As the grid is modernized, devices are becoming smarter. This requires a more reliable communications path to carry important information and, often, critical data about the electric power grid. This data can be used for grid stability, safety, emergency operations and even disaster recovery, meaning it must be delivered promptly, securely and without risk of competing with less critical information.
The amount of data being generated is continually increasing, and for the data to be useful it must make its way to a control system. This generally has been accomplished via radio frequency (RF) systems. While utilities have used licensed networks for many years, the deployment of advanced metering infrastructure (AMI) introduced the use of mesh radios using unlicensed spectrum.
Once a field area network is deployed, the logical next step is to add distribution automation (DA) devices to operate on this network. For many reasons, the continued use of this deployment strategy, which relies on unlicensed spectrum, may not be the best choice any longer. As part of the changing dynamics within field area network (FAN) design and implementation, it’s important to explore one aspect of the FAN — the choice of which type of spectrum on which to build your network.
The Difference Between Licensed and Unlicensed
Across the U.S., RF spectrum is a finite, and thus scarce, resource that is regulated by the Federal Communications Commission (FCC) to be used for the public good. The spectrum is broken into bands, with each designated for a specific use. For an unlicensed spectrum, these bands can be used by anyone as long as some general rules are followed, like using an FCC-approved device and keeping the transmit signal level low so that you do not broadcast your transmission signal to a large area.
Controlling your broadcast transmission range is a key point for unlicensed spectrum because you are experiencing interference from others and introducing interference to others using the unlicensed spectrum. Utilities using an unlicensed frequency spectrum to operate their AMI and FANs are doing so in spectrum that is available for anyone to access and use.
In addition, the spectrum can be used for more than just utility applications; it also can be used for common household conveniences such as cordless telephones and baby monitors. These systems have been designed with transmission techniques that attempt to avoid interference, such as using frequency-hopping patterns to continuously move around small bands of spectrum or by retransmitting the data upon discovery of interference.
Due to the noise caused by others using the same unlicensed spectrum, the receive level of your signal must be stronger compared to radios operating in licensed spectrum bands. Combined, the limited transmit power and strong signal needed at your receiver means that communication is achieved over relatively short distances, sometimes as little as over a single neighborhood block.
For applications such as AMI, where you have many devices relatively close to one another, this strategy is great for allowing many users access to the same spectrum. To get these messages from the closest radio device to the control center, these network designs ultimately result in end-device messages being stored and forwarded many times from radio hop to radio hop, which can result in system operators waiting, sometimes many seconds, for status or the execution of commands. Even worse, these systems can time out, which results in multiple retransmissions, thereby adding increased and unpredictable delay and jitter to the network and to control applications.
Unlicensed field area networks can introduce ongoing maintenance tasks and introduce frustration to the utility end user due to the unpredictable nature of when and where service-impacting interference occurs. Interference may not exist during design and initial implementation but can crop up over time, forcing a partial redesign to optimize the network by adding repeater locations, for example.
Conversely, most licensable spectrum is reserved and used for systems or applications that are dedicated to a single use and in which the operator has sole and exclusive use of band across a geographic area for a licensed time period. The FCC keeps a record of licensed spectrum requests that have been granted and has a technical procedure for preventing and/or resolving interference between licenses. If a band of spectrum has been granted for a specific application, such as utility multiple address data nodes, utilities can access the spectrum by filing for a license and paying a nominal administrative fee.
For spectrum that has been dedicated for general use but is facing more demand than supply, such as the public cellular network, the FCC uses an auction process to allocate the spectrum to the highest bidder. While getting a spectrum license takes time and costs money, when considering the overall operating costs, the consistency that you get from having sole and exclusive use makes it worth the effort.
The Last Mile
Communications have always been important for power plants and transmission substations to monitor the electric grid. As power plants and transmission networks have evolved and gotten smarter over time, utilities have been pushed to gather control and monitor data from the distribution network, also known as the last mile of the electric grid.
Utilities are putting smart devices on distribution poles and even on houses to monitor and provide control of grid elements as far as the electric grid will reach. The problem is that the distribution network and customers’ homes aren’t connected to the same private networks that the power plants and transmission substations are already using.
It is very costly to build a new private infrastructure from the last mile network devices to the already-connected power plant or substation. Therefore, the last mile is often reached by a radio communications technology using unlicensed radio networks. As the amount of data needed increases in places that are not already connected to the rest of the network, the use of licensed radio networks — instead of unlicensed — should be considered.
Choosing Licensed Networks
For years, unlicensed radio communication has been the choice for utility last mile communications networks for AMI and DA. This is primarily due to the consistent environment in which you have to operate. Lately, a more technically demanding communications network is required because several factors have changed:
- The quantity of devices that need to be connected has grown.
- The amount of information that needs to be collected from each device has increased.
- Faster communication is required for all devices (i.e., latency decreases).
- Throughput requirements have grown (i.e., faster speeds — 9.6 Kbps vs. 5 Mbps).
- Security requirements have increased.
- The desire to reduce or eliminate interference has increased.
Countless licensed options on the market provide utilities a solution to the changing communication dynamics in the last mile. Licensed networks provide:
- Consistency, due to the licensed band being free from other potential interference sources.
- Improved range, because you can transmit a stronger signal and achieve greater receive sensitivity because the band is free from noise created by interference from other parties, resulting in overall performance in throughput speeds and latency.
- Increased security.
- Options for broadband or wideband channel capacity (as opposed to narrowband).
By obtaining a private, licensed spectrum, you can see that your network is consistent, secure and controlled only by you.
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