Wireless Communication Options: Part 4

[Joel_Weder]

Cellular Modems
The cost of cellular modem data plans has in most regions of the world fallen dramatically with the proliferation of cellular infrastructure and low-cost hardware. Many countries where technology uptake has occurred more recently have to some extent skipped over installation of landline telephone systems, finding it far less complex and expensive to install a few cell towers instead.

Also, the technology itself has matured significantly. 5G networks are becoming more common, offering massively greater data rates than older systems. Every smartphone sold today includes the capability to send and receive data, and most can even act as a mobile hotspot for sharing their data plan with other devices nearby. (though there is a resurgence of interest in phones without data capability) The manufacturers and vendors of cellular data modems can leverage off this growth to offer devices with greater capabilities at lower cost.

There are several well-entrenched players in the market, sufficiently well-funded to develop new models relatively quickly. The cellular market varies significantly in different regions of the world, making it challenging to develop offers that can cover all or at least the largest markets, so those larger vendors do tend to dominate except in niche areas. However, the benefit of mass production is a great reduction in the hardware cost for the end user.

When considering installation of cellular modems to provide communication with remote sites, typically the cost of the hardware (CapEx) is a minor issue. Instead, the ongoing monthly cost for data usage (OpEx) is the primary concern. With significant competition in the marketplace, and a great proliferation of wireless towers across the landscape, the cost of data plans has (in most areas at least) become quite acceptable to many industrial/municipal/commercial customers.

It is however quite possible, even with a 5G system, to develop a SCADA system's communication protocol interface in such a way that it generates far more traffic than necessary, making the monthly bills unacceptably high. Also, even though many wireless modems include Ethernet capability, they cannot provide anywhere near the same throughput as even a 10 Mbps LAN connection. Developing the communication interface must be done with efficiency in mind.

Another major concern for those considering the installation of a cellular wireless modem system is coverage area. Some systems are well within the coverage patterns of their service provider's towers. For example, most sites of a municipal water system are likely to be well within a coverage area. However, many systems are in remote areas far from cellular towers. Or they may be in low-lying areas (e.g. in a ravine near a water source) in a spot where cell coverage is impossible. In such cases, the end user must consider other options.

Yet another item to think about is reliability of such a system. Some customers can accept occasional outages for brief periods, but it must be considered that a cellular data network could be down for days or even weeks after a major event or disaster. (e.g. a hurricane or forest fire) Or the system may just go down for a short time due to a minor technical issue. Many end users who require a highly reliable communication link will avoid cellular modems for this reason, unless they include some form of redundancy. Which leads to the next topic...

Redundancy

Operators of remote systems frequently specify some degree of redundancy in their communication links. It may be required to provide redundancy at the SCADA Host site, and likely also at any repeater sites. In some cases, the end user requires full redundancy all the way out to remote locations, which means every site will require an alternative means of communication. The cost of any such system will inevitably be much greater, though various options may be considered to manage the expense.

With licensed narrow-band systems, redundancy may be accomplished by providing “hot standby” hardware. The active radio's health is monitored by a controller, which can switch communication to a standby unit rapidly when necessary. It may then generate an alarm (e.g. via SNMP) to alert the system operator. This hardware is most commonly installed at the central repeater and/or SCADA Host locations, and only rarely at remote sites.

In a license-free network (e.g. 915 MHz or 2.45 GHz), redundancy can be provided through the inclusion of alternate upstream paths, either through a mesh-type system or a pre-configured redundant path design. In a Mesh system, a wireless device which detects that its upstream link has failed may automatically re-route its traffic to another device which has a path upstream. Mesh technology however requires the wireless devices to be constantly talking to each other, learning and re-learning the best path upstream. This can use a lot of system bandwidth, as well as increase power consumption. In a system using pre-configured alternate paths, there is no additional load on the system until the preferred path fails. The device then spends a few moments connecting to an alternate upstream device. This typically includes a capability to perform self-healing, by monitoring the primary channel and reverting to it when available.

Another option for redundancy is to purchase devices which include radio modules operating in two different bands, possibly even using different technologies. For example, both narrow-band licensed and ISM band license-free radios. Or one module might be a cellular modem, or a satellite IIoT module (eg Swarm). Note that redundancy may be provided with fully-duplicated wireless networks of any type, but unless the devices operate on widely separated frequencies this may not suffice to avoid interference blocking the desired band.

The ability to automatically select the least-cost path at the RTU or other end device is of interest to some as well. For example, if the RTU discovers that the primary communication channel has failed, it might provide built-in ability (with or without additional logic) to switch to an alternate such as cellular or satellite. Use of this alternate path may incur additional cost, so the system would only use the alternate path as long as necessary.

One last item to mention is Dynamic IP Routing, sometimes called adaptive routing. In a wireless system which includes IP Routing capability for defined network paths, some of the more advanced devices (typically licensed narrow-band radios) are capable of automatically changing the route for a given destination based on the current state of the communication system.

In the early days of wireless data communications, simply having a wireless link was impressive. Outages might have been undesirable, but were expected. Today, current experience with wireless communication by most is that it's fast and highly reliable. System designers must be innovative and creative in ensuring their customers are satisfied with their system.