Ian Verhappen is a contributor and blogger for Control and Control Design. He has 25+ years experience in instrumentation, controls and automation. You can email him at [email protected].
Have you ever been on a public Wi-Fi hotspot such as a coffee shop or airport with the signal strength indicator on your mobile device or laptop indicating a nice strong signal but then not being able to log on the network? The likely reason is that there are more people trying to get connected than you have connections available. Though frustrating, this situation is not critical. However if the same thing were to happen when a wireless industrial device needs to connect to send a process update or alarm, the consequences could be much more severe.
As engineers, we're expected to anticipate these sorts of situations and plan accordingly, not only for the conditions when the system is installed but also for the reasonably foreseeable future. So how might we be able to accomplish what is a seemingly impossible task? The answer, though to some an enigma is management.
Much like we have area classification drawings to help us plan for the hazardous conditions that might be expected to occur, if we divide our facility into areas based on the approved usage of the license or ISM bands, we will be able to manage the number of devices making demands on any one system.
The ISM bands in North America are separated into three primary frequencies:
- 900 MHz: operating between 902 to 928 MHz with a 26 MHz bandwidth and, as of 2006, 30 channels
- 2.4 GHz: operating between 2.400 to 2.4835 GHz with a 100 MHZ bandwidth with 16 channels
- 5.8 GHz: operating between 5.725 to 5.875 GHz with a 150 MHz bandwidth with 14 channels.
As we also know, the 2.4 GHz bandwidth suffers from the most congestion in part because that is where the IEEE 802.11 a-n radios (Wi-Fi, cellular); IEEE 802.15.4 (industrial wireless protocols including WirelessHART, ISA100.11a, Zigbee, and Bluetooth) all operate and as a result is where we are most likely to run into difficulties.
Also read: Lots of Network Choices. Choose Your Design Wisely
So how will bandwidth classification drawings solve the problem?
A set of bandwidth classification drawings can be developed for your facility so that you can specify that, within certain areas of the plant, all or portions of frequencies are assigned to specific applications. For example, the 5.8 GHz frequency will be used for Wi-Fi communications in conference rooms and public areas and then turn off the 2.4 GHz radios in these areas. This will also have the side benefit of controlling network access and reduce cybersecurity risks, as well. The 2.4 GHz frequency in the office environment will then be available for staff or cellular use.
In the plant environment you can continue by specifying that 5.8 GHz is for Wi-Fi and those potential future roaming operators, while 2.4 GHz is reserved for process control/automation applications.
Because signal distance is proportional to frequency, the 900 MHz band can then be reserved for the longer connections across a facility, such as between the access points in a tank farm or in the process plant.
If separation by frequency is insufficient, the resolution of the separation can be managed to the channel level; however, if you are planning on or already experiencing this level of demand, remember that because all channels overlap you cannot assign adjacent channels to adjoining physical areas; they must typically be three channels apart. Of course, with careful planning you can keep all the neighbors three channels apart and still use all the channels available. That is definitely when you need the bandwidth classification drawing to help keep track of it all.
As you can see, this information can be presented on a single drawing or as a series of drawings with each frequency on a different drawing so that you can represent the sphere or area that is assigned to a particular application, use, or group.
Though not a very technical solution to a technical problem, managing networks by assigning them in advance in this or a similar way not only provides a path to resolve any conflicts should they arise, but also provides clarity on who or what applications take precedence, should a situation develop.
