Public Safety Advocate: Public Safety Networks LMR and FirstNet Working Together

There is still a lot of confusion out in Public Safety and elected official land about the future of Land Mobile Radio (LMR) as FirstNet is deployed, and there is still the issue of the eleven major metro areas slated to lose the T-Band and their LMR systems if we cannot convince Congress to make some changes to the law. I was happy to see that during the recent webinar held by Mission Critical Communications, Chester County, PA speakers were questioned as to whether LMR will be replaced by FirstNet, they responded with a resounding NO! (Chester County was the first countywide fire dispatch system I designed and installed in the early 1970s.) However, it seems there is still an issue of spreading the word, not so much to the public safety community but to IT departments and elected and appointed officials responsible for the budgets, both capex and opex, that fund LMR systems.

It is no wonder some people are confused since we keep seeing comments, press releases, and experts talking about how soon Mission Critical Push-To-Talk (MCPTT) is coming to FirstNet, and how some still believe the off-network 3GPP standard known as ProSe will be a factor. However, even the developers of the public safety system in the United Kingdom have realized they won’t be able to provide reliable off-network PTT over LTE. For that reason, they are planning to use Tetra radios with simplex or talk-around. The more the experts talk about the technology issues with Mission Critical PTT, the more they seem to be losing sight of the fact that the technology over the network is only part of the issue. If the network itself is not mission-critical, PTT over the network cannot be mission-critical, even if it is called “Mission Critical PTT” (MCPPT).

It is, I believe, vitally important that those in government who make decisions about funding LMR radio systems, upgrading them, keeping them running and operational understand the dilemma the standards body, labs, and pro-MCPTT folks have created for the public safety community. There have been instances where public safety officials are presenting their next year’s budget and they are questioned by the budget committee about why public safety still needs to invest in LMR technology. The public safety officials usually make a good case for why, but one or more of the budget committee members have heard from technology experts that MCPTT will be real and will be rolled out in 2018. They simply do not understand that every day public safety bets their lives on their communications systems.

When AT&T won the FirstNet RFP it shocked many of us by stating that in addition to Band 14 (the FirstNet spectrum) it would offer up the use of all its existing LTE spectrum across the United States. Further, AT&T has stated that as it continues to increase the number of cell sites and starts deploying 5G small cell services mainly in metro areas, these radio channels will also become an automatic part of the FirstNet network. This helps solve the issue of network congestion when an incident is limited to a small area that is covered by only one or two cell sectors, but it does not automatically turn the network into a mission-critical network. In reality, the terms “mission-critical” or “public safety grade” are a misnomer since with LMR systems this refers to the ability for the LMR system to degrade from its fully operational status to several levels of fallback during major storms.

Let’s look at the past three hurricanes as examples of bad, worse, and worst-case scenarios. In the case of the Houston storm and floods, most of the LMR and broadband networks remained up and operational or were quickly brought back online. Moving to Florida, while most of the state’s LMR systems and many cell sites remained operational in areas such as Key West, there was a very different scene in some cases with both LMR and broadband services being off the air. Here, too, the agencies and networks went to work quickly to bring communications back online. The worst case was Maria hitting a number of islands in the Caribbean hard, the worst hit, of course, being Puerto Rico. Not only was the damage to all of the communications infrastructure severe, since most of the power lines, phone lines, and fiber connections run on telephone poles above ground, the backhaul to all of the systems was mostly non-existent. There were a few cell sites that stayed up and many LMR systems survived the storm or were quickly put back into service but simplex was the order of the day in most places.

Differences between LMR and Cellular

LMR radios have been used in public safety since the 1930s. They started out as one-way to the vehicle and then two-way. Simplex (off-network) was the first type of push-to-talk used and as LMR advanced, the premise of simplex was always a part of radio system designs. We went from base radios at the station house to remote stations on towers, then moved to repeaters so everyone on the channel could hear all of the traffic. When handhelds were first deployed they did not have the same range as high-power mobiles so we started using multiple receivers spread out in the repeaters’ coverage areas. There are now satellite receivers connected at the repeater with a device (voter) that measures the signal received and passes only the best signal through to the repeater.

Next up was transmitter steering or the use of multiple transmitters to increase coverage and they were “voted” on depending on where the user was located. This also became the way most cities operated with citywide channels and then district or zoned channels for local calls and incidents. Through all this, simplex or off-network was always part of the design. After that, we went to simulcast systems where multiple transmitters were turned on at the same time to cover broader areas without having to change transmitters. If memory serves me correctly, the first Motorola simulcast system was installed in Orange County, CA and one of my partners, Barney Dewey, was one of the tech engineers on the project. So far, all of these systems were analog voice.

The current state of the art for LMR is similar to cellular systems and is called “trunked radio.” These are multiple radio sites with multiple channels all controlled by a back-end smart system. Some of these systems are analog and some are digital (P25 is the preferred standard). There are various types of trunked systems today but the most common use 5, 10, 15, or 20 channels at a site. The first channel is the home channel and that is where idle units rest. When there is a dispatch or a call, the group involved in that call is quickly moved to one of the other channels and the home or control channel remains free. This technology has enabled a much larger number of field units to share the same radio system and since the groups are defined and independent from each other, many systems include police, fire, EMS, public works, and other city or county agencies. Again, these networks were designed from day one with degradation capabilities, moving them from trunked systems to repeaters if the system back-end fails. Push-to-talk off-network is still a vital part of these systems because it is used heavily between units at a scene to penetrate buildings and because when a system fails completely, simplex communications survives.

Cellular and Broadband

Cellular systems were first deployed in the United States in 1973. They were analog and channelized. The difference with cellular is that each site had many channels and other sites surrounding a site were on different channels. The back-end of the system kept track of which cell site a mobile was in and then changed the channel when the device went into another cell. GSM and CDMA changed analog to digital. GSM used channels the way generation one did while CDMA used a very different technology that combined signals into a single larger swath of frequency. CDMA made use of 1.25 MHz of spectrum to begin with and GSM used 30-KHz channels. Once we reached third-generation cellular, the idea of individual channels went away, and in fourth-generation technology the competing 2G and 3G technologies were replaced with a common worldwide standard known as Long Term Evolution (LTE). LTE can make use of a host of different amounts of frequencies, and even frequencies in different bands can now be aggregated to provide more bandwidth. LTE was designed as a digital data technology with the use of IP packets. That is one reason so much voice is still carried over 2G and 3G networks. LTE has added Voice over LTE and many carriers make use of it. Voice over LTE will be how we use dial-up from now on. This is the main reason AT&T’s idea of including all of its LTE spectrum is a real boon to public safety. Data rates and capacity are dependent on the amount of spectrum being used. Band 14 (FirstNet) was allocated 20 MHz of spectrum but with the new AT&T model, that can be anywhere from 20 to 120 MHz of spectrum in a given area.

Making use of LTE for voice meant voice had to be converted from analog to digital packets (IP packets). However, unlike data packets that can arrive at the receiving end in a different order and be shuffled into the right order, Voice LTE packets must arrive in an ordered sequence, therefore, LTE gives Voice over LTE priority status on the network. Adding PTT over LTE is, again, the art of converting the spoken audio to digital IP packets and sending them to the receiving end or ends since PTT can be sent one-on-one the same as dial-up voice or it can also be sent to groups of users so they all hear the transmission at one time. The takeaway here should be that from the beginning, simplex was what enabled LMR systems to operate and it is still a vital part of the architecture, while PTT over LTE is a bolt-on packet-based technology that is treated as simply more packets over the network. It’s important to understand the difference in the way PTT is handled over LTE versus LMR and how off-network PTT can be implemented, if it ever. The major issue with LTE is that the transmit and receive side are so far from each other that you would have to add a second receiver or transmitter to the device and then increase the power to a few watts, which would really drain the handheld’s battery in hours.


All of the above and more I have not discussed in this blog, indicates to me that FirstNet may not ever become the only public safety network. It is possible because technologies are moving at a faster and faster pace but there is a lot of work to do and it is not at all helpful for the technical folks to make blanket claims about technologies that are not proven or for which the standard has not been finalized and then extensively tested in the field. Further, as I have said before, PTT over FirstNet will be used when the public safety community has tested it and trusts it—not when technologists say it is ready.

PTT over LTE works great to provide interoperability between departments with different LMR channels. It makes sense and has been proven, so that is not an issue. However, there are several reasons I am not a fan of FirstNet becoming the only public safety network. The first is the PTT issue, second is the current lack of redundancy and, finally, having a single network means the network might be more vulnerable to interruptions, either by nature or malice. Let’s move slowly with “Mission Critical” PTT over LTE and get the appropriate applications and video services up and running. Let’s make use of the various PTT over LTE systems already in the market and proven, tying them together for inoperability and connection to LMR systems. Let’s get a handle on the training and day-to-day use issues because they will change from what we now believe to be the case.

FirstNet is real. It is exciting to see it come to life but we cannot afford to get ahead of ourselves from a technological point of view. Public safety needs to be 100-percent onboard and comfortable with FirstNet. Even with users’ smartphone experiences, this system will still work and be used differently and will require specialized training to be successful and not simply tried and declared a failure because the public safety community’s expectations were set too high.

Andrew M. Seybold
©2017 Andrew Seybold, Inc.


1 Comment on "Public Safety Advocate: Public Safety Networks LMR and FirstNet Working Together"

  1. Brett Larson | March 23, 2018 at 6:41 pm | Reply

    Hello Andrew. Thanks for all of your great articles. They have been very helpful for me to learn about the space.

    I have one overarching question, that I am sure you get a lot. If not in 5, 10, or even 20 years, why couldn’t cities and counties eventually stop investing in LMR, if FirstNet can be “hardened” with fallbacks and with improvments to MCPTT?

    You referenced in a different article that LTE eventually developed VoLTE so that network operators could stop maintaining the 1G and 2G. It seems like a natural comparison to say that eventually a MCPTT and fallbacks will be developed that will allow cities/counties ot stop maintaining their LMR infrastructure.

    Really appreciate your articles and thanks for answering my question!

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