IEEE 802.15.4 is a wireless communications protocol that runs at the seemingly unimpressive maximum speed of 250 kb/sec. That's bits, not bytes. Most people that have heard of wireless communications usually think of IEEE 802.11 or Bluetooth , since those are the most mass-market consumer protocols available. So why in the world would we need another wireless protocol that runs at a turtle's pace?
The reasons behind IEEE 802.15.4 lie in its application domain: short range wireless networking. One of the biggest obstacles to using wireless devices is power consumption, since there is usually no power cable available to a mobile wireless device. 802.11 was designed for large data transfers with a tethered device like a wireless router spewing out gobs of RF power. On the receiving end, the laptops with Wi-Fi connections usually have a large (ie: big-ass by consumer mobile standards) battery that can run an 802.11 transceiver for hours at a time. [2]
The 802.15.4 spec defines three frequency ranges that it's allowed to play in:
Of those frequency ranges, the 2.4 GHz is the most common frequency range because it's a worldwide standard frequency for industrial, scientific, and medical equipment. In other words, it's open and doesn't require a license to use. 868 MHz is an open band in most of the EU and 915 MHz is an open band in North America and some Asian countries. Zigbee is only defined for the 2.4 GHz bands so you technically don't need to understand the 868/915 MHz bands, however radios are starting to come out that address these lower bands. Since there is almost no software change to use these lower bands, and you get added benefits, I though it would be worthwhile to add them into this discussion.
One of the main differences between the 802.15.4-2003 spec and 802.15.4-2006 spec is how the PHYs are defined. In the 2003 spec, the bit-rate was directly tied to the frequency range of the PHY. For 868 MHz, the bitrate was a meager 20 kbps. 915 MHz got you 40 kbps, and 2.4 GHz had the highest bitrate at 250 kbps. That, coupled with the fact that 2.4 GHz is almost a worldwide standard frequency for the free ISM band, is one of the principal reasons why you see that most of the PHYs available now are at 2.4 GHz.
| Frequency (MHz) | Modulation | Bit Rate (kbps) |
|---|---|---|
| 868 | BPSK | 20 |
| 915 | BPSK | 40 |
| 2400 | O-QPSK | 250 |
When the 802.15.4-2006 revision came around, one of the main changes they made was to the PHY definitions since there was such a low uptake of radios in the 868 and 915 bands. They modified the PHYs so that it was possible to use the same modulation (O-QPSK) and spread spectrum (DSSS) scheme in all bands. They also modified the bitrates so they were higher in the 868 and 915 bands. If using an O-QPSK DSSS transceiver, you could then get 250 kbps in either the 2.4 GHz or 915 MHz bands and up to 100 kbps in the 868 MHz band.
| Frequency (MHz) | Modulation | Bit Rate (kbps) |
|---|---|---|
| 868 | BPSK | 20 |
| 915 | BPSK | 40 |
| 868 | ASK | 250 |
| 915 | ASK | 250 |
| 868 | O-QPSK | 100 |
| 915 | O-QPSK | 250 |
| 2400 | O-QPSK | 250 |
Along with Wi-Fi, this band is also shared with cordless phones, Bluetooth, proprietary protocols, and microwave ovens. Yep, you heard it. There's a chance you can drop a connection by heating up that day-old coffee. Actually, the research into coexistence finds that the noise from microwave ovens is negligible, but hey, you never know.
Anyways, on the hardware side, the important information to know aside from the band of operation is probably the number of channels and the frequency of operation. Aside from that, the other important parameters that I normally keep track of is the Tx power, Rx sensitivity, and the power consumption of the chip. You can check out my 802.15.4 chip comparison sheet here .
The 802.15.4 spec defines a PHY interface that consists of two types of services: a data service and a management service. In reality, this interface is basically useless to software writers because the PHY interface will be defined by the radio that you are using. Hence it basically just ends up being taken care of by the hardware or in the very low, low, low level driver. In any case, I still think that it might be beneficial to add some verbiage on the basics of the PHY interface. s
The PHY data service consists of :
On the PHY management service side, there are just a few services that need to be taken care of:
802.15.4 PHY Frame Format The PHY frame format just consists of a synchronization header to mark the start of the frame and a PHY header. The sync header is usually added by the hardware to mark the start of the frame so there's no need to worry about it. The PHY header consists of a single byte which holds the size of the frame.