[AusNOG] Powerline Adapters

Paul Brooks pbrooks-ausnog at layer10.com.au
Tue Feb 21 09:37:52 EST 2012

On 21/02/2012 8:37 AM, Skeeve Stevens wrote:
> Hey all,
> Re: http://media.netcomm.com.au/public/assets/pdf_file/0012/89886/NP504-Spec-Sheet.pdf
> Does anyone know conceptually how these things work?
Spec sheet says they follow the IEEE 1901 standard, aka 'Homeplug AV' -
http://en.wikipedia.org/wiki/IEEE_1901 and http://en.wikipedia.org/wiki/HomePlug
Its an international standard, and devices from different manufacturers will
inter-operate - I have the 200 Mbps versions from Netcomm and TP-link talking to each
other through my house.

Note this is a different competing standard from the G.HN that I outlined at AusNOG a
year or so ago, which promised gigabit speeds but seems to have got bogged down in
interop testing.

> At home I have hard core walls where wireless is practically useless, so a while ago
> I installed the 100mb version of the Netcomm power line devices. My life has been
> happy every since, albeit at 100mb - which is a bit poopie.
> I will be buying the above to get more speed to the server I access at home which
> isn't in my office, but…. I wanted to know if anyone understood HOW these things
> actually work.
> In an ethernet environment you have a cable, it speed is dedicated to that cable.
> But these devices are not point-to-point - but maybe they communicate that way?. At
> home I have them in 4 different rooms, and they all talk to the server… but I don't
> know how, or, most importantly, how well these units operate.
> If these new devices are 500Mbps - what does that mean. Does it mean on a
> point-to-point link with only two of them, it is 500Mbps. What happens when you add
> a third, fourth or fifth unit.

Easiest way it to think of them as sorta-kinda like multi-point VDSL2, with extra
smarts. They do multiple frequency channels, and like xDSL each channel runs at a
different bit-rate depending on the amount of noise it needs to cut through.
Much like xDSL2, two units might get 500 Mbps between themselves if they are very
close to each other, but their synchronisation speed will fall away the further apart
they are by cable distance - particularly if you are crossing circuits so the signal
has to travel all the way to your circuit-board and back again. I have two rooms which
are on different circuits, and the electrical wiring for one seems to take an
extremely long-winded path such that a unit in that room can barely talk to the rest.
When you have multiple units, they each broadcast signal strength per channel
information to each other, and collectively negotiate a combination of frequency
channels per peer that optimises the aggregate information rate. In your case, each
one will divide the theoretical 500 Mbps aggregate across each of the other three
units, roughly in proportion to signal strength. They can get a bit asymmetric - I
have one pair which negotiates 65 Mbps in one direction and 38 Mbps in the other.
In your case, if you currently have 4 units of the 100 (85?) Mbps version, then they
have probably negotiated roughly 15 - 30 Mbps per pairing, so while you have C and D
plugged in, you'll probably max out at ~30 Mbps between A & B, even if there is no
traffic between C and D.

> So… (A) being the main unit in the lounge with the server attached, and (B) being
> home office 1, (C) being home office 2 and (D) being bedroom (we have a Boxee box! ;-)
> I guess the question is, what is the speed when B,C,D are all talking to A, and what
> is happening is A and B are talking and C and D are talking - do they 'interfere'
> with each other?
Yes - the more units you have plugged in, the lower the allocated speed per pair.
They'll sort themselves out :-) Best thing to do to see what is happening is to load
up the management software on a laptop connected to one of them, which will give you a
screen with the signal strengths and negotiated bandwidth to each of the other units.
Plug each of the units in and out in various combinations and watch the peer
bandwidths be renegotiated.
Then run some TTCP floods between pairs and let us know you get on.

> I'm really interested to know how these work, because I could see them being useful
> in that running 500Mbps for up to 200m could be a very interesting last ditch, or
> interim solution in the right situation.
> I am sure people here have had some experience with these kinds of things. They
> claim they 'just work' and given I have 4 of the units at home with never a single
> problem or bit of config done, I can attest to that.

They don't work across 3-phase power, where different rooms are attached to different
phases. Normally most homes with 3-phase power have the entire dwelling on the same
phase, with the extra 2 phases used just for an airconditioner, so everything works,
but I did run across a house where the sparkie of some renovations decided to run the
electrics of two new rooms off a different phase 'to balance things up'. Consequently
the playstation in the brand new rumpus room could not talk to the media store or
Internet gateway in the study.

Do enable the encryption and 'pair' them.

These with an integrated 802.11n basestation are also extremely useful -

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