Public Safety Advocate: Lack of Broadband PTT Interoperability, T-Mobile, Ligado

Recently, there has been a flurry of articles in the press, on Twitter, and on LinkedIn touting the advantages of Push-To-Talk over Broadband (PTT over LTE) and stressing that PTT over broadband is ready for primetime, which appears to be true with a few caveats. It is also true that multiple FirstNet-Certified broadband PTT applications are available, including three that currently interoperate with each other. However, there is no requirement for all PTT solutions to interoperate with one another, therefore it is unlikely we will see complete inter-vendor interoperability anytime soon. In a sense, this lack of inter-vendor interoperability is defeating a basic purpose for FirstNet, which is to provide a fully interoperable Nationwide Public-Safety Broadband Network (NPSBN) to enable any agency to call for assistance from any other agency in the United States and for the responding agency be able to communicate en route to keep in touch with its own Emergency Communications Center (ECC) and with the requesting agency.

The public-safety community knew for years that the lack of Land Mobile Radio (LMR) interoperability was creating agency-to-agency coordination issues and in some cases even resulted in loss of lives and property. Thus, more than 25 years ago, the quest for an NPSBN began. Congress created FirstNet in 2012, but it did not go live until 2018. Today there are a million-plus FirstNet users with more joining weekly. Push-to-talk is still the most heavily used FirstNet feature, and companies are working diligently to more easily and quickly integrate FirstNet PTT with LMR PTT. 

Push-To-Talk on FirstNet

The articles touting PTT over broadband are generally correct in that audio quality is good and PTT access speeds are good. FirstNet’s new PTT offering (FirstNet PTT) is very fast with great audio, and meets the 3GPP Mission-Critical Standard (MCPTT). At the moment, FirstNet PTT is lacking some MCPTT features including the ability to cross-connect with LMR PTT, but this will come and the emergent product should be a robust, functional PTT system. The 3GPP standard for off-network, unit-to-unit communications modes has been met by MCPTT with LTE ProSe. However, anyone who has worked with LMR off-network communications will tell you that today’s version of ProSe does not begin to meet distance-between-units requirements for off-network PTT. ProSe unit-to-unit communications are measured in yards while LMR PTT is measured in miles. Even so, the MCPTT flavor of PTT appears to be well thought-out and implemented. 

If FirstNet PTT were the only PTT available on FirstNet (Built with AT&T), I would have no concerns. However, the list of FirstNet-Certified PTT applications and other non-certified products using FirstNet continues to grow. The 3GPP standard states than in order to be 3GPP-compliant, the PTT system must both be hosted within the network and provide for off-network communications. At present, this would include FirstNet PTT and Kodiak’s (Motorola) Enhanced PTT (EPTT) if its claims to meet the 3GPP requirements prove true. However, last time I looked, Enhanced PTT does not include ProSe or any other off-network solution, thus it does not meet 3GPP MCPTT standards. Note that while both of these PTT solutions are hosted within the network, they are not able to interoperate with each other.

Broadband PTT offerings that are not hosted within the network are called “over-the-top applications.” While over-the-top applications servers do not necessarily reside within the physical FirstNet network, they are compatible across multiple LTE networks. ESChat, the over-the-top vendor with the greatest number of FirstNet customers, was recently awarded a large PTT contract with the Department of Justice (DOJ). Tango Tango provides over-the-top PTT service to mostly smaller departments, as does Orion, and BeOn and L3 Harris have recently begun offering service over FirstNet. FirstNet-Certified-over-the-top PTT applications include ESChat, VIA by JPS, Tango Tango, and Orion. (L3 Harris BeOn is not currently certified.) All these applications interoperate with all LTE wireless networks, and ESChat, VIA, and Tango Tango currently interoperate with each other. FirstNet PTT and Kodiak EPTT are network-integrated PTT applications and are FirstNet-Certified, however, they cannot interoperate with each other or with other LTE PTT network applications. In summary, today it appears there are seven different flavors of PTT running on FirstNet. 

That takes us back to the fact that FirstNet was created to provide a fully-interoperable nationwide broadband platform. With so many PTT vendors competing for FirstNet customers, the PTT portion of FirstNet looks like LMR’s lack of interoperability. The TCCA keeps running “plug-fests” and insisting that many PTT vendors meet the interoperability criteria as set forth, yet none of this interoperability has made its way to FirstNet. The 3GPP needs to resolve the issue of interoperability between “FirstNet-Certified” 3GPP PTT applications the over-the-top PTT applications mentioned above. 

For the record, the 3GPP does not normally pass standards for applications, it passes standards that directly affect LTE and now 5G networks. Perhaps the industry has placed too much emphasis on the first 3GPP standard for an application and now needs to turn its attention to what public safety requires in the field for day-to-day and incident-based push-to-talk services, including interoperability among and between the different flavors of PTT that have been certified by FirstNet (Built with AT&T). 

Where Do We Go from Here?

We now have really good PTT on FirstNet and, in fact, on most LTE networks. We have really good PTT on LMR but in many different portions of the spectrum. The goal should be to figure out how to have all or most of these cross-communicate with the other PTT offerings. One solution would be to simply mandate a single PTT vendor’s product, but that would cause a real problem for many public-safety agencies that have their favorite PTT systems installed and working. Next would be to install two PTT applications on a FirstNet device. I have the Motorola system and ESChat set up on mine. The problem with that is if you do not look at your phone quickly enough you can’t be sure which PTT network is calling. Time is of the essence during an incident and having to figure out which PTT to use delays communications.

That leaves the most obvious choice, which is to require all vendors’ PTT on FirstNet to interoperate with all the other vendors’ PTT— without undue delays in set-up and PTT access time. Several vendors on FirstNet have already figured out how to work with the others so the current interoperability hold-up is not technical. Since vendors are not keen on sharing, it is, if I dare use the word, political. For many years, the P-25 standard was not truly a standard because vendors built to the standard and then piled on nice additional features and functions. While they can say they built to the standard, only their P-25 hardware will work with their system. This is no longer the case today but it took far too long to resolve the issue.

In the case of PTT on FirstNet, we do not have the luxury of time to see how things shake out. We are in the midst of a nationwide pandemic, wildfire and hurricane seasons are upon us, and the mid-west continues to be pelted with tornadoes and floods. During these times, more incidents will become multiple-agency events. As of today, while the FirstNet network is common to many agencies, the flavor of PTT used by a responding agency over the network may not be compatible with the PTT used by the requesting agency. 

In light of the fact that Southern Linc’s own Ericsson Mission-Critical PTT is currently unable to communicate with FirstNet PTT MCPTT or other vendors’ PTT, and Verizon’s Kodiak PTT is not compatible with FirstNet’s Kodiak PTT, it quickly becomes evident we have an interoperability issue on par with what we faced for years in the LMR world. Politics aside, these interoperability issues must to be resolved. 

Perhaps a short-term solution would be to combine FirstNet’s new FirstNet PTT, which meets MCPTT standards, with an over-the-top PTT application. This would enable a greater number of agencies to cross-communicate using PTT on FirstNet. Further, the over-the-top application could be quickly and easily downloaded to be used by any responding agency for the duration of the incident. While this would not provide 100% compatibility, it would provide better interoperability than what FirstNet PTT users have today.

T-Mobile and Ligado

When offered a great deal remember, caveat emptor. T-Mobile seems to have made good on its pre-merger promise of free 5G service to the public-safety community. However, when you compare characteristics you will find many things you benefit from with FirstNet are glaringly missing. First and foremost is full pre-emption and priority. T-Mobile’s answer as published in a FierceWireless Communications article shows how much T-Mobile does not understand about pre-emption and priority. T-Mobile uses “Wireless Priority Service” (WPS) with a separate enrollment process through the Department of Homeland Security (DHS). All first-responder voice lines are eligible for WPS. T-Mobile highly recommends that WPS be added for optimum voice performance. Each first-responder organization must sign up with DHS to have a DHS Point of Contact (POC).

WPS is a long-standing service that provides dial-up priority. However, “WPS priority” is not the same as “pre-emption and priority,” which means priority to the extent of pre-emption from the device, to the cell site, over the network. T-Mobile mentions nothing about a separate core, whether it will throttle data during a major incident or, in fact, if it will provide more than what it calls “priority on the network.” The FierceWireless article states, “According to public safety and FirstNet advocate Andrew Seybold, the Government Accountability Office (GAO) released a ruling in April where the GAO determined T-Mobile could not meet public-safety requirements as established in a Request for Qualification issued by the Court Services and Offender Supervision Agency (an independent federal agency).” It appears T-Mobile does not plan to provide a way to identify members of the first-responder community, while FirstNet has its own PLMN-ID, which is a SIM-card number that identifies the user as a member of the first-responder network. And there is no mention in the T-Mobile announcement about Quality of Service.

I am sure those at T-Mobile who hatched this plan were not around in 2010 to 2012 when T-Mobile and Sprint filed comment after comment with the FCC asserting public safety did not need the 10 MHz of additional spectrum it was seeking for what is now FirstNet. Nor do they remember the website “Connect Public Safety Now” that was sponsored and populated by T-Mobile and Sprint that was anything but friendly to the public-safety community. I guess the folks at T-Mobile would just as soon we would all forget how much time and effort they spent in their attempt to wrestle the D Block away from pubic safety so it could be auctioned without any public-safety special access requirements. 

The public-safety community—not Congress, the FCC, or The FirstNet Authority—determined that the network would be a single, nationwide network under the oversite of The FirstNet Authority. As a result, any issues, omissions, or other items that do not meet the terms of the contract will be handled by only one vendor. There will be no finger-pointing or blaming another network operator for causing a problem and there will be a fast, efficient way to identify and resolve issues. No broadband network vendor other than AT&T showed any interest when the network was put out to bid and no other broadband vendor responded. Now that FirstNet has become a huge success for the public-safety agencies, everyone else wants a piece of the pie they had spit out.  


Several times I have stated that I believe the FCC Commissioners made two very bad decisions in April. The first was to permit the entire 6-GHz critical-communications microwave band to be used for unlicensed WiFi 6 systems. The second, which has the potential to cause havoc to the public-safety community, the military, the FAA, and every person who carries a smartphone, was to permit Ligado Networks (previously LightSquared) to build a “low-power” network adjacent to spectrum used to receive signals from the GPS satellites. 

We fought this battle in 2012 when LightSquared wanted to use the spectrum for a high-power LTE broadband system. That battle was fought and finally won by those who understood the technology used in GPS receivers that listens for a very slight amount of signal and that they can lose the ability to hear those signals if the noise level on the spectrum is raised even a little. I have included a link to one of my “Tell It Like It Is” columns that contains a quote I want to share with you. “What this really says is: We don’t care if we interfere with GPS receivers; it is not our fault but the fault of the receivers, which should not be afforded any protection in any event; this is our spectrum and we want to use it for a purpose for which it was not intended regardless of the outcome. I cannot believe that this document was well received at the FCC, and to me it shows how desperate LightSquared is becoming as it digs a deeper hole for itself, or rather a number of holes.”

In 2012 we had an FCC that listened to its engineers, responded to industry engineers, and even ran interference tests. That FCC concluded there was no way to protect GPS signals from spillover caused by using the LightSquared, now Ligado, spectrum for other than what is was designated. This FCC has approved Ligado’s request and as LightSquared before, Ligado seems to be saying damn the torpedoes, full speed ahead. Our Department of Defense (DOD), the military, and others filed objections to no avail. Now that this has been approved by the FCC, it can only be overturned by Congressional action. Will it be overturned? I’m not so sure, but if we start seeing levels of interference that disrupt our GPS system it will mean no first responders, military agencies, the FAA, or any companies or citizens will be able to rely on the accuracy of GPS locations displayed on their devices. It will probably take this level of interference to convince Congress to intercede and undo the damage that has been done.

Winding Down

By the date states and territories had to decide whether to opt-in to FirstNet or build their own FirstNet-compliant network, all fifty states and all U.S. territories had opted-in. (Note: opting into FirstNet did not obligate all agencies within the state or territory to use FirstNet; local agencies can choose to join or not join FirstNet.) According to an article in MissionCritical Communications in May, FirstNet (Built with AT&T) has launched in three Pacific territories: American Samoa, Guam, and the Commonwealth of Northern Mariana Islands. 

FirstNet is providing portable sites and working with local broadband network operators that are making FirstNet available to these territories. Guam was the first territory to opt in and AT&T has contracted with NTT Docomo to build Guam’s FirstNet-compatible network. Docomo, one of the largest broadband network operators in Japan, provides cell service to Guam and signed on to provide FirstNet services. The new portable cells sites have been delivered to the three Pacific territories because typhon season is approaching and FirstNet wanted to ensure these territories will have portable sites in place when they need them.

Meanwhile, AT&T and the Fraternal Order of Police (FOP) have formed an education partnership to show law enforcement in the field how to get the most benefit from using FirstNet. This educational partnership should provide a good model for other public-safety agencies and their member organizations. It is especially vital as Next-Generation 9-1-1 (NG911) is rolled out that those in the field know how to access all the data, video, and other information the combination of NG911 and FirstNet can provide during and after incidents.

As the public-safety community continues in its efforts to ensure the safety of all those living and working in the United States, we are seeing new ways to deploy assets including several 911 centers providing work-at-home capabilities for some in an effort to respond to incoming calls while keeping those who tend to the calls safe. As we come out of this pandemic, I am sure some of the accommodations being implemented by the public-safety community will be reviewed, assessed, and will become permanent changes in the way public-safety communications and services are delivered.

Meanwhile, it is important for all citizens to understand that the public-safety community is working through the hazards as it continues to protect life and property. All of us need to abide by rules put in place in our states, cities, and counties not only for our own wellbeing but also for the wellbeing of those providing the services we need to survive.  

Until next month…

Andrew M. Seybold
©2020, Andrew Seybold, Inc.


1 Comment on "Public Safety Advocate: Lack of Broadband PTT Interoperability, T-Mobile, Ligado"

  1. Ryan Poltermann | May 28, 2020 at 7:46 pm | Reply

    Hi Andy! Some thoughts, but the short of it is that interoperability between carriers is a better choice for public safety than interoperability between MCPTT and proprietary PTT systems. Now, if they want to directly support MCPTT standards, then by all means!

    “All these applications interoperate with all LTE wireless networks, and ESChat, VIA, and Tango Tango currently interoperate with each other.”

    I can’t speak about VIA, but the reason ESChat and Tango Tango can interoperate is because Tango Tango uses ESChat (as you mentioned back in February). I’d be interested in seeing a full list of who’s interoperable with whom, and what backend they’re using. There are always two halves to these systems, the infrastructure and the application. EFJ portables have had interoperability with the proprietary Motorola SmartZone system, so the question is do we want to take the same steps backward to a proprietary and what could become a de facto standard. There’s already an interoperable standard that exists, and it’s MCPTT.

    As for the ETSI plugtests, the NDAs prevent a lot of folks from being able to say what impact they had one way or another. But in general, the more recent ones have been testing features in 3GPP Releases beyond FirstNet’s current version. As an example, the upcoming plugtest will be testing Release 15.

    This is both by design for the plugtests and a nature of the beast that is a nationwide network the size of the US. Plugtests are meant to catch interoperability issues before they’re deployed. And, the sheer scale of the network means that it has to move more slowly than smaller countries. I almost wonder if that’s part of the reason other countries like South Korea and Canada will be going to multiple carriers.

    With respect to Guam, it should be pointed out that AT&T’s position in terms of carrier interoperability sways with its financial interests. Guam isn’t the only place where a partnership is happening; they’re selling the network in Puerto Rico, too. At least we know the number for that, about two billion dollars.

    We can’t bury our heads in the sand and say that agencies should only use one carrier; there are agencies right now whose needs aren’t met by any carrier. And in a disaster, it won’t matter which PTT app they’re using if their only network is down (ProSe/D2D/Sidelink is another story). Tornadoes, hurricanes, wildfires, any natural disaster doesn’t choose sides or care who owns what cell tower. Somehow we keep losing sight of that.

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