Back in the 1980’s when they were trying to establish themselves in the British market, Zanussi ran a campaign for their products using the advertising strapline “the Appliance of Science“. I was reminded of it this week when I was reading a white paper from another appliance manufacturer – GE. Not because it had anything to do with science; in fact just the opposite – it was about the most unscientific paper I’ve ever come across.
It was written to promote GE’s view on which wireless standard should be chosen for the Home Area Network (HAN). These are designed to connect devices around the home to a smart meter or a home gateway that has access to information about your current energy tariffs. GE thinks the best choice should be ZigBee because “ZigBee is better than Wi-Fi”. One of the paper’s authors is an active editor for the ZigBee Alliance Smart Energy Profile, so that’s not surprising – he’s entitled to be enthusiastic about the technology he’s part of. And it may be that ZigBee is a good choice. But GE’s analysis doesn’t provide any evidence as to why it might be. Instead it provides an evidence-free quasi-analysis that does ZigBee more harm than good.
We’ve had a year of hype as different wireless standards vie for the crown of being chosen as the de facto one for smart metering. Much of that obscured the facts which need to be considered to make that choice. In the last few months I thought the industry had settled down and was beginning to a bit more logical. This rant from GE suggests that some of those involved in the debate still have a lot to learn. If you want to see how not to make a reasoned argument, download and read the GE white paper. I’ll highlight what is so wrong about it.
There is no shortage of wireless standards that would like to be picked. The current runner setting the pace is ZigBee, but Wi-Fi and Bluetooth are there as well, plus a bunch of prominent also-rans, including Z-Wave, Wavenis and Wireless-MBUS, along with a clutch of rank outsiders. Most of these standard would admit that they still have some way to go. None of the standards are yet fully fit for purpose for smart energy, but most of them recognise their deficiencies and are working on fixing them. There’s a valid argument that none are really good enough and the industry should take the best bits of each and come up with something new, but we probably no longer have time for that approach.
Because none of them are perfect, it’s proving difficult to make a choice, as government led groups around the world are discovering. Many different factors need to be weighed up, of which the most important are range, power consumption, robustness to interference, security, interoperability and IP ownership. Throughput and latency, which are important in other wireless applications aren’t particularly relevant for smart metering, as so little data is transferred, none of which is time critical.
In their white paper, GE is quite content to ignore robustness, security, interoperability and IP ownership, which makes the job a lot easier for them. And they limit the contenders to Bluetooth, ZigBee and Wi-Fi. So they’re making their task relatively easy by not being very representative. Yet they still manage to mess it up.
They start their analysis with range. Range is one of the most misunderstood parameters in wireless connectivity. For any of the standards operating at 2.4GHz, if the radios are transmitting at the same power, with the same antenna, the range is predominantly determined by the receive sensitivity. That depends on the ability of the receiver to extract a signal from the noise, which is a factor of the symbol rate (the rate at which you modulate the signal), how many symbols you use to encode each piece of data, and to a lesser degree the modulation method, modulation index, the degree of error correction used for the packets and the amount of receiver diversity you use. For more about how these all work together, have a look at my book – “Essentials of Short Range Wireless“.
That’s obviously all a bit too complicated for the GE team. Instead they make the observation that the Bluetooth range on their mobile phone is only a few metres. On that basis, they discard it from further consideration. There’s a reason the range is limited on most phone, which is because phone manufacturers understand power consumption. For them, the most important parameter is battery life. As most Bluetooth applications only need a range of a few metres – typically from the phone to a headset, they cut the transmit power down to minimise the power draw. 2.4GHz radios tend to be exponential in their power draw. So a device transmitting at 4dBm will consume 30mA, whilst one transmitting at 14dBm will probably take up 250mA. For a phone manufacturer, 30mA is acceptable, 250mA is not. They do the same for Wi-Fi implementations, which is why your phone’s Wi-Fi range doesn’t match that of your laptop. But I suspect the people writing this paper haven’t yet worked out how to use Wi-Fi on their phones.
You can see that GE engineers might not appreciate this subtlety – after all, they’re not experts in power management, as their devices don’t have to run on batteries. It’s ironic that the poor efficiency of household appliances is one of the reasons we need a smart grid in the first place. If you have a look on the energy bible site for dishwashers, the best performing GE product comes in at position 147 out of 605. And over half of their products fall into the 16% of worst performing dishwashers. (I chose dishwashers as there’s less variation in size, so it seems a fair comparison.) Hence GE can hardly claim to be qualified to pontificate about power consumption. Although that doesn’t stop them.
That’s not the only deficiency this paper has. They state that Bluetooth is based on the IEEE 802.15.1 standard. It’s not. Bluetooth writes its standards independently. It contributed an early version to the IEEE, but that’s five versions out of date. They also refer to the Bluetooth Alliance. A quick check of the website, or any of the specifications, all of which are publicly available, would show them it’s the Bluetooth Special Interest Group. There’s no such thing as the Bluetooth Alliance. Maybe they’re confused because Alliance rhymes with Appliance.
But on with their range experiment, which is now down to deciding between ZigBee and Wi-Fi. The first bad point they find against Wi-Fi is the fact that ZigBee “is the lowest power”. Putting aside the fine detail that power consumption depends on topology and application, I’m afraid that’s not true either. Like for like, Bluetooth low energy, Z-Wave and Wavenis are all lower power than ZigBee. But they’ve already been excluded, so according to GE, they don’t count.
But then Wi-Fi stumbles again, because according to GE, ZigBee is the only standard that supports mesh and ZigBee is generally used in a mesh. Unfortunately for GE’s analysis, ZigBee is not the only standard for mesh. Mesh can be run over most networks. There are plenty of commercial mesh networks running over Bluetooth. And ZigBee isn’t the only wireless standard that has specified mesh. There’s another one. It’s called 802.11s, which is part of the 802.11 set of standards which underpin Wi-Fi. In fact, most ZigBee implementations don’t use mesh. The standard includes it, but the bulk of commercial deployments only use ZigBee in a star network or cluster tree topology.
Then comes the issue of cost. Apparently Wi-Fi chips are around $2 more expensive than ZigBee. That will come as a shock to the companies who bought around 700 million Wi-Fi chips last year, a volume that helped drive the price down. In contrast, less than 10 million ZigBee chips were sold, according to IMS, which means they’re typically more expensive than Wi-Fi. And both are a lot more expensive than Bluetooth, which is now shipping for less than $1.
Now we get the really weird bit. GE claims that an additional processor is needed to run the protocols for these chips and for Wi-Fi that costs $11 more than it does be for ZigBee. My experience is that people using embedded Wi-Fi and Bluetooth are writing their application code directly on these chips. After all, most of these chips already contain two or three processors. So developers don’t need to use another separate microprocessor. And certainly not one costing over $10. So GE doesn’t seem to know how to design products cost effectively, and are using that inadequate engineering to try and tell the rest of the world what to do. But maybe that’s why GE appliances cost so much.
Having tried to bludgeon us into submission with a concerted display of ignorance, they now change tack to prove their point with an experiment. They take a ZigBee chip (a good, low power one) and compare it with an 802.11n chip, which they claim is low power. In one sense they’re right, the 802.11n chip they use is designed to be low power. Or at least low power for 802.11n, which is capable of constantly pushing several hundred Mbps of data across a link. If they had wanted to do a valid comparison, they should have chosen an 802.11 chip designed for low power sensor applications. In other words, they should have used one operating at 1Mbps using DBPSK modulation, such as the ones from Gainspan and G2 Microsystems.
Instead they perform a comparison of bicycles against jumbo jets and come to the surprising conclusion that bicycles are lower power. Which means that in GE’s eyes, ZigBee wins.
In a final act of madness, they do a calculation of the savings that a customer would make if an appliance used ZigBee instead of Wi-Fi. They don’t separate out the power taken by their unnecessary external microprocessor, (they never tell us what that is), so it’s all very dubious. But they claim that choosing ZigBee rather than Wi-Fi would save a customer a massive 4.2 kWhr per appliance per year.
Let’s go back to those energy bible figures. If you sort them by annual power usage, GE dishwashers take up 48 of the bottom 60 places in terms of energy guzzling consumption. They consume around 324 kWhr per year, against the most efficient dishwasher on the list – the Fisher & Paykel DS605, which takes only 157 kWhr per year – less than half the power of the GE appliances. So if they spent their time concentrating on designing decent appliances rather than writing silly papers like this, the GE team could save the user 167 kWhr per year, which makes pontificating about the choice of radio based on a 4.2 kWhr saving seem rather irrelevant. If you’re beginning to think that GE engineers who live in glass houses shouldn’t throw stones, you’re probably right.
What is worrying is that this has been submitted to the study groups looking at wireless for smart grid as a serious paper. Every proponent of a wireless standard will be biased to some degree – that’s to be expected, as you’d expect the people working on a standard to believe in it. But this paper trivialises the whole process. As I said, it ignores important points like robustness, IP, interoperability and security. One of the most thorough analyses I’ve seen regarding these points is in a paper put out by the Bluetooth SIG, which is worth reading. In comparison, this GE white paper is little more than an evidence-free rant.
I don’t think that this does the cause of Smart Energy any good. Nor does it bring any honour to the ZigBee Alliance, who I suspect are embarrassed by it. And it certainly does nothing to enhance anyone’s opinion of GE. If this is the level of due diligence they apply to technology it’s no wonder their appliances sit at the bottom of the league tables for energy efficiency. On the basis of this report I’ll keep on buying Fisher & Paykel and Zanussi.