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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  • metageek - Thursday, September 01, 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 01, 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 01, 2011 - link

    Thanks. Reply
  • ranster - Thursday, September 01, 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 01, 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 01, 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 01, 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 01, 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 01, 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 01, 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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