US public safety LTE networks

The USA is undertaking a major project to build a nationwide public safety broadband network using a commercial provider, but it faces some unique challenges in realising its ambitions, as James Atkinson reports.

The USA is at the forefront of nations pioneering the migration of public safety agencies from narrowband land mobile radio (LMR) wireless communication platforms to broadband LTE systems.

In 2012, Congress passed legislation to build the first-ever Nationwide Public Safety Broadband Network (NPSBN) and set up the independent First Responder Network Authority (FirstNet) to oversee its implementation and operation, along with granting $7bn in funding.

Mobile carrier AT&T was awarded a 25-year, $6.5bn contract to build and maintain the public safety network in March 2017 and was granted 2 x 10MHz of 700MHz (Band 14) spectrum for both public safety and non-public safety use to complement its existing 4G LTE spectrum holdings. AT&T is providing FirstNet access to its infrastructure and is spending $40bn to maintain and improve its network.

Unlike its fellow mission-critical (MC) LTE public safety network pioneers in the UK and South Korea, the USA faces some unique challenges, not least of which is its sheer size. Implementing the ubiquitous coverage that first-responders require across all its vast states and territories is a tall order and an expensive one compared with the geographically much smaller UK and South Korea.

And then there are the high numbers of first-responders that need to be migrated. There are upwards of 60,000 public safety agencies in the USA and more than 10,000 LMR networks. Rough figures estimate there are between 750,000 and 850,000 sworn police officers, 826,000 licensed and credentialed Emergency Medical Services (EMS) professionals and around 1,216,600 career and volunteer firefighters.

The other catch, and it is a very significant one, is that although all 56 states and territories have allowed FirstNet/AT&T to deploy the network in their states, there is no requirement for state and local public safety agencies to use the network. AT&T must attract users to the network to ensure it is self-sustaining, as required under the legislation. But other mobile carriers, Verizon in particular, are offering similar services, which may affect FirstNet/ AT&T’s enrolment efforts.

Other factors affect enrolment. Some public safety agencies have expressed reluctance to join FirstNet, citing uncertainties with the resiliency, reliability and security of the network, coverage and cost. Others want to wait until they can access the mission-critical voice services they are used to on LMR networks.


Ryan Poltermann, innovation architect at Commdex and vicechair, LMR-LTE Integration and Interoperability Working Group of the National Public Safety Telecommunications Council (NPSTC) – an organisation of 15 public safety organisations, along with federal, state and local government representatives – believes that coverage is the main influencer of choice.

“In the US, coverage remains the factor for selecting a carrier. This coverage issue may preclude one or more carriers from even being a choice to public agencies, and it is worth mentioning there are biases that would prevent switching even if the coverage were equivalent or better,” says Poltermann.

AT&T currently offers an Enhanced PTT service from Motorola Solutions’ Kodiak Networks and says it will provide a 3GPPcompliant mission-critical push-totalk (MCPTT) service in early 2020. It has also selected a second MCPTT provider, but has not yet revealed who this is.

“This is a standards-compliant, mission-centric solution that’s being purpose built for public safety,” says an AT&T spokesperson. “It’s designed to further advance first-responders’ communication capabilities with reliable, highperformance calling. We’ll have more to share in the coming weeks and months.”

AT&T’s main rival Verizon has not taken this lying down and is busy competing for public safety agency subscriptions by also offering priority and pre-emption voice and data services. It also offers Motorola’s Kodiak Networks carrier-integrated PTT service, as well as a variety of over-the-top (OTT) PTT service providers such as ESChat. Sprint also offers Kodiak.

T-Mobile has said that if its proposed merger with Sprint is finally approved, it will offer free 5G services (once it has rolled out a 5G network) to first-responders through its Connecting Heroes Initiative. It has not stated whether priority and preemption services will be available or if it intends to offer a 3GPP-compliant MCPTT service.

This provides a scenario where four (or three) carriers are, or will be, offering MCPTT, carrier-integrated PTT and OTT PTT services. Plenty of choice for public safety agencies, then. Some see this multiplicity of choice as a good thing as it ensures competition; others think it dilutes the FirstNet ideal of a single, nationwide mission-critical network for all first-responders.

“I believe we should expect multiple vendors providing Release 13-compliant MCPTT solutions in 2020 in the US,” says TJ Kennedy, former president of FirstNet and now one of the co-founders and principals of The Public Safety Network consultancy. “I am excited to see a competitive marketplace.”

However, Poltermann worries that this multiplicity of choice may have consequences for the public safety community. “We face distinct challenges that other nations won’t have. Because of the lack of an integrated approach, the agencies are free to choose whichever communications method they wish. This causes issues not only for interoperability, but also from a collective bargaining standpoint. This lack of collective bargaining means that the features public safety desires may not be received.”

Certainly, FirstNet/AT&T will need to offer a more attractive service if it is to succeed in its ambitions and stave off competition from other rivals. A FirstNet spokesperson says: “FirstNet is driving innovation in the public safety broadband marketplace in the United States. The network is driving competition and choice, and delivering dedicated public safety services like pre-emption that did not exist before FirstNet.

“We worked hand-in-hand with the public safety community in all 56 states and US territories to understand their coverage and capacity needs for the network. This is unique to FirstNet.” And as the spokesperson points out, it is still early days. FirstNet is just “two years into a five-year deployment based on individualised state buildout plans, and AT&T continues to be ahead of schedule”.

Kennedy says: “The speed at which this has been delivered across the country has exceeded expectations. It is an opportunity to leverage open standard solutions to create true operability for all public safety communications by leveraging MCPTT/Data/Video.”


Interoperability, or operability, as Kennedy prefers to call it, is critical. FirstNet was largely conceived because of the interoperability issues faced by public safety agencies responding to the 9/11 attacks when their different LMR networks were unable to communicate with each other.

But what does interoperability mean here? Does it mean the MCPTT, carrier-integrated PTT and OTT PTT options available on AT&T should be interoperable with each other? Or that the MCPTT and OTT PTT services offered by other carriers should be interoperable with AT&T’s FirstNet service: ie, have access to the FirstNet secure core?

This is what lies at the heart of the Boulder Regional Emergency Telephone Service Authority (BRETSA) petition to the FCC, which has attracted support from other jurisdictions, as well as Verizon among others. BRETSA wants the FCC to make a declaratory ruling that FirstNet/AT&T should provide access to the FirstNet core to ensure operability between all firstresponders who have chosen another provider and want the freedom to continue to do so. FirstNet and AT&T oppose this and want the petitions dismissed.

Kennedy observes that in the FirstNet model, the RFP required multiple mission-critical PTT solutions to be made available on FirstNet and to ensure there was full operability across those solutions. “This is an important element which means that the MCPTT solutions running on FirstNet will be tested and operable with each other. This also provides competition for MCPTT services on FirstNet and will drive innovation and competitive pricing.” If this is the case then that will ensure operability between PTT solutions on FirstNet, but it does not ensure operability between carriers.

Below: If public safety agencies use other mobile operators besides FirstNet/ AT&T then it will be necessary to ensure interoperability between carriers to allow different agencies to communicate with each other at major incidents – the issue is largely political and commercial, rather than technical. Adobe Stock/csfotoimages.

“It is in the interest of public safety to have interoperability,” says Poltermann. “While I am of the mind that having all users on one network makes things a lot easier and safer, the reality is that there’s multiple networks. Multiple carriers with interoperability using open standards and allowing for flexibility in UE and applications is best for US public safety.” Hence he wrote in support of BRETSA. But he is concerned that the cheaper OTT PTT clients are a significant threat to the take-up of MCPTT.

“There’s a lot of unknowns about implementation,” he says. “We currently face interoperability issues on the current carrier PTT platforms based on the ‘customer’. If one agency pays for a device and another one pays for the other, they can’t talk to each other through PTT. We’re worried about it rippling into MCPTT.”

Kennedy is more optimistic and thinks the superior service MCPTT will offer will ensure a good take-up. “I believe MCPTT will be successful since it will meet all the functional demands being met by P25 today for most departments. Open standard solutions are important.”

Ken Rehbehn, directing analyst, critical communications at Omdia (IHS Markit), argues that data services on LTE have some important differences that make the interoperability question less pressing. “First, by moving to LTE we automatically gain an interoperable environment thanks to the 3GPP protocol set. The degree of connectedness then becomes a political, not a technical, question.

“More fundamentally, unlike past voice systems, the data system of 3GPP underpins access to the cloud, which is by its nature completely interoperable thanks to standardised web interfaces. This diminishes the argument about interoperability, but it does not eliminate it entirely. If the cloud is isolated in a walled-off portion of the internet, interoperability becomes a question of cloud access instead of radio access.

“The FirstNet core, for example, is largely isolated from the broader internet. A PTT voice solution hosted on FirstNet core will, as a result, not be interoperable to users without access to the FirstNet core. So, the problem is mainly political, but there is still a potential trace of the technical interoperability issue,” says Rehbehn.

MC LTE at scale

Where Rehbehn sees a potential problem in the future is with the transition to MCPTT over LTE at scale, as this will require LTE network broadcast capability – 3GPP Evolved Multimedia Broadcast Multicast Services (eMBMS).

“We need eMBMS to enable very large talk group communities. Broadcast eliminates thousands of redundant voice packets arriving at handsets at different times. Without eMBMS, we cannot achieve MCPTT scale. The issue there is that broadcast mechanisms are poorly defined for operation across network boundaries,” he says.

The danger is each network becomes an island and agencies on FirstNet for MCPTT and others on Verizon or T-Mobile are not able to communicate during a major incident. “That brings us back to a very bad starting point, and prevents wholesale fleet migration to MCPTT,” says Rehbehn. “In my opinion, the missing eMBMS features will ultimately force all public safety agencies in the US to move to FirstNet or remain on classical Project 25 systems forever.”

This brings us to the crux of the matter behind the BRETSA petitions, because as Rehbehn points out: “For FirstNet, there is a commercial imperative to not fix this eMBMS limitation. This issue is not a technical problem, it is a commercial one. AT&T is a business and it potentially has a captive audience, so why would it risk losing them to a competitor by opening up [a] hole in the wall allowing other carriers with first-responder subscribers to access its FirstNet core and services?”

Direct mode

A further challenge to the migration to MC LTE services, and one that faces every country looking to follow suit, is the lack of an adequate direct mode, radio-to-radio solution in 3GPP – Proximity Services (ProSe) or Sidelink, as 3GPP calls it.

As Poltermann points out, the coverage between a P25 site and a cellular site is distinctly different (particularly P25 on VHF). “The overall coverage issue also comes into play with ProSe/Sidelink, because there are large areas of the country with inadequate coverage. We’re a bit concerned about ProSe, and we’ll have to see how it plays out.”

Rehbehn goes further, saying: “The lack of a viable direct mode/ ProSe solution for MC LTE stands as the biggest barrier to wholesale fleet migration to MCPTT.” While the current 3GPP ProSe might be adequate for outdoor direct mode, Rehbehn is doubtful it will be powerful enough to provide indoorto-outdoor connectivity. In addition, ProSe has had little support from silicon vendors, with only Samsung saying it will implement it.

Alternatives involve either pairing LTE devices with some form of other device to provide direct mode services, or integrating two radios into a single device. Rehbehn says: “L3 Harris has made some interesting developments here, combining LMR and LTE in cost-effective general purpose radios for both vehicles and handheld devices. Motorola has the APX NEXT P25 Smart Radio.”

Poltermann says this makes carrier support for interworking between LMR and LTE extremely important. 3GPP completed a lot of work on the interworking function (IWF) in Release 16, but further work is ongoing in Release 17. In addition, ATIS/TIA in the US are still working on how to implement it in P25 systems, so a standards-based IWF is still a little way off. “We’ll have to see just how many vendors implement it,” says Poltermann. “The ripple-down requirements means that it will take some time to appear.”

Public safety agency engagement

One disadvantage AT&T faces is that it is subject to oversight and public scrutiny in a way that Verizon, T-Mobile and Sprint are not. It also has the weight of the public safety community’s expectations to contend with, and it seems this might need some attention.

Poltermann observes: “FirstNet’s strategy is not particularly clear. While a roadmap has been published, the roadmaps don’t provide timelines or even solid goals. To be a little harsh, FirstNet communication throughout the whole process has been lacking and we all hope that this will change soon.”

Rehbehn points out that FirstNet is locked into a very long-term contract with AT&T with confidential terms and conditions. “Unfortunately, the opaqueness of the contract between FirstNet and AT&T does not help agencies gain confidence. We do not know what the KPIs are or how AT&T is stacking up.”

A Government Accountability Office (GAO) report published in January 2020 tends to agree. While GAO found that AT&T was on track or ahead of its milestones, it suggested that FirstNet needed to tighten up its oversight and be more transparent. It also noted that public safety officials “were dissatisfied with the level or quality of information received from FirstNet” as regards AT&T’s progress or FirstNet’s oversight. GAO also thought FirstNet could do more to try and gauge end-user satisfaction.

FirstNet is not contractually obliged to share such information, but GAO pointed out that key practices call for communicating appropriate information to relevant stakeholders and reporting on monitoring results. Sharing more information about the oversight FirstNet conducts could improve public safety stakeholder sentiment for and support of the programme, it stated.

In response to this, the FirstNet spokesperson says: “The FirstNet Authority has accepted the GAO’s recommendations to further enhance the FirstNet Authority’s contract oversight and stakeholder outreach processes and is currently working to implement them as part of our public safety engagement programme.”

Next steps

Looking ahead at how things might develop, Kennedy says: “I predict most public safety agencies in the US will start to transition to MCPTT in 2020 and most others will add it no later than 2021. I see MCData and MCVideo naturally following on next. It will be the de facto standard for public safety to communicate across LTE.

“There will be significant focus on situational awareness tools and integration as well as user experience in 2020. You will also see MCPTT and more integration with other PTT systems in 2020.”

However, other agencies are still purchasing P25 systems, and as Poltermann points out, they are expensive. “This amount of investment means there’s an expectation of using the devices as long as possible. There will be a transition period of course, but for the US it could be a decade or two.”

Nonetheless, as the AT&T spokesperson says: “FirstNet represents an unprecedented public-private investment in infrastructure that makes America a leader and public safety a national priority.”

This is undoubtedly true, but if the political and commercial realities of the US market mean public safety agencies continue to use different carriers for MCPTT then a solution to the current interoperability stand-off between FirstNet/AT&T and other providers will need to be resolved.