|The Differences Between 802.15.4-2003, 2006, a, c, d||| Print ||
|Written by Akiba|
|Sunday, 23 March 2008|
There is still a lot of confusion between the different versions of the 802.15.4 specification.When you mention it, most people immediately think of the original 802.15.4 spec which is otherwise known as 802.15.4-2003. This was the spec that laid the foundation for low-rate wireless PANs (LR-WPANs). The specification described the radio, modulation, bitrate, headers, protocol, and services.
The original 2003 spec had three different radios to choose from when implementing it: 868 MHz, 915 MHz, and 2.4 GHz. By far, the 2.4 GHz radio dominated the product offerings, so much so that there are a many new papers discussing Zigbee co-existence with other 2.4 GHz radio technologies (802.11, Bluetooth).
The main reason that the 2.4 GHz spectrum came to dominate was that 2.4 GHz is a free spectrum in almost the entire world, and also the 2003 spec mandated that the 2.4 GHz would be allowed to run at 250 kbps while the 868/915 MHz frequencies had to run at lower frequencies due to their allowed modulation schemes. For reference, the 2003 spec limited the 868 MHz frequency to run at 20 kbps and the 915 MHz band to 40 kbps using BPSK modulation. Unfortunately, the 868/915 MHz radios never recovered from this and there is still a scarcity of radios at these frequencies.
In 2006, the LR-WPAN working group introduced revisions to the original 802.15.4 specification. The original name of the revisions was called 802.15.4b however since alphabet letters are less exciting than numbers, the industry ended up calling this the 802.15.4-2006 and relegated the old spec to the similarly innovative name of 802.15.4-2003.
The biggest contribution the 2006 spec did was to give more flexibility to the radios. See pix below. New modulation schemes were introduced for the radios in the 868/915 MHz spectrum, most likely because nobody was using them. One of the mistakes that they made in the 2003 spec is that the modulation schemes were different between the 2.4 GHz and 868/915 MHz frequencies which meant that not only did you need to change the local oscillator for the modulation frequency, you also needed to change the modulator/demodulator block in the chip when you processed the signals. This basically meant that you would need separate chips to support the 868/915 MHz and 2.4 GHz frequencies or you would have a huge chip with a large portion of the radio unused all the time.
Some of the other modification they made to the 2006 spec are as follows. These are taken from the Introduction section in the 802.15.4-2006 spec:
There is also an amendment to the 802.15.4-2006 spec called 802.15.4a which was approved in 2007. I suspect that the 802.15.4a Task Group was started before the 802.15.4b (802.15.4-2006) TG which is why they got the 'a' moniker. The 802.15.4a amendment defines alternative PHYs for certain applications that require:
written by Dmitry Eremin-Solenikov, July 18, 2009
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