Releasing a bottleneck in the home network, Pt2 – at home with HomePlug

As promised the next instalment of what happened when I upgraded my home Internet access from ADSL to FTTC, and found that I had some interesting bottlenecks existing in what is a fairly simple network.

Last time, I left you hanging, with the smoking gun being the HomePlug AV gear which glues the “wired” part of the network together around the house.

HomePlug is basically “powerline networking”, using the existing copper in the energised mains cables already in your walls to get data around without the cost of installing UTP cabling, drilling through walls, etc. As such, it’s very helpful for temporary or semi-permanent installations, and therefore a good thing if you’re renting your home.

The HomePlug AV plant at Casa Mike is a mix of “straight” HomePlug AV (max data rate 200Mb/sec), and a couple of “extended” units based on the Qualcomm Atheros chipset which will talk to each other at up to 500Mb/sec as well as interoperate at up to 200Mb/sec with the vanilla AV units.

One of the 500Mb units is obviously the one in the cupboard in the front room where all the wires come into the house and the router lives. However, despite being the front room, it’s not the lounge, that’s in an extension at the back, so the second 500Mb unit is in the extension, with the second wifi access point hanging off it so we’ve got good wifi signal (especially 5GHz) where we spend a lot of our time. The other 200Mb units get dotted around the house as necessary, wherever there’s something that needs a wired connection.

So, if you remember, I was only getting around 35Mb/sec if I was on the “wrong side” of the HomePlug network – i.e. not associated with the access point which is hardwired to the router, so this was pointing to the HomePlug setup.

I fired up the UI tool supplied with the gear (after all, it’s consumer grade, what could I expect?), and this shows a little diagram of the HomePlug network, along with the speed between each node. This is gleaned via a L2 management protocol which is spoken by the HomePlug devices (and the UI). I really should look at something which can collect this stuff and graph it.

HomePlug is rate adaptive, which means it can vary the speed dependant on conditions such as noise interference, quality of the cabling, etc., and the speed is different for the virtual link between each pair of nodes in the HomePlug network. (When you build a HomePlug network, the HomePlug nodes logically seem to emulate a bus network to the attached Ethernet – the closest thing I can liken it to is something ATM LAN emulation, remember that?)

The UI reported a speed of around 75-90Mb between the front and the back of the house, which fluctuated a little. But this doesn’t match my experience of around 35Mb throughput on speed tests.

So where did my thoughput go?

My initial reaction was “Is HomePlug half-duplex?” – well, turns out it is.

HomePlug is almost like the sordid love child conceived between two old defunct networking protocols, frequency-hopping wifi and token ring, after a night on the tequilas, but implemented over copper cables, using multiple frequencies, all put together during an encoding technique called Orthogonal Frequency Division Multiplexing (OFDM).

Only one HomePlug station can transmit at a time, and this is controlled using Beaconing (cf token passing in Token Ring) and Time Division Multiplexing between the active HomePlug nodes, orchestrated by the concept of a “master” node called a “Central Coordinator”, which is elected automatically when a network is established.

When you send an Ethernet frame into your HomePlug adaptor, it’s encapsulated into a HomePlug frame (think of your data like a set of Russian Dolls or a 1970’s nest of tables), which is then put in a queue called a “MAC frame stream”. These are then chopped up into smaller (512 byte) segments called a PHY block, the segments being encrypted and serialised.

Forward error correction is also applied, and as soon as the originating adaptor enters it’s permission to transmit (it’s “beacon period”), your data, now chopped down into these tiny PHY block chunks, is striped across the multiple frequencies in the HomePlug network. As they arrive at their destination, acknowledgments are sent back into the network. The sending station keeps transmitting the PHY blocks until the receiving node has acknowledged receipt.

Assuming all the PHY blocks that make up the MAC frame arrive intact at the exit HomePlug bridge, these are decrypted, reassembled, and decapsulated, coughing up the Ethernet frame which was put in the other end, which is written to the wire.

The upshot of this is that there’s a reasonably hefty framing overhead… IP, into Ethernet Frame, into HomePlug AV MAC frame, into PHY block.

Coupled with the half-duplex, beaconing nature, that’s how my ~70Mb turned into ~35Mb.

The thing to remember here, the advertised speed on HomePlug gear is quoted at the PHY rate – the speed attainable between HomePlug devices, which includes all the framing overhead.

This means, where HomePlug AV says that it supports 200Mb/sec, this is not the speed you should expect to get out of the ethernet port on the bottom, even in ideal conditions. 100Mb/sec seems more realistic and this would be on perfect cabling, directly into the wall socket.

Talking of ideal conditions, one of the things that you are warned against with HomePlug is hanging the devices off power strips, as this reduces the signal arriving at the HomePlug interface. They recommend that you plug the HomePlug bridge directly into a wall socket whenever possible. Given my house was built in the 1800s (no stud-walls, hence the need for HomePlug!), it’s not over-endowed with mains sockets, so of course, mine were plugged into power strips.

However, not to be deterred, I reshuffled things and managed to get the two 500Mb HomePlug bridges directly into the wall sockets, and voila: Negotiated speed went up to around 150-200Mb, and the full 70-odd Mb/sec of the upgraded broadband was available on the other side of the homeplug network.

Performance is almost doubled by being plugged directly into a wall socket.

In closing, given everything which is going on under the skin, and that it works by effectively superimposing and being able to recover minute amounts of “interference” on your power cables, it’s almost surprising HomePlug works as well as it does.

This HomePlug white paper will make interesting reading if you’re interested in what’s going on under the skin! 

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