Hands-On Powerline Networking: How Well (Or Not) Are Latest-Generation Devices Working?

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.