P25 (Project 25) two-way radio technology remains a popular technology choice for mission-critical communications users. The vast majority of systems are found in the US and Canada, but there are some in Australia, New Zealand, South America and the UK.
According to research published by the US-based Project 25 Technology Interest Group (PTIG) in May, there are a total of 2,351 P25 systems worldwide including 1,017 trunking and 1,334 conventional systems (see box, overleaf).
It seems that despite the advent of Push-to-talk over Cellular alternatives, few, if any, mission-critical users wish to rely wholly on LTE broadband technology yet.
“Mission-critical push-to-talk (MCPTT) is top of the mind with a lot of industry folk, but customers just focus on the mission at hand. P25 systems are important to them as they are the lifeline for fire, police and other first-responders. New things don’t move the needle much with them,” observes Chris Lonnett, vice-president of North America government, central region sales at Motorola Solutions.
Anatoly Delm, director of product marketing at Motorola Solutions, points out that while FirstNet on AT&T and other US carriers offer PTT voice solutions, they all position themselves as providing the data complement to mission-critical LMR voice services.
“The US approach is that mission-critical broadband services are very complementary to LMR products, and that has been our focus in product development on both the device and on the infrastructure and system side,” says Jeremy Elder, systems infrastructure and consoles product line manager at Harris Public Safety & Professional Communications.
“We are working on interworking solutions between LMR and LTE as we see both technologies continuing in parallel for some time yet. In the near term, we see P25 as a very stable market. We certainly see a healthy demand for both devices and infrastructure, and that is why we are still enhancing our P25 portfolio.”
James Teel, director, product line management – applications and services at Harris Public Safety & Professional Communications, argues that MCPTT is a hook that will lead to MCData and MCVideo. “When that happens we will see voice services augmented with video and data such as AVL and in-building location enhancements. All our P25 radios have Bluetooth and Wi-Fi and we are working with a third party to make that indoor location capability even better, and we are also investing in video analytics capabilities.”
Harris made an early decision to invest in multi-mode converged devices at a time when many doubted there would be a market sustainable enough to justify investment in dual LMR-LTE terminals.
“We have had the XL-200P P25-LTE portable radio for some time and it has been very successful,” reports Elder. “It shows that there is a market for an agnostic device.” At IWCE 2019 in March, Harris unveiled the XL-185 Mobile, which it claims is the first vehicular P25-LTE mobile radio.
“I think it is proven now that converged or hybrid devices are what is needed – at least today – and that is where we have driven our strategy,” says Teel. “The XL-200P is in the traditional two-way radio form factor that public safety users trust, with LTE baked in as well.”
Florida-based BK Technologies clearly thinks so too, as it is launching a hybrid device in the shape of its BKR 9000 Multi-band Radio this year, supporting P25 VHF and UHF bands and LTE 700MHz, 800MHz and 900MHz.
Motorola Solutions has so far chosen to develop P25 and LTE terminals, such as the LEX L11 rugged smartphone, separately. Lonnett says: “Critical communications users like the traditional radio form factor, but administrators and covert operatives use cellular commercial devices as standard,” he says. “It’s about the right form factor for the job role.”
It is also about routing the right traffic over the right network. A case in point being Motorola’s decision to team up with Sierra Wireless to offer the latter’s AirLink LTE in-vehicle routers, so first-responders can send broadband data packets on the move. Its MC-Edge Intelligent Gateway provides a hub for P25, LTE and LoRa IoT data to be transmitted over already available and reliable P25 networks.
P25 standard
While manufacturers explore these avenues, the US Telecommunications Industry Association (TIA)’s TR-8 (Mobile and Personal Private Radio Standards) Engineering Committee continues to improve and augment the P25 standard. Key work items at present include Group Regrouping, Key Fill Interface extensions beyond Key Fill Device, Link Layer Security Services and interworking between P25 and LTE.
Group Regrouping allows a dispatcher to group multiple talk groups on different channels onto the same channel rather than patching them across to each other. Andy Davis, chair of TIA TR-8, reports that the current focus is on the procedures for exchanging the necessary supporting messaging for group regrouping.
A second review of Key Fill Interface, which enables encryption and encryption key management, has been completed and another revision is being drafted to include comment resolutions. The revised document is expected later this year.
“Link Layer Encryption has seen the most activity over the last few months,” says Davis. “This is a fairly large effort as it is designed to encrypt the whole air interface. It impacts on FDMA and TDMA voice and data services for trunking and conventional systems.”
The air interface encryption will complement the already standardised voice and data payload encryption. The new work will also now protect the control signalling and caller identities, while applying a second layer of encryption for the encrypted voice and data payload. Various aspects of link layer encryption key management services are still under discussion. “There is a lot to do,” acknowledges Davis. “It has taken time, but we should see some publication later this year.”
P25-LTE Interworking
TIA is also working with ATIS (Alliance for Telecommunications Industry Solutions) to deliver a standard to enable interoperability and interworking between P25 and 3GPP LTE. The two organisations are creating a study document for the interworking function (IWF) to define behaviours for services that are common to both technologies, such as group, individual, broadcast calling, encryption and emergency calling.
“3GPP has already defined an IWF,” points out Davis. “You could describe it as a black box with the standard 3GPP interfaces on one side and the standard P25 interfaces on the other side. If you take the example of group calling, a call can be initiated by either technology, so what we are looking to define is the sequence of messages necessary to enable that voice service. You have a standard message coming in one side and a standard message coming out the other side, but in between those messages need translating.”
The study work is divided into two phases. Phase 1 covers P25 conventional and trunking call types. Joint LMR-LTE and analogue FM interworking will be addressed as part of Phase 2. Phase 2 will also tackle individual voice calls and control services, and it will also allow P25 to do key management of 3GPP devices in one way or another.
“We have identified a couple of ways as to how that is possible to do,” says Davis. “Some of this architecture-level work has identified that there is some additional work for 3GPP to do. We will move onto Phase 2 in the next month or two. We may see some more challenging areas where we or 3GPP will have to modify parts of the existing standards.”
As expected, the P25 ISSI (Inter Sub-System Interface) will provide the primary interface to the main function for P25-LTE interworking. “The ISSI interface defines how you connect two discrete P25 systems. We will now make modifications to the ISSI standard to define how you connect a P25 system with a 3GPP system. The idea is that as far as the P25 system is concerned, the 3GPP system will just look like another P25 system and vice versa,” explains Davis.
Leveraging ISSI and CSSI
TIA is also continuing to work with the Department for Homeland Security on its P25 Compliance Assessment Program (CAP). CAP is a programme for testing P25 critical functionality and reporting the results in a consistent way. CAP has now added ISSI and CSSI (Console Sub-System Interface) tests to the programme. In June, it released a draft bulletin outlining the proposed tests, which is now out for public review.
Davis observes: “ISSI and CSSI are fairly complex to set up and configure, as we have tried to accommodate the pre-ISSI/CSSI operations that already exist. But now customers want to interconnect systems. The features are standard, but each deployment of ISSI or CSSI has a unique network configuration and may use different sets of standard features, so there is still some uncertainty.”
Harris’s Elder agrees there is a definite trend for more customers to leverage ISSI to integrate P25 systems. “Nearly every major system we are shipping now has a requirement for an ISSI system. We are seeing it being used for mutual aid, so other agencies coming in to help or when there are big natural disasters. There is a lot of uptake on this now.”
Motorola Solutions’ Lonnett says: “We certainly sell ISSI, but I think customers have struggled to implement it in a meaningful way. Hence the release of our Critical Connect product last year. Critical Connect is an ISSI-based solution, but managed in the cloud by Motorola.
“It takes the power and potential of ISSI, but limits the customer’s responsibility to just one connection to our cloud service, and we can manage as many interconnections to neighbouring systems as the customer wants, and to commercial carriers’ PTT systems.”
Delm stresses it is important to note that it is an on-demand service. “ISSI is hard to set up, but with Critical Connect you can connect in minutes if you require a multi-agency response, and then you can tear down the connections as soon as they are no longer required.”
Harris’s Teel adds: “We are also seeing ISSI as a precursor for interoperability with the broadband world and we have seen some leverage of the ISSI to facilitate this.” He cites the city of Parma in Ohio, an existing customer with a Harris VIDA (Voice, Interoperability, Data and Access) core. Parma has taken advantage of the ISSI in the Harris core to interconnect with AT&T’s Enhanced PTT solution, so P25 users and AT&T users with the ePTT app on their smartphone can talk to each other.
Statewide systems
According to PTIG’s figures (see box), there is a definite trend towards larger, statewide systems. In July, the State of Michigan announced that there were now more than 100,000 mission-critical radio users on the Michigan Public Safety Communications System (MPSCS) from almost 1,900 agencies.
Ken Rehbehn, directing analyst, critical communications at IHS Markit, says: “What is important about P25-wide systems in states like Michigan is that it illustrates a significant trend, namely the growing importance of regional networks. Historically P25 has been deployed by local agencies with a fairly small footprint. That worked in analogue days with fairly simple radios.
“Now we are seeing larger procurements driven by interoperability requirements with technically challenging network infrastructure that is denser and more feature-rich.” He adds that the goal of federal grant programmes is interoperability. They also favour regional applications that encompass multiple small agencies because these can benefit more than one agency, while also delivering economies of scale as a secondary benefit.
Rehbehn points out that agencies end up with more functionality and excellent interoperability, and a sustainable economic model, if they join a larger, interconnected system. “It is a sensible way to progress. Otherwise you have a lot of small fragmented systems, which do not deliver on the promise of interoperability and require far more core network infrastructure, which increases costs.”
Michigan’s Motorola statewide P25 system has been in place for 25 years, and over that period has slowly added more jurisdictions. In contrast, Iowa’s state system only went live less than a year ago, but already the capital, Des Moines, and 20 counties have signed up. “Michigan took a long time to get everyone on board, but I have never seen so many jurisdictions sign up so fast as with the Iowa P25 system,” says Lonnett.
North Dakota has also just signed a $90m contract with Motorola Solutions for a state system following a competitive tender. “It is a collaborative procurement with both state and local jurisdictions participating at the same time and it is designed with both in mind,” says Lonnett. “So, customers continue to invest in P25 systems.”
The DMO issue
Another reason public safety is not abandoning P25 is the lack of an adequate talk-around or direct mode operation (DMO) feature on LTE devices. “DMO is still an issue for MCPTT,” says Elder.
“3GPP’s ProSe equivalent is not really being adopted by the chipset vendors and there are all the problems with lack of range.”
3GPP has recently indicated that 5G New Radio’s Vehicle-to-Everything (V2X) capabilities might provide the basis for an improved DMO solution for public safety. Rehbehn has his doubts, pointing out that first-responders need DMO to transmit in and out of buildings and over long range in forest fires.
“Direct mode is a more demanding set of requirements than V2X,” he argues. “Abandoning P25 solutions is not practical until that DMO gap is closed. Reliance on one device with an LTE direct mode capability is some way off.”
A more immediate solution is that offered by the Sonim LMR Enabled Detachable (SLED) P25 accessory paired with the Sonim XP8 rugged PTT smartphone. Rather than incorporating a P25 radio into the XP8, a separate P25 accessory provides the direct mode solution.
There are other reasons public safety continues to rely on P25 for mission-critical voice for the time being. Davis cites the much longer equipment budgetary cycles for public safety agencies, as well as their desire not to have to depend on others for their critical communications.
“Pricing and equipment lifecycles for FirstNet remain to be seen. Reliability is still a reason to go for P25, but as FirstNet proves itself on that front, people will finally move over,” says Davis.
Rehbehn concurs, noting: “P25 continues to serve a very important role in the public safety community thanks to its strong interoperability feature set. The data limitations remain a significant issue and many agencies operate LTE capabilities in parallel.”
FirstNet may be gathering steam, but it is this kind of parallel deployment of traditional P25 voice services and LTE voice and data services that we are likely to see operating in public safety for some years to come.
