Since being ratified in 2003, the wireless communications standard IEEE 802.15.4 has enjoyed a steadily increasing uptake rate. New and inventive industrial, medical, security and other specialized applications continue to be regularly introduced, employing either open or proprietary protocols riding atop the lower level network layers prescribed by the standard. With mature tools, cost-reduced silicon, and significant design momentum behind the standard, 802.15.4 enabled products are now set to flood the consumer market. ZigBee RF4CE is the primary protocol that will power this next wave of deployment.
While 802.15.4 defines channels in the 868 MHz, 915 MHz, and 2.4 GHz bands, the two lower bands are typically employed only in region-specific and special cases. The upper channels, 11 through 26, residing in the 2.4 GHz Industrial, Scientific and Medical (ISM) band are the heavy favorites around the world.
You may recognize this band of frequencies as a favorite for other consumer appliances as well, including: wireless phones, WiFi routers, baby monitors, Bluetooth cell-phone peripherals and, of course, the ubiquitous microwave oven. So how then do ZigBee RF4CE (RF for Consumer Electronics) devices establish responsive and reliable communications channels in this often crowded environment?
There are bullies in the 2.4 GHz neighborhood that regularly try to threaten 802.15.4 communications. Microwave ovens in close proximity are among the most common offenders. In spite of sufficient shielding to keep the oven's user safe, enough stray energy often leaks from oven doors to create a significant aggressor signal, from a low-power radio network's point of view.
ZigBee RF4CE addresses this situation by applying brains over brawn. RF4CE packets are typically quite compact " requiring only a time-slice of only a few millisecond in which to successfully transmit. Since microwave ovens rarely operate at 100% duty cycle, it is a simple matter for an RF4CE transmitter to monitor the channel, wait for a clearing (which will typically be found once per mains power cycle), then rapidly transmit its data packet.
The time-sharing strategy employed by 802.15.4 is called Carrier Sense, Multiple Access Collision Avoidance (CSMA/CA). In CSMA/CA systems, the transmitter first listens to ensure that the channel is clear. If the channel is clear, the transmitter sends out its brief burst of data. If the channel is not clear, the transmitter holds off for a random interval of time and tries again; repeating until transmission is successful. Whether or not reception was successful is left to higher layers of the network model; in the case of RF4CE, an ACK frame is sent by the receiving node.
If you are skeptical about the effectiveness of such an approach, consider that all wired Ethernet connections are based on the closely related strategy of CSMA/CD (Collision Detect). The CSMA strategy works very well.
802.15.4 defines three methods for determining if a channel is clear enough for a transmission; these are Clear Channel Assessment Modes 1, 2 and 3 (CCA1, CCA2, and CCA3):
- In CCA1, the transmitter listens to the channel only for "energy". If the Energy Detect (ED) level exceeds a predetermined value, the channel is considered busy and transmission will be held off for a later attempt.
- CCA2 is a more sophisticated, but arguably a less-conservative strategy, in which the transmitter first listens specifically only for an 802.15.4 compliant carrier. If the Carrier Sense (CS) process returns an affirmative result, the transmitter will again hold off transmission.
- In CCA3, the transmitter can be set to assess a clear channel on either the AND condition of CCA1 and CCA2 queries, or the OR condition of the CCA1 and CCA2 queries.
In nearly all real-world scenarios, CCA1 is the most time-efficient, power-efficient and conservative approach to accomplishing the carrier sense portion of a CSMA type transmission.
In the modern connected home, we can expect 802.11b,g wireless routers to often coexist with 802.15.4 networks. In such situations, the same time-sharing scheme used with the microwave oven is again employed. Unlike the microwave oven, however, both the wireless router and 802.15.4 networks can be directed to operate at thoughtfully selected channels within the 2.4 GHz ISM band. This intelligent spectrum sharing is an additional strategy to ensure robust communications, even among multiple coexistent networks of both types.
In Figure 1, we see that North American WiFi channels 1, 6, and 11 are defined in such a way as to leave 3 MHz of clear bandwidth between them. Zigbee RF4CE's 2 MHz wide channels are centered at 2425, 2450 and 2475 MHz, slotting in nicely within the WiFi-free regions of the spectrum. (ZigBee RF4CE employs only channels 15, 20 and 25, of channels11-26 available in this band.) There are similar guard bands in European WiFi channel assignments.
Figure 1: WiFi channels and RF4CE channels in the ISM band
(Click on image to enlarge)
If the WiFi environment isn't controlled as to be limited to occupying only channels 1, 6, and 11, competition between the two types of networks may occur. Still, since WiFi networks do not run at 100% duty cycle, 802.15.4's CSMA/CA strategy ensures robust and reliable communications during the frequent quiet moments of such networks.
There are two more "brains over brawn" features of 802.15.4 communications in the 2.4 GHz band. First is the modulation type used, Offset Quadrature Phase Shift Keying (OQPSK). As with any PSK modulation scheme, there is no information encoded into the amplitude of the signal. So long as the receiver is successful in resolving the phase change information of the carrier, the transmitted data will be recovered. Since ZigBee RF4CE networks are deployed in unpredictable and changing physical environments, immunity to changes in signal amplitude is a valuable attribute.
The second modulation strategy employed is "spreading". Spreading is the general term used to describe how a few data bits of information are mapped onto a much longer sequence of transmitted bits (chips) prior to transmission and how, upon reception, that sequence of received chips is mapped in the opposite direction back into a few data bits. A "best fit" process used to make the whole process immune to some number of incorrectly received chips.
More specifically, 802.15.4 uses Direct Sequence Spread Spectrum (DSSS) spreading. In DSSS, the data to be transmitted is considered four bits at a time; and every group of four bits is mapped onto a symbol. Each symbol is then associated with one of sixteen 32-chip sequences which are defined in the standard. Though these chip sequences look rather like random sets of 1's and 0's, they were carefully selected to ensure that there is little mathematical similarity among them. If a few chips are incorrectly received, the re-mapping process in the receiver can usually arrive at the correct symbol by making a best fit estimate to the known chipping table. Spreading adds a good bit of interference immunity and robustness to the data link.
Finally, there is the oldest strategy of all in making yourself heard: speaking more loudly. On the one hand, every radio transmitter, regardless of application, should attempt to be a good neighbor and only transmit at a power level just sufficient to the task. However, if the environment demands it, 802.15.4 radios are required to be able to broadcast to a power level of at least 0 dBm (1 mW), and have at least 30-dB range in output power control.
Early settlers in the 2.4 GHz ISM band are generally primitive devices, with a poor sense of community. Microwave ovens, baby monitors, and similar devices just broadcast away as they see fit. As the value of this piece of spectral real estate became more fully recognized, industry interests have worked cooperatively to arrive at time and frequency sharing schemes that allow for a much harmonious co-existence of several classes of product.
We're all about to witness a tremendous roll out of ZigBee RF4CE-enabled products as the newest occupants in this neighborhood. Careful planning of the specifications, and clever design of clear channel detection, frequency, modulation and power control will ensure a pleasant consumer experience and a greatly enhanced perception of value for ZigBee RF4CE enabled products.
There is a linked list of related, relevant articles after the "About the Author" section below. For more information, go to http://www.freescale.com/zigbee.
About the Author
Doug Shade has enjoyed the privilege of working for Motorola/Freescale Semiconductor since 1986 and currently serves Freescale's Microcontroller Systems Group as a Senior Member of the Technical Staff, with focus on validation of digital, analog, mixed signal and RF designs.
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