Power-line communication

This article explains carrier-frequency data transmission in the low-voltage network. For message transmission over high-voltage lines, see there.

Powerline Communication (PLC) or Powerline for short, also called PowerLAN by some manufacturers, refers to a technology that uses existing electrical lines in the low-voltage network to set up a local network for data transmission, so that no additional cabling is required.

As of 2019, there are three dominant global standards:

  • IEEE-1901-FFT, which emerged from the HomePlug AV manufacturer standard, uses OFDM with fast Fourier transformation as modulation,
  • IEEE-1901-wavelet, originated from the manufacturer standard HD-PLC, especially widespread in Asia, uses OFDM as modulation on wavelet basis,
  • ITU G.hn, a vendor-independent standard, also uses OFDM with fast Fourier transform as modulation.

IEEE-1901-FFT and IEEE-1901-wavelet use a very similar MAC frame format preceded by a beacon, and differ mainly in the modulation used and the QoS gradations (4 vs. 8). ITU G.hn, on the other hand, uses largely the same modulation methods as IEEE-1901-FFT, but has a MAC frame format that does without a beacon and instead uses a map communicated in the respective preceding frame (Figure), in which the structure of the following Mac frame is communicated to all receiving stations. The three standards are therefore incompatible with each other and cannot exchange data with each other. This is particularly confusing for users of power network adapters when a manufacturer offers devices with different standards that have a similar design on the outside, but are nevertheless incompatible with each other, such as the devices in the dLAN series from Devolo, which are based on IEEE-1901-FFT, and the devices in the MAGIC 2 series from the same manufacturer, which are based on ITU G.hn.

Adapters conforming to the Homeplug or IEEE 1901 FFT standard can transmit data at a maximum of 2000 Mbps with a range of up to 300 m, while adapters conforming to the ITU-G.hn standard can transmit 2400 Mbps gross with a range of up to 500 m.

Hardware

Technically, the transmission is realized with the help of adapters that are connected to the power grid on the one hand and to an end device to be networked (e.g. a PC, a printer, a game console or a webcam) on the other hand via a built-in Ethernet connection. There are different designs according to different PowerLAN standards from various manufacturers, for example as an intermediate plug or in combination with a WLAN access point. For professional use, more powerful devices are also offered that provide transmission options via additional media (e.g. coaxial or twisted pair cables) as well as functions for data prioritization and hierarchical network topologies.

PowerLAN bridges can only communicate if they are located on the same line conductor. To enable communication via different external conductors, there are also PowerLAN hubs, which are mounted on a mounting rail in a group distributor, for example. In addition, there are also phase couplers that connect the outer conductors for the carrier signals. For some time now, Powerline adapters with integrated PoE have also been available on the market, which then control the PoE end devices connected behind them with data and power via the Powerline carrier signal.

How it works

Technically speaking, PowerLAN is a carrier frequency system that is implemented via adapters. These are plugged into a socket and connected to a terminal device (e.g. a PC, printer or games console) via a built-in Ethernet interface. The data signal of the connected terminal device is modulated by the transmitting adapter in the high-frequency range, usually between 2 MHz and 68 MHz, onto the power line and demodulated again by the receiving adapter. From the point of view of the power grid, Powerlan signals are disturbances which, if installed correctly, lie within the tolerance limits for electromagnetic compatibility and have no effect on the power supply.

With PowerLAN, the electrical lines available in a household with 230 V voltage as well as 50 Hz or 60 Hz are additionally used for the transmission of data. With the help of Orthogonal Frequency-Division Multiplexing (OFDM), which is already used in other transmission methods (e.g. xDSL or WLAN), a large number of signals are simultaneously phase and amplitude modulated onto a carrier frequency on the transmission side (frequency division multiplexing). Depending on the transmission standard, the available frequency spectrum is divided into channels in order to reduce the susceptibility to interference and to enable appropriate countermeasures (error correction and interleaving methods). The modulated data is then sent via the power line to the receiver, where the carrier frequencies are separated from the power line again by bandpass and demodulated.

The devices based on the Homeplug standard, which are widely used primarily in the private sector, achieve typical gross transmission rates of 14 Mbps (Homeplug), 85 Mbps (Homeplug Turbo), 200 Mbps (Homeplug AV), 600 Mbps (IEEE 1901) and 1200 Mbps. The maximum range of Homeplug adapters on power lines is 300 meters. The Homeplug AV (200 Mbit/s) and IEEE 1901 (600 Mbit/s) standards are compatible with each other.

The low-voltage networks used are often three-phase networks with three outer conductors, neutral conductor and protective conductor, whereby the outer conductors (phases) are usually distributed to different areas within residential units. PowerLAN uses the phase/neutral conductor pair and, more recently, the protective conductor. Depending on other factors, such as cable length, attenuation and, if applicable, sources of interference, the data modulated onto the power line via PowerLAN is available at least on circuits of this phase within the residential unit. However, since the transmission takes place in the high-frequency range, crosstalk occurs, among other things, due to lines laid in parallel, as a result of which the signals are also available in other conductors. This is accompanied by an attenuation of the signal strength, which is reflected in a reduced range and a lower transmission bandwidth. Phase couplers can be used for a desired, preferably undamped signal bridging between two circuits.

dLAN 200 AVplus adapter from devolo with plug-in loop-through and up to 200 Mbps transfer rate
dLAN 200 AVplus adapter from devolo with plug-in loop-through and up to 200 Mbps transfer rate


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