Plaster Networks Powerline PLN Isolators

Building a More Reliable Powerline Network through
the Selective Use of Isolators

Introduction

Powerline networks offer a convenient, cost-effective solution that uses the existing home wiring to network devices across the house. The HomePlug AV standard can support Ethernet data rates up to 100Mbps, with the expectation of even higher rates in the near future. Yet most outlets in the home are not able to achieve these peak data rates because of inhospitable electrical loads from appliances and other devices that share the electrical circuits.

Depending on the application, a “slower” outlet on the powerline network may not be of notice to a user. E-mail and web-browsing are typically more forgiving to slow network speeds and dropped network packets, because a moment’s delay in downloading a new e-mail or accessing a website is “within the norm.”

Other types of applications such as audio/video streaming and HDTV are much more sensitive to decreases in network performance. In particular, a slow network link can result in a jittery video or unexpected pauses and breaks in image and sound. These symptoms are much more noticeable and much less acceptable to the end user.

Since audio/video is a major application for technologies such as HomePlug AV, the corresponding requirements for streaming data tend to set the bar for powerline network technology. And in fact one of the key considerations for these applications centers not on achieving the fastest performance from any one outlet, but rather on delivering sufficient network reliability and performance across all the outlets in the home – and on the ability to deliver a reliable HD video stream to any room.

Achieving a higher level of reliability for powerline networking requires enhanced technologies that can effectively identify and isolate the sources of electrical noise within the home that degrade network performance. This paper discusses how Plaster Networks has approached this challenge. The Plaster Networks (PLN) solution includes a remote management and diagnostics service that automatically identifies potential sources of powerline network interference and then notifies the user. The service will commonly prescribe an isolator (power-passing filter) to help reduce the interference and improve overall reliability and performance of the network.

Advances in Powerline Networking

Powerline networking technology has made significant advances over the last decade in delivering broadband connectivity using existing home wiring. Today’s powerline technology incorporates advanced features such as TDMA (Time Division Multiple Access) and CSMA (Carrier Sense Multiple Access) for shared network access, OFDM (Orthogonal Frequency Multiplexing) and TCC (Turbo Convolutional Codes) for robust data transmission, as well as 128-bit AES (Advanced Encryption Standard) for security.

Figure 1: The HomePlug standard has evolved to provide sufficient bandwidth for the delivery of high definition video within the home.

New advances will continue to improve the speed of powerline networks. However, even with these faster data rates, noise and other interference can still degrade overall performance. The requirement to accommodate noise on the powerline circuit is rather unavoidable because the electrical wires were never originally intended as the basis for a computer network. Unlike a dedicated Ethernet network using hard-wired Cat 5 cabling, powerline signals must share the electrical wires with a multiplicity of electrical devices, ranging from small cell phone chargers to major appliances.

To put matters into perspective, the effect of interference and noise on a powerline network is very different from the interference experienced with a Wi-Fi network. While the effects of powerline interference are real, the resulting performance impact is significantly less than one might experience with a Wi-Fi network. As a result, powerline has proven to perform significantly more reliably than Wi-Fi. Second, as we shall see later in this paper, powerline networks now have new capabilities for identifying and isolating potential sources of interference. An easily-installed power-passing filter will eliminate noise on the powerline circuit. Wi-Fi does not offer an equivalent filter.

Effects of Noise and Attenuation on the Powerline

Powerline networks generally work very well for high-speed connectivity; however, users may occasionally see decreased performance caused by interference from other devices on the network. When powered on, these devices inject radio-frequency noise into the home wiring that can potentially interfere with the powerline network signal. Many users have experienced a similar effect when an electric hair dryer causes the television reception to become snowy or fuzzy. The hair dryer is emitting RF frequencies that are disrupting the display on the TV image.

The source and level of powerline noise can vary over time, which occasionally causes frustration to users. For instance, a dishwasher may introduce a periodic electrical load at the end of each day, at the same time that the household may choose to watch a movie over the network. In this case, turning on the dishwasher may elicit a groan from the TV room – something of “real-time” alert for diminished network performance. By contrast, a cell charger may generate a constant source of interference when a user leaves the unit plugged into the wall. In this case because the noise is always present, the user may not know that the charger is impacting the network performance. The user has no “real-time” alert and therefore has no easy way to detect the performance drop.

Appliances and other electrical devices can impact network performance even without being turned on. The wiring in these devices acts as an open transmission line at the HomePlug AV frequencies (1.6- 28 MHz). These open transmission lines can reflect signals back into the wiring. As a result even if electrical appliances are “off,” they can still have an effect on the performance of the powerline network. For instance, even a small space heater switched “off” may significantly reduce overall network throughput. In this case, the performance degradation occurs just by connecting the power cord to the electrical circuit.

Lastly, powerline networks can experience performance drops caused by large capacitors used in electronic switching power supplies. The capacitors serve to contain locally generated noise within the device, in compliance with FCC and equivalent foreign specifications. However, the capacitors also have the effect of reducing the HomePlug AV signal, because of low capacitive reactive impedance at the frequencies used by HomePlug AV.

Figure 2: The chart on the left demonstrates the negative impact of a network router power adapter plugged into the same outlet as a powerline Ethernet adapter. When the router’s power supply is plugged in, network throughput drops by 39%. Using a Plaster Networks isolator eliminates the effects of the router power supply (as shown on the right).

It should be emphasized that these noise sources are the exception rather than the norm. The tremendous dynamic range of the HomePlug AV signals is very forgiving with respect to noise. But when such noise does occur users can deploy an isolator (filter) to compensate. Powerline isolators provide an effective means to maintain powerline network performance and reliability.

Next, we will discuss the importance of selecting an isolator with the proper filtering characteristics. In a nutshell, not all isolators are the same.

Selecting an Electronic Filter for Powerline Circuits

Just as electronic devices, such as computers, benefit from filters (usually built-in) that reduce noise coming FROM the electrical lines, powerline networks can benefit from filters that reduce noise entering INTO the electrical lines. These filters serve to isolate the electrical circuits from noise created by audio/video components, household appliances, computer equipment, and more.

Installing Isolators to Reduce Powerline Network Noise and Interference

Figure 3: Electrical devices such as appliances, toys, and electronics can generate RF noise on the powerline circuit.
Isolators effectively block this noise from entering the powerline.

Isolator design can have a significant impact on how effectively the unit eliminates noise from the powerline circuits. The filter must be able to work with a wide variety of electrical devices, ranging from small chargers to large appliances. Because the full current for the connected appliance must pass through the inductor(s) of the isolator, the isolator should use heavy wire to carry the load (up to 15 Amps). Also, the ferromagnetic material in the inductor core should provide enough capacity so as not to saturate with current.

The major performance metric of the Isolator is the attenuation offered to unwanted signals. This attenuation is measured in decibels, a logarithmic measure. An attenuation of 10 decibels decreases the level of the noise by a factor of one-tenth. Thus, 30 dB is an attenuation factor of one thousand, while 60dB attenuation reduces noise power by a factor of one million. Most filters are designed for about a 30 decibel attenuation factor, while other filters attenuate noise by up to 60 dB over the HomePlug AV frequency band.

Figure 4: The Plaster Networks IX2 isolator provides a two-stage, fourth-order filter design that decreases noise by a factor of up to one million in the frequency range used by the powerline network. The robust, dual-plug design supports loads up to 15 Amps with 60dB of attenuation of noise in the 1.6 to 28 MHz band.

Locating Electronic Filters for Optimum Performance

So far, we have discussed how electrical devices generate noise in the 1.6 -28MHz range, and how that noise may degrade the performance of a powerline network. We have also identified important considerations for selecting a proper filter to isolate this noise from reaching the powerline. Next, we shall discuss specific examples of where to locate an isolator for best results.

To start, consider how noise propagates across the electrical circuit. Appliances and other devices typically generate a strong noise signal across a wide frequency spectrum. In general, the noise level attenuates relatively quickly along the powerline. In contrast, the network signal generated by the powerline adapter transmits over a narrower band with a much lower level of attenuation, with the result that the signal can transmit well beyond the range of the noise.

By analogy, consider student listening to a speaker in a lecture hall. The student may hear noise (or interference) from other students who are whispering around him. If the whispers are several rows in front of or behind the listener, then they probably will not interfere too badly with the lecture; however, if the whispers come from the neighboring seat, then they may well overshadow the voice of the speaker. Even though the volume of the whisper is much softer than the voice of the speaker, the proximity of the whisper still overshadows what the speaker has to say.

Applying the same principle to powerline networking, a HomePlug AV adapter plugged into the same outlet as a “noisy” appliance will experience significantly more interference from that appliance, as compared to another adapter plugged into an outlet across the room.

Figure 5: Powerline network interference depends strongly on the distance between the network adapter and the source of the noise. The diagrams above show the performance degradation caused from a hairdryer at varied distances from the powerline adapter.

For this reason, many powerline Ethernet vendors recommend relocating an adapter to another outlet when experiencing performance issues. However, relocating an adapter is not always a viable option for many users. Particularly in older houses, you may only have one outlet next to the device that you would like to connect to the network - such as a DVR (digital video recorder) or game console. In many cases, installing an Isolator provides a more convenient solution than relocating the adapter.

Combining Network Diagnostics with Powerline Filters

Above, we described how to locate isolators to achieve peak performance, in many cases by pinpointing electrical devices (1) that generate significant noise and (2) that are located next to a powerline adapter. However, users may not have a reliable way to determine “noisy” devices. Also, because the household circuits are hidden inside the wall, the users have no way to determine, for example, if the outlets in adjoining rooms are on the same circuit (and thus susceptible to interference from each other).

As a result, in the past users have had no practical way to determine the degree to which an isolator will improve network performance. The only recourse was trial an error: moving adapters and filters around the home and hope to notice an improvement. Of course, the user could “guess” at where to install an isolator in an attempt to improve the results. With the variety of devices and the total number of outlets in the house, resorting to trial and error seldom results in success. What’s more, the user has no easy or accurate way to measure any changes in performance.

However, a remote network management and diagnostics service can effectively pinpoint adapters that may be experiencing high levels of interference. The service can subsequently recommend isolators for the devices that may be limiting the throughput of the network at specific adapter locations. The Plaster Networks service uses an advanced set of diagnostic tools that aid in determining where a powerline network may benefit from an isolator. The service evaluates each node in the network for the presence of asymmetrical transmit and receive data rates. When the ratio of receive to transmit data rate capacity is significantly less than one, then the service flags the adapter as potentially benefitting from installation of an isolator. The culprit is usually other electrical devices plugged into the same outlet socket as the HomePlug AV adapter (or a nearby socket on the same circuit).

Figure 6: The Plaster Networks Service automatically detects instances where a powerline network may benefit from an isolator by monitoring differences between the send and receive transmission rates from each adapter. A significant variation in these two rates indicates potential source of noise on the powerline near that adapter.

The Plaster Networks solution offers two levels of diagnostics capabilities. The first level comes with a local administration console built into the Plaster Networks adapters themselves. Users may access the software from any PC on the home network using a standard web browser. Unlike other powerline adapters, the Plaster Networks units do not require the user to install special PC software. Through the local console, users can measure the throughput rates from one node on the network to any other node. Thus, the Plaster Networks solution provides the user with a method to measure the performance gains from installing an isolator (by measuring the throughput before and after installing the filter).

The Plaster Networks Service offers a second and more powerful level of diagnostics that automatically detects sudden changes in performance caused by noise on the powerline network. The Service works remotely, over the Internet, and can successfully identify intermittent noise by its spectral and time of use characteristics. For instance, the Service can automatically detect the times of day when appliances and other devices on the electrical circuit may be causing some level of interference. In some cases, the service may detect interference when the appliance is simply plugged in.

Figure 7: The Plaster Networks Service automatically detects changes in network performance caused by noise on the powerline. The service can even diagnose the problem remotely and notify the homeowner to install an isolator to remove the noise from the network.

Having detected a potential source of interference, the Service can notify the user with a recommendation for installing a Plaster Networks isolator that will improve the performance and reliability of the network link. As a result, the Service mitigates intermittent network degradation caused by installing or operating appliances on the powerline. The result is much more reliable network performance, with fewer interruptions that can impact applications such as streaming HD video.

Summary

In this paper, we have demonstrated how powerline networks have evolved into a suitable solution for home broadband networks – including the ability to stream high definition video throughout the home.

Yet despite the continuing advances in technology, these networks are still subject to interference caused by noise on the powerline. This noise results from the fact that the powerline is a shared medium that was not originally designed for high speed networking. In most cases, the HomePlug AV technology can readily accommodate this noise to deliver a suitable network connection that offers much greater reliability over other technologies such as Wi-Fi. However, to deliver the highest level of reliability across the greatest number of homes, the HomePlug technology benefits from the intelligent application of isolators (power-passing filters).

Combining powerline Ethernet adapters with a suitably designed isolator can help ensure peak performance even in the most challenging powerline environments. Plaster Networks offers a two-stage, fourth-order filter with 60dB of attenuation in the HomePlug AV frequency band (1.8-28MHz).

The effectiveness of the isolator is determined not only by the filter design, but also by knowing where to install the unit. In general, consumers will benefit by installing an isolator on “noisy” electrical devices located near a powerline adapter. However, given the variety of electrical devices in the home and the number of available outlets, choosing the right location can become somewhat of a guessing game.

The Plaster Networks Service includes advanced diagnostic tools that can help pinpoint the optimum location(s) for installing isolators. The Service uses detailed powerline network statistics and advanced signal processing to help pinpoint the location of “noisy” electrical devices. The Service subsequently prescribes an isolator that will help improve the reliability and performance of the powerline network for delivering consistent broadband throughput across the widest range of home powerline environments.

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