Cut the wires, cut the costs
July 1st 2011

A wireless monitoring system can offer benefits over traditional systems, including reduced installation costs and installation time and the ability to solve new application problems.Rhys Bowley, applications engineer at National Instruments UK & Ireland, explains

To derive the most value from a wireless system, you need to evaluate wiring and installation costs, technology capabilities and application requirements.

It is easy to understand why a wireless approach to remote monitoring is lower cost in an industry like nuclear power, where installation costs are estimated as high as £3700/m. Fortunately, not all applications have such high installation costs. How do the costs of wireless and wired systems compare for other applications such as warehousewide control or structural monitoring of a 3km bridge? The answer depends on the application, but you can conduct a baseline analysis if you assume the following: Software investments are equal Costs of line or battery power is not included Networking infrastructure is the same for wired or wireless implementations The cost remaining is the actual cost of copper or fibre cables and repeaters and installation compared to the cost of wireless hardware. A review of cable suppliers on the Web shows that standard CAT 5 Ethernet cable ranges from £150 to £270 for a 1km bulk roll. The specified distance for 100BASE-TX Ethernet is 100m, so reaching 1km would require 10 repeaters at a cost of £20 for each nonindustrial unmanaged repeater. Therefore, a 1km run of copper CAT 5 would cost as much as £470.Making the same considerations with fibre-optic cable repeating every 400m we are left with a cost of £1100 per 1km. Costs significantly increase for outdoor cable and weather proof repeaters.

Wired or wireless? Wireless technology must first address the application requirements. One of the main reasons to select a wired protocol is bandwidth and reliability. Standard wired 100BASE-TX Ethernet is usually faster than wireless Wi-Fi and IEEE 802.15.4, which provides the basis for ZigBee.When we consider gigabit Ethernet at 1 Gbit/s, the bandwidth advantage of wire is clear. If bandwidth is not a major requirement, the cost savings combined with installation flexibility can make wireless an appropriate choice for your application. But which wireless technology? Bandwidth, range and power Three key factors to consider are: bandwidth, range and power requirements.When you compare wireless protocols like Wi-Fi and ZigBee,Wi-Fi has the advantage in bandwidth with a maximum bit rate of 150Mbit/s, while ZigBee has the advantage in distance and power requirements. This is a typical tradeoff made in wireless protocols. This bandwidth and power trade-off is obvious in systems such as laptops or smart phones with integrated Wi-Fi that typically operate for a matter of days between recharging and provide high-speed data transfer, compared to a wireless sensor network based on ZigBee technology that might operate for years on standard AA batteries and transfer reduced data between sleep states.

Technologies based on ZigBee can yield up to a 10 times improvement in distance over Wi-Fi. At a maximum distance of 300m and a bandwidth capability of 250Kbit/s, ZigBee is suitable for low-speed, long-distance remote monitoring applications, while Wi-Fi is suitable for shorter-distance, higher-power and higher-bandwidth applications.

Network Topology In addition to total distance, ZigBee can offer interesting options for network topologies. A Wi-Fi system is typically configured in a star topology with a centre access point and clients up to 80m from the access point. An 802.15.4 network, like ZigBee, supports star, cluster or mesh networking topologies.

Meshing is the ability for an end node or device to route packets back to the gateway.

In the mesh topology, the entire system is often synchronised to sleep and wake at the same interval. The high power requirements in battery-powered mesh networks today result in systems that are required to wake for one to two seconds and then sleep for hours before waking again. As protocol enhancements progress, it will be possible to improve this performance.

Eliminating wires offers measurable reductions in cabling costs and Wi-Fi and ZigBee both offer solutions to the challenges faced in various applications.Wi-Fi provides a bandwidth advantage, whilst ZigBee performs better over longer distances with lower power usage.

Whichever protocol best fits your application, when you cut the wires, you cut the costs.

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