Cut the Wires, Cut the Cost

A wireless monitoring system eliminates wires and offers several benefits, including reduced installation costs and installation time and the ability to solve new application problems. To derive the most value from a wireless system, you need to evaluate the wiring and installation costs, technology capabilities, and application requirements.

 Wired and Wireless Cabling Costs

It is easy to understand why a wireless approach to remote monitoring is lower cost in an industry such as nuclear power, where installation costs are estimated as high as $2,000 USD per foot. Fortunately, not all applications have such high installation costs. What about other applications such as remote monitoring, structural health monitoring, or asset protection? How do the costs of wireless and wired compare? 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, solar, or battery power is not included
• Networking infrastructure to enterprise is the same for wired or wireless implementations

The cost remaining is the actual cost of Ethernet with copper or fiber cables and repeaters compared to the cost of a wireless installation. A review of cable suppliers on the Web shows standard CAT 5 Ethernet cable ranges from $260 to $430 USD for a 1,000 m bulk roll. The specified distance for 100BASE-TX Ethernet is 100 m, so reaching 1,000 m would require 10 repeaters at a cost of $30 USD for each nonindustrial unmanaged repeater. Therefore, a 1,000 m run of copper CAT 5 would cost about $580 USD. With multimode fiber, 100BASE-FX specifies a distance up to 400 m at 100 Mbit/s. A 1,000 m of fiber cable will cost about $1,300 USD. Three repeaters are necessary at $150 USD for each fiber repeater. In total 1,000 m of fiber-optic cabling costs $1,800 USD. Figure 1 shows quoted cabling costs and repeaters from various vendors. Cabling costs increase for outdoor CAT 5 cable and weatherproof repeaters.  

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Figure 1: Ethernet cabling costs vary from $260 to $1,300 USD per 1,000 m.

 An overview of the cabling costs, Table 1, shows a range from about $580 USD for copper Ethernet cable to $1,800 USD for fiber cabling per 1,000 m. This cost does not include measurement devices or PC or embedded control hardware.

Table 1: Ethernet cabling costs vary from $260 to $1,300 USD per 1,000 m.

Choosing the Right Technology – Wired or Wireless

Although the ability to eliminate cabling costs with wireless installations presents potential cost savings, wireless technology must address the application specifications. One of the main reasons to select a wired protocol is bandwidth and reliability. Standard wired 100BASE-TX Ethernet is faster than both wireless IEEE 802.11 or Wi-Fi and IEEE 802.15.4, which provides the basis for ZigBee. When gigabit Ethernet at 1,000 Mbit/s is included, the bandwidth advantage for Ethernet is clear. If bandwidth is not a top requirement, the cost savings combined with installation flexibility can possibly make wireless the right choice for the application. But which wireless technology is appropriate?

Table 2. This table compares bandwidth, range, and power for Ethernet and wireless technology.

Bandwidth, Range, and Power Requirements

There are three key factors to consider when evaluating wireless technologies: bandwidth, range, and power requirements. When you compare wireless protocols based on IEEE 802.15.4 and IEEE 802.11, 802.11 has the advantage in bandwidth with a maximum bit rate of 54 Mbit/s, while 802.15.4 has the advantage in distance and power requirements. This is a typical trade-off made in wireless protocols. Wi-Fi offers higher data rates, which require additional encoding; extra data requires additional radio traffic resulting in increased power consumption by the radio. 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 IEEE 802.15.4 technology that might operate for years on standard AA batteries and transfer reduced data between sleep states.

For technologies based on IEEE 802.15.4, this trade-off in bandwidth also results in up to a 10X improvement in distance. At a maximum distance of  300 m and a bandwidth trade-off from 54 Mbit/s to 250 Kbit/s, protocols based on IEEE 802.15.4 are ideal for low-speed, long-distance remote monitoring applications, while Wi-Fi is ideal for shorter-distance, higher-power, and higher-bandwidth applications.

Network Topology

In addition to total distance, protocols based on 802.15.4 offer a couple of options for network topologies. A Wi-Fi system is typically configured in a star topology with a center access point and clients up to 80 m from the access point. Wi-Fi installations support repeaters or routers to extend distance and can be configured in a cluster or tree, but do not support meshing, which is the ability for an end node or device to route packets back to the gateway. An 802.15.4 network supports star, cluster, or mesh networking topologies. In the mesh topology, the routers are either always powered on, which is referred to as powered routing, or the entire system is synchronized to sleep and wake at the same interval. The jitter and battery inefficiencies introduced in battery meshing systems today result in systems that are awake 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.

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Figure 2: Star, cluster tree, and mesh networking topologies. 

Choosing between Wi-Fi and ZigBee

Eliminating wires offers measurable reductions in cabling costs with outdoor shielded cabling costs as high as $558 USD per 1,000 m; however, the application requirements must be met before this cost reduction. If wireless does address the application requirements, there are two distinctly different wireless standards commonly selected: IEEE 802.11 or 802.15.4. The trade-offs between wireless protocols typically come down to bandwidth, distance, and power. Wi-Fi has the bandwidth advantage while 802.15.4-based protocols perform better in applications that require longer distances and lower power. IEEE 802.15.4 protocols also offer additional network flexibility with a mesh network topology, which routes packets from end nodes to the gateway through the shortest path available.

–Robert Jackson 

Robert Jackson is a senior industrial and embedded product marketing manager at National Instruments. He holds a bachelor’s degree in engineering from Oklahoma State University.

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