I saw the recent TI announcement of their 6LoWPAN series of products, and it got me thinking about Part 15 devices in general.
"6LoWPAN" is "Internet Protocol Version 6 over Low-Power Wireless Personal Area Networks." IPv6 allows a total of 2^128 unique addresses (compared to the mere 2^32 that were available in IPv4). Thatís about 3.4 x 10^38 available IPv6 addresses, compared to about 4 billion (4 x 10^9) IPv4 addresses.
Visionaries foresee a world where everything is connected to everything else by a giant IPv6 network. OK, fine.
Letís assume that some fraction of these devices will be connecting via wireless means in unlicensed spectrum, known in the U.S. as ďPart 15Ē, or ISM (Industrial, Scientific, and Medical) bands. The best-known ISM band is 2400-2483.5 MHz, available pretty much worldwide and used for WiFi (802.11b/g, etc.). In the U.S., another popular band is 902-928 MHz and there are similar sub-1 GHz allocations in other parts of the world (868 MHz in Europe, for example).
I worry that all sorts of equipment manufacturers will jump on the wireless-connectivity bandwagon in the Part 15 bands. Some applications will not be well-suited to these bands. Why, you ask?
Hereís a quote from the Part 15 rules.
ď15.5 General conditions of operation.
(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.Ē
That means that 6LoWPAN devices, like all other devices operating in the Part 15 bands, using a wide variety of protocols and signal formats, must not create interference to any other devices, so the transmitters must be squeaky clean. Protocol etiquette and good RF design practice can reduce if not solve this problem.
It also means that these Part 15 wireless IPv6 nodes must accept any interference from ANY OTHER RADIATOR. Doesnít matter if it is another 6LoWPAN device, a nearby authorized radio station, a nearby wireless device using another protocol in the same band, an unintentional radiator, or whatever.
My office is at one end of my house. I use Wi-Fi (802.11b) from the office to the wireless hub at the other end of the house. The kitchen is between the hub and the office, and the microwave oven leaks enough of its 1100 Watts of 2.4 GHz RF to clobber the connection. If someone uses the 2.4 GHz cordless phone from the kitchen in the family room where the hub is located, it clobbers the hub and I am off-line. ďClobberĒ is my term meaning ďcause sufficient interference to disruptĒ. Even if the interferer is off-channel, itís possible to cause problems by driving the receiverís first stages into nonlinearity or even compression. These interference cases are annoying, but I can live with them. And some day Iíll improve the frequency coordination in my house and buy some new devices. The 5.8 GHz family room cordless phone does not bother the hub, so when the kitchen phone finally dies, itíll be replaced by another 5.8 GHz phone.
I think some of the applications that will try to use Part 15 RF bands for IPv6 connections will fail because of interference issues. If the visionaries are correct, and we someday have several billionÖor trillion/quadrillion/quintillion wireless nodes trying to coexist, interference issues are inevitable. Itís not impossible to make these things work, itís just really hard, involving filtering, antenna patterns, and knowing the worst-case local RF environment. Consider the consequences of using unlicensed spectrum for your application very very carefully and plan accordingly.