high speed broadband – Mike's Twitter Overflow

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!

80 down, 20 up, releasing a bottleneck in the home

A couple of weeks ago, I upgraded the Internet connectivity at home, from an ADSL service which could be a little bit wobbly (likely due to poor condition on some of the cabling) and usually hovered between 2Mb and 3Mb down, to FTTC – reducing the copper run from about 3.5km down to about 200m.

The service is sold as “up to 80Mb/sec” downstream, with upload of up to 20Mb/sec, which turns out to be achievable on my line, as my ISP’s portal reported the initial sync as 80Mb, and this gives around 75Mb of usable capacity at the IP layer once you’ve knocked off the framing and encapsulation overheads.

I eagerly headed off to thinkbroadband.co.uk and speedtest.net to run some tests. They confirmed I’d only get 40Mb/sec until I replaced my trusty but ageing Cisco 877 – that’s one bottleneck I already knew about and had a replacement router coming. But, never the less, I was happy with a >10x uplift on the previous downstream speed, and off I went happily streaming things, as can be seen from my daily usage…

Yes, some of that usage in the first day or two would have been repeatedly running speed tests in giddy abandon at the bandwidth at my disposal, but the daily usage is now generally higher.

There’s a number of reasons that could be behind that, but I suspect that among the most likely are services which support variable bit-rate video delivery, which include things such as YouTube and BBC iPlayer will be automatically upping to the higher quality stream.

The new router arrived on the 9th, and it was off with the speedtests again… and that’s where I found an interesting bottleneck in the house.

I could happily get 75Mb/sec in one room – where the router and main access point was. However, in the lounge, which is in an extension at the back of the house, I could only get around 30Mb/sec, despite having an access point in the same room.

I’ve ended up with multiple access points in the house, because the original “cottage” was built in 1890 and has fairly thick walls made of something very, very tough (from experience of hanging up pictures) which is also largely impervious to radio waves it seems, while the extension is attached to the “outside” of one of the original external walls, as well as being the furthest point away from where the Internet access comes into the house. This meant that I wasn’t left with much choice but to infill using a second wireless AP.

But both APs are of a similar spec and support 802.11a/b/g/n, and I was connecting on the less congested 5Ghz spectrum on both. So, where was the bottleneck?

The attention turned fairly quickly to the HomePlug AV network which I was using between the front and back of the house. It hadn’t caused me much concern in the past, but now it was prime suspect in my quest to wring the maximum out of my shiny new upgraded circuit.

Finding the longest piece of cat5 cable I have (a big yellow monster of a cable), and running that through the middle of the house to the AP, revealed that my suspicions were correct, but I also knew that the bright yellow cable snaking through the kitchen couldn’t stay there.

In the next few days I learned more about HomePlug than is probably healthy, and that will form the basis for my next article…

DR still in the doldrums – An Open Letter to Digital Region

A few months ago, I wrote about what I percieved to be going wrong with Digital Region, the local-authority backed superfast broadband wholesale network in South Yorkshire.

It seems that matters have not improved since then: a Sheffield-based hosting company, KDA, has written an Open Letter to Digital Region, which pretty much confirms that everything which was true several months ago is still true today, and goes on to suggest that there’s enough experience and skill in the tech community in South Yorkshire to turn this around, if only those in charge were willing (able?) to change tack and allow the community to steer the organisation.

It’s also alluded that a cut-price disposal of the network assets, which should rightly be the South Yorkshire taxpayer’s, for a cut-price may already be in hand, and that a failure of DR will be associated generally with the South Yorkshire tech industry, tarring it’s (generally good) reputation.

DR shouldn’t be the way it is – DR should be more agile than the large telcos, and find it easier to be more focused on the needs of the local userbase, but it isn’t. It seems to be strangled by inflexibility and bureaucratic behaviour, which needs to change if it’s to survive, and deliver the promise that the local authorities set out to achieve. But, at the moment, I’m doubtful that this will happen. The peppercorn sell-off probably feels like an easy way out, however much it’s short-changing South Yorks residents and business in the process.

You can read the full text of the Open Letter here.

Regional Broadband, the Lords Select Committee and Google Fibre in the UK

Some of you may be aware of the Google Fibre project which is an experimental project to build a high-speed FTTH network to the communities in Kansas City. They chose Kansas City from a number of different communities who responded to Google’s “beauty contest” for this pilot, because they had to pick just one and felt that it would have the greatest effect and be the best community to work with.

Like many other Community Broadband projects, Google point out that what the large incumbent telcos sell as “high speed internet” is seldom “high speed” at all, and is commonly sub 4Mb/sec. Google estimate that during the pilot, the cost of works for lighting up each subscriber premises may be as much as $8000 – though this is cheaper than the £10000 that it’s rumoured to cost to deploy high speed broadband to a rural subscriber in the UK.

So, this got me thinking, what could Google potentially bring to the UK with a similar sort of project?

It strikes me that one of the ways that Google could help the most is by facilitating the existing community benefit-based FTTC/FTTH groups to build networks in their communities, which right now can be frustrated by lack of access to public money from the super-fast broadband deployment fund (aka BDUK).

A significant amount of BDUK money is going to BT as the incumbent, or needs complex joint-venture constructs (such as Digital Region – though that was not a BDUK-funded project), because these organisations firstly have whole departments dedicated to handling the paperwork required to bid for the public funding, and secondly because they have a sufficiently high turnover to bid for a sufficient amount of public money to deliver the project. These are hurdles to community led companies, who will most likely just drown in the paperwork to bid for the funding, may not have all the necessary expertise either on staff or under contract, and likely don’t have the necessary turnover to support the application for the funding.

Meanwhile, the House of Lords Communications Select Commitee have issued this request for evidence (.pdf) in respect of an inquiry into whether the Government’s Super-fast Broadband strategy (and the BDUK funding) is going to be able to repair “digital divides” (and prevent new ones), deliver enough bandwidth where it’s needed, provide enough of a competitive market place in broadband delivery (such as a competitive wholesale fibre market), and generally “do enough”.

Could this be where Google enters stage left? As opposed to running the project in it’s entirety, they partner up – managing things such as the funding bid process on behalf of the communities, possibly acting as some sort of guarantor in place of turnover, as well as providing technical knowhow and leveraging their buying power and contacts?

This would at least give an alternative route to super-fast broadband. Right now, BT are winning a lot of the County Council led regional/rural fast broadband deployment projects, sometimes because they are the only organisation able to submit a compliant bid.

It remains to be seen if the money will benefit the real not-spots, or just prop-up otherwise marginal BT FTTC roll outs. Don’t get me wrong, I’ve no axe to grind with BT, but is the current situation, with little or no competition, ultimately beneficial to the communities that the awarded funding is purporting to benefit?

This is certainly one of the questions the House of Lords enquiry is looking to answer.