Imagine a hospital so well connected that a computer turns off lights and dials down the heat in rooms it senses are unoccupied. A patient regaining mobility can wander the halls alone and the computer will monitor vital medical alerts. And in the sports lab, a hip-pack computer records leg loads measured by transducers in an athlete's shoe for each stride on a treadmill workout. Some of this is possible now with lots of wires and specialized equipment. But the coming ZigBee hardware and networks promise this and more, simply and inexpensively.

A closer look

Many medical applications would benefit from standards-based wireless technology that is reliable, secure, and runs on low power. Established standards for wireless applications, such as Bluetooth and IEEE 802.11, allow high transmission rates, but at the expense of power consumption, application complexity, and cost.

ZigBee networks on the other hand, are primarily intended for low-duty-cycle sensors, those active for less than 1% of the time. For instance, an off-line node can connect to a network in about 30 ms. Waking up a sleeping node takes about 15 ms, as does accessing a channel and transmitting data. Applications such as reading the pressure in an oxygen tank can send the reading once per hour from a sensor which would then return to sleep. The low-power demand extends battery life in remote sensors. The network name comes from the zigzagging path a bee (a data packet) takes to get from flower to flower (or node to node).

The ZigBee network is defined by the ZigBee Alliance and based on the IEEE 802.15.4 standard (zigbee.org). The standard's charter says its purpose is to “investigate a low data-rate solution with multi-month to multi-year battery life and very low complexity. It is intended to operate in an unlicensed, international frequency band”. Potential applications include hospital management, medical equipment, remote-patient monitoring, interactive equipment, and remote controls. The scope of the task group is to define the physical layer (PHY) and the media access controller (MAC).

Because system cost is always a factor, an integrated single-chip approach is preferred by semiconductor manufacturers developing IEEE 802.15.4-compliant transceivers. The standard at the physical layer determines the RF architecture and topology of ZigBee-enabled transceivers in fabrication.

For short-range wireless devices, key elements also include the Network/Security layers for sensor and control integration. The ZigBee Alliance is now defining characteristics of these layers for star, mesh, and cluster-tree topologies. The performance of these networks will complement the IEEE standard while meeting requirements for low complexity and power.

The standard defines two physical layers representing three license-free frequency bands that include 16 channels at 2.4 GHz, 10 at 902 to 928 MHz, and one channel at 868 to 870 MHz. Maximum data rates for each band are 250 kbps, 40 kbps, and 20 kbps respectively. The 2.4 GHz band operates worldwide, while the sub-1 GHz band operates in the Americas, Europe, and Australia/New Zealand. The IEEE standard conforms to regulations in Europe, Japan, Canada, and the U.S.

The analog portion of the receiver converts signals from RF to digital baseband. Synchronization, despreading, and demodulation are handled in the digital portion of the receiver. The transmitter's digital part does spreading and baseband filtering, while the analog part modulates and converts to RF.

The receiver architecture is a compromise between performance, cost of silicon area and external components, and power consumption. Transceivers use direct-conversion receivers (DCR) or Zero-IF architecture because it needs no image frequency and IF filtering. Further advantages come from channel-select filters which are low-pass instead of band-pass filters. Also, the baseband frequency is the lowest possible. DCR architecture also provides lower cost, complexity, and power consumption.

The transmitter architecture is also direct conversion. It needs only one baseband path because of the BPSK modulation. This differential architecture minimizes common-mode noise. The output can be single-ended or differential. Single-ended outputs have lower costs, an on-chip TR switch, and eliminate the need for an external balun (balance or unbalance).

Channel allocation in the sub-1 GHz bands sets the required bandwidth and frequency resolution. This had major impact on the PLL topology.

A goal was one PLL circuit for the 868 to 870MHz and 902 to 928 MHz bands using a fixed crystal frequency. A fractional-N PLL architecture meets this requirements. In addition, the software controlled fractional-N PLL can meet future worldwide spectrum expansion from 860 to 930 MHz.

A few network features

ZigBee's self-forming and self-healing mesh-network architecture lets data and control messages pass from one node to another by multiple paths. This feature extends the network range and improves data reliability. It may also be used to build large, geographically dispersed networks with smaller networks linked to form a ‘cluster-tree’ network. ZigBee's security toolbox ensures reliable and secure networks. Data transmissions are protected by access-control lists, packet-freshness timers, and 128-bit encryption.

There are too many features of IEEE 802.15.4 to cover in detail, but a few more important ones are in the PHY, such as receiver-energy detection, link-quality indication, and a channel-clarity assessment. The standard supports contention-based and contention-free channel access methods with a maximum packet size of 128 bytes, and a variable payload up to 104 bytes. Also employed are 64-bit IEEE and 16-bit short addressing to support over 65,000 nodes per network. The MAC provides network association and disassociation, has an optional superframe structure with beacons for time synchronization, and a guaranteed time-slot mechanism for high-priority communications.

To keep costs low, the ZigBee Physical Device distinguishes the type of hardware based on definitions of reduced (RFD) and full-function devices (FFD). An IEEE 802.15.4 network requires at least one FFD to act as a network coordinator.

An RFD has minimal RAM and ROM resources and is intended to be a simple send-receive node in a larger network. A reduced stack size needs less memory and less expensive ICs. ZigBee RFDs are generally battery powered, but even these can search for available networks, transfer data from its application as necessary, determine whether data is incoming, request data from the network coordinator, and sleep for extended periods to reduce battery consumption. RFDs only talk to FFDs, a device with sufficient system resources for network routing.

The FFD, usually line powered, can serve as a network coordinator, a link coordinator, or another communications device. FFDs can talk to other FFDs and RFDs. A logic device in the network recognizes deployed physical devices. Logic devices can be coordinators, routers, and end devices. A coordinator initializes a network, manages network nodes, and stores network node information. A router passes messages between paired nodes. An end device acts as a leaf node in the network and can be an RFD or FFD. And application devices distinguish the type of device from an end-user perspective as specified by application profiles.

A few applications

ZigBee networks carry different types of traffic with unusual characteristics, such as data that is periodic, intermittent, and repetitive low-latency.

• Periodic data is information usually defined by the application such as a wireless sensor or meter. It typically is handled using a beaconing system in which a sensor wakes at a set time, checks for the beacon, exchanges data, and returns sleep.

• Intermittent data is either application or external stimulus, such as a wireless light switch. Data can be handled in a beaconless or disconnected system. In disconnected operations, the device only attaches to the network when communication is required, saving energy.

• Repetitive low-latency data uses time-slot allocations as needed by security systems. These applications may use guaranteed time slots. It is a quality-of-service method that gives each device a specific duration as defined by the PAN coordinator (a network organizer) in the Superframe to do whatever it requires without contention or latency.

Reading a meter, for instance, represents periodic traffic with data from water or gas meters transmitted to a line-powered electric meter which passes the data over a power line to a central location. For this operation, the RFD meter wakes up and listens for the beacon from the PAN coordinator. When received, the RFD requests to join the network. The coordinator accepts the request. Once connected, the device passes its meter information and returns to sleep.

For further reading

More on ZigBee networks and the Alliance can be found at: Homepage of IEEE 802.15 WPAN Task Group 4 (TG4), http://grouper.ieee.org/groups/802/15/pub/TG4.html

Ed Callaway, P. Gorday, L. Hester, J.A. Gutierrez, M. Neave, B. Heile, V. Bahl, “Home networking with IEEE 802.15.4: A developing standard for low-rate wireless personal area networks,” IEEE Communication Magazine, vol. 40, no. 8, pp. 70-77, August 2002.

Homepage of ZigBee Alliance, http://www.zigbee.org/

B. Razavi, RF Microelectronics, Prentice Hall 1998.

D. Pozar, Microwave and RF Design of Wireless Systems, 2001.

Göpfert, L. and the ZMD Engineering Team, A Fully-Integrated 900MHz CMOS RF Transceiver Including Digital Baseband for IEEE 802.15.4/ZigBee Application.

P. Kinney, ZigBee Technology: Wireless Control that Simply Works, White Paper of October 2, 2003.

Frenzel, L., A Supplement to Electronic Design, Wireless Control That Simply Works, January 12, 2004.

A brief glossary for the nonIT

Basebands: RF stages of the receiver and transmitter.

Beacons: Define the boundaries of a superframe.

BPSK: Binary Phase Shift Keying is an effective modulation technique for zero IF architectures. In BPSK modulation, the phase of the RF carrier is shifted 180° in accordance with a digital bit stream.

Contention based: Radios compete for access to time slots.

Direct conversions receivers: The direct conversion receiver uses a mixer and local oscillator to perform frequency down-conversion with a zero IF frequency.

Fractional-N PLL: These synthesizers are used in wireless communication applications as a local oscillator to generate accurately defined frequencies.

Packet freshness timers: Sequential freshness is a security service that uses an ordered sequence of inputs to reject frames that have been replayed.

PAN: Personal Area Network.

PLL: (phased-locked loop) topology is an electronic circuit with a voltage or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal. PLLs are frequently used in wireless communication, particularly where signals are carried using frequency modulation (FM) or phase modulation (PM). Phase-locked loop devices are more commonly manufactured as integrated circuits (ICs).

Spreading (and despreading): In Direct Sequence spread spectrum, all messages are modified by different pseudo-random spreading codes so undesired messages look like noise. The real message can be extracted using the original spreading code.

Stack: The middle layers of the OSI model from Network Layer to the Application Framework Layer.

Superframe: A structure allowed by the IEEE 802.15.4 standard. The network coordinator defines boundaries of a superframe with network beacons to commence and terminate. The superframe is divided into 16 equally sized slots, similar to time division multiple access protocols.

Zero-IF architecture: see direct conversion receiver described above.