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Understanding PTToC and What It Means for Critical Comms

Just a decade ago, LMR was the only standard for critical communications. Communications technology has brought about new advancements as organizations around the world seek to understand how these will influence their communications. Tait Americas President (Solutions Marketing) Bruce Mazza explores what Push-to-Talk over Cellular (PTToC) means for mission critical organizations today and in the future.

by Bruce Mazza / May 22, 2017

PTToC allows users to replicate radio features like instant group communication with a simple button push. This smartphone application presents an opportunity for staff without radios to access communication features that radio provides, from their smartphones.

WiFi networks are pervasive now, and offer excellent broadband coverage in many places. As long as a PTT user is connected to the internet – through a cellular network, WiFi or another bearer – they can communicate through the PTT app on their smartphone.


Mission critical systems must guarantee a certain level of service, which cannot be guaranteed on cellular networks today. In particular, availability and latency cause problems for mission critical users.

  • Public carriers are more prone to failure during large emergencies, which can clog public networks, even take them down. However, mission critical networks are built for availability, and even if the network fails, users can still communicate through simplex mode. That type of redundancy simply does not exist with PTToC.
  • On LMR, the time between pressing the button and when a call is received is typically 200 milliseconds. On a public cellular network, mission critical users don’t have control of that timing. Too many people active on the network can cause unacceptable delays. But while PTToC cannot be mission critical today, it can still offer benefits for mission critical organizations.
  • Mission critical systems provide dedicated coverage and capacity, with priority and emergency calling features which cannot be replicated on cellular networks today.



Even mission critical organizations have a percentage of staff who do not require critical communications. Often, the additional cost to connect them to the network can’t be justified.

Public transport is a good example. Many staff on a train don’t have push-to-talk device radios today, but they do carry mobile devices for mobile ticketing or other data tasks. If there is a safety issue, a security incident or emergency, a push-to-talk application on that device means a ticketing agent can alert dispatch or the conductor through the group communication network.


Another big advantage of PTToC is the IP connection. As long as your smartphone is connected to the internet, you can connect to your critical network from anywhere in the world. Imagine you’re a police chief who’s off-duty on a golf course in Hawaii, and you learn about a major event in your home town. Through the IP connection, you can instantly join your organization’s communication, ask the right questions and talk to your people, or just listen in to make sure the situation is under control.


Cellular and WiFi signals will work where private radio data LMR system signals may not. In large buildings like malls or hospitals, and underground locations like utilities substations, WiFi gives your people an extra layer of coverage that helps fill black spots.


Integrating mission critical radio with PTToC, with solutions like UnifyVoice, can create a “network of networks” for the best of both worlds. Several techniques are already in use.

Increasingly, simple analog gateway devices are integrating a PTToC domain into a land mobile radio domain, so that audio can pass between push-to-talk app users and users on the radio system.

That audio gateway can also be transformed with protocols such as ISSI for P25 and AIS for DMR. These intelligent IP based software gateways pass both audio and information like the subscriber unit ID or emergency call-out signaling from unit to unit.

The next step will truly unify the push-to-talk and the land mobile radio for the end user. If a user pushes the PTT button on the radio or speaker mic, the device will select whether the signal will transmit over LMR or broadband. This decision is automatic, depending on the strongest available signal. This will be the ultimate integration technique because the user doesn’t need to decide between devices. They can use a traditional device and gain access to the “network of networks” for the best coverage possible.


The standards body that manages the mission critical PTToC is the 3rd Generation Partnership (3GPP). Their primary goal is to replicate LMR features in the cellular world. Through the standards, a number of enhancements will be made for PTToC. These include:

  • There won’t be a one-size-fits-all mobile interface client, because specific interfaces will be mapped to people’s roles. You’ll have one interface for a security guard, another for a policeman, and another for an undercover detective.
  • A radio can operate in direct mode, where one radio user can talk to another without any base station or network. Today that feature does not exist in mobile phones because they need to talk to a tower. But this upcoming standard will let you communicate mobile phone-to mobile phone, if you’re within range — an important feature for firefighters on a fire ground.
  • Priority calling will give specific frontline workers higher priority on public networks than regular public network users.

These Mission Critical PTT (MCPTT) features were ratified by 3GPP in March 2016 and will be in place in cellular network equipment as a software module within a couple of years, although realistically, it will be longer before they have an impact.

And the new standards still won’t solve some other inherent disadvantages of public cellular networks, such as the lack of backup generation for 4G towers and the radio base stations. So if the power goes down, so does the cellular communication. Typical LMR towers, however, have anywhere from two to five days of backup power generation at each site.

So even though it is not yet mission critical, there are plenty of good reasons to invest in PTToC. By unifying critical communication networks, you can give your users choice and advanced coverage while they work with the reality of multiple networks. PTToC brings added redundancy and a simpler interface that will improve their safety, efficiency, and user satisfaction.

Learn about UnifyVoice, the Tait solution that provides operational and administrative staff with enhanced access to instant communication and information via PTT capability.


This article is taken from Connection magazine. Read the article, and others like it, here.

For more thought-provoking articles from industry experts on the current trends in critical communications, read Connection issue 8, out now!

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