Measurement Speeds and Wireless Throughput

1. Measurement Speeds and Wireless Throughput

Wi-Fi offers higher bandwidth and as the IEEE 802.11 wireless protocol can support much 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 for 4 channels at 51.2 kS/s the required throughput is 6.6 Mbit/s.  There is some additional overhead for Wi-Fi packets, but clearly the sample rate of 51.2 kS/s requires the bandwidth of Wi-Fi.

 NI WSN is well suited for higher channel count applications.  As an example an NI WSN application with 8 nodes and 4 analog and 4 digital channels per node at 1 second sample interval requires 5.2 kbit/s. The 82 Bytes per sample packet includes packet header information, 4 analog input channels, 4 DIO channels, and channel information such as link quality and battery voltage.

For larger topologies such as a network with four routers and 32 end nodes the total throughput is 44.6 kbit/s.  An important note is that in this topology the 32 end nodes communicate through one of the four routers so the network traffic is doubled from these end nodes.  To calculate throughput in this extended topology multiply the number of nodes in this case, connected directly to the gateway by 1 hop and the number of end nodes connected to a router and then gateway by two hops and add the results.

2. Distance Requirements

Next you need to determine the distance from your measurement to your network access. If the distance is greater than 30 m line of sight, then you need repeaters for Wi-Fi. Even if distances are less than 100 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 100 m, then IEEE 802.15.4 offers an option with a maximum distance of 300 m line of sight, and with routers the total distances can be extended.

3. Network Topology

Then you need to select the right network topology. To address this, consider 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 you need additional distance, a tree topology for which you can use either Wi-Fi repeaters or IEEE 802.15.4 routers helps extend your distance. If network reliability is important, then with an IEEE 802.15.4 mesh network an end node can route packets through multiple routers to a gateway. 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 9 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.

After answering these four questions, you can more easily select the wireless technology that is right for your application. Addressing your application requirements is the first step. For any wireless installation, you should analyze the RF performance at the deployment site. Site surveys conducted by professionals ensure adequate coverage, network performance, and the ability to scale as you add more sensors.

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.  Then data is sent wireless to a gateway with a real-time PC for storage and connectivity to IT infrastructure.

NI Wireless DAQ and Wireless Sensor Networks

If wireless meets your application requirements, you then need to decide 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. National Instruments offers measurement devices for Wi-Fi with NI Wi-Fi data acquisition (DAQ) hardware and for IEEE 802.15.4-based wireless sensor networks (WSNs) with the NI WSN product family.