Currently, wireless technologies are used in a relatively small (6%) but rapidly growing (32%) share of industrial communication applications based on 2017 figures. In the first part of this series, we focused on how network performance affected a wireless application. We learned that to effectively deploy wireless for an application, it is critical to understand whether the technology meets the requirements of the application.
To test this, we used Ethernet as a benchmark for comparing the technologies, since this shows the highest rate of current and future adoption. If you missed the first part of this series you can find it here. Continuing our best wireless practices for industrial application series we will focus on signal integrity.
Once it has been ascertained that wireless networks have the performance needed in optimal circumstances, the second key consideration is whether the environment where the network will be deployed is conducive to hosting wireless.
The three factors to keep in mind are supported range of the technology, negative influence of obstructions in the field of transmission, and interference from other wireless signals in the environment.
Regarding range, it is key to keep in mind the supported maximum ranges for wireless technologies. It is also critical to remember that while the network standard may support specific ranges, not all devices may be able to transmit that range due to radio or antenna constraints.
With 2.4 GHz-based 802.11 networks, the maximum range is 400m for most industrial radios, while with 5 GHz-based 802.11 networks, the generally recognized range is 200m. Bluetooth, which also transmits at 2.4 GHz, can support up to 300m generally, with 1000m achievable in some cases.
When discussing maximum ranges that wireless networks can be transmitted over, these numbers are often qualified as “clear line of sight” measurements. This means that the field between the transmitter and receiver are completely clear of obstructions. Additionally, one must account for a Fresnel Zone when determining whether a clear line of sight exists.
A Fresnel Zone is an elliptical field that exists between a wireless transmitter and receiver, and must remain clear to prevent destructive interference with waves emitted from the transmitter. The radius of this zone is determined from the distance between transmitter and receiver in combination with the wavelength of the signal, which is proportional to the frequency.
Even with objects in the Fresnel zone, a signal may still be strong enough to transmit, but there may be negative effects at the far edges of the supported range.
When looking at the influence of obstructions on wireless signal integrity, not all materials are created equally and not all wireless signals handle obstructions uniformly.
For example, 5 GHz wireless networks will see substantially higher signal loss (-dB) than 2.4 GHz networks when passing through obstructions, though neither performs well when faced with materials such as concrete and solid metal. A 2.4 GHz radio transmitting at 20 dB will see as much as 50-75% signal loss when dealing with metal or concrete.
The final consideration is the effects of interference from devices transmitting within the same wireless spectrum. 2.4 GHz is currently the most frequently used unlicensed band for wireless transmission with it being used by a diverse number of devices including microwave ovens, telephone handsets, wireless microphones, 802.11-based wireless devices, Bluetooth-based devices, and 802.15.4 wireless devices.
2.4 GHz has become popular because it is free, has reasonably good range, and can use smaller antennas. The downside to this is that this band can become congested very quickly, with that congestion having serious negative effects on network performance.
With 2.4 GHz 802.11 Wi-Fi, the band is divided into 14 different 20 MHz channels, however not all channels are available globally and only four of these channels are non-overlapping (Channels 1, 6, 11 and 14). In simple terms, if Channel 1 is congested, it is not as simple as moving transmission to Channel 2, since the two adjacent channels overlap. 2.4 GHz Wi-Fi has the highest probability of network congestion.
Bluetooth also transmits at 2.4 GHz but handles interference differently. It employs the use of Adaptive Frequency Hopping, which rather than transmitting purely through a single 1 MHz channel, it creates a sequence of unique channel hops for each network and if interference is detected on a channel, that channel is removed from the sequence.
Lastly, 5 GHz 802.11 Wi-Fi has a higher number of non-overlapping channels available to it and fewer device types transmitting at that frequency. Weather radar and military communications occur at 5 GHz but occur at channels outside of normal indoor networks.
What 5 GHz loses in terms of range and immunity to obstruction effects, it makes up for in throughput and network scarcity. The downside risk to this is that as more Wi-Fi networks implement 5 GHz radios (ex. 802.11 n/ac), there is a risk of this becoming congested as well.
Wireless has its place in the industrial world, but it is important to know where it is good and where it is lacking. It can be used effectively for a controller to I/O communications, but it is important to understand whether throughput, latency and jitter are acceptable within the performance requirements of the application.
For example, an application requiring network responses within 1ms is not likely to be satisfied using wireless technologies as they exist now.
Where wireless technologies may ultimately see their biggest impact is in how we use multiple platforms to visualize and manage equipment in factories, and as a core technology in enabling the Industrial Internet of Things.