Public Safety Advocate: P25 and MCPTT Feature Comparison

Over the years, the National Public Safety Telecommunications Council (NPSTC) has developed and published a large number of documents to provide Land Mobile Radio (LMR) and broadband network operators, public-safety users, and vendors with information that will assist them in providing features and functions needed by the public-safety community. For the most part, NPSTC committees are structured to enable anyone interested in a specific topic to participate in a committee’s activities and development of final work products.

Once a committee has developed a work product, it is vetted by the executive management and board of directors before being released. If it is deemed to be pertinent to, or affecting FirstNet (the Nationwide Public Safety Broadband Network, NPSBN), it is usually sent to the Public Safety Advisory Committee (PSAC) that is made up of public-safety professionals. The PSAC turns the work over to The FirstNet Authority board of directors, which then reviews and releases the final documents and makes them available to any broadband network within the United States and the rest of the world.

Sometimes these work products help shape development and deployment of a technology, feature, or function. In 2014, well before the FirstNet contract award, NPSTC released a report entitled, “Defining Public Safety Grade Systems and Facilities.” I played a small part in the development of this report in my then-role as vice-chair of the APCO broadband committee (now disbanded). Our APCO committee developed the section of the report that addresses site hardening, security, and access. This and many other portions of this report have served as a reference to both The FirstNet Authority and FirstNet (Built with AT&T).

This week I will discuss another report entitled, “Public Safety Broadband, Push-to-Talk over Long Term Evolution Requirements,” which was published in 2013 during the pursuit of a NPSBN, and another report published in 2018. Pertinent portions of this report were used by the 3GPP standards body as it developed Mission-Critical Push-To-Talk (MCPTT) standards, or as I prefer, “Public-Safety Grade Push-To-Talk (PSGPTT)” standards. 

During a recent conference call, a state organization pointed out that with a handheld or mobile unit over a P25 Trunked system, when users push PTT and then let go, there is a positive response from the network that the unit is within range. Prior to pushing the PTT button, there is an LED alert if the unit is out of range and when pushing the PTT button there is an audible alarm if the user is out of range. . However, when using a new MCPTT device provided by FirstNet, there is no confirmation light or audible beep indicating the unit is in range of the FirstNet network. They told me they have already notified FirstNet, and I asked for and received permission to discuss this issue in an Advocate

I have spent a lot of time looking for a detailed, point-by-point comparison of P25 and 3GPP MCPTT features and functions. I have not found one but I have found many P25 features/functions and a list of 3GPP MCPTT features/functions I could present in a side-by-side comparison. I am sure someone has already done this so I decided to limit this discussion to features and functions that do not seem to be comparable between P25 Trunked PTT and MCPTT over FirstNet (Built with AT&T) or LTE. 

Analog FM

It is interesting that public-safety agencies still using Analog FM (and tehree are a lot more than many people realize) think there is no real indication of a mobile unit being within network range when the PTT button is pushed. However, if the analog system is a repeater or simulcast system, there is usually a “squelch” tail that can be heard once the PTT button is released. Some radio systems use “reverse burst,” which I believe was invented by Motorola, which. essentially eliminates the squelch tail but the squelch tail can still be initiated by the field unit. The theory is to shift the Private Line (PL) or Channel Guard sub-audible tone by 180 degrees after the PTT switch is released. The length of the reverse burst is usually 150 to 200 milliseconds, and since the unit’s receiver has been “slammed shut,” even if there is a squelch tail it won’t be heard. 

However, when in off-network or simplex mode, there is no squelch tail or other indication of whether you are within range of the mobile unit you want to talk to or, in fact, any mobile unit. In most cases, when in off-network or simplex mode, mobile units are fairly close to each other and chatter on the simplex channel serves as a good indication they are within range.

P25 and P25 Trunked 

With P25 systems, dispatchers can use a feature known as “radio check.” With radio check, the dispatcher or Emergency Communications Center (ECC) can determine if the field radio is turned on and if it is in range. The issue is more often from the field to the ECC. When a radio in the field is keyed (PTT button is pressed), users will hear an acknowledgement tone (ACK) if within range, registered on the network, and the selected network or channel is not busy. If the radio is out of range, users will hear a long, low-pitched tone. Some radios, especially mobiles, will also activate a red alert LED when out of range. If the system is busy, users will hear what sounds like a telephone busy signal when the PTT button is pushed.

DMR and other Digital Radios

Digital Mobile Radio (DMR) and other digital systems appear to have the same functions as P25 but since most public-safety agencies are either still on Analog FM or one of two forms of P25, I have not included their specific operation here.

FirstNet MCPTT and Non-MCPTT

I checked with several approved FirstNet push-to-talk vendors and was assured that out-of-range indication capability is indeed alive and well in many of their PTT systems. The technical name for this capability is “Floor Control” and it functions in a number of ways. First, if you are not in range of one of the several PTT systems on the FirstNet network, one set of “you have the floor tones,” usually the reverse of the tone sequence, which tells users they are in range and can go ahead and speak. Another tone sequence is a “revoke,” indicating either you are not authorized to access a channel or there is no channel available. 

When trying to contact a group of PTT users and they are all available and on FirstNet, you will receive a “you have the floor” set of tones. However, if your group is on both FirstNet and P25 and they are bridged with ISSI, things can work a little differently. In this case, if you request to talk to the group, a page is sent out to all FirstNet PTT users and then a connect to ISSI command is sent. If there are no bearer channels available on the P25 system, you will receive a revoke tone for the interconnection between P25 and FirstNet but you will still be able to communicate with the PTT users on FirstNet. 

FirstNet (Built with AT&T) recently announced its first push-to-talk product that meets all 3GPP standards for PTT services. Advocate readers know I believe ProSe, the 3GPP off-network solution, is simply not satisfactory for public safety in its present form. Two people can yell to each other farther than ProSe can talk over LTE off-network. That means MCPTT and, in fact, all approved PTT systems and applications on FirstNet, need not bother with off-network today. What ProSe becomes in a few years is anyone’s guess but for this discussion, it is not in consideration for public safety.

Apparently, FirstNet had explained that the units as sent out do not include an out-of-network tone or LED indicator, but they are the first release of a product that will be upgraded over time and I suspect the out-of-network and revoke functions will be included in the next upgrade. In the meantime, if you are testing FirstNet PTT devices, you need to carefully review the devices and their functionality and then ask your FirstNet contacts to provide a feature and function roadmap for you. 

There are a few points I would like to make again. This is the first time the 3GPP standards body has been involved in the applications space and the the 3GPP is made up mostly of cellular vendors and network operators with very little representation from within the public-safety community. I fully expect that once MCPTT devices and services are put into operation, they will have to be upgraded based on recommendations from the public-safety community. Input from only one agency or public-safety expert in only one area of the United States is not sufficient for vendors or standards bodies to be able to spec and build devices that are acceptable to public-safety users everywhere in the United States or elsewhere. 

FirstNet and PTT Today

Today, there are seven PTT applications running on FirstNet. One is a 3GPP standards-based MCPTT offering (FirstNet PTT) and I hear FirstNet will soon announce a second approved MCPTT vendor. It is not clear whether these will be be interoperable, so we will have to wait to see if the rumors are true. We do know that the current version of FirstNet PTT will be upgraded within the next six months or so to include most of the features and functions we expect, so for now I am considering it a full-blown contender. 

Three over-the-top applications—ESChat, Tango Tango, and VIA provided by JPS—are fully interoperable with each other. Hopefully, there will be a time when all seven are fully interoperable, but for now we have to deal with only three of the seven being capable of talking to each other.

Back on topic, most of the seven PTT solutions do provide out-of-range or out-of-network indicators, thus I assume the FirstNet PTT service will soon be upgraded to provide indicators. The number of “bars” at the top of the screen is only a guesstimate of how much signal is available and it is not very accurate. I have experienced times with no bars and being able to text, and other times with full bars and not being able to make a voice call. As more public-safety people use PTT over FirstNet with some from the P25 world, I believe they will expect the two types of PTT (LMR and LTE) to share common features and functions. 

At this time, I do not believe LMR and LTE PTT can or will be 100-percent alike. For example, when using a FirstNet phone with PTT, with most PTT applications you can call up a map of where other FirstNet users in your group are located. The LMR PTT system knows where users (if GPS in built into the radio) are but the FirstNet side of the bridge does not. The other way around, today, an ECC can shut down a P25 device rendering it useless if it is lost or stolen, but I do not believe any FirstNet PTT device can be shut off in the same way. However, they can be disabled with a call to someone at FirstNet. (ESChat can stun or kill an account disabling the PTT function but not all phone functions.)

While some types of bridges do not permit unit numbers and locations to cross from the FirstNet side to the P25 side, some bridges do. Other options that provide perhaps better and less expensive LMR-to-FirstNet (LTE) bridges are coming. Again, if I could compile a P25 and MCPTT features/function chart, we would be able to see the differences. 

Expecting different vendors’ technologies and their applications to be identical is not practical now or into the future. It took many years to develop a P25 standard, but even today vendors are in the habit of adding a few of their own bells and whistles so other vendors’ P25 devices do not provide identical functionality. With FirstNet, as long as all the users are on a single, common broadband network, all of approved devices will work on the network in the same way. Most applications will be the same, even between iOS and Android devices. We do not yet have the same degree of similarity and interoperability for PTT, CAD, and other applications, but we do have a consistent, nationwide network and we are creeping forward with on-network interoperability. 

I believe whatever PTT is or becomes available on FirstNet (LTE) should be modeled after what the public- safety community is accustomed to on the LMR side of the house. All alert tones, lights, and indicators should be the same moving from an LMR to a FirstNet device and back again. Any differences should not under any circumstances prevent a first responder anywhere in the nation from picking up an LMR or FirstNet device, pushing a button, and being able to talk and be heard!

Winding Down

No, the T-Band giveback has not been repealed yet and the FCC is having to move forward with preparations for auctioning this spectrum. On LinkedIn on Monday, Lisa Fowlkes, Chief of the Public Safety and Homeland Security Bureau at the FCC, posted the following: “Today, the FCC released a Notice of Proposed Rulemaking to meet Congressional deadline of the T-Band, as required under the Spectrum Act of 2012.” Since the FCC Chairman has already asked Congress to repeal the T-Band giveback portion of the law that created FirstNet, I assume the Chief of the Bureau is doing what is required of the FCC, which is to prepare to notify public-safety users (Congress forgot to mention existing business-radio users) that they need to prepare to vacate T-Band spectrum.

I think this is a case of lack of attention from Congress. The Chairman of the FCC, the GAO, the T-Band organization, NPSTC, the International Association of Chiefs of Police, the International Association of Fire Chiefs, and many more have requested that Congress pass the bill to repeal the T-Band giveback. Bills have been sitting in both chambers and I am told the House has included the repeal in its infrastructure bill, but now Congress has taken its summer break. Meanwhile, some of the largest cities in the United States and their suburbs, which include many small, underfunded public-safety agencies, are facing having to vacate spectrum that is vital to them but which, if auctioned, will not result in any bidding. I have explained why in several previous Advocates.

As we all know, communications are critical to the public-safety community. Everything first responders do is based on reliable communications up and down the chain of command. Dispatchers take and route 9-1-1 calls, field units respond and request additional services, after-the-fact incident reports must be submitted, and scenes must be cleaned up. 

We cannot afford to relinquish the T-Band. We are running out of time and Congress must be made to realize we need passage of the T-band repeal quickly. This is an election year and those who serve us in Congress need to hear from YOU soon regardless of your party or affiliation! Public safety has enough to worry about today with the virus pandemic, demonstrations, and in some cases, lack of support and threats to their funding. These eleven major metro areas should not be forced to deal with yet another issue at this time. 

Until next week…

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


4 Comments on "Public Safety Advocate: P25 and MCPTT Feature Comparison"

  1. I’m confused. You seem to have equated floor control with out of range indication. They are not the same thing and should prompt a different response from the user. You don’t have the floor, however indicated, means someone else is talking or about to talk to your talk group, you should hear them, when they are finished you will get another chance to talk. Out of range means you have no coms, so do whatever your SoP is for loss of coms.

  2. Rex, thanks and you are not the one confused, but it is I! I stand corrected and will adjust the article to reflect and correct my errors. Thanks

  3. James Stefano, Jr. W2COP | July 10, 2020 at 6:19 pm | Reply

    GREAT article! So it appears we have another case of the cart before the horse with the proliferation of some great PTT apps but no Interoperability across the board. Exactly what the NPSBN was trying prevent. MCxxx 3GPP is “okay” but will lack many feature-sets OTT apps provide and won’t be able to provide, and doesn’t work across disparate network topologies.

    Two key things have to happen which NPSTC wants and needs to tackle, a national identification schema for agency, device and user IDs, and a creation of nationwide common talk groups (most likely broken out at the county level) so that all these different platforms/agencies can communicate with one another. Not to mention the encryption and authentication that needs to go along with that. When you compare the speed of apps and features being rolled out to the burden being placed at the federal level come up with all these standards, the latter could seem like forever to happen. Nevertheless, it HAS to happen to some extent.

    My feeling is that all stakeholders including vendors should be in this aggressively. However everyone seems busy these days and good talent in this area with public safety experience is hard to come by. We need input from IT experts at the onset to lay the foundations early on and get this right. We will need funding at the federal level for these initiatives and on-going costs to cover cloud server hosting (and redundancy) for the common talk groups … along with constant network monitoring to ensure 24/7 availability.

    We did it with NCIC and NLETS decades ago, it can be done again. It’s all pure IP and data when it comes down to it. Cloud hosting is cheaper than ever these days. I would have no qualms about it being bid out with metrics and deadlines like the FirstNet financial model to get it done, and quickly. However we need the standards first. We may have to settle for a framework and let the final vendor IT experts work out the details, but we should have started years ago well knowing what was coming. Too late to pull back the reigns now. Maybe FirstNet Phase II? Lets ensure the Interoperability portion remains an open network so all interested agencies can hop on board at minimal cost for the connectivity and all vendors can share a piece of the pie. I think there is something similar at a larger scale, it’s called “The Internet”, there are no boundaries and it really works!

  4. James-thanks for the kudos and the comments. I agree, with all of what you said, it has been done before and it needs to be done in this case. AT&T won the contract to build and manage the FirstNet network but they are really not responsible for how and what data flows over it, rather that is more the purview of the public safety community. What ever goes over the networks needs to be vetted by FirstNet for security and compatibility issues but it remains the job of public safety to get data flows coordinated and interoperable.

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