IEEE Standard 802.15.4

By Jon Adams

You’ve probably heard about ZigBee, the robust, lightweight wireless networking protocol that’s due for release in the next few months (www.ZigBee.org). ZigBee is based on the open IEEE Standard 802.15.4, which was ratified and released to the public in May 2003. This powerful standard is critical to the success of ZigBee, and it is a good time to explore the functionality of this standard to better understand why ZigBee provides such excellent reliability.

Frequency bands
IEEE 802.15.4 defines specific RF frequencies, modulation formats, data rates, and coding techniques. It also specifies how individual packets are structured, and the interaction between two ends of a data link. IEEE 802.15.4 specifies 27 RF channels in the three frequency bands shown in Figure 1.

Figure 1: Freq Bands

2400 MHz ban
Let’s look first at the 2400 MHz band, which is tremendously valuable because it allows unlicensed operation nearly anywhere in the world. There are 16 RF channels in this band. The channels start at 2405 MHz, and are spaced 5 MHz apart up to 2480 MHz.

The channels do not directly coincide with Wi-Fi channels. Therefore, IEEE 802.15.4 systems can coexist with Wi-Fi systems with little physical separation. The data rate is fast enough to allow very short (time domain) packets, yet slow enough to make sure that the required energy per bit is compatible with the goal of very long primary battery life and good range.

Lower bands
Transceivers that provide low-band operation support both a single channel (868.3 MHz) in the 868 to 870 MHz European unlicensed band, and 10 channels in the Americas 902 to 928 MHz Industrial, Scientific, and Medical (ISM) band.

Modulation technique
IEEE 802.15.4 relies upon a very robust modulation technique known as Phase-Shift Keying (PSK), instead of Frequency-Shift Keying (FSK). FSK is a far less efficient, but simpler to implement modulation technique that is used in Bluetooth and many other applications that range from toys to cheap two-way data solutions.

The 2400 MHz band uses Offset Quadrature-PSK, while the lower bands use Binary-PSK. Both modulation modes offer extremely good low bit error rate (BER) performance at low Signal-to-Noise Ratios (SNR). Figure 2 compares the performance of the 802.15.4/ZigBee modulation technique to Wi-Fi, Bluetooth, and other proprietary FSK modulation formats.

Figure 2: BER versus SNR

In all cases, both forms of PSK are anywhere from 7 to 18 dB better, which directly translates to a range increase from 2 to 8 times the distance for the same energy per bit, or an exponential increase in reliability at any given range.

Application impact
What all of this means to the end application is reliable, robust ZigBee performance that outclasses other wireless methods. This is important, as ZigBee is dependent upon IEEE 802.15.4 for getting the command from the light switch to the light, or from the security sensor to the alarm. IEEE 802.15.4 is the reason why other systems that use less robust modulation techniques cannot compete with ZigBee when it comes to reliable operation.

Packet frames
Also part of the Physical layer, and above the choice of modulation, data rates, and channelization, are further components that ensure that IEEE 802.15.4 manages to get the message through in environments that would cause other common protocols to fail.

There are four frame types for IEEE 802.15.4 transmissions:

1. Data
2. Acknowledgement
3. MAC Command
4. Beacon

Frame commonality
All frames have the following common components:

Preamble – All of the frames begin with a 40-bit preamble that helps the receiving station to pick the transmission out of noisy environments.

Frame Length field – Tells the receiving station exactly how long the frame is.

Sequence Number – An 8-bit value that is incremented each time a device transmits a new, unique frame.

Frame Check Sequence (FCS) – Each frame ends with a 16-bit mathematical sequence that allows the receiving station to validate that the packet was received without error.

Acknowledgement frame
An IEEE 802.15.4 system uses the Acknowledgement frame to tell the transmitting station that the data arrived successfully. This takes place as shown in the following data exchange between stations A and B:

1. A checks the RF channel to ensure that another station is not transmitting.
2. A transmits a data frame addressed to Station B.
3. B receives the frame, and uses the FCS to make sure that the packet was received without error.
4. At 192 microseconds after the end of A’s transmission, B responds with the Acknowledgement packet which is addressed to A, and contains the Frame Number specified in A’s original frame. This allows A to know that B received the data without error.
5. If this Acknowledgement is not received successfully, then the process begins anew with A transmitting the Data Frame a second time.

Even with multiple retransmissions, the entire process takes well under 10 milliseconds, making for a timely, robust and efficient data exchange process.

Robust communications

IEEE 802.15.4 is a communications system that contains robust mechanisms that start at the RF channel itself. They include the careful choice of radio frequencies, the use of the most robust and simple modulation techniques, and the use of communication frame features to make sure that the message can get through. Finally, the acknowledgement frame closes the loop, which allows the sender to know that the message got through. ZigBee wants to make sure that data that must get through, does get through, and ensures this with the approach that only IEEE 802.15.4 provides.

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Jon Adams is chair of the ZigBee Alliance's Qualification Group, and is the director of Radio Technology and Strategy for the Wireless and Mobile Systems Group of Freescale Semiconductor, a wholly-owned subsidiary of Motorola. Jon speaks and presents regularly on ZigBee, UWB, and the future of embedded wireless for machine-to-machine communications. Contact Jon at jta@freescale.com.

The ZigBee Alliance is an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked monitoring and control products based on an open global standard. Contact the alliance directly for membership and event details.

ZigBee Alliance
Tel.: 925-275-6607
Fax: 925-275-6691
Web site: www.zigbee.org