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DIDO: snake oil or wireless salvation?
Parsing the Perlman paper
According to Steve Perlman, his new wireless technology, DIDO, opens up a brave new world of wireless data that promises infinite and endless capacity for everybody. That's got some people excited (so excited that they can do no more than repeat his claims verbatim), while others have suggested that snake oil may be on sale...
Having read the DIDO white paper (PDF) a few times by now, I’m not prepared to call it one way or the other.
I can, however, say three things: first, the white paper itself doesn’t provide anything like enough detail for any technologist to assess whether its claims are true or not; second, the white paper is presented in such a way that makes it look like snake oil, whether it is or not; and third, the claim that this somehow invalidates Shannon’s Law is just silly.
Although journalists have pounced on the document, faithfully reproducing its diagrams and reciting its claims, it provides pitifully little detail about how the wireless system works.
Here's what the white paper claims:
1. DIDO uses a centralised data centre to process wireless signals. This central processor first tests the link quality between access points and clients, including identifying users who are sharing a radio channel, then calculates how to modulate the signals that will be sent to the access points (and therefore the users), and then actually sends the signals.
2. Because of this, each wireless user can use “the full data rate of shared spectrum simultaneously with all other users”, and therefore, “Shannon's law does not apply”.
3. Because the intelligence – modulation and channel management – is abstracted into the “cloud”, DIDO uses much simpler consumer kit than is needed by WiFi or cellular systems.
4. Its simple radios give DIDO systems latency of between 1 ms and 3 ms.
5. The mathematics behind DIDO is too long and complex for mere mortals to understand.
As I said, none of this is given enough meat for any complete dissection of the claims. The white paper looks like something you would draft if you wanted to create an irretrievably confused media who, after making the “TL;DR” decision, would settle for reproduction of the claims (along with a few diagrams), rather than trying to work out whether it made sense.
Still, let’s make the effort.
Put the access point in the cloud
Regardless of the hidden mathematics behind DIDO, the key to its deployment is that it moves the intelligence of access points into the cloud – specifically, the “DIDO data centre”, which mediates all communications between user and content provider.
Let’s leave aside the question of how to distribute enough data centres around the globe to replace the intelligence of every access point in every home and business in all the world. The big issue for lots of internet users is the idea that if they’re going to use a wireless connection, they will have to hand all of their communications off to one organisation for pre-processing.
The potential for
snooping data-gathering is immense.
From a technical point of view, there’s another issue. DIDO claims millisecond latency, which in my mind is at odds with the requirement for a data centre to pre-process your communications.
It’s quite true that the process involved in managing the air interface imposes a latency penalty on wireless data systems. But there’s no free lunch here: DIDO relocates that processing latency (into its data centre). The air interface gets lower latency, but the path between user and content gets higher latency.
'Shannon’s Law does not apply'
It sounds like a perpetual motion machine, or a “100 mpg fuel additive pill”, doesn’t it?
Certainly, it is a trap to snare the unwary, and it has snared them in spades.
As far as I can tell, and from my understanding of Shannon’s Law, it still applies.
Shannon’s Law defines how much information a medium can carry, given variables such as the available power, the amount of spectrum devoted to the channel (a 40 MHz block of spectrum has a higher limit than a 5 MHz block), and the amount of noise present in the channel.
Even if DIDO achieves what it says, it doesn’t do so by invalidating Shannon’s Law. It merely adjusts the behaviour of one of the parameters in the equation: the available channel capacity under given conditions. It claims to eliminate competition between users as a parameter, but the equation itself still applies to a DIDO channel.
Yet another issue I have with the claims made for DIDO is in this statement: “[DIDO access points] can transmit at higher power, if necessary, without the concerns that WiFi or cellular has of overlapping APs or cells at the same frequency causing too much interference”.
While it’s true that, for example, the power transmission from a Wi-Fi access point is limited as part of the attempt to prevent interference between devices, that’s not the whole story. Power is also limited to prevent interference with other devices using the same spectrum (cordless phones, for example), and for reasons of safety.
The DIDO white paper is taking a singularly insular view of the world: whatever spectrum it happens to be using, no other applications of that spectrum will matter. It’s no surprise, then, that spectrum regulators like the FCC have yet to embrace the technology.
DIDO and Australia’s National Broadband Network
The DIDO story landed like a grenade into Australia’s National Broadband Network (NBN) debate, so I think it’s necessary to draw the threads together, even at the risk of boring non-Australian readers.
The idea that DIDO instantly makes all fibre networks (including the NBN) obsolete overlooks a host of issues that aren’t answered in the Perlman white paper. I’ll deal with them very briefly here.
Standardisation: As it now stands, there isn’t any. DIDO is a single-vendor solution. A proprietary wireless system might be okay for the individual user, but what’s good for one individual isn’t good for a whole country.
Commercialisation: DIDO is, right now, an invention. A commercial product is a long way away indeed. From here to a “DIDO-everywhere” future is a matter of years, not weeks: not only would the system need to be funded and commercialised, it would need to fend off whatever the rest of the world is able to produce by way of catch-up.
Regulation: Between now and commercialisation, DIDO has to jump the regulatory hurdles facing any new wireless protocol: acceptance by spectrum authorities. That means DIDO's backers have to convince spectrum regulators such as Ofcom, the FCC, Australia's ACMA and others not only that it works, but that it can "play nice" with other legitimate spectrum users.
As for DIDO itself, if Steve Perlman wishes to convince the world that it’s a genuine breakthrough, a lot more detail is needed. ®