Public Safety Advocate: LMR, FirstNet PTT Operational Concerns

I recently became aware of growing concerns related to interconnecting Land Mobile Radio (LMR) networks with any LTE network including the FirstNet (LTE) network. Some LMR operators appear to be worried about the amount of new traffic that will end up on their networks if they allow LMR-to-FirstNet connections.

Traffic on LMR networks has always been a concern, so the frequency coordinators at the FCC came up with rule-of-thumb criteria to determine the number of users a channel can support. Analog FM, P25 conventional, and P25 trunked channels differ in their capacities. P25 trunked systems came to be when it was found that a shared trunking system could handle more users per channel than an analog FM channel. However, it seems this sharing has led to some anxieties and perhaps misunderstandings of how tying an LMR network to FirstNet or any LTE network might increase traffic on the LMR network.

Some concerns about channel loading are simply a misunderstanding of how this type of integration affects channel limits for analog FM channels, P25 conventional channels, and channels on P25 trunked systems. How the connection is made between the LMR network and FirstNet makes a difference. The Inter Subsystem Interface (ISSI) is preferred in the P25 trunked world. However, depending on the LMR vendor, adding ISSI to an existing P25 trunked system can increase the network cost by as much as $500K or as little as $40K. 

The Public Safety Technology Alliance (PSTA) sub-committee on LMR/LTE interoperability has recognized the Digital Fixed-Station Interface (DFSI) as another interface technology for P25 trunked systems. DFSI was originally designed to interface consoles to P25. While not yet fully ready for prime time, the PSTA has recommended DFSI (after some minor modifications) as another way to interface P25 conventional and analog systems. Radio over IP (RoIP) is a third way to make the connection between LMR and LTE. RoIP requires a donor radio, which is a radio on the LMR network that is connected to the RoIP gateway which, in turn, is connected to the FirstNet/LTE network. 

In addition to the type of LMR network and how the LMR and LTE networks are connected, channel loading is also influenced by whether the LMR network is a local, city, or county network, or a shared regional or statewide network. Experts in the field of LMR/LTE integration tell me that the same person or organization that handles devices on the LMR network should also handle the interface between the network and the devices that will be interconnected. 

There are some assumptions that generally hold true. The first is that when an LTE device is given to a person in the field, it is either a replacement for or in addition to an LMR radio. In the first case, traffic loading should not increase since the LTE device is replacing the LMR radio. In the latter case, it is not likely that the person in the field will use both devices in Push-to-Talk (PTT) mode at the same time. Again, the channel loading will remain the same.

On the other hand, new devices may be issued to new hires and people who don’t normally carry LMR handhelds. These new users could drive up traffic. Traffic could also increase during incidents when neighboring or other agencies are called in to assist. If they are capable of PTT over FirstNet, they could be given access to the LMR/LTE bridge so all personnel on the scene would be able communicate with each other. This can be managed as though the incoming users were on the same LMR radio channel. The goal of FirstNet has always been to provide communications interoperability between agencies. 

Use Cases

Let’s start with local city or county networks that are used by a single agency. These networks are usually made up of more than a single radio channel, and most analog and P25 conventional systems are made up of multiple channels. Typically, when Fire Service is dispatched to an incident, it is assigned a primary channel on the network for coordination and a simplex (off-network) channel for on-scene personnel. When Law Enforcement is dispatched, it typically uses a standard “working” channel. 

I would not recommend tying the primary dispatch channel to FirstNet since dispatch traffic in both directions is liable to be heavier on a non-dispatch-centric channel unless it is used to provide listen-only capabilities for those with LTE devices. This would increase the load on FirstNet but not on the LMR network.

Connecting an LMR system to FirstNet/LTE should not increase LMR traffic unless additional LTE devices are using the network. Again, if most of the LTE devices replace LMR radios, or if the LMR radio and LTE device are issued to the same person, traffic loading should remain the same. By using a digital system in addition to a specific channel, it is possible to assign the LTE units to groups and limit the number of groups to manage the amount of traffic on the network. Some LMR system mangers seem to believe that when using a donor radio to interface an LMR system with FirstNet, the volume of traffic coming over the donor radio from the FirstNet/LTE side will overwhelm the LMR network. Once again, if the balance of devices on the LMR and LTE networks remains nearly the same, there should be no increase in traffic loading.

P25 Trunked Systems

If ISSI is used for the LMR/FirstNet connection, it is installed at the network core and fully supports groups and users. In this configuration, load on the P25 system will actually be reduced since the FirstNet/LTE network will host the over-the-air interface to the subscriber device. Loading of the P25 core will remain the same since it manages the LTE devices as though they were P25 devices. 

Today there are several combination FirstNet/LMR devices available and these are preferable for everyday use of FirstNet-to-LMR connections. In fact, they might even reduce traffic on the LMR system when it has been assigned to a FirstNet-only group and users must select another group before joining the LMR network. This method of handling interconnections between FirstNet/LTE and LMR networks falls under the normal management style of the network. 

Any increase in traffic can be managed by managing the groups and the number of users within a group. Incoming out-of-jurisdiction users can be assigned to a group while involved in an incident as if they were on the same P25 trunked network. The volume of increased traffic could result in some system delays but most networks are under the control of the dispatch center or Incident Commander and are run with a heavy hand to minimize unneeded or unwanted traffic. I have included a link to a recent ESChat video that shows examples of how P25/ISSI calls work across multiple carriers and discusses the use of groups as well as emergency calls. (Note: I am not under contract with ESChat. I have written white papers for the company but ESChat is not an ongoing client.)

In another example, a Sheriff’s department had issued LTE units to the Sheriff and his staff. With the LTE device, a group can be set up to enable group members to listen to activity on the LMR system or listen and talk via their FirstNet device back through the LMR radio system. A group can also be set up to enable staff to talk among themselves without using any LMR bandwidth. This type of closed group would work for upper echelon, detectives, swat teams, and other units that would otherwise be assigned to a talk group or radio channel for their exclusive use during incidents.

For example, the Sheriff’s staff could be set up to listen to the LMR dispatch channel for their organization, but they would not be able to talk to dispatch without changing to a different group. If the staff wanted to conduct a more private conversation, it could do so by using a FirstNet-only group. Detectives are a perfect fit for this type of set-up. When they are working a case with other detectives, they can use a closed FirstNet group to talk with other detectives and avoid LMR traffic. When they are undercover, communicating through LTE devices would not draw as much attention from the people they are investigating as would LMR handhelds. 

When we set up Santa Barbara County Sheriff and the City of Santa Barbara Police Department, one group was assigned to the Sheriff and his staff so they could talk directly with the Police Chief and his staff without involving either department’s LMR system. 

Winding Down

I received some flack as well as some great comments about last week’s Advocate and I have posted every comment, both pro and con. This is how I do things. No one gets everything right all the time. I do my best to verify my information but when I err, I say so and double down to do better next time. 

Remember, if you would like me to write about a particular topic, send an email to I will consider all suggestions. The number of subscriptions to the Advocate has grown substantially since was set up. Many of the new readers are from the ECC and dispatch world. Please feel free to let me know what you are interested in and what you would like me to write about as it relates to public-safety communications.

Again, I will point out that since my topics vary week-to-week, some weeks will not be as interesting to some subscribers as others. My underlying mission is to present ideas and concerns, describe wins and losses, and most important, give everyone something to think about. I enjoy writing weekly Advocates and I always appreciate your feedback, whether positive or negative.

Until Next Week…

Andrew M. Seybold
©2019, Andrew Seybold, Inc.


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