Understanding technology capabilities and application requirements is important when selecting a wireless technology for an application. The reasons to choose wireless include reduced installation costs, installation and deployment flexibility and the ability to address new applications. Before selecting wireless, first ensure the bandwidth available with wireless meets the application requirements.
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 requirements. Two of the main reasons to select a wired protocol are bandwidth and reliability. Standard wired 100BASE-TX Ethernet is faster than both wireless IEEE 802.11g, or Wi-Fi, and IEEE 802.15.4, which provides the basis for ZigBee. When gigabit Ethernet at 1 Gbit/s is included, the bandwidth advantage for Ethernet is clear. If the application does not require a bandwidth above 100 Mbit/s, then the cost savings combined with installation flexibility make wireless an effective option.
Table 1. Bandwidth, Range and Power Comparison 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. Comparing wireless protocols based on IEEE 802.11 and IEEE 802.15.4, Wi-Fi 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 significantly 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. A wireless sensor network based on IEEE 802.15.4 technology 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 IEEE 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 30 m from the access point. While standard Wi-Fi installations support repeaters or routers to extend distance and can be configured in a cluster or tree, they do not support meshing, which is the ability for a node or device to route packets back to the gateway. Many 802.15.4-based wireless sensor networks (WSNs) support star, cluster tree and mesh networking topologies (see Figure 1).
Figure 1. Star, cluster tree and mesh networking topologies
Top Four Questions to Ask When Selecting a Wireless Technology
- Which measurements do I need to address my application?
- What are the distances from my measurements to my data center or enterprise connection?
- Which network topology do I need?
- What is the system power source?
1. Measurement Speeds and Wireless Throughput
Wi-Fi offers higher bandwidth, as the IEEE 802.11 wireless protocol can support higher sample rates than IEEE 802.15.4 based protocols. Measurement type, number of measurement channels and measurement speed will determine the throughput requirements.
For high-speed measurements, Wi-Fi offers additional bandwidth. For instance, 24-bit high-speed acceleration data is sent in 32-bit packets, and the required throughput is 6.6 Mbit/s for four channels at 51.2 kS/s. Wi-Fi packets require some additional overhead, but clearly the sample rate of 51.2 kS/s requires the bandwidth of Wi-Fi.
Wireless sensor networks (WSN) are well suited for higher channel count applications. As an example, a WSN application with 36 nodes and four analog and four digital channels per node at 1 second sample interval requires 24 kbit/s. The 82 Bytes per sample packet includes packet header information, four analog input channels, four DIO channels and channel information such as link quality and battery voltage.
2. Distance Requirements
Next determine the distance from the measurement to network access. If the distance is greater than 30 m line of sight, then Wi-Fi needs repeaters. Even if distances are less than 30 m, RF interference sources including trees or buildings can reduce the achievable distance. To ensure a reliable system, a site survey is recommended for all wireless installations. If required distances exceed 30 m, then IEEE 802.15.4 offers an option with a maximum distance of 300 m line of sight. Routers can extend the total distances.
3. Network Topology
Make sure to select the appropriate network topology by considering the location of access points or gateways and the maximum distance from the network infrastructure to an end node or device. One topology option is a simple star network where a central access point has several end devices connected;. This is an ideal configuration for Wi-Fi as long as the distances from access points to devices are less than 30 m. If additional distance is needed, a tree topology for which either Wi-Fi repeaters or IEEE 802.15.4 routers can be used helps extend distance. If network reliability is important, then an end node can route packets through multiple routers to a gateway with an IEEE 802.15.4 mesh network. This provides network reliability in case a router fails.
4. Power Availability
The final consideration when deciding between wireless technologies is power availability. For two- to three-year battery deployments at lower bandwidths, IEEE 802.15.4 is ideal. The central gateway and embedded PC require either 10 to 30 VDC power or solar power; however, end nodes function for several years on standard AA batteries. In Wi-Fi, an access point generally requires power while the end devices are typically powered by DC or solar power for extended operation.
Answering these four questions will help in the selection of the appropriate wireless technology. Addressing application requirements is the first step. For any wireless installation, analyze the RF performance at the deployment site. Site surveys conducted by professionals ensure adequate coverage, network performance and the ability to scale as more sensors are added.
Applications for Wi-Fi-based Wireless Data Acquisition
The higher bandwidth of Wi-Fi at 54 Mbit/s enables wireless data acquisition systems to address high speed waveform measurements such as strain and acceleration. The trade-off for higher bandwidth is power. An example wireless data acquisition application is short term strain and stress tests for products in the design or early deployment phases. This might include a new machine like an agricultural harvester. Power is available from the engine, and the wireless communication enables faster deployment and flexibility for measurement installations. Measuring the strain on different components for the harvesting machine allows engineers to verify the design and validate the wear and performance calculations performed during the early design phases.
Applications for IEEE-802.15.4 based WSNs
The low power and longer distance available with IEEE 802.15.4-based networks fits well for longer-term remote measurement applications. One example is environmental monitoring. The ability to easily distribute several nodes up to 300 m from a gateway and further extend this distance through mesh routers makes WSN ideal for monitoring the environmental conditions for a corporate effluent treatment pond. The system can easily measure the pH, dissolved oxygen concentrations and water level of the pond. The battery operated end nodes are easily installed close to the water's edge without the requirement of local power or communication wiring. Data is then wirelessly sent to a gateway with a real-time PC for storage and connectivity to IT infrastructure.
Wireless Data Acquisition or Wireless Sensor Networks
If wireless meets the application requirements, the decision rests between two wireless technologies: Wi-Fi or IEEE 802.15.4-based networks. The trade-off between wireless protocols typically comes down to bandwidth, distance and power. Wi-Fi has the bandwidth advantage while IEEE 802.15.4 based networks perform better in applications that require longer-distance coverage and lower power. IEEE 802.15.4-based protocols often deliver additional network flexibility with a mesh network topology, which routes packets from end nodes to the gateway through the shortest path available.
September 2009, Pumps & Systems