know. My roommate's laptop is an HP dm1z, also not upgradable, and my Droid X is stuck with the antenna it shipped with as well. It's not my hardware, it's my environment. WiFi has limitations, like it or not. Reply
  • bdipert - Thursday, September 01, 2011 - link

    Different tools for different tasks, jigglywiggly. Powerline can make a pretty good 'backbone' technology if, as I state in the article, you want to 'dispense with burrowing through dirty, spider- and snake-infested crawlspaces and drilling holes in walls and floors in order to route Cat5e cable around'. Wi-Fi conversely can be effective across intra-room and few-room spans...and with mobile devices. Reply
  • Paedric - Thursday, September 01, 2011 - link

    Thanks for the article first, that's something I've been interested in for quite some time.

    However, I have a question; you tested it in a "perfect" environment by disabling interfering devices, to test the potential of the system, but what happen if it is not the case?
    Is the performance hit really noticeable?

    I don't want to rout a cable across the whole house, but I'm not really keen on turning off the fridge, lights, and unplugging devices every time I want to connect to the internet.
    Reply
  • Denithor - Thursday, September 01, 2011 - link

    I have the TRENDnet TPL-303E2K Powerline AV Adapter Kit installed in my home, connecting my wireless router in the living room to my office computer about 50 or 60 feet away. Couldn't get a solid enough wireless signal in the office for any kind of gaming, hooked up this kit and within literally 2 minutes was playing everything just fine.

    There's no need to unplug or turn off anything. It just works...
    Reply
  • gariig - Thursday, September 01, 2011 - link

    I bought my parents the same TRENDnet that Denithor has (crazy coincidence) because their wireless router and extra computer are on the other side of a ~2000 SQ FT house. Works flawlessly for normal computer usage (e-mail, Youtube, etc) and printer sharing. I don't know how well it works for large file transfers but I'd imagine you'll at least get 100 mbps Reply
  • bdipert - Thursday, September 01, 2011 - link

    It depends. That's the only meaningful answer I can offer. That's why, after much gnashing of teeth and back-and-forth waffling, I decided to do my testing with everything turned off and disconnected. Otherwise, if (say) I had an especially noisy refrigerator motor, my results might have unfairly undershot some alternative typical-refrigerator reality. Obviously, my data wasn't the absolute best case...as I mentioned, I stuck with DHCP address assignments for the two Endpoints, instead of hard-wiring static IP addresses, and I concurrently ran all available powerline networking adapters although only three were in active use at any point in time, and I chose outlets out of functional meaningfulness to me, intentionally ignoring whether or not they spanned multiple breakers, or jumped across phases, in the process. But I also don't think it would have been right to turn on all potential interference sources, then do the tests.

    With that said, I regularly sling ~20 Mbps Windows Media Center streams (HD ATSC recordings) around my LAN, including through powerline spans, with no problem.
    Reply
  • leexgx - Thursday, September 01, 2011 - link

    just would of been nice if you had done an short test with stuff on to see how it is handled them (just 1 page short tests) as you did it with every thing off

    you could of had an laptop with you to monitor each power plug speeds when stuff came on, last power plugs I used the speeds stated seem close to bandwidth useable (-50 ish % for overhead)

    I found power plugs to be very reliable and how they handle packet loss as well most of the time (last time I played with them)
    Reply
  • Joe Martin - Thursday, September 01, 2011 - link

    Does it work for streaming video or not? Very hard to read article. Reply
  • bdipert - Thursday, September 01, 2011 - link

    It's impossible for me to provide a simple answer to such a question without either undershooting or overshooting the spectrum of possible realities. First off, there's the bandwidth potential of any two powerline nodes in YOUR particular setup to consider...only you can measure and ascertain that. Then you've gotta determine what you mean by 'streaming video'...are we talking about a 20 Mbps encapsulated MPEG-2 (ATSC) HD stream coming from a Windows Media Center server, for example, or a heavily compressed sub-1 Mbps H.264 standard-definition video stream? Protocol? Etc... Reply

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