“The Case for Commercial PTT Apps in Public Safety” – MissionCritical Communications

August 14th, 2017

Posted In: Industry Insights

The Case for Commercial PTT Apps in Public Safety

Originally published in MissionCritical Communications, August 2017

By Todd Johnson and Nick Falgiatore


First responders long have depended on hardened land mobile radio (LMR) systems—featuring public safety-grade radios and infrastructure—to provide their mission-critical communications needs.  These networks and devices are entirely different from those offered by commercial providers, as the latter are not designed to stand up to the rigors of first responder use. Moreover, while public safety networks are engineered to provide sufficient capacity during events that require a multijurisdictional response, commercial networks often are overloaded during a major incident, emergency or otherwise.

One thing that public safety and commercial networks have in common is push-to-talk (PTT) functionality.  For decades, PTT in the public safety sector was provided exclusively by agency-owned LMR systems. However, commercial PTT services have emerged more recently in the public sector as a whole—public safety, public works, public transportation and public utilities—driven by the following factors:

  • Commercial PTT technology has evolved dramatically, due to the evolution of cellular networks, the emergence of smartphones and new PTT-enabling technology, and the advancement of quality of service (QOS) for Voice over Internet Protocol (VoIP) services.
  • Given the high cost of public safety-grade radios—a Project 25-compliant portable digital radio can cost up to $5,000 per unit, compared with a smartphone that sells for about $800—there has been an increased interest in leveraging commercial PTT technologies for non-mission critical users, such as those in public works, public transportation and public utilities, but also local government officials, school personnel, animal control agents, and more.
  • Even the first responder community is leveraging commercial PTT to supplement their public safety-grade radios, usually by command staff who do not have day-to-day need for an LMR device, but also by law enforcement, fire and emergency medical personnel who have to venture into areas within the jurisdiction that do not receive adequate LMR coverage, but do receive reliable commercial coverage.

Consequently, the line between public safety and commercial PTT is becoming increasingly blurry.

The Evolution of Commercial PTT

The first commercial PTT service in the United States was launched in 1996 by the former Nextel, which was acquired by Sprint in 2005. The service leveraged Integrated Digital Enhanced Network (iDEN) technology unveiled by Motorola two years earlier. The platform—which leveraged Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) technology, as well as speech compression—was unique in that it enabled both one-to-one and one-to-many transmissions, the latter aspect being analogous to the traditional walkie-talkie. Nextel’s service originally was targeted to companies that were engaged heavily in dispatch operations, such as taxi services and commercial trucking, as a less-expensive alternative to traditional LMR-based paging networks. Later, Nextel moved into the consumer space, marketing the service to those who desired both cellular voice and paging.

After the acquisition, Sprint phased out Nextel’s iDEN-based service starting in 2010 and ending in 2013, in favor of its own PTT offering that operated on the carrier’s Code Division Multiple Access (CDMA) platform. Meanwhile, several other commercial PTT services emerged, notably from the two largest cellular carriers, Verizon and AT&T, but also from smaller regional entities, such as SouthernLinc, which still operates on the iDEN platform. Among the four major national carriers, only T-Mobile currently does not offer commercial PTT service.

While commercial PTT service has been attractive to public works, utilities, transportation, schools, et al, use cases also have been identified that directly benefit first responders.  Today, cellular coverage is equal to or better than that provided by many public safety LMR systems.  Due to the explosive growth that has occurred in many communities, the radio frequencies used by LMR systems often are very congested and in some cases overloaded. The result is that many LMR systems—even those installed as recently as a decade ago—are unable to provide the rural and in-building coverage that is provided by most of today’s cellular networks, which is a problem for first responders. In addition, public safety radio systems are designed to operate only within an agency’s jurisdiction, leaving responders unable to maintain communications when responding to events outside the jurisdictional footprint.

The emergence of PTT applications that integrate with traditional LMR systems is helping to address these shortcomings. By augmenting their LMR devices with a PTT application on their smartphones, first responders can maintain communications with their agency’s radio system when their agency-issued radio cannot.

Another use case involves command-level personnel, e.g., police and fire chiefs, who do not need to have a radio issued to them because they do not engage in emergency response on day-to-day basis, but do need to communicate with incident commanders when a major event occurs. Here too the PTT application that interfaces with the agency’s LMR system is quite useful. This also applies to personnel who might be responding from outside the jurisdiction—creating instant interoperability—and government officials, such as the mayor, who need to be kept abreast of developments.

Clearly, public safety agencies are not abandoning their LMR systems in favor of commercial PTT offerings. But just as clearly, such offerings are serving as an effective adjunct to first responder networks. And because PTT today simply is just another data application, and because the vast majority of first responders use their smartphones while on the job, the marriage between commercial PTT and the public safety sector is a natural fit.

Public Safety PTT Apps

There are essentially two classes of PTT apps that integrate with LMR systems: those that are available from traditional LMR vendors and those that are available from third-party providers. The apps offered by Motorola Solutions and Harris Corporation fall into the former category.

Motorola’s WAVE and Harris’s BeOn apps both enable public safety agencies to extend LMR talkgroup traffic to user-provided smartphones operating on commercial data or Wi-Fi networks. While there are some fundamental architectural differences concerning how each vendor provides their PTT service, the functionality is similar in that both applications allow the radio system to pass along emergency identifications (IDs), PTT IDs, and alias information.  This is appealing because it allows users of the PTT application to access the same information and emergency features as primary public safety radio system users.

The primary limitation of these solutions is that they are proprietary and typically sold to buyers of large and expensive Project 25 (P25) digital trunked radio systems, leaving smaller agencies without a viable alternative.  While these solutions can be scaled to interface with smaller conventional systems, the cost usually is cost prohibitive.

Fortunately, the latter class of apps exists, featuring offerings from commercial wireless carriers such as AT&T, Verizon, and Sprint, and third-party vendors such as ESChat. These vendors all have unique service offerings and capabilities, but fundamentally provide the same core solution: a PTT system integrated with the agency’s LMR channels.  Further, these solutions are not proprietary to the radio system vendor—which enables competition within the procurement process—and have price points better suited to smaller systems.  The market for these applications continues to evolve, with Motorola’s purchase of Kodiak – the vendor responsible for developing the PTT applications utilized by AT&T and Verizon – being the most recent development.

Regardless of their source, all PTT applications need a way to interface with the agency’s public safety radio system in order to exchange two-way traffic between the user’s smartphone or tablet device and the LMR system.  Excluding Harris BeON application, which is integrated with the vendor’s P25 controller, the two primary integration modes are via an Inter-RF Subsystem Interface (ISSI)—which is a component of the P25 standards suite—or a conventional interface.

The ISSI originally was developed to integrate two P25 radio systems.  The PTT application vendors have leveraged this interface as a means of passing radio traffic between a P25 system and a PTT service by emulating the signaling provided by a P25 system to make the system think it is communicating with another Project 25 system.  The primary benefit of an ISSI interface is that a large number of talkgroups can be passed from the system to the PTT service without additional equipment required for each talkgroup.  In addition, an ISSI supports additional features—such as unit IDs, emergency alarms, and encryption—that are not available through other interfaces.  There are two primary limitations of ISSI interfaces. One is that such interfaces only are available for trunked P25 systems. Another is that list pricing from the radio vendors typically is quite high.  Consequently, agencies buying a new 25 system should be sure to secure ISSI functionality upfront, when greater discounts and purchasing incentives are available.

The alternative to an ISSI interface is a conventional interface, which typically requires a donor mobile radio to pull the audio from one radio channel.  The PTT vendor then interfaces with the backplane of the radio, typically through a 4-wire audio interface, although the PTT vendor can utilize any data available through the ports on the back of the radio.  Conventional interfaces are well suited for agencies that only may use a few channels, or only may need a few channels interfaced to their PTT service.  However, conventional interfaces do not allow each end-user device to have a unique ID or utilize other features, such as emergency alerts.


It is likely that the day will come when public safety communications will rely on a PTT application riding on a broadband network—think the Nationwide Public Safety Broadband Network (NPSBN) being implemented under the auspices of the First Responder Network Authority (FirstNet)—but until that time arrives, there are many benefits that can be realized by interfacing an existing public safety LMR system with a commercial PTT service.  The following key takeaways should be considered:

  • PTT service may extend the coverage for public safety users and/or eliminate the need for a radio altogether for command-level personnel.
  • Regardless of radio system manufacturer or system size, commercial PTT solutions are available from a variety of vendors. Consider publishing a request for proposals (RFP) to solicit for PTT service, in order to ensure the best pricing and the ability evaluate different service offerings.
  • When investing in a commercial PTT service, agencies should make sure that they understand the interface being offered, and the pros and cons of that interface.
  • Agencies planning to purchase a new P25 system should secure an ISSI upfront as part of the package, to ensure the best pricing and the ability interface to a commercial PTT solution in the future.
  • Ensure that whatever PTT solution you implement has a long-term plan for migration to mission-critical PTT, the 3GPP standard currently under development and scheduled for implementation next year, which will provide a common interface for PTT applications.

Todd Johnson and Nick Falgiatore are senior technology consultants at Mission Critical Partners, Inc., a public safety communications consulting firm headquartered in Port Matilda, Pennsylvania. They can be emailed at toddjohnson@mcp911missioncriticalpartners.com and nickfalgiatore@missioncriticalpartners.com, respectively.


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