The ZigBee standard is based on the IEEE 802.15.4 standard. All ZigBee applications are also 802.15.4 applications. The ZigBee part of the standard guarantees interoperability between equipment from different manufacturers and also supports complex, self-healing "mesh" networks with as many as 65,000 nodes The ZigBee standard is continuing evolve and some of the application frameworks and profiles have not even been defined.

However, it is not always necessary to implement a ZigBee wireless network. For many applications 802.15.4, which is simpler and easier to deploy, is sufficient. If the application doesn't require interoperability with equipment from other vendors and can be implemented in a point-to-point or star network, 802.15.4 may be the best way to go. If the higher ZigBee application layers haven't been defined yet, 802.15.4 is the only way to go. The application may be migrated to ZigBee compliance once the appropriate application framework and profile has been ratified.

Whether the network is 802.15.4 or ZigBee, it has several types of nodes: control nodes, full function nodes and reduced function nodes. Each node minimally has a radio, a microcontroller and media access control software that manages the interface between the radio and the rest of the system.

In most wireless network applications, the network IS the application. In the case of 802.15.4 or ZigBee enabled industrial control applications, the primary application is still the industrial control system and the wireless network is the means it uses for communication. Thus, when a ZigBee functionality is added to industrial control system, two separate systems are being designed in parallel. This might pose an obstacle to ever using wireless networking since the vast majority of industrial control engineers are not, and do not want to become RF experts. (

Fortunately, many vendors of 802.15.4/ZigBee radios and controllers have recognized this fact and are offering highly integrated system-level solutions. So, for the most part, engineers do not need to become RF experts. They should, however, learn a little bit about RF parameters and what they mean in terms of system complexity, cost and power consumption.

Receiver Sensitivity and Power Output The main RF parameters to use when evaluating a ZigBee/802.15.4 radio are receiver sensitivity, transmit power, and link budget.

Receiver sensitivity is the minimum power, in decibels (dBm) at which a radio can reliably receive data. A large (and negative) dBm number indicates "higher" receiver sensitivity. The higher the negative dB number for receiver sensitivity the farther apart the radios may be spaced, and the fewer radios are needed, with obvious implications for costs.. The 802.15.4 standard specifies a minimum receiver sensitivity of -85 dBm for 2.4 GHz radios and -92 dBm for 900 MHz radios. All vendors of 802.15.4 radios exceed these standards, offering radios with receiver sensitivities that range between -90 dBm and -100 dBm. Although 10 dBm may not seem like a very big difference, it has huge impact on line-of-sight range and system costs.

Improving a radio's receive sensitivity from -94 dBm to -100 dBm effectively doubles the line-of-sight range of the radio. For example, if a radio with -94 dBm receive sensitivity has a 100 meter range, increasing that sensitivity by just 6 dBm, to "100 dBm, extends the range to 200 meters. Perhaps more important, higher sensitivity can reduce or eliminate the need for expensive, power hungry, power amplifiers (PAs), thereby reducing system complexity, cost, and power consumption. For this reason, engineers should select a radio with the highest possible receive sensitivity.

The second important factor that drives the range of a radio is transmit power. The higher the transmit power of a radio, the longer its range for a desired signal magnitude. The 802.15.4 standard requires radios to have a minimum output power of -3dBm, or 0.5mWatts. Radios on the market today have output power of between 0 dBm (1 mWatt) and 3 dBm (2 mWatts). Higher is better. In fact, a radio with higher transmit power is less likely to require external components, such as power amplifiers, which can add as much as $1.50 to Bill of Material (BOM) cost. In addition, power amplifiers consume a lot of power, compromising the battery life of end-nodes. The link budget
Both receiver sensitivity and transmit power will influence the line-of-sight range of a transmitter/receiver pair. The better the receiver sensitivity and the higher the transmit power, the higher the range. Even in buildings, without line-of-sight connections, the combination of high transmit power and good receiver sensitivity will improve the robustness of radio links.

The sum of the absolute value of the receiver sensitivity and output power is called the "link budget" and is related to the range of operation. For example, Chipcon's CC2420 2.4 GHz 802.15.4 radio has transmit power of 0 dBm (1mWatt) and receive sensitivity of -94 dBm, while Atmel's Z-Link radio has transmit power of 3 dBm (2 mWatts) and receiver sensitivity of -100 dBm. The Chipcon radio's link budget is 94 dBm and the Atmel radio has a link budget of 103 dBm.

Under the same conditions, if the range of the Chipcon radio were 100 meters, the range of the Atmel radio would be 280 meters. Therefore, a difference in link budget of just 9 dBm increases the range by nearly 3 times. This means that about 1/3 as many nodes would be required to cover the same network area, using the Atmel radio as using the Chipcon radio.

A hypothetical network that required 1,000 nodes with the lower link budget radio would require only 357 radios using the Atmel radio with the -103 dBm link budget to obtain the same coverage. At $10 per node, the cost of a $10,000 system falls to $3,570 using the radios with the higher link budget.

Industrial control engineers do not need to be RF experts to add ZigBee or 802.15.4 wireless functionality to their applications. However, they should be aware of the effect of receiver sensitivity, transmit power and the link budget on both range and system cost. In all three cases, the higher the negative dBm number, the longer will be the useful range, and fewer nodes will be required to cover a given area.

In addition to the link budget, the number of external components also can affect system cost and application board space. External components are typically passive filtering and crystals to generate clock signal that the system requires to operate to published specifications. The number of external components should be published in vendor documentation. In this case, lower is better for both a cost and application footprint. perspective.

Author Bio: Chris Baumann is Director of Atmel's BiCMOS Products business unit. Prior to his joining Atmel in 1989, he held various positions at Texas Instruments and Honeywell. Mr. Baumann received his B.S. degree in Electrical Engineering from the University of Notre Dame and his M.S.E.E. degree from the University of Notre Dame. cbaumann@cso.atmel.com