The P25 two-way radio standard is more than 25 years old but continues to be widely used by public safety agencies, particularly in the USA. James Atkinson takes a look at its current development programme and finds out what the future holds for it

The P25 (Project 25) digital two-way radio communications standard was born out of a confluence of advancing technology and some unfortunate experiences with the existing analogue trunked systems.

The lack of interoperability between the analogue trunked radio equipment provided by the three main vendors had at times hindered the effectiveness of incident response. At the same time, digital wireless technology was looming, and with several US states looking to roll out major systems, it was felt that the development of an interoperable open digital radio standard for public safety agencies would be highly beneficial.

An initial meeting in 1989 of local, state and federal public safety representatives and industry resulted in the formation of the Project 25 Steering Committee, as agreed by the Association of Public-Safety Communications Officials (APCO), the National Association of State Telecommunications Directors (NASTD) and various agencies of the US Federal Government. The P25 Steering Committee comprises four members of APCO, four NASTD members, five from federal agencies and up to eight end-user representatives. The P25 suite of standards has been created and maintained by the US Telecommunications Industry Association (TIA)’s TR-8 (Mobile and Personal Private Radio Standards) Engineering Committee, although other sources can contribute documents to the standard too. The P25 standards are published as the TIA-102 suite of documents.

“The goal was to create standards-based interoperability, as opposed to the market approach, which was about vendors differentiating themselves with proprietary offerings. The aim was to promote interoperability among vendors. Vendors can then pick and choose which P25 interfaces, services and features best suit their markets,” explains Andy Davis, chairman of the TIA TR-8 Engineering Committee and TR-8.8 (Broadband Data Systems) Engineering Subcommittee.

P25 goals
The Steering Committee set out five goals: competition in life-cycle procurements; graceful migration (backwards and forwards); interoperability; spectrum efficiency; and user-friendly equipment. A decision was made to develop the standard in phases. P25 Phase 1 included the development of the Common Air Interface (CAI), first for conventional and then for trunked systems. The chosen vocoder uses the IMBE (Improved Multi-Band Excitation) voice codec developed by Digital Voice Systems, Inc (DVSI).

Phase 1 equipment is based on 12.5kHz FDMA specifications, designed to provide an easy migration from existing analogue FM systems and licensing regimes. The standard does not include analogue or mixed digital/analogue operation, but most manufacturers have chosen to enable this. The Phase 1 standards were completed in August 1995.

The Phase 2 suite of standards was published in 2012, which among other things added the TDMA trunked CAI, based on DVSI’s AMBE+2 (Advanced Multi-Band Excitation) voice codec. Being FDMA and TDMA, the two protocols are not compatible, but Phase 2 systems can use a dynamic transcoder to translate the voice codecs and enable interoperability between Phase 1 and 2 equipment.

P25 Phase 2 was primarily designed to provide greater spectral efficiency. The AMBE+2 vocoder uses a more compressed bitstream to provide two TDMA voice channels in the same 12.5kHz bandwidth. Known as 6.25kHz bandwidth equivalence, this enables P25 Phase 2 transmissions to meet FCC demands for voice calls to take up less bandwidth.

There are seven other P25 standard interfaces, including the Inter Sub-System Interface (ISSI), which determines how the RF subsystems work with each other and connect into wide area networks; and the Console Sub-System Interface (CSSI), which specifies the messaging between a console subsystem (a dispatcher’s console, for example) and the P25 RF subsystem.

Davis highlights the high level of end-user involvement and their influence over services and technologies within the P25 standard. However, this level of influence means that P25 manufacturers feel obligated to continue their support for older equipment in the field and ensure that 25-year-old radios can talk to their modern counterparts. “We cannot leave older device users behind,” explains Davis. “We have to enable them to communicate with newer devices. That has affected the evolution of the P25 market, as the equipment is expected to last 10-20 years-plus, and that is quite a challenge.”

Current major work items
Most new additions to the P25 standard are generally minor, but major work items are still being developed by TIA TR-8 to meet changes in market demand and advances in technology.

The three current major work items are: group regrouping (ISSI/CSSI); key fill interface extensions beyond key fill device to mobile and portable radios (to key management facility, to authentication facility, and to another key fill device); and link layer encryption security service.

Group regrouping allows a dispatcher to take multiple talk groups that would normally occupy multiple channels at a site and group them together, thereby enabling them to all use the same channel, which is spectrally more efficient than the alternative of patching them across.

P25 has faced some security issues in the past and the other two major work items are aimed at addressing some gaps in the standard’s security design and implementation.

“The security with P25 has been challenging – unlike TETRA, where it is baked into the background as part of the system, for P25 it is an optional extra,” says Ken Rehbehn, founder and principal analyst at CritComm Insights.

This has led to the development of proprietary solutions. “Because of this it has not been easy to achieve interoperability when these security systems have been put in place,” observes Rehbehn. “It works fine within individual agencies, but inter-agency interoperability can be tricky.”

TIA TR-8 is addressing these issues. The key fill interface is used for enabling encryption and encryption key management. Encryption scrambles the voice or data according to a key. If two parties have the same key, they can scramble and descramble their communications.

“But I might want to use one key with you and another one with my supervisor, not just to provide privacy from eavesdroppers, but also to keep the conversation private from other agencies,” explains Davis. “Shared secret keys need to be rotated or changed to avoid being comprised.

“A key fill device is a handheld device that allows you to change keys via a wireline interface. We are extending this service to not only reprogram radios, but also to share with the infrastructure. For example, to enable authentication services to verify that you are who you say you are.”

This work aligns with the Department for Homeland Security’s P25 Compliance Assessment Program (CAP), outlined in 2017, which demands that “…emergency responders must exchange communications seamlessly across disciplines and jurisdictions to successfully respond to incidents emergencies”.

CAP is a programme for testing critical functionality and reporting the results in a consistent way. It requires participants to include P25-compliant AES 256 encryption in products being tested. The aim is to encourage everyone to use the same algorithm, to boost interoperability between agencies.

Davis adds the third major work item, link layer encryption security service (LLSS), is an extension of this growing need for encryption. “At the moment you can encrypt the voice and the data payload, but it will not encrypt your identity or the control message. LLSS will encrypt virtually everything that goes over the air. If you do not have the right key, you will not only not hear voice or see data, but you will also not see who you are talking to or what the control message is.”

Narrowband/broadband interworking
The other major area of focus is to develop a standard way for P25 and broadband LTE systems to interwork. Work on this is being undertaken by the international standards body 3GPP, which writes the specifications for cellular mobile standards such as 4G and 5G.

3GPP said in June 2017 that it is currently evaluating and studying further mission-critical (MC) related topics for LTE Release 15 and beyond, including: “Interworking between the 3GPP defined MC system and legacy systems such as TETRA or P25, for voice and short data service.” Normative work is due for completion in June 2018, with the full Release 15 scheduled for publication in September 2018.

Davis reports that TIA TR-8.8 and ATIS (Alliance for Telecommunications Industry Solutions) in the US are looking at 3GPP mission-critical features that have been added to the LTE standard and, when common with P25 LMR features, defining how the common features may interwork/interoperate. “Certain things need to be translated such as protocols, identity, media translations and encryptions.

“We are looking at what MC LMR and MC LTE features there are in common and trying to map them together. Interworking will be a major work area for us and it is obviously very important in the US for FirstNet.”

The FirstNet network will provide a national mission-critical broadband network for first-responders in the US. But for the time being at least, it will only deliver broadband data services. First-responders will continue to use P25 for mission-critical voice services. “Finding a successful way of interworking between the two technologies is therefore quite important,” observes Davis.

Manufacturer innovation
LMR manufacturers have been aware of the need to connect traditional two-way radio systems to other kinds of networks for some time and have already put solutions onto the market.

Steve Penny, P25 architectural design engineer at Tait Communications, says: “Tait is working on convergence both within LMR and with broadband from many angles. We have developed products like UnifyVehicle and UnifyVoice, along with inter-operation gateways to allow interconnection between systems. Each of these developments has a long roadmap of enhancements and development planned, including taking advantage of interworking between networks, particularly broadband networks such as cellular and Wi-Fi.

“Our primary goal has been to transfer the key content of voice and identity between differing technologies. Our next focus was adding further value to the customer via such functions as emergency signalling and individual calling. We continue to add auxiliary features such as location information to further enhance situational awareness.”

Motorola Solutions has been part of P25 since its inception and produced its first P25 ASTRO 25 products back in 1991. Its current development trajectory is similar to Tait’s, as John Kedzierski, corporate VP, Motorola Solutions Infrastructure Products & Systems, explains. “In terms of our activities and our R&D, the focus is now on enhancing data capabilities.

“Narrowband technology can never deliver data in the way a broadband system can. So, when it comes to responding to major incidents and natural disasters, we want to provide more than voice services. Our WAVE application extends our interoperable platform beyond the P25 coverage area and enables interconnection with cellular and other networks.”

Kedzierski points out that the latest Motorola P25 products all use software-defined radio technology and IP-based platforms, which makes it much easier to support multiple technologies and applications. Motorola Solutions’ aim is to enable customers to focus on their mission by driving seamless interoperability across networks as invisibly as possible.

“Technologies are different, but we want to handle the complexity by creating a platform and developing middleware that can handle different sorts of radio users and which also provide interfaces to data and back-office systems, along with features such as location tracking and access to video users. The access is enabled via common APIs and interfaces.

“The goal is to help customers avoid having to operate multiple platforms,” says Kedzierski. “We want customers to select the right tool for the job and not have to compromise.”

In the long term, Tait sees P25 as a secure, adaptable communications service, which is only one of many communications services offered by the ‘box’ installed in a vehicle for communications. “P25 is excellent for instant, deployable communications,” says Penny. “It is an excellent tool for firefighters in Australia. While in the station and en route to an incident, the trunked P25 radio network is used to communicate and monitor location of the fire trucks.

“On-scene, either a repeater or unit-to-unit broadcast in ‘conventional’ mode is used to maintain local communications, including location of fire trucks. Vehicles can relay the location broadcasts back to dispatch via separate cellular modem and satellite modems installed in the vehicle.

“In the near future, solutions like this will be deployable without external cellular or satellite modems, as they can be integrated via UnifyVehicle, along with other new services that have been requested by public safety agencies.

“Tait sees a gradual move in the public safety world towards a situation where end-users choose a radio technology based on the media content it supports. P25 already allows roaming between multiple systems, which is facilitated via ISSI connections between P25 networks. The 3GPP interworking function will also offer a suitable solution for the cellular MCPTT service.”

P25 market
The majority of P25 networks are found in North America, though they have also been deployed in Australia, New Zealand, Brazil, India and Russia. But is P25 a growing market?

“P25 is a slightly growing market still,” says Rehbehn, “but the curve will continue to flatten out as the years go by, so it is a challenging situation. But voice communications for first-responders remain vital. As important as adding mission-critical data support is, the ability to broadcast to a group of related workers underpins tactical operations.”

A speaker in a TIA video on the history of P25 notes that in 2015, 49 per cent of US first-responders were still using analogue systems. Motorola Solutions’ Kedzierski reveals there are quite a few cases of analogue migration to P25 where the customer is keeping analogue going for basic radio users. “This is why two-way radio is around to stay. Not because it is better, but because it is different. It is harder to provide the necessary propagation and coverage using anything else in the US.”

He reports that new customers are still rolling out big P25 systems and entering into 20-year agreements. “LMR systems are very reliable and very economic to make, and a P25 system can cover a wide area with far fewer number of sites than 4G. There continues to be a large market for digital migration in the US, so there are no concerns from us about the longevity of P25.”

You might expect P25 manufacturers to say that, but Rehbehn provides a concurring independent view.

“I’m bullish about the continuing prospects of P25 and LMR in general. It is not a cynical view to say LMR will be around for quite some time. For the next five years we will largely see digital migration to P25 continuing and mostly to Phase 2 trunked TDMA.

“I see P25 and LTE as complementary technologies for some years yet and P25 and TETRA will remain essential elements of public safety communications for the foreseeable future. After 2025, we may see convergence on an effective substitute, but that substitute must fully address talk around (direct mode) requirements, which LTE is struggling to find a solution for,” says Rehbehn.

Asked when he thinks the P25 standard will be ‘done’, Davis replies: “It will be done when you bury the last radio. It constantly evolves to meet changing needs and technologies. We won’t be done until people stop using it.” Judging by the views expressed here, that date is still quite some way off.

Author: James Atkinson