Public Safety Advocate: Networks, Devices, Push-To-Talk

Over the next few months, but not every week, I will be expanding on this week’s article about Land Mobile Radio (LMR) networks and commercial broadband networks, with an emphasis on the benefits and drawbacks of each as they relate to public-safety communications. In this series I will review infrastructure, handsets, and accessories with a focus on how Push-To-Talk (PTT) communications work over each network type and why I believe use of both network types in an interoperable hybrid model offers public safety the best option for ubiquitous coverage and agency-interoperable PTT communications.

LMR networks, whether Analog FM, P25 Conventional, or P25 Trunking, have each been designed and built for a mission-specific communication function. Commercial broadband networks, on the other hand, have been designed and built to support a broad range of applications and functions. FirstNet (Built with AT&T) incorporates enhancements to AT&T’s commercial network that include a dedicated Core Network (CN) for public safety, Quality of Service (QoS), and Radio Access Network (RAN) priority and pre-emption.

P25 LMR networks and commercial broadband networks have been built to established standards, providing a guarantee that compliant networks will support compliant subscriber devices (LMR radios and broadband phones, tablets, and data cards). LMR PTT offerings are more tightly controlled and delivered as a tested set that includes infrastructure, radios, and accessories. There are more variables with broadband, primarily because handset devices range from basic phones to smartphones and tablets and there are two popular operating systems (Google’s Android and Apple’s iOS). This can be a challenge for broadband PTT service providers as they do not control the infrastructure, device hardware, and operating system elements.

LMR Devices

Let’s start with what I will call purpose-built devices for land mobile radio systems. Today there are three primary types of LMR systems being used within the public-safety community and a few other types of systems are used on a much smaller scale in non-public-safety markets throughout the United States. Much of the rest of the world uses Tetra, a technology that more closely resembles cellular communications though the core user technology is purpose-built devices. 

“Purpose-built devices for LMR systems” refers to handheld and mobile devices that have been used by the public-safety community since before World War II. LMR technologies have been upgraded and advancements have been made, but these purpose-built devices are generally designed to serve one function, which is to provide push-to-talk communications on a one-to-one and one-to-many basis. Vendors build these devices to established standards and each vendor designs and certifies its devices and push-to-talk software. Because they meet established standards criteria, all these purpose-built devices work on compliant LMR networks. Compliance with standards provides inter-vendor interoperability and handheld or mobile purpose-built device compatibility on any compliant LMR network.

Taking this one step further, LMR device vendors create their own user interface in newer LMR devices that have screens. However, many devices have no visual user interface and their features and functions are controlled by rotary switches, multi-function buttons, and push-to-talk buttons. This works since the sole purpose of these devices is to provide PTT capabilities over LMR systems. For the most part, applications cannot be added or deleted and once a device is programmed for the radio channels it will use, there is not much else to do. LMR vendors certify their devices as meeting FCC requirements and certify that add-on accessories such as speaker/microphones are compatible with their radios.

As mentioned, there are at least three different types of LMR technologies in use today. Let’s start with LMR Analog FM. Contrary to what a number of vendors would want you to believe, Analog FM is not dying. In many parts of the country, Analog FM is being replaced by one of several new digital forms of LMR, but these are still all about push-to-talk voice.

Those who grew up in the LMR world with Analog FM and devices without screens or other bells and whistles know one radio channel in a given area can support only one PTT session at a time. Radios we used then, and many that are available today, have a radio rotary channel (frequency) switch mounted on the top of the radio. When users need to change radio channels or want to move to a different channel after a dispatch to clear the frequency, they are instructed to tune to a specific channel. They then use PTT communications on that channel and only users on that channel, within coverage, can hear the conversations.

As an aside, in the 1970s, most LMR radios were crystal-controlled and only a few channels were built into the radio. In the late 70s, I believe Motorola was first to add a channel switch small enough to fit on the top of a handheld radio along with the volume control and one or two other controls. This channel switch enabled a whopping sixteen channels to be built into a radio, thus more PTT conversations were possible within a jurisdiction. The limitations of this switch became evident when groups and zones were developed and radios became programmable with many more radio channels. Typically, sixteen channels were still used for both zones and groups. Today, most LMR radios are capable of many more radio channels, zones, and groups. 

New Model radio with Emergency and Zone select ABC switch

Even today, many LMR radios have no screens or small screens to indicate the current channel and many still have a sixteen-channel switch. However, that switch can serve multiple purposes today. It can still be used to control radio channel settings, it can be used to change zones or groups of channels, and it can be used to move from one “talk group” to another in radios with that capability. Even today, purpose-based LMR radios are for PTT and nothing more. Some PTT LMR radios now provide voice notifications whenever a channel, zone, or group is changed so users do not have to look at the rotary switch and other buttons.  

Motorola LMR handheld no screen

L3Harris Multiband LMR plus FirstNet LTE

Channels Becoming Groups

LMR Analog FM and P25 (digital) conventional radios continue to follow the one-channel, one-PTT session model. Trunked systems introduce the concept of dynamically allocated frequencies. Users do not select the channel they want on handheld and mobile devices because those devices are “camped” on a control channel until someone starts a conversation or requests a channel. At that time, all devices on the same channel (group) are “commanded” to a specific frequency (and timeslot). The radios may stay on that frequency for as little as one second after the user has released the PTT button, at which time the radios return to the control channel. Therefore, it may be that each time a user presses the PTT button throughout a conversation, all the radios will switch to a different frequency. From the users’ perspective, the rotary knob can be described as a “channel selector” on a conventional system and a “talk group” selector on a trunking system

With both LMR Analog FM and P25 Conventional systems, the channel is selected by the user. This is where the channel switch comes into play. Before radios were capable of literally thousands of channels divided into zones, when users were told to move to channel 4, without looking at the radio, they would reach down and turn the channel switch counter-clockwise to a stop at channel 1, then count three “clicks” forward and they would know they were on channel 4. This process has become much more complex now that users have to move to the appropriate zone and then tune to the desired channel within the zone. 

This is still manageable in many systems because the sixteen most used channels for an area are normally programmed as zone 1, channels 1-16, and other zones are used for larger incidents or to provide interoperable communications with neighboring agencies, statewide, and even nationwide. However, LMR channels remain fragmented on at least five different portions of radio spectrum and until very recently, no vendor made a radio capable of operating on more than one of the five available spectrum segments. 

Purpose-built radios are designed and built for one function: push-to-talk that is interoperable when radios are on the same channel as others in the area. LMR units work both on- and off-network. Off-network is vital for on-scene communications and must be available when all or some users are within network coverage or when all users are out of network coverage. LMR devices have become more complex but they remain single-purpose and single-function radio communications devices.

FirstNet (LTE) Devices

FirstNet was designed to augment LMR systems and to serve as one common nationwide network where every user from every agency can have devices that provide interoperability with any other users or agencies on the FirstNet network. However, devices deployed by FirstNet or other LTE broadband networks are different. The first type of mobile device within the FirstNet LTE world is a commercial broadband smartphone or tablet containing applications and features and functions required by John Q. Citizen. Other devices offer all the features and functions of a smartphone or tablet but may include a few additions to render them more adaptable to mission-specific use. 

Such additional features and functions can include support for LTE Band 14 public-safety broadband spectrum in addition to all of AT&T’s existing LTE spectrum. The radio license for Band 14 was issued by the FCC to The FirstNet Authority and The Authority authorized ONLY FirstNet (Built with AT&T) to use Band 14 spectrum. Many devices for Band 14 have been ruggedized for the harsher environments encountered by the public-safety community. Further, some smartphones have added a push-to-talk button on the side of the device, capability for adding a speaker/microphone and, in some cases, louder receive audio so users can hear voice traffic in a noisy environment. Sonim Technologies was the earliest provider of FirstNet-specific phones with all these features. Companies such as Motorola, Kyocera and, it is rumored, Samsung have or will offer the same feature set as Sonim devices.

Sonim Ruggedized Phone with PTT button on the side

Apple’s current iPhone models have been certified and include Band 14 capability but they do not include a PTT button on the side of the device. Apple and some other FirstNet-ready vendors do not handle PTT as well as they should. Some concerns are about how to alert users of an incoming PTT call while they are engaged in a phone call, and a few other matters that can make a significant difference in device and network operations in normal public-safety operations. I will go into more detail about this in an upcoming Advocate.

PTT for LMR and FirstNet

PTT over LMR has been in use since the 1930s. Generally, once an LMR purpose-built device is approved by the FCC and made available, there is no question as to whether it will communicate with other vendors’ LMR devices on LMR radio channels. However, moving into the broadband world, there are many issues impacting PTT interoperability that have not yet been addressed. Let’s start with the fact that today there are at least five FirstNet-certified PTT vendors with another rumored to be announced this month and most are interoperable with the others. This means there are major compatibility issues with broadband PTT and while some vendors want to become interoperable, several don’t.

Next, the 3GPP standards body has released a poorly-named Mission-Critical Push-To-Talk standard (MCPTT). Even though “Mission Critical” has a negative connotation within the public-safety community, the rumored PTT vendor for FirstNet is claiming it will be fully MCPTT-compliant and several network-based PTT systems claim they will become compliant. However, it appears being compliant is not the same as being interoperable. This is a major difference between LMR purpose-built PTT devices and FirstNet devices that must retain all the bells and whistles consumers want. As of today, the various flavors of PTT are not compatible.

In order for public-safety PTT to be effective, it must operate over LMR networks and be interconnected to FirstNet. There are several ways to provide LMR/FirstNet (LTE) interoperability but some do not provide all the functionality and one can cost an exorbitant amount unless all vendor options are explored. It should be noted that the three vendors supporting advanced FirstNet/P25 Trunking interoperability use the Project 25 Inter RF Subsystem Interface (ISSI).

ISSI interoperability planning between P25 systems is a complex element of mutual-aid system design. Interoperability planning for P25 trunked systems with FirstNet-Certified PTT solutions requires the same level of strategic planning but offers unparalleled inter-system interoperability. 

The issue we currently face is how to homogenize LMR purpose-based PTT devices with more generic broadband smartphones where PTT is one of many applications rather than a baked-in primary application. Purpose-built LMR devices require few if any upgrades for PTT. However, in the broadband world, application updates, device operating system updates, and even new feature sets come up several times a year. Getting interoperability right once does not guarantee it will stay right even six months down the road. Look at the number of iOS and Android updates over the last six months and then look at which 3GPP LTE release is being used by each broadband network, how many features were included in each LTE release, which networks chose to implement which portions of the releases, and how many times an approved PTT application has been updated in the same timeframe. There has already been an incident where one major smartphone vendor made an upgrade to its operating system and caused PTT on its devices to stop working in the background and users of one of its devices missed PTT traffic directed to them.

In conclusion, I am not saying the LMR PTT model is the only model that should be considered. I am saying that there are huge differences between LMR purpose-built PTT devices and application-based PTT solutions currently being deployed in the LTE word. As I continue this series of articles it will become apparent that there may be a better way to implement PTT on broadband devices that may, perhaps, make it easier for both LMR and LTE PTT services, features, and functions to work together and bring compatibility to FirstNet-to-FirstNet PTT and LMR-to-FirstNet PTT.

Winding Down

This week we have seen the first occurrence of what could result in public-safety staffing problems. In Kirkland, Washington, EMS and fire personnel as well as two police officers are now in quarantine because they responded to and then transported a person who had the COVID-19 virus. They did not have protective clothing nor were they able to pre-test the patient before treating and transporting. These first responders are out of service for a few weeks. If the virus continues to spread, this situation could lead to first responder shortages and staffing issues, including in Emergency Communications Centers (ECCs).

First Responders have to be protected and they need to be able to pre-screen patients to determine the likelihood of the patient being infected. Neither fire, EMS, nor law enforcement agencies have the funds to purchase the necessary gear and test devices. Once Congress passes a funding bill, what will it take for public-safety agencies to qualify for emergency funding when so many will be vying for these funds? Perhaps this project should be undertaken by the Department of Homeland Security (DHS) or at least FEMA, using some of their pre-allocated funds for emergencies. (Editor’s note: Today FEMA announced a plan to do this!)

Meanwhile, I am looking forward to IWCE at the end of the month and I am confident Las Vegas will be prepared. This is always a great show and this year should be no exception. I hope you will continue with your plans to attend, perhaps making some slight adjustments such as carrying disinfectant wipes, hand sanitizer, and forgoing traditional handshakes and hugs that are natural when greeting people you have known and worked with for years. It should be a safe event for all and I, for one, continue to look forward to participating.

Until next week…

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


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