Sending Signals Underwater
Can I transmit sensor data wirelessly underwater?
Signed, Bubbling with Curiosity
Wise Guy: Yes, you can. But with traditional (that is, radio frequency or RF) wireless communications, where you use frequencies above, say, 10 kHz, the attenuation factor presented by water is quite severe. On a good day it's 10 dB/ft. at a carrier frequency of 100 MHz.
RF is not your only option, though. Wireless communication is also possible via optical frequencies. A couple of decades ago there were some interesting laser communications experiments that involved relaying blue light (to match the bluish cast of water) from a ground-based laser off a reflector on the space shuttle which then illuminated a multi-kilometer footprint on the ocean's surface. Under that illumination footprint was a submarine with a receiver to detect this communication signal. The experimenters speculated that relatively large bandwidths were "available" for data transmission far in excess of the data rates associated with extremely low frequency (ELF) communications.
If you don't have a nice blue laser available, you can realistically turn to acoustic communications. In this realm the carrier frequency is relatively low, that is <20 kHz (+/-), with long bit intervals. As whales and sonar systems can attest, these signals may propagate for long distances underwater, but bit distortions frequently occur due to multipath and other effects. Considerable research and development effort has gone into devising error-correcting codes that can minimize some of these distortions.
For more on attenuation and multipath see the December 2005 installment of Wireless Works at www.sensorsmag.com/sensors/article/articleDetail.jsp?id=312305)
Are RFID Viruses For Real?
Not long after Bluetooth became widely accepted, the threat of Bluetooth viruses arose. Recently I read about how RFID tags can spread viruses. Can they, and should I be worried?
Signed, M.I. Vulnerable
Wise Guy: In theory, there is a method by which a tag can be modified to carry a virus—that is, a binary bit sequence. When interrogated by the reader, the hacked tag would then transmit this binary sequence to the reader. It's difficult to understand how this bit sequence could then be injected into the RFID database and cause havoc, but in theory it is indeed possible.
Are 802.15.4 Products Interoperable?
At Sensors Expo last month I saw numerous vendors using IEEE 802.15.4 radios in their products. Does this mean that I can get a set of gear from one vendor and mix it with gear from another vendor?
Signed, All Mixed Up
Wise Guy: Well you can do that. But will it all work together? That's highly unlikely.
All 802.15.4 radios are meant to provide low radiated power (a "small footprint") on a low data rate communication channel, and all boast limited energy consumption. But there are different options for using the 802.15.4 specification. In fact, the specification allows for operation at different carrier frequencies.
Therefore, Vendor A's radios may be tuned to one frequency band, while Vendor B's radios are tuned to a different frequency band. This helps with coexistence in the same way that two radio stations broadcasting at 97.9 on your FM dial make trouble, but if one broadcasts at 97.7 and the other at 98.1 they can coexist peacefully. However, it most assuredly does not allow the vendors to "work together" in the sense that Vendor A's data are quite literally being carried across Vendor B's network.
The situation is even more complicated because there is no steadfast rule that says the vendors must communicate using the same protocol. Protocols are analogous to languages. Vendors A and B may be using the exact same radio at the same frequencies and following the same hopping sequence—but they may also be communicating in different languages. The result? Confusion, pure and simple.
The remedy is a standard communication protocol such as Bluetooth or ZigBee. (I use the term "standard" loosely, though, because the standards are really just specifications, not requirements.) Now if Vendor A and Vendor B agree to adhere to the same specification, then their gear can indeed work together, and you can arrive at your hoped-for goal: true interoperability among their gear.