RF in Action: Wireless switches offer unlimited benefits
Todd Hanson, Honeywell Sensing and Control
6/20/2011 1:32 PM EDT
Making Wireless Worry-Free
The IEEE 802.15.4 wireless PAN standard defines a low-duty wireless signaling scheme targeting basic instrumentation and control applications. The standard has been in place for a number of years with several generations of RF devices already deployed. As a result, the IEEE 802.15.4 specifications are both robust and field-proven to eliminate crosstalk between network links as well as resistant to interference from out-of-network users in the same frequency band such as Bluetooth™ and portable phones.
Because the 802.15.4 is a PAN standard, operation is distinct from and independent of familiar wireless LAN systems. There is no exchange of information or other interaction between a PAN and the LAN, so there is no need for corporate IT department involvement. Further, the PAN systems are independent of one another. So, for example, if two construction cranes with 802.15.4 switch networks are operating at the same construction site, there is no risk of interference or interaction.
802.15.4 radios operate in the 2.4 GHz band using low power, short-duration signaling. Because the radio spectrum and power levels are approved for unlicensed operation worldwide, system developers do not need to be concerned with such things as compliance testing.
How to Design-In Reliability
802.15.4 supports a simple star configuration network configuration with each switch in the network communicating with a single monitor/receiver. Communications links are point-to-point between switch and monitor; there is no signaling between switches. This network architecture keeps the switches isolated from one another so there is no opportunity for the behavior of one switch to affect the behavior of any other.
To enforce the isolation of their communications links, 802.15.4 radios can be set up to require registration with their network monitor. The registration or “pairing” process is simple and needs be performed only once to establish the linkage. This process takes less than 5 seconds. Once established, the “pairing” persists even if power is lost to the monitor or a battery is replaced in the switch. This not only keeps system maintenance simple, it ensures that the network is self-healing. The monitor is able to automatically re-establish any dropped link to a registered switch. Replacement or reconfiguration of the switches can be done through a clearing and re-pairing process.
Three Ways to Ensure 802.15.4 Security
Security is always a concern. The above pairing process follows the security techniques outlined in IEEE 802.15.4. When registration is activated, the monitor provides the switch with several pieces of information. The first is the monitor’s individual 16-bit network identification number, which the monitor chooses at power-up to avoid conflict with other networks in the vicinity. The second is a unique 16-bit address for the switch, which the monitor creates. The third piece of information is a 128-bit encryption key that the pair will use to encode future communications. Once the information is exchanged, the switch and monitor use it to address and encode the signals they exchange.
The combination of addressing and encryption ensures the uniqueness and security of each switch’s communications channel. No two monitors have the same network address, so monitors and switches will not respond to signals originating in other networks. A monitor will not confuse signals within its network, nor will switches respond to signals intended for other switches. No switch has another switch’s address, so switches will not react to one another’s signals.
Even if an addressing error occurs somewhere, the network will not react to the erroneous signal because the encryption keys for decoding the message will not match. The use of encryption further ensures that no unregistered node can successfully insert erroneous signals into a network or decode signals from a network, making the network secure from both eavesdropping and hacking.
Once a switch and monitor have been paired, the two continually exchange information as long as both have power. The switch sends a switch position message to the monitor each time the position changes, accompanied by a “health” status message that indicates battery level and RF signal strength. In addition, the switches send out “health status” messages at regular intervals. This interval is typically 30 seconds but is configurable to meet specific installation needs; shorter intervals can reduce battery life.
Using a 802.15.4 radio network to connect limit switches to a central controller frees industrial equipment developers and users from the costs and restrictions of wired switch solutions and expands the range of possible uses and placement of switches. It also reduces the wire complexity within a control cabinet. The commissioning and lifetime cost of switches drops while reliability and integrity rise.
About the Author
Todd Hanson is a Director of Strategic Marketing and Wireless Solutions at Honeywell Sensing and Control (S&C). In this role, Todd is responsible for S&C’s wireless solutions as well as the overall S&C industrial business. He actively collaborates with other Automation and Control Solutions businesses to formulate a One Honeywell strategy where S&C supports the other businesses to help drive total Honeywell solutions.