Powerline Networking 101

What Is Powerline Technology?

Powerline is a networking communications technology adapted for use over existing electrical power lines, hence the name. The Institute of Electrical and Electronics Engineers (IEEE) and HomePlug Alliance organizations created working groups and standards for Powerline technology that can be applied to electrical grids as well as in-home circuits. In fact, when you hear the terminology Smart Grid, many of the proposed industrial applications utilize Powerline technology. On the consumer front, we would most likely recognize the application of Powerline technology by the electricity industry in the form of smart meters. While not every smart meter uses Powerline, those that do are an example of the electricity industry utilizing existing power lines to exchange data, such as utility companies receiving updates from your smart meter regarding your electricity usage. That is if you opt in to such a service, of course.

Trendnet’s Powerline 1200 AV2 adaptersTrendnet’s Powerline 1200 AV2 adapters

When you pause and think about that though, the potential for using power lines to transmit data could mean that, eventually, electric utility companies may become an alternative to Internet service providers in providing connectivity to households. While it may not come to pass in more densely populated areas, providing Internet via Powerline to rural households is much more likely. I have several friends who live in such areas; they've mentioned that ISPs are hesitant, if not downright unwilling, to bring fiber or cable to their homes. While that may be frustrating from a consumer point of view, to the ISP, the return on investment just isn't as high on providing Internet to one remote family compared to wiring up a neighborhood. The alternatives are usually satellite or DSL.

Although it might be exciting to discuss combining your electricity and Internet delivery, for the purpose of this article, let's bring the focus back to in-home applications. Using Powerline allows you to take advantage of existing electrical wiring for your networking requirements, circumventing the need to place an Ethernet drop in every room for wired connectivity. Take a quick look around and count how many electrical outlets you see. Each one of those is a potential network uplink if you use Powerline!

Slow down a minute, though. You can't just pop in a Cat 5e or Cat 6 cable into an electrical outlet and start watching YouTube videos. There has to be a Powerline adapter in place to convert the 802.3 Ethernet protocol into the newest Powerline standard, called HomePlug AV2, for transmission over the electrical wiring.

“Wait,” you say, “What happened to the first specification for HomePlug AV?” For that answer, we can glean the history of the standard from the master observer of all things related to use of communications signals, the National Security Agency.

In a 2001 article titled Data Communications via Powerlines, long before the HomePlug standard, there were four Powerline protocols battling for contention to be the High Speed Powerline Communications (HSPLC) solution. The protocols and the companies backing them included PowerPacket by Intellon, Plug-In PLX by Intelogis, Digital Powerline and AN1000 Powerline from Adaptive Networks. All were developed to transmit data at high speeds, while compensating for inherent issues with using electrical wiring for transmission like high attenuation, interference and signal mismatches.

HomePlug 1.0 (left) and HomePlug AV (right) LogosHomePlug 1.0 (left) and HomePlug AV (right) Logos

A committee was formed under the name HomePlug Alliance, composed of the computer and networking equipment industry's big players. In the first year of the 21st century, two weeks before the summer solstice, the coin toss showed "heads" and thus, PowerPacket was chosen to be the power-line communication standard. In reality, the decision was most likely based on PowerPacket's usage of Orthogonal Frequency Division Multiplexing (OFDM) for better performance and more resilient signal transmission, but we'll cover OFDM in depth in the next section.

Presumably for consumer and industry brand recognition, the PowerPacket name was dropped in favor of HomePlug 1.0. At that stage, real-world data transmission rates capped out at around 5Mb/s (even as they were marketed up to the theoretical max of 14Mb/s), and transmissions operated between the 4 and 20MHz frequency range.

But was 5Mb/s enough to watch the 10-hour loop of Nyan Cat in 1080p HD while playing Call of Duty 2? Of course not! I'm sure the HomePlug Alliance realized this as well. Thus, they established the next Powerline specification in 2005, HomePlug AV.

HomePlug AV

HomePlug AV is the standard that many Powerline products on the market today still use. It provides theoretical maximum speeds of 200 Mb/s while operating in the 2 to 28MHz frequency range.

In order to efficiently handle the transmission and receipt of data, there is logical separation between the control and data-handling mechanisms. In enterprise networking, you'll often hear this referenced as "control plane" and "data plane," respectively. The head honcho of control mechanisms is a process called the Central Coordinator. It is accompanied by a control process called the Connection Manager. Think of these two as the chief executive officer, responsible for making decisions, and the chief operations officer, responsible for executing those decisions. Information flow is not just one way, however, as the Connection Manager feeds data back to the Central Coordinator so future decisions can be adjusted as necessary.

HomePlug AV’s control mechanismsHomePlug AV’s control mechanisms

The Connection Manager has two staff members at its disposal: MAC and PHY. First up we have the Physical Layer, which is referenced in the OSI Model as Layer 1 and in Powerline parlance as PHY. The PHY handles management of the channel and information rates, reported in the HomePlug AV spec as 200 Mb/s and 150 Mb/s, respectively. You might wonder what the 50 Mb/s gap represents. According to the Shannon-Hartley theorem (math…yawn), there is a set speed for maximum channel capacity through which information can be transmitted without having to correct for errors. This speed in the HomePlug AV spec is that 200 Mb/s rate. Losing 50 Mb/s is a result of the need to compensate for transmission errors. Performance at the PHY is achieved via windowed Orthogonal Frequency Division Multiplexing (OFDM) and Turbo Convolutional Code (TCC). 

OFDM is a channel-management mechanism that can split the available spectrum into sub-spectrum sizes for data transmission. The key benefit in OFDM is multi-path transmission. We don't have to wait for a particular lane to clear because we have multiple lanes from which we can choose to send data. OFDM’s efficiency is why it is used as the channel-management mechanism in Wi-Fi specifications 802.11g/n/ac instead of Direct-Sequence Spread Spectrum (DSSS), used in 802.11b.

To visualize the difference between DSSS and OFDM, think of a swimming pool and swimmers lining up to do laps. In DSSS, the entire pool is dedicated to one swimmer and other swimmers have to wait for the pool to clear before entering, whereas OFDM splits the pool up into swim lanes so multiple swimmers can enter the pool at the same time. However, the more lanes that are created, the more disturbance each swimmer could feel from neighboring swimmers. So, to account for the "splash over", each lane has dead zones on either side to give the signal room to be transmitted without interference from signals in other lanes. TCC is the error-handling algorithm, which is responsible for achieving maximum transmission while accounting for the inherent noise in the transmission medium.

Visualizing OFDM vs DSSSVisualizing OFDM vs DSSS

Following the PHY is the Media Access Control (MAC) layer, referenced in the OSI Model as Layer 2. It's at this layer Quality of Service (QoS) features are handled via Time Division Multiple Access (TDMA) and Collision Sense Multiple Access/Collision Avoidance (CSMA/CA). This layer is also where the Central Coordinator asserts order throughout the Powerline network through the use of three control regions: Beacon, CSMA and Contention-Free.

First, the Central Coordinator sets a Beacon Period in which it broadcasts a schedule to all Powerline adapters, instructing each node its permitted time frame to send traffic, whether that traffic is Contention-Free or CSMA. When establishing the Beacon Period, the Central Coordinator syncs it to the AC Line Cycle, which is when the AC current "wave" is pulsed down the wire. Each node then specifies through the Contention-Free region its QoS requirements to meet traffic demand. If the Central Coordinator can handle the request, it instructs the Powerline adapters to choose the transmission frequency. This "Tone Map", along with an estimation of channel usage, is sent to the Central Coordinator so it can determine the lifespan of the connections. When persistent bandwidth isn't required, perhaps for interactive types of traffic (think telnet or ssh), allocated time in the Beacon Period may be used by a Powerline adapter to send traffic using CSMA. Because the QoS mechanisms are reliant on timing, once the Central Coordinator broadcasts the Beacon packet, the MAC synchronizes his swatch upon inspection of the associated timestamp.

Now that we've discussed the performance-handling techniques, let's talk about the sync settings and how these Powerline adapters become aware of each other. You'd be right if you surmised that this mechanism must be (ahem) centrally coordinated.

When you first plug in a Powerline adapter, it listens for a logical network. If one is present, it attempts to join. Otherwise, it establishes itself as the Central Coordinator and begins broadcasting a Beacon. As other Powerline adapters are added to the logical network, each node that hears the Beacon adds its respective information to a Discovered Station List. If a Powerline node hears information about another logical HomePlug AV network, it adds that information to a Discovered Networks List. As any good manager does, the Central Coordinator checks in periodically with each Powerline adapter to retrieve these lists so it can build and update the network topology.

Each Central Coordinator, aside from tracking performance, holds sway over the logical HomePlug AV network from a security stance via use of a Network Membership Key. You can set multiple Network Membership Keys on different Powerline adapters in case you wanted to use admission control to determine which Powerline adapters are able to participate in their respective logical networks. In theory, this is similar to when you VLAN network segments in a switch, separating the packet paths logically.

As the topology is updated over time, the Central Coordinator determines whether another Powerline adapter would be better suited to take over the Central Coordinator role depending on capability, number of discovered stations, number of discovered networks and most influentially, user selection. While I can't pin down further details, I'm pretty sure that process is something like this.

In the event no activity is observed, such as when attached devices are powered off, the Central Coordinator instructs the nodes to enter power save mode.

Now let's learn what amazing things HomePlug AV2 has in store for us!

HomePlug AV2

The immediate observable difference between HomePlugAV and HomePlugAV2 is the marketing of gigabit-class speeds. To clarify though, the gigabit speed is at the PHY layer which really just means you get gigabit connectivity, not gigabit throughput. Other key advances include:

  • Increased bandwidth through use of higher band spectrum
  • MIMO
  • Inherent repeater functionality
  • Power save mode

Underneath these advancements, the core mechanisms such as the PHY, MAC, and Central Coordinator remain the same.

Features and Data Rates of HomePlug AV and HomePlug AV2Features and Data Rates of HomePlug AV and HomePlug AV2

Having almost three times the band in which to operate allows for multiple HD streams and more bandwidth-intensive usage. HomePlug AV2 also utilizes beamforming, which is what allows for better transmission channel adjustments such as OFDM. In addition, MIMO enables the Powerline adapter to use any two wires in a three-prong outlet for transmission, whereas HomePlug AV always uses the line-neutral pair. Extended coverage is also more likely without having to purchase an extra Powerline adapter since each AV2 node has inherent repeater functionality.

Line-Neutral Pair (left) vs. Three-Prong (right) outlet configurationsLine-Neutral Pair (left) vs. Three-Prong (right) outlet configurations

From an efficiency perspective, Powerline adapters built according to the HomePlug AV2 specification have a sleep mode to prevent unneeded power draw. Until activated to transmit data, a Powerline adapter enters a sleep state with specified times to awake to check for any data transmission requests. The HomePlug AV2 logo has been revised, so you won't be able to tell just by the logo whether the Powerline adapter you're purchasing maps to that specification. Be sure to look at the specs listed on the side or back of the box for verification!

Powerline Standards

While there are excellent resources available on the Internet to learn about Powerline technology and its associated standards, there are many instances of misinformation as well. In addition, terminology isn't consistent among vendor marketing and product packaging, so how are consumers or even those interested in writing about the subject supposed to maintain accuracy in the related discussions?

In my own research, the earliest publicly available documentation easily accessible is that excerpt from the NSA that I cited. From there, the HomePlug Alliance was elected to be the governing body over Powerline specifications. To date, the only specifications defined by the HomePlug Alliance are:

  • HomePlug 1.0
  • HomePlug AV
  • HomePlug AV2

When you see other terms like "Powerline AV" or "Powerline AV2", these are marketing terms that are technically misleading if the cited specifications on the packaging don't list "HomePlug AV" or "HomePlug AV2". Why would a vendor label its product with a term that doesn't specifically match a certification? It may be one of these reasons:

  • Status Pending -- Product certification is in-process, and as we discuss in our How We Test Powerline article, the HomePlug logo and certification reference can only be used under stringent requirements. While the product is being processed for certification, the vendor could use terms that hint at being capable of operating under the newest specification.
  • Delivery Delay -- Product certification would take too long before a newer product was going to be released, so the expense to get the existing product certified doesn't have a good return on investment.
  • Sell it now -- A vendor could decide that the product operates well enough through their own implementation of the standard that they could push it to market without seeking certification from the HomePlug Alliance.

Don't be misled! Remember that there only three approved HomePlug certification standards published by the HomePlug Alliance. For more information about the requirements under which a vendor can display the HomePlug Certification Mark, check out the related discussion in our How We Test article. If a product is marked otherwise, that product is either non-compliant or is pushing marketing terminology.

Products For Home Networks

Most vendors that produce home networking products sell Powerline products of their own, usually as individual units or in kits that include at least two Powerline units. Assuming that the outlets using the Powerline adapters are on the same electrical circuit, a home user just needs to connect their first Powerline adapter to their router. Once connected, and depending on the manufacturer, there may be some management console-based configuration needed (or not, as some vendors have simplified the setup process to just a push of a button).

Common components that make up a Powerline adapter on a Netgear Powerline AV 500 PassThru AdapterCommon components that make up a Powerline adapter on a Netgear Powerline AV 500 PassThru Adapter

Amongst the major players in the Powerline game, network speeds range between 200 and 2000 Mb/s and can reach out to distances from 300 to 500 square meters. Powerline adapters, depending on the make and model, can also include a variety of additional features including:

  • Wi-Fi range extension to help extend the reach of your wireless network.
  • Pass-through power outlet to help make up for the loss of an available power source.
  • One to four 10/100/1000 Mb/s Ethernet jacks that can go out to a network ready device, such as a computer or a network switch.
  • Up to 128-bit AES encryption to protect your data.
  • Energy-saving capability that powers down the Powerline adapter when it’s not in use.
  • LED lights that can display power, connection status and network activity.
  • Factory reset button to restore the Powerline devices to their default settings.

When using Powerline at home, there are a few factors to consider. Since homes come in all different shapes, sizes and age, the wiring behind the walls may not provide the best level of conductivity, and can prove to be a limiting factor in performance. Additionally, some larger home appliances like laundry washers and dryers may reduce network speed due to power fluctuations caused by their demands. Also, if the home is already using HomePlug 1.0 Powerline products, HomePlug AV and AV2 will not be able to communicate with devices using the older standard, although they can co-exist on the same circuit. Lastly, it’s not recommended to connect a Powerline device to a power strip or surge protector as the filtering circuits within the strip can interfere with the product’s network communication.

MORE: Networking in the Forums

Matthew Matchen is an Associate Contributing Writer for Tom's Hardware. Follow him on Twitter @matchemm

Follow us on Twitter @tomshardware, on Facebook and on Google+.

This thread is closed for comments
    Your comment
  • wtfxxxgp
    I love these things. Saved me headaches and hassles to get my online gaming setup going without having to run long cables or relying on crappy wifi dongles
  • Xivilain
    This is one of those great niche products that not a lot of folks hear about. Definitely a look at for people with "bomb shelter" style basements with concrete and metal materials, where WiFi cannot reach.
  • videobear
    From Newegg customer reviews, performance of even the latest powerline equipment is far inferior to ethernet or even wifi. Plus the units have massive quality control issues. Not interested.
  • Onus
    Interesting about the valid specification names. I'll have to check the compliance level of what I'm using. I have a kit with one ethernet connector on one end (plugs in near my router), and four on the other end (my wife uses one, and my testbench gets the others): http://www.newegg.com/Product/Product.aspx?Item=9SIA24G15V0949 which is no longer available, but looks similar to http://www.newegg.com/Product/Product.aspx?Item=N82E16833124482 which is. They run more reliably and faster than a cheap PCI wireless card, which is better than a USB dongle. This is despite the fact that they are plugged in on different sections of a manufactured home, which means they're going through an additional junction box between the sections.
  • coupe
    "Assuming that the outlets using the Powerline adapters are on the same electrical circuit, ..."

    I think this part should be brought to attention more. Most people who are looking to implement a powerline setup might be confused about this limitation.
  • TechyInAZ
    I always thought that powerline tech was cool! And it is cool, but to run it well you need a house with nice electrical wiring. One of my houses had two 220V (or was it 120V, I forgot) outlets, so the powerline Ethernet wouldn't work on one half of the house lol.

    If you plan to use these, make sure you know where your powerlines are going.
  • Redraidr12
    @videobear Mine works wonderfully.
  • shadycuz
    "Assuming that the outlets using the Powerline adapters are on the same electrical circuit, ..." I think this part should be brought to attention more. Most people who are looking to implement a powerline setup might be confused about this limitation.

    They work across Circuit breakers. Most houses only have one Circuit. Breakers "break" that circuit down into manageable pieces so if a device acts up, your whole house doesn't loose power, just that room.
  • Supermuncher85
    Yeah love these things. Just keep in mind that if you do have a backup generator, it will not jump between circuits. Lesson I learned the hard way.
  • quadrider21
    Great artical, I'd like to know a little more about what the security button does.
    Do all three of these standards support security measures?
  • chill1221
    From Newegg customer reviews, performance of even the latest powerline equipment is far inferior to ethernet or even wifi. Plus the units have massive quality control issues. Not interested.

    This was the experience that my family had when they tried employing this technology. Ended up throwing it away.
  • Onus
    I do believe these units are a mixed bag. I tested some other ones (500Mb/s) that died fairly quickly (one in hours, the other in days). When working though, speed was very good. The current 200Mb/s ones I am still using have been flawless (not needing even a single reset, despite power outages), and they are faster than a cheap PCI wireless card was.
  • digitaldoc
    I have a few sets of these powerline adapters. I can confirm that they come with 128 bit encryption out of the box. They also will cross the circuit breakers without any issue in my setup. The signal cannot go beyond the electrical meter.
  • joex444
    I'm assuming this "distances from 300 to 500 square meters." is a typo and that "square" should be removed. Though at that point one must remember that the distance of wire between two outlets can be hard to estimate, particularly if one doesn't know whether they're the same or different circuits.
  • lawprime
    Most excellent article! Barges and other big boats use Marine Band Radar which is adjacent to the 2.4GHz used in WiFi and often interferes. Back when 5GHz, beam forming, and MIMO were not options, Powerline Ethernet really helped me keep my network flowing. But I once shared my good experience in response to a
    Slashdot request for help and I got an rather rude follow-up from another reader who said that these devices use large-scale house wiring to broadcast radio signals that completely screw up nearby Ham Radio reception. I have never heard this complaint before nor since. Any Hams out there who could weigh in?
  • mutatio
    I use my older Netgear setup, rated at 500Mbps, plugged into Monster power strips that allow for surge protection without interfering with the networking. It has worked reliably for at least 4+ years. It works well enough that it's how we hook up our Sony HDTV and home theater system for streaming. Even with an 802.11a connection Netflix will stagger and seize up. With the powerline/ethernet combo we don't run into any streaming hiccups. I am excited about the AV2 standards, but Netgear doesn't have the same equipment offered. The one we have (XAVB5001) is nice because it gives you a router-like box on the receiving end for multiple ethernet ports (TV, HTS, Apple TV, etc.).
  • f-14
    From Newegg customer reviews, performance of even the latest powerline equipment is far inferior to ethernet or even wifi. Plus the units have massive quality control issues. Not interested.

    i don't trust alot of newegg reviews especially when they claim to be 'expert' users and are building their first pc or their problem is easily identified and cured in a google search. most of the reviewers are extremely lazy and expect their ddr2 ram to fit in ddr4 ram slots and whine and complain how something didn't work because their expertness forgot to program manual settings listed right in the freakin manuals of their products.

    most of these phony 'expert' reviewers are easy to spot, none of their system specs or settings are listed.

    "DADE L.
    3/13/2015 6:10:12 AM
    Tech Level: High
    Ownership: more than 1 year
    Verified Owner
    3 out of 5 eggsWorks really well while it works
    This review is from: LINKSYS PLSK400-NP Powerline AV200 4-Port Network Adapter Kit, Up to 200Mbps
    Pros: I was able to get a LAN connection upstairs in my house which was super awesome. Things run really well and are a breeze to set up.

    Cons: Sometimes disconnects for me. Probably once every hour or so. It really hurts my experience playing games as the outage normally kicks me off of whatever game I'm playing. I often end up just going back to WiFi.

    Other Thoughts: Mine probably just had something wrong with them - they were still super helpful and I do really like them otherwise." <=====3 most likely he plugged it in on the same circuit as his chest freezer or refrigerator or window air conditioning unit, note his lack of specs and no mention of large appliances on his circuit? not even a mention of a 5-5-5-15 ram clock timing or anything experts KNOW to include.

    i bought this same kit for my brother an my parents, my brother used to have an issue with it until i saw it was plugged into his circuit with his chest freezer when i line tested the circuit (i thought the instructions were clear and simple, he never bothered to read them) i've personally used these for a few years as well in places i couldn't run cat5e due to steel beams in the walls and subflooring and i added another with built in wifi extension, no software was required and worked great, but to enable the security to the wifi you need to download the software and use the software to set the security every time that adapter loses power or it loses it's settings. the whole system kicks butt over the modem & routers G/N wifi from both netgear and linksys and actiontec.

    there is limited bandwidth tho as it can run 1080p on netflix and youtube an ipad and iphone at the same time, mmo gaming is acceptable with about 20 in ping added on (w/o using the wireless) but you simply can not online game and run netflix at the same time with out lag spikes.

    i've analyzed the network several times when my brother has a party and 16 cellphones connected to its wifi isn't a problem unless you're trying to online game at the same time even via a direct cat5e connection to the modem with ple400 a plsk400 and a plwk 400 wifi. and people using more than 1 wireless devices so don't plan on gaming if more than one person is using the wifi, the lag will infuriate you.

    these are the homeplug av standard, i will look into finding him a replacement homeplug av2 standard set up as soon as i find one with wifi i can gift him for x-mas or his birthday which ever comes first. ( i use family like lab rats to test lots of things heh heh heh SSSHHHHHHhhhhhh!!!)

    over all good article, i'm not an expert, just a jack of trades but more dangerous than the average user as many of my cisco network admin friends have trained me like a minion/apprentice to help them for lots of projects and enough to build, program and maintain my own networking nightmares. basement and house to house gaming was glorious FU STEAM & Origin!
  • Alex Atkin UK
    "Assuming that the outlets using the Powerline adapters are on the same electrical circuit, ..." I think this part should be brought to attention more. Most people who are looking to implement a powerline setup might be confused about this limitation.
    They work across Circuit breakers. Most houses only have one Circuit. Breakers "break" that circuit down into manageable pieces so if a device acts up, your whole house doesn't loose power, just that room.

    No actually they are correct. Notice the wording, each "circuit breaker" isolates a "circuit". Powerline is only designed to work on a single circuit, what you are described is the whole phase.

    While yes they CAN work across circuit breakers (and thus across different circuits) as long as they are on the same phase, its not guaranteed and usually degrades performance. They are not sold for that purpose.
  • ralanahm
    Thanks for the idea. I had tried one many years ago that was worse then dial-up so I ran cat5 across the floor in apartment and now I think I will get one again.
  • gorfmaster1
    I have had a few of these and they can save you from having to run Ethernet to areas where wireless is poor. I would recommend this to anyone who is looking at running cable as a potential alternative. plus it has an inherent "hub" type setup on some allowing up to ~8 devices in one building which is very nice IMO.
  • Onus
    I had not considered the phasing issue, but that's a very good point. These ought to work across circuits, but L-N will never work across phases. Perhaps AV2 addresses this by using N-G (unless they're shorted in the breaker box).
  • none12345
    No tests? I dont trust that this stuff works as advertised, i want to see how fast it actually goes. How it does with multiple users with loads. Latentcies introduced under various load patterns etc.

    Ive been curious how good this tech has been for awhile. But never bothered to try it. Id love to see some tests....
  • Onus
    I know you're looking for more thorough testing, but you might get a glimmer from the speedtest.net results in my mini-ITX roundup at http://www.tomshardware.com/reviews/asrock-h81m-itx-h97m-itx-ac-gigabyte-b85n-phoenix-wifi,4124-6.html ; the wired tests were through my Powerline adapters. Far from comprehensive, it is a glimpse.
  • shrapnel_indie
    While not every smart meter uses Powerline, those that do are an example of the electricity industry utilizing existing power lines to exchange data, such as utility companies receiving updates from your smart meter regarding your electricity usage. That is if you opt in to such a service, of course.

    You may not have an option with this technology. Where I live, they upgraded everybody to the smart meter.