Wireless network options for industrial applications

ZigBee as a standard, or 802.15.4, is the industry’s current choice for many low power, low data rate wireless communication applications. However, there are situations where 802.11 WLAN fits in very well for high data rate traffic, and many applications now require longer range and more battery life. This article discusses options for wireless industrial networks.



In wireless networks, system resources increase as the data rate increases. Cellphones or 802.11 WLAN protocols will not work for most embedded applications due to power consumption and code size requirements (typical 802.11 WLAN networks require up to 1MB of program memory for functioning node). However, many tasks, such as remote monitoring of temperature, pressure and actuation, do allow for the sizeable power consumption and code size of 802.11 WLAN.ZigBee protocols place relatively small demands on code space (32KB to 70KB) and have a moderate range (10m to 100m). These characteristics make ZigBee a strong candidate for industrial networking. One of the big advantages of ZigBee is its “mesh” capability. Mesh networks allow messages to be passed from node to node. If any of the nodes fail or drop out, the message can still reach the destination. That being said, mesh networks require sophisticated handling of packets and therefore, more program memory (Figure 1).



ISM-BAND PROPRIETARY NETWORKS Proprietary networks operating in the ISM (industrial, scientific and medical) bands, including applications such as remote temperature monitoring, pressure and actuation, are best supported using ISM band proprietary networks. The range or reach of technologies using ISM band proprietary networks is much greater than what can be realized with ZigBee, Bluetooth or WLAN. Figure 2 is a comparison of reach vs. technology for several wireless networks.



ZigBee and ISM band proprietary network power consumption is nearly the same as for remote monitoring of temperatures, pressures, and actuation type data. A ZigBee node can operate for approximately one year on a pair of AA batteries, while nodes using proprietary ISM band protocols can easily last up to 10 years using the same power source.



The worldwide frequency for ZigBee 802.15.4 systems is 2.4GHz using DSSS (direct sequence spread spectrum) QPSK as a modulation scheme. ZigBee radios are also allowed at 915MHz DSSS in the Americas and 868MHz DSSS in Europe. These frequencies use BPSK (binary phase shift keying) as a modulation scheme. The majority of ZigBee devices are currently 2.4GHz. The 2.4GHz band has become increasingly crowded due to its worldwide usage for many wireless standards and the usage of microwave ovens. The less crowded ISM bands of 915MHz, 868MHz or even 433MHz offer an alternative to crowded 2.4GHz wireless options.



The 2.4GHz frequency has a shorter antenna wavelength than does 915MHz or lower frequencies. This is why many WLAN routers require two antennas (or even three antennas for 802.11g at 5.6GHz).



Reflections and multi-path cause nulls in the 2.4GHz transmissions. Networks realized using lower frequencies like 915MHz do not exhibit as much of the multi-path and nulling, and therefore, work very well with a single antenna.



Many applications at 915MHz or below can be realized with an onboard strip line PCB antenna. This reduction of antennas helps decrease overall system cost and is another reason why networks outside of 2.4GHz are often chosen for their low cost and longer range industrial networks.



MODULATION SCHEMES

Options in the ISM band include proprietary RF networks using OOK (on-off keying), ASK (amplitude shift keying) and FSK (frequency shift keying) modulation schemes. These offer benefits that engineers simply cannot overlook. Transceivers in the 310MHz to 950MHz band can perform many of the RKE (remote keyless entry) and two-way wireless network protocols in the ISM band.



As an example, Micrel offers a generic C-Source code, an FCC 15.247-compliant protocol for frequency hop spread spectrum (FHSS) using the MICRF505 FSK transceiver chip. This software provides for FHSS modulation in a 250kHz bandwidth using 25 frequency hops. The chip has an onboard power amplifier (PA) which allows for -3dBm to +10dBm transmission to an antenna without an external transmit/receive switch. Using the onboard PA of the set to 10dBm, ranges of up to 300m are easily achieved at data rates of up to 200Kbps.



CONCLUSION

In wireless industrial networks, the application and environment ultimately dictate what type of network wins. Frequency, protocol, and power consumption are key elements in the decision process. Protocols such as 802.11, ZigBee, and even proprietary schemes such as MicrelNet can all co-exist, each addressing specific needs in the industry.



About the author Vince Stueve has more than 20 years combined management and high tech experience. He has spent the past 10 years as a field applications engineer with increasing responsibility for supporting customers with emphasis on analog, telecom, networking, and RF products. He has been with Micrel Inc. for three years where he currently works as a field applications engineer with responsibility for supporting customers in the South Central Untied States, Mexico, and South American markets. Prior to joining Micrel, Stueve worked for Texas Instruments and Abbott Labs as a design engineer, and at Avnet and Insight as a field applications engineer. He received his BSEE from Wichita State University and is the author of a number of technical magazine articles. For more information, contact Micrel Inc., 2180 Fortune Dr., San Jose, CA 95131; (408) 944-0800; www.micrel.com.



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Figure 1, Figure 2