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