Public Safety Advocate: Networks with a Purpose

Fixed and wireless networks are all built for a purpose. This might be for a single purpose such as Land Mobile Radio (LMR) public safety systems or it might be for a broader purpose such as cellular networks built to serve a company’s customers be they individuals, businesses, machine-to-machine (Internet of Things, IoT), and other uses. The design of the network, the coverage it provides, and the services enabled to run over the network will differ. Some wireless networks are used to communicate from one continent to another, satellite networks are designed to deliver specific content from one location to another, usually remote locations, and satellite systems for voice and slow-speed data enable people in unpopulated areas, on oil rigs in the gulf, or on ships to communicate via dial-up telephony among other purposes.

This week’s Advocate is about several types of purpose-built networks including Land Mobile Radio (LMR), commercial broadband wireless networks, and the new FirstNet hybrid commercial/public safety system. All three of these networks are currently being used by the public safety community. The LMR systems are purpose-built to cover a specific geographic area such as a city, county, region, and more and more, an entire state. These networks are designed, planned, and built to carry primarily Push-To-Talk (PTT) audio for the public safety community from the dispatch center to the field, from unit to unit, and from group to group. While some LMR networks do handle data, especially compared to wireless broadband, they are very slow and not capable of much more than text services.

Land Mobile Radio

From the day they are designed, LMR systems are considered to be as close to mission-critical as possible. Each and every element of the network might not be fully compliant with the NPSTC Public Safety Grade report but LMR networks also support various forms of fallback or, as I prefer to say, graceful degradation. P25 trunking systems can revert to single-site repeater systems, and further to simplex or unit-to-unit functionality. Likewise, a simulcast network can gracefully degrade in the same manner. A simple analog FM repeater system can degrade to base-to-mobile and mobile-to-mobile operation when needed.

Land mobile radio systems are also “managed,” usually by a dispatch center. If a city or county near you still dispatches in the clear (unscrambled analog FM) and you listen in, when you hear a fire dispatch it typically names the equipment responding, the location and type of incident, and then both the command channel and a Tactical (TAC) channel for use on the scene. The command channel may be the same as the dispatch channel or a different channel that is still available at the dispatch center. The TAC channel will be called simplex or sometimes talk-around and it is designed for local communications at the scene.

Law has the same access to tactical channels, usually for SWAT teams and other special-purpose teams at an incident. During a major wildfire, for example, there may be several command channels in use and literally dozens and dozens of tactical channels for the various smaller groups attacking the fire. When the dispatcher is not managing and assigning channels, the incident commander (IC) can, but normally IC will request additional channels from the dispatch center and it will assign channels to the incident. LMR systems only permit one person to speak on a given channel at a time and no one can respond until the speaker is done. There are also emergency buttons on some systems for someone in need of assistance, and sometimes there are ways certain users can have priority status on a digital network. All-in-all, this almost-military channel management has been successfully used for many years.

LTE Commercial Networks

LTE or 4G broadband networks were and are built for the purpose of serving customers over a large area, usually most of the United States or another country. They are designed to provide the maximum possible amount of service in each cell site and within each cell sector. They are exceptionally smart networks where the size of a cell can be made larger or smaller, adjacent cells can be matched to increase coverage when needed, and much more. Each cell site can consist of multiple portions of the LTE spectrum to add more capacity to a given cell and to enable these spectrum portions to be “stitched” together to provide faster data speeds and add capacity.

LTE networks are purpose-built to provide as many customers as possible with as much data speed and capacity as possible. LTE was designed as a data-only technology. Voice has been added in the form of push-to-talk (a small market segment) and Voice over LTE (VoLTE), which converts standard analog voice to data packets and sends them over the network. At the other end, these packets are reassembled and the voice is converted back to speech. The reason LTE engineers added Voice over LTE was so network operators would not have to maintain their 2G and 3G networks for voice traffic. This in turn allows network operators to decommission first their 2G networks, then their 3G networks, and to then deploy LTE in that spectrum. This is very important to the carriers since LTE is much more spectrally efficient than 2G and 3G systems and, of course, provides for more users in the same amount of spectrum and better LTE experiences for everyone.

LTE networks as they are built today are hardened to some extent. Many sites have battery back-up and generators, but an LTE network has no fallback modes at the moment. If a cell site is down and adjacent cell sites cannot provide coverage in the area, a smartphone is not very smart and simply does not work until the network is restored. LTE networks can and have failed on a nationwide basis (not recently, but in the early days of LTE during major network software upgrades). LTE networks can fail over a large swath of their coverage due to flooding, storms, wildland fires, and service outages that impact a broad area. Finally, of course, individual cell sites can go down for a number of reasons including the power source being cut, antennas being damaged, something cutting the fiber to the site, and a number of other reasons.

It should also be noted that commercial operators have vastly more resources available to them to bring their networks back up on the air. During a potential hurricane, they generally stage people and supplies just outside the area so they can make a quick entrance once the dangers have passed and put the network back into service. If the event is not pre-planned or known about, network operators still respond quickly and during many of these incidents provide Cells on Wheels (COWs) and other temporary methods of restoring services. In Puerto Rico, they are using COWs, tethered drones, and balloons provided by Google.

FirstNet: The Ecosystem

I have briefly discussed both LMR and commercial LTE networks. FirstNet is different in many ways. We were expecting someone to come in and build out a totally new network using only FirstNet band 14 spectrum and for it to take all of the 5 years allocated for the build. The good news is this is not what is happening. When AT&T was chosen to be the network builder and operator, it offered all its existing LTE spectrum starting as soon as a state opted in, the build-out of band 14 over time, AND as AT&T builds out its own broadband network, the new sites, 5G nodes, and inbuilding systems will automatically be added to the spectrum public safety agencies are able to access.

It should be stated again, as it has by multiple organizations, multiple times, that FirstNet will NOT replace LMR anytime soon. Public safety is being told by both FirstNet the Authority and FirstNet the Ecosystem to keep and enhance its LMR systems. FirstNet should be viewed as another network to add capabilities to the public safety community where they have not previously had complete, pre-emptive access. My vision for the next several years is to continue with LMR systems and upgrade them as needed, to let AT&T build out FirstNet for data and video services, and to interconnect LTE and LMR systems during incidents to enable better interoperability for voice communications. PTT over LTE is NOT mission-critical today and it does not support all the features and functions needed by the public safety community.

The way I am looking at the FirstNet build-out is that it starts with existing AT&T LTE spectrum assets but with full pre-emption for public safety users. These users also have a unique PLM-ID or identifier that will ensure they are the only ones permitted on the FirstNet system. Over time, band 14 will be added, as will more cell sites and 5G microcells. Today, FirstNet traffic is using the same transport and core as commercial users. However, within a month or so, AT&T will turn on the FirstNet core and only FirstNet traffic will be routed from a cell site to the new core, sharing the same backhaul but traveling in one tunnel while consumer data is traveling in another tunnel and the two never intersect.

AT&T agreed in its RFP response to FirstNet the Authority to make sure the network is upgraded to public safety grade as it is being built out and configured. Meanwhile, the 3GPP standards body is working on ways to provide various levels of fallback for LTE networks including the ability for a single cell site to stay up and running even if the fiber to the core has been destroyed. These changes will take a while. In the meantime, FirstNet the Ecosystem is being built and sites are being hardened and there are more ways to provide redundancy in the LTE world.

I wrote about some of the ways LTE network operators can add what looks like redundancy in a previous Advocate. These include umbrella sites that are hardened and can replace a number of smaller cells below them (for example, a mountaintop site overlooking a highway with small cells along it). The ability to expand and contract cell coverage areas to help with traffic loading can also be used if some cells are out of service. Again, AT&T has a large number of COWs, drones, and other devices that can be quickly moved as well as disaster recovery teams that are tasked to work on the system only in areas where the network is down or damaged due to a storm or other disaster.

Comparing Coverage

When the public safety community looks at what areas it wants and needs for broadband coverage, the tendency is to compare its existing LMR coverage with what AT&T is providing today and has shown to be willing to build out over the next 5 years. This is understandable, but most LMR systems are 20 or more years old, have been upgraded a few times, and coverage has been increased to provide better coverage for inbuilding and less populated areas. For example, many systems have added satellite receivers (not the satellites themselves but remote receivers). These are used to increase the talk-back range of handheld devices. When designing a public safety network, the ideal situation is to match outbound and inbound signals. However, it is more usual that the network will transmit farther than it can hear. Thus, the use of extra receivers increases the inbound signals.

LMR systems have been in operation since the 1930s while LTE has been in service since late 2010. Further, LTE systems have been designed and built to provide, usually, nationwide coverage while LMR systems are local in nature and design. It is easier when a city or county controls its own radio system to add another receiver or a new radio site, or otherwise add to the LMR coverage than it is to convince a commercial network operator to add a cell site, especially if the population of the area does not warrant the investment.

The FirstNet network has a mandate to build out metro, suburban, and rural areas in the same increments and in many cases to cover even areas where it may not be feasible for a commercial network operator to provide service. So far, I have seen a willingness on the part of AT&T to work with local jurisdictions in a number of ways to add coverage. The first is by agreeing to add more FirstNet cell sites to a given area. The next is working with cities or counties and have them fund some additions they want to the network. The last is to work with cities or counties that have applied for broadband funds from their state or federal organizations.

It is also important to keep in mind that like all LTE/5G networks being deployed, the AT&T network and, therefore, the FirstNet system, will never be finished. The 5-year goal set out by FirstNet is to ensure the project stays on track but the contract is for 25 years and AT&T’s network won’t remain static during that time. That means FirstNet’s won’t either. As areas that are, today, sparsely populated become populated, and new highways are built, smart cities will continue to evolve and with all of this comes the continued enhancement of FirstNet.


All networks are purpose-built. The purpose may be different, the customer base may be different, but they are all purpose-built. I consider FirstNet as it is being deployed by AT&T to be a network that starts with a purpose-built network designed to provide the general public with broadband access, adds many of the public safety network attributes, and over time becomes, in reality, a hybrid network. In areas where there is a need to use the public safety spectrum on a secondary basis and in rural areas where AT&T already provides coverage, that coverage becomes FirstNet coverage, and where there is no coverage and AT&T builds out new network facilities for public safety, this is also beneficial to AT&T’s commercial customers.

We will end up with three types of networks: Those purpose-built for LMR voice communications, those purpose-built to serve citizens and business users, and the hybrid network that will be enhanced in a purpose-based way to provide a more robust, more secure network with guaranteed pre-emption access for the public safety community. One interesting final observation is that fiber or microwave that is required for a purpose-built network can also be used for other purpose-built networks. A few strands of fiber to the public safety LMR site, a few more to the purpose-built FirstNet site, and more strands feeding local libraries and schools as starting points for networks purposed to provide broadband connectivity where there is none today. Each network has a purpose or it would not have been built. But that does not mean there cannot be other purposes added over the top of the primary purpose and such is the case with the AT&T network, which also becomes the FirstNet Ecosystem.

Andrew M. Seybold
©Andrew Seybold, Inc.


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