Is PTT the Next Interoperability Challenge?
Tuesday, April 17, 2018 | Comments
Like many in public safety, I’ve been watching with great interest as manufacturers develop new and improved devices intended for use on public-safety networks that expand on previous versions’ functionality, survivability and overall suitability. Device features such as improved ruggedness, user-replaceable batteries and positive-locking accessory side connectors are major improvements as device manufacturers zero in on products that will work in the public-safety world.

Proponents of mission-critical push to talk (MCPTT) services discuss how we can move from existing LMR systems to Long Term Evolution (LTE)-based MCPTT services with its many improvements. We especially hear that by moving to cellular PTT networks, we will all be interoperable. In fact, the word “interoperable” has become a marketing buzzword, with little regard for what true interoperability actually should be.

Unfortunately, what we see today does not support the concept of interoperable communications. There are a number of PTT applications and services. Some are generic to a particular carrier network, and others are “over-the-top” PTT applications that are carrier agnostic. None of these services can talk to each other without the use of external gateway devices intended to patch together specific talkgroups.

Once recent implementation to address the problem of off-network, direct mode PTT could further complicate interoperability. One vendor that makes excellent rugged devices recently added a proprietary PTT modulation scheme in the unlicensed 900 MHz Industrial Scientific Medical (ISM) spectrum.

Here we go again — yet another proprietary silo.

The devices are not directly interoperable with LMR equipment, at least without using additional interoperability equipment at added expense and complexity, and with yet another possible point of failure. Putting aside the downsides of using unlicensed spectrum for public safety, using a proprietary method on nonprotected, non-public-safety spectrum is another impediment to true interoperability. From the public-safety point of view, the idea is to get as many responders as possible on common communications platforms, not to see how many different platforms we can come up with.

The fear is that as vendors rush to capture PTT market share, each handset manufacturer will come out with its own latest-and-greatest direct mode PTT solution. This is not the way to address the problem of off-network, simplex communications, which is so critical to public safety when multiple agencies have to respond quickly to an incident and need to communicate immediately. Having to set up gateway devices takes valuable time and introduces the possibility of errors that can cripple communications systems when they’re needed most.

Existing 700 and 800 MHz bands contain a plethora of interoperability channels. Using another band means the devices are not inherently interoperable with any of the hundreds of thousands of existing 700/800 MHz radios in use by public safety nationwide, raising a host of questions. Will device manufacturers offer proprietary technology to other cellular device and LMR manufacturers so they can produce equipment that interoperates? How many other handset vendors will come out with their own direct mode method that is incompatible with everything else? And how long will it take before everyone can use a compatible PTT method?

Public safety has spent decades trying to make sure that one radio has the capability to talk to any other radio. We especially want radios that interoperate at the lowest possible level, basically radio to radio if that’s what the situation calls for. Multiband and multimode radios are dropping in price, and there are a large number of national interoperability channels in all bands that will allow agencies to easily talk directly with each other as long as the frequencies are programmed into their radios, which many agencies are doing. The interoperability channels are required to be either analog FM or Project 25 (P25), and radio equipment is built to be able to match those requirements. These are older technologies, but they provide consistent communications paths. These tools are solving the issue of voice interoperability and formerly incompatible radios.

Public-safety LMR is not perfect; we still have interoperability challenges. However, this is not so much a technology issue as it is governance and awareness — basically a human issue. But it’s disheartening to see cellular PTT falling into the same technology trap we’ve been fighting in LMR for decades. We hoped the lessons we learned the hard way in LMR would have been taken to heart by the cellular industry as it enters public safety.

Commercial Network PTT
This brings us to commercial network PTT services. As it stands, a PTT user on service A cannot talk to another user on service B, unless they have some sort of extra interoperability device to patch together specific talkgroups. Public safety has been fighting this battle for years with LMR manufacturers. As each releases its newest digital voice format, it adds yet another silo that needs to be addressed. And again this would necessitate extra equipment and complexity to tie systems together so that adjoining agencies can communicate with each other.

The frustration is that although public safety is moving to LTE, a worldwide standard, and the global standard for MCPTT over LTE is complete, we are still told that we cannot talk across networks or PTT services without adding interoperability equipment for specific agencies. Additionally, there could be different cellular MCPTT systems being offered on the same carrier network. Again, we have to ask why we are implementing yet more silos.

Even with all the advances in technology and the adoption of standards, we are no better off with commercial PTT than we are on proprietary LMR systems, as carriers either can’t or won’t support seamless MCPTT communications between networks.

Commercial networks and equipment can do amazing things. Users can make telephone calls, send texts, and even send live streaming video between different devices on different networks, without worrying about who is on what network. These are advanced methods of communicating.

Yet public safety, which uses fairly simple LMR radios to communicate in a simple method, is a thorny problem for the LTE world. Commercial carriers are trying to solve a simple problem by applying ever more complex solutions. The 3GPP MCPTT standards are an example; even with standards, we see incompatible and noninteroperable PTT services.

Whether commercial MCPTT will ever be able to replace LMR systems is not yet clear. It might just be that LMR radio is the best way to do MCPTT; time will tell. In many cases, the newest technology is not necessarily the best technology. In public-safety communications, keeping it simple is usually the best way to go. Commercial aviation, one of the most high-tech industries in the world, still uses two-way-radio to communicate with pilots and control the movements of aircraft. It’s simple and truly interoperable; anyone is capable of talking to anyone else as the situation requires.

In spite of all the talk about interoperability, the cellular industry is struggling with that concept when it comes to MCPTT. Until recently, commercial carriers have not had to worry about interoperable PTT. However, if the industry is serious about MCPTT, we need to figure out how to implement true interoperability and embrace the overall philosophy. We don’t need “silos of excellence;” we need simple, consistent communications. For those in the cellular industry, remember that it’s not interoperable if it works only with your products and only on your network.

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Bill Springer has 38 years of experience in public-safety communications, including radio, telephone and data networks. He can be reached at

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On 4/20/18, Leon van der Linde said:
Why doesn't FirstNet set down rules as they did with the LMR industry? If you want to be in the supply game, your cell phone must look like this, have these icons on the screen, your screen must look like this, and your software and firmware must look like this and operate like this, etc. Then you force the cellphone industry to start making products that look the same and operate the same.
It worked for P25. Maybe you need to do it for MCPTT. If you get only two or three suppliers then that is it. Other manufacturers will wake up and come to the party. They always want to be part of the cake. Just dangle the carrot in front of their nose. You set the standard, and they need to comply.


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