Power-Outlet LAN: In-home power lines carry audio and data

Published:2011/8/18 0:59:00 Author:Li xiao na From:SeekIC

Jan Bulting

Mains (or ’power-line’) signalling has matured from the early stages of the single 130-kHz carrier with simple modulation for a primitive 1-channel on-off control. Today, off-the shelf equipment is available that allows you to use the mains wiring and power outlets in your home or office to convey analogue or digital audio as well as PC data at impressive speeds. The power-outlet LAN is here, but how does it work?

When it comes to sending analogue or digital signals over in-home mains wiling, i.e., excluding professional systems designed for the national power grid (like the highly controversial Power Line Transmission) a bewildering number of manufacturers claim to have the ’very best’ for you at ’incredibly low’ prices. In this article we will look at the basic operation of a system that, although fairly pricey, we thought performed adequately. Other products not mentioned here may employ similar methods of operation.

Reshuffling the pack(ets)



Transmission of analogue signals over the mains wiring is now a thing of the past as the technology is fraught with difficulties and invariably yields poor results. Today, we seem to be talking digital only. However, there is still a rather capricious aspect to tackle: the transfer characteristic of the mains wiring between sender and transmitter, and that，unfortunately for all digi-whiz-zos. is i00T: analogue in nature- Figure 1 illustrates that different attenuation characteristics must be taken into account for ever/ outlet-to-our-let path in the home or office. Attenuation may be quite high at certain frequencies used by the signalling system (4.3 to 20.9 MHz). To make the characteristic even more unpredictable, loads connected to certain power outlets may actually cause not only a dynamically changing transfer function but also added noise (electric drills, tube lights etc.). The PowerPacket system employed by InteUors for their HomePlug Power Alliance 1.0 compliant products is capable of continuously and automatically detecting in-band frequencies subject to heavy attenuation. As shown in Figure 2. the available frequency band may be used by up to 84 carriers spaced 200 kHz apart. The actual signal encoding method, ODFM is not discussed here, but we move on straight to the crux of the system — see Figure 3: Carriers dropping below a certain threshold set up in the receiver are switched off, and transfer information tells the transmitter to reshuffle ready-encoded ODFM signals across other, suitable, carriers within the band. By regulation, signal levels on the mains wiring must be reduced by several dBs on frequencies inside radio amateur bands.



Is it safe?

The Devolo dLAN Audio and dLAN Ethernet units we used for this article are CE certified and carry all relevant approvals regarding electrical safer/ and isolation for use on 230-V domestic and office mains outlets. Great, but is my data safe from my neighbor’s curiosity, supposing he is using a similar dLAN? Alter all, our homes are connected to one and the same electricity grid? The answer is that your electricity meter and associated circuitry in the metering cupboard will act as a filter that largely prevents those 4-20 MHz signals leaking onto the electricity network and from there onto the neighbor’s mains wiring, although this cannot be entirely ruled out in unfavorable situations where a degree of phase coupling is present. More importantly, however, 56-bit DESpro encryption is used in combination with passwords so your data should be pretty secure.

What speed? How many extensions?

Devolo claims a data speed of between 5 and 14 Mbits/s depending on noise levels and, of course, the effective distance between units connected to form a LAN. Such speeds are sufficient for DSL distribution in the home. Although in theory up to 253 units may be connected into a power-outlet LAN, Devolo say that in practice bottleneck situations may occur when more than about 10 units convey data simultaneously.

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