More and more, system engineers are taking advantage of industrial wireless technologies to reduce the amount of cabling in their designs. There are some issues to be aware of, however, when replacing dedicated connections with wireless links:
1. Need latency tolerance. Today’s wired Ethernet connections are full duplex. This means that each end device can both transmit and receive at the same time. On the other hand, wireless technologies such as 802.11a/b/g/n are half duplex. This means that when any one device is transmitting, all other devices must wait. Make sure that your application is designed to be tolerant of the latency introduced due to the half-duplex nature of wireless.
2. Control multicast traffic. When implementing wireless technology in factory automation projects, be aware of any multicast traffic coming from PLCs or producer devices. Multicast traffic is handled differently than unicast traffic by wireless access points. Multiple devices can receive multicast traffic, while unicast is destined for only one device. Wireless access points transmit multicast traffic at a minimal rate to ensure that all listening clients will be able to receive the traffic. This results in low aggregate bandwidth over the wireless AP as it has to lower its transmit rate down from the maximum.
3. Low bandwidth requirements. Make sure that your application’s bandwidth requirements are low enough to be satisfied by the lower rates. Many designers overlook these points and experience problems when moving to wireless solutions. Being aware of the limitations of wireless technology can ensure that your upfront design will work in a wireless deployment.
4. Don’t take shortcuts with wireless. Consider the entire system design and the support lifecycle of the system before choosing technology and vendors. Time spent up front on site surveys, path loss calculations and fade margin will pay dividends when it comes time for installation. Design in fade margin. Wireless is very reliable when well designed, but if you don’t design in appropriate fade margin you’ll have problems in the future.
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