By now you know I believe the term mission-critical Push-To-Talk (PTT) is being misused as a label for what is available over LTE today. This does not mean this will always be the case but in order for those pushing public safety toward “mission-critical PTT over LTE” to better understand the issues, perhaps a course in PTT 101 might be beneficial to all of the LTE community who have not actually been on an incident where push-to-talk communications is what gets the job done. Below I have tried to provide a public safety push-to-talk primer as well as some comments about how PTT can be transitioned to FirstNet over time.
In order to fully understand the requirements of the public safety community when it comes to PTT services, it is important to understand the different types of PTT and the different requirements of each of these. PTT is not simply the ability to talk between devices quickly and easily without having to dial a phone number. Basic PTT functions certainly include that feature but they also include the ability to talk one-to-many, to change who is in the one-to-many or group and, of course, one-to-one communications. In older analog push-to-talk systems using Land Mobile Radio (LMR), it is not possible to segment users into groups as in the world of digital PTT, but many public safety agencies still use analog FM systems. The group function, while not supported with a single radio channel, is supported by using multiple radio channels that are assigned to districts or on an as-needed basis. The use of different analog radio channels provides the same type of user segmentation found in the digital PTT world of P-25 and other digital technologies in use.
There are many different types of push-to-talk uses and each is vitally important in its own right. The first type is for interoperability (see below), the second is dispatch for an incident.
Here the call taker receives a 9-1-1 call, and then the dispatcher (perhaps the same person) determines the type of emergency, the appropriate agency or agencies, and associated units including any supervisory personnel. The dispatcher then selects the correct radio channel on analog systems or the corresponding “talk group” on a digital repeater or digital trunked system, pushes his or her PTT button, and gives a voice dispatch. Sometimes this is preceded by tones that open up receivers, pagers, or other devices that are muted until they receive a tone. Other times dispatches might be preceded by a single tone that indicates a dispatch is about to be made.
When using analog, the dispatch is normally sent to every unit listening on the dispatch channel in addition to units that will be responding. On digital systems, the dispatch is sent to an entire group or groups of personnel in the field. It is vital to the safety of those responding to the incident that others who may not be directly involved are made aware that the dispatch has been put out. If the incident only calls for a single patrol car response, patrol cars surrounding the incident in progress often start making their way closer to the scene in case back-up is needed.
If the incident is a fire, those not involved still need to be aware of the fire and the potential for the incident to require a second, third, or more alarm or that special equipment might need to be called. Officers in the field need to know what is happening in real time to be able to help manage resources and those directly involved need to know which units are responding and when they will arrive at the scene.
Usually, the first unit on the scene, still on the main or dispatch channel, will give a situation report that needs to be heard by the dispatch center, other responding units, and units nearby as well as those in command of the area involved in the incident. It is not unusual to hear a report from the first-in engine company at a fire that, for example, “there is a two-story home fully involved, unknown if anyone is still in the residence, keep all units dispatched to the scene coming and/or send additional equipment.”
Moving to FirstNet Broadband
Dispatch PTT is the second most logical type of PTT that could be included in the FirstNet broadband system. You might notice I said “could be included” as opposed to moved completely to FirstNet. There is, I believe, a transition phase that needs to be implemented. During this phase, the dispatch is sent to existing LMR radios using either the dispatch channel or group selection, and also simultaneously over the FirstNet broadband system using the group designator for the appropriate type of response.
If the LMR and FirstNet networks are tied together using ISSI for P-25, IP bridges, or some other method for analog, and the dispatch console software is changed to make dispatching as easy for the dispatcher as it is today, this transition will also provide conditions to learn what else needs to be included in the way of features and functions for dispatch on FirstNet broadband. It will also enable senior staff to continue to carry only a single device but to be aware of dispatches. Further, this would provide the ability to send additional data and graphic information to the field at the same time the voice dispatch is being made.
Eventually, perhaps after FirstNet’s broadband network has been upgraded to a mission-critical status, all dispatch voice could be moved to FirstNet along with automatic data dispatch to provide additional information. That does not mean ALL push-to-talk functions would be moved over at this point, only the dispatch portion of PTT and only when FirstNet has proven to be mission-critical for alerting and dispatch.
PTT on the Scene
On-the-scene PTT usage is perhaps the most difficult for those not familiar with PTT to understand. This is partially because there are a number of different ways on-the-scene or incident PTT is being used by various departments. Further, it is sometimes used in different ways between law, fire, and even EMS personnel. If you listen to a dispatch on an analog LMR system for fire, for example, you will normally hear the dispatcher provide channel information at the end of the dispatch, something like, “your command channel is Command 12 and your tactical channel is TAC 5.” This type of jargon is confusing to those not involved in dispatch and incidents, but the dispatcher is telling responding units to switch their radios over to Command 12 for wide-area communications and to TAC 5, a tactical (simplex, off-network) channel, for local coordination.
In the world of digital PTT, the changes are usually in group numbers and the tactical channel is either called a “talk-around” channel (meaning it is on the output of a normal voice channel) or still a separate tactical channel. The reason the dispatcher will move responding units off the main or primary dispatch channel is to keep this channel as free of chatter as possible so the dispatcher can be assured he or she can send another dispatch without having to wait for the channel to clear. If and when these functions are ever moved over to FirstNet, there will be other ways to make sure the main dispatch channel remains open for other dispatches while dispatched units can still communicate both across the network and on a local basis.
This entire sequence becomes much more complex for two reasons. First, some departments simply do not switch to the tactical channel on the incident. It is easier to not have to change a channel on a radio. However, it also makes little sense for one unit to transmit from the incident to a radio site miles away and have that site repeat it to a user on the other side of a fire truck, for example. That is one of the primary reasons for tactical communications.
I can tell you from years of experience that the departments that don’t move to tactical channels during incidents receive the most complaints from those in the field that they cannot effectively communicate with other units on the same incident. Think about you and another person being within viewing range of each other but perhaps out of shouting range so you use your cell phone to talk to each other. That conversation is transmitted from one phone back to a cell site and then into the network and then back to the same or perhaps even a different cell site and sent out to your friend’s phone. That is what we do today, but as you can see it becomes counterproductive on the scene of an incident.
The other reason incident communications might not be as easy as some people think to fit into a broadband system is that as an incident grows in size, the need for incident command structure channels grows just as quickly. During a wildland fire, incident command grows very quickly with the addition of logistics, divisions, and more. Each group is usually assigned its own tactical or simplex radio channel so the radio traffic stays within that group and does not interfere with other groups. However, there is usually one central command channel where the head of each group listens and talks back to the incident commander and others. In this type of situation it is not unusual for division commanders to listen to both their tactical and the command channel or to have a second person listening to one channel while they listen to the other. (This is often done using a single radio that scans between the two channels stopping on the one that has voice traffic on it or the one that has been set to priority.)
There are some ways in which LTE broadband could be applied to these scenarios I am sure. However, the idea of being able to truly communicate off-network over the distances required in wildland situations, inside buildings, into sub-basements, and in other places where the network does not provide coverage has not been solved by the LTE community. So far, the solutions I have seen put forward are not based on what is needed in the public safety community but might work inside a mall where teens can share voice and videos with each other. I am not convinced it is even possible to provide an LTE device with the power and flexibility to meet the needs of the public safety community for off-network operation.
To be clear, off-network operation is a requirement even when in range of a network. Thus the unit-to-unit communications systems need to support distinct types of off-network communications:
- Off-network while still in range of the network
- One unit in network coverage and another unit or units outside network coverage
- Both units or multiple units out of network coverage
Add to this that with broadband devices, the network controls the “channel” of the device, with analog FM, the channel is selected by the user, and in digital systems the group selection is made by the user.
I believe the ability to move back and forth between a network and a simplex or tactical conversation must be controllable by the person in the field. And it should be easy. Frankly, the new LMR handheld radios that contain hundreds of channels and/or groups can be confusing to the end-user community. If someone figures out how to make a broadband device that can provide the needed unit-to-unit range, perhaps it will be time to resurrect the older type of radios that had a 16-position rotary switch on top of the radio. Most users did not have to look at the radio, they simply reached down and counted the “clicks” when they changed channels. If we ever reach the goal of a single device, we could go back to it being a choice of on- or off-network and if off-network, which channel to use. This also needs to be completely understood: Being able to move off-network does not mean all off-network units can be on the same channel. In many different types of incidents, different groups are on different off-network channels to ensure that the voices being listened to are those of the people doing the same task. How many different and unique channels is a question that is not easy to answer. In some wildland fires, all available tactical channels are in use and more might be needed.
One of the first uses for PTT over FirstNet has already been demonstrated in the field. That is to tie multiple LMR channels and the FirstNet network together so incoming personnel can use their LMR system to communicate via the connection to FirstNet with those on the incident who may be using different LMR systems. One of the best examples of how this works is the LA-RICs system that was used along with county fire and sheriff’s radios for the past two years during the Rose Parade. The LA-RICS system tied multiple agencies together on the LTE system even though they were using very different LMR systems.
Today, and as FirstNet is rolled out, interoperability is the best and highest use of PTT over FirstNet that I see. The caveat here is that until the network has been hardened, which will take time, interoperability over FirstNet should not be considered as a truly mission-critical system, but in most cases it will work and stay up as long as needed.
The convergence of push-to-talk requirements as they are now being used on land mobile radio systems with the FirstNet LTE network will have to happen over time. First, as I mentioned, providing interoperability between different LMR systems. Then, perhaps, FirstNet will evolve so normal LMR dispatch is also conducted over FirstNet simultaneously, and perhaps in the future only on the FirstNet system, but I see that as a number of years into the future.
In my mind, the primary issue is whether FirstNet’s broadband network and devices will ever be able to truly support the need for off-network one-to-many communications (tactical, simplex, talk-around, peer-to-peer) or if the best solution will be to keep off-network communications on LMR systems and use FirstNet for everything else.
It is not possible to see as far into the technology future as some of these issues demand but at the end of the day it does not matter what various technology laboratories, engineers, or vendors say about which capabilities are ready for prime time and can be accommodated over the FirstNet network. What is more important is when public safety believes and trusts the network with their lives because that is what they do every day with their existing LMR radios. Don’t ask them to give up their lifeline until they believe the next-generation lifeline is as good, if not better.
Andrew M. Seybold
© 2017, Andrew Seybold, Inc.