I have at least a few readers for which the sound of a man's voice saying "government cell phone detected" will elicit a palpable reaction. In Department of Energy facilities across the country, incidences of employees accidentally carrying phones into secure areas are reduced through a sort of automated nagging. A device at the door monitors for the presence of a tag; when the tag is detected it plays an audio clip. Because this is the government, the device in question is highly specialized, fantastically expensive, and says "government cell phone" even though most of the phones in question are personal devices. Look, they already did the recording, they're not changing it now!
One of the things that I love is weird little wireless networks. Long ago I wrote about ANT+, for example, a failed personal area network standard designed mostly around fitness applications. There's tons of these, and they have a lot of similarities---so it's fun to think about the protocols that went down a completely different path. It's even better, of course, if the protocol is obscure outside of an important niche. And a terrible website, too? What more could I ask for.
The DoE's cell-phone nagging boxes, and an array of related but more critical applications, rely on an unusual personal area networking protocol called RuBee.
RuBee is a product of Visible Assets Inc., or VAI, founded in 2004 by John K. Stevens. Stevens seems a somewhat improbable founder, with a background in biophysics and eye health, but he's a repeat entrepreneur. He's particularly fond of companies called Visible: he founded Visible Assets after his successful tenure as CEO of Visible Genetics. Visible Genetics was an early innovator in DNA sequencing, and still provides a specialty laboratory service that sequences samples of HIV in order to detect vulnerabilities to antiretroviral medications.
Clinical trials in the early 2000s exposed Visible Genetics to one of the more frustrating parts of health care logistics: refrigeration. Samples being shipped to the lab and reagents shipped out to clinics were both temperature sensitive. Providers had to verify that these materials had stayed adequately cold throughout shipping and handling, otherwise laboratory results could be invalid or incorrect. Stevens became interested in technical solutions to these problems; he wanted some way to verify that samples were at acceptable temperatures both in storage and in transit.
Moreover, Stevens imagined that these sensors would be in continuous communication. There's a lot of overlap between this application and personal area networks (PANs), protocols like Bluetooth that provide low-power communications over short ranges. There is also clear overlap with RFID; you can buy RFID temperature sensors. VAI, though, coined the term visibility network to describe RuBee. That's visibility as in asset visibility: somewhat different from Bluetooth or RFID, RuBee as a protocol is explicitly designed for situations where you need to "keep tabs" on a number of different objects. Despite the overlap with other types of wireless communications, the set of requirements on a visibility network have lead RuBee down a very different technical path.
Visibility networks have to be highly reliable. When you are trying to keep track of an asset, a failure to communicate with it represents a fundamental failure of the system. For visibility networks, the ability to actually convey a payload is secondary: the main function is just reliably detecting that endpoints exist. Visibility networks have this in common with RFID, and indeed, despite its similarities to technologies like BLE RuBee is positioned mostly as a competitor to technologies like UHF RFID.
There are several differences between RuBee and RFID; for example, RuBee uses active (battery-powered) tags and the tags are generally powered by a complete 4-bit microcontroller. That doesn't necessarily sound like an advantage, though. While RuBee tags advertise a battery life of "5-25 years", the need for a battery seems mostly like a liability. The real feature is what active tags enable: RuBee operates in the low frequency (LF) band, typically at 131 kHz.
At that low frequency, the wavelength is very long, about 2.5 km. With such a long wavelength, RuBee communications all happen at much less than one wavelength in range. RF engineers refer to this as near-field operation, and it has some properties that are intriguingly different from more typical far-field RF communications. In the near-field, the magnetic field created by the antenna is more significant than the electrical field. RuBee devices are intentionally designed to emit very little electrical RF signal. Communications within a RuBee network are achieved through magnetic, not electrical fields. That's the core of RuBee's magic.
The idea of magnetic coupling is not unique to RuBee. Speaking of the near-field, there's an obvious comparison to NFC which works much the same way. The main difference, besides the very different logical protocols, is that NFC operates at 13.56 MHz. At this higher frequency, the wavelength is only around 20 meters. The requirement that near-field devices be much closer than a full wavelength leads naturally to NFC's very short range, typically specified as 4 cm.
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