Feature Wireless Technology
band. This provides greater volume, lower costs and more opportunities for integration of the RF transceiver with a low cost microcontroller core. For applications that require a hard
Wireless automation for process control T
Several different wireless technologies have evolved to support flexible links for industrial networking and process control applications, with a range of capabilities. Heiko Luckhaupt of RS Components explains
here is a range of unlicensed fre- quencies available, varying from the sub-1GHz variants in different parts of the world to the unified 2.4GHz
real time capability such as monitoring critical chemical or nuclear processes, a low latency solution is vital. Wireless versions of fieldbus protocols such as WirelessHART provide a way to deliver that real time performance in a more flexible manner with wireless links. While it is based on the popular unli- censed band at 2.4GHz used by other technologies - notably WiFi and ZigBee - it also includes security and interfer- ence protection that makes sure every packet of information is delivered within a certain time. Within the 2.4GHz global unli- censed band ZigBee is emerging as a robust yet flexible industrial protocol, allowing mesh networks to develop so that data can hop from one node to the next. Modules such as the 699-7301 Atmel ATZB-A24-UFLR, the 771-6349 Digi International XB24-Z7PIT-004, or the 615-8818 Telegesis ETRX2 are low cost ways of delivering wireless con- nections for process control. ZigBee has also now added support for IPv6 so that these nodes can be accessed directly from the internet.
ZigBee IPv6
After several years of discussion, a ver- sion of the ZigBee mesh networking standard that supports the new IPv6 internet protocol has been launched. IPv6 extends the address space of today’s IPv4 protocol, which has essentially run out of numbers to iden- tify all the devices on the internet. Moving ZigBee
to support IPv6
requires changes to software in the net- work layer and, as it is also designed to work with the ZigBee Smart Energy pro- tocol (version 2, SE2) at different fre- quencies, can need changes to the MAC and PHY layers in a device. Revision 0.7 of the SE2 is being discussed in the industry and takes ZigBee into other fre- quency bands, not just the unlicensed 2.4GHz band. This supports smart power grid, power and water utility applications as well as industrial
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machine-to-machine (M2M) applications such as process control. ZigBee IP is the first open standard for an IPv6- based full wireless mesh networking solution and provides seamless internet connections. The key is that, unlike ZigBee today, ZigBee IP net- works do not need gate- way devices to connect to the internet and so will work directly with other Ethernet and Wi- Fi devices. The specifica-
tion adds network and security layers and an application framework to the existing IEEE 802.15.4 standard to pro- vide a more scalable architecture but with the same proven, end-to-end security using TLS1.2 protocol, link layer frame security used in ZigBee. This is based on the AES-128-CCM algorithm and support for public key infrastructure using standard X.509 v3 certificates and ECC-256 cipher suite. A key change is that ZigBee IP contin- ues to support global operation in the 2.4GHz frequency band but now also supports regional operation. Because there is no need for a gate- way on the network, ZigBee IP had to support the development of discov- ery mechanisms with full application confirmation. Devices can be added anywhere in the network, and paired with devices perhaps on other net- works, so the discovery, authentica- tion and pairing layers are significantly more complex. Interference will be an increasingly important challenge with IPv6-based systems that are more spread out and working with other networks in the 2.4GHz band such as Bluetooth and WiFi. As a result ZigBee IP products have access to 16 separate, 5MHz channels in the 2.4GHz band so that band management can be used to avoid interference.
ZigBee already incorporates an IEEE 802.15.4 CSMA-CA mechanism that reduces the probability of interfering with other users, plus ZigBee uses automatic retransmission of data to ensure network robustness. Because
Above: the ZigBee IP network topology for process control
the duty cycle of a ZigBee product is usually extremely low, relatively few packet data units are transmitted, reducing the likelihood of an unsuc- cessful transmission and making the protocol more robust. The 802.11 WiFi standard (a,b,g,n) is cost effective but there can be issues of interference from other networks. While it lacks real time support it can be well suited to monitoring applications such as streaming a video feed from a remote camera using modules such as the Microchip RN171-I/RM (765-2881).
WirelessHART WirelessHART enables users to quickly and easily gain the benefits of wireless technology while main- taining compatibility with existing HART devices, tools and systems. The wireless version includes several features to provide built-in 99.9% end-to-end reliability in all industrial environments such as channel hop- ping to avoid interference and pro- vide coexistence with other wireless networks. Systems can be developed using flexible 2.4GHz transceivers such as the 769-2687 Atmel AT86RF233-ZU as the RF transceiver.
Security
Security is also a key consideration for an industrial network, and WirelessHART employs the latest secu- rity techniques to provide the highest levels of protection available via 128bit AES encryption and a unique encryp- tion key for each message, as well as device authentication. Each device in the mesh network can serve as a router for messages from other devices.
Conclusion
The range of wireless frequencies and protocols available for process automation, coupled with the differ- ent performance requirements down to hard real time responses, has cre- ated a fragmented industry. The emerging ZigBee IP technology opens up all the regional frequency bands alongside the 2.4GHz band and gives the flexibility to connect equipment up in new ways. This requires higher security levels, and the new protocol combines higher performance with more flexibility and the promise of an
effective cost reduction pro-
gramme as devices ship-in ever higher volumes.
RS Components
uk.rs-online.com T: 08457 201 201
Enter 213 FEBRUARY 2014 Automation
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