Why P25 technology remains a good fit for public safety communications

Public safety agencies in many countries depend on two-way radio communications for daily and incident communications. Digital land mobile radio (LMR) technology is particularly critical and ensures police officers, firefighters and emergency management teams can respond quickly and efficiently when emergencies arise.

P25 for public safety

One digital LMR standard that is particularly beneficial for public safety communications is Project 25 (P25) technology. Developed with state, local and federal government representatives and Telecommunications Industry Association (TIA) governance, P25 has gained worldwide acceptance for public safety, security, public service and commercial applications. Radio equipment that demonstrates compliance with P25 can meet a set of minimum requirements to fit users’ needs. In fact, the P25 standard was initially created for public safety professionals, although the technology is also used globally by utilities, mining companies, transportation agencies and other mission-critical infrastructure entities.

P25 systems can operate in conventional or trunked modes, with two phases of the technology. Phase 1 is based on frequency-division multiple access (FDMA) technology. It was designed to use the same bandwidth as a narrowband analog FM 25 kHz channel, meaning they could be used side by side and narrowband FM channels could gradually be replaced by P25 digital channels.

P25 Phase 2 is based on time-division multiple access (TDMA) technology and is more spectrally efficient than Phase 1, providing two effective channels per 12.5 kHz bandwidth. In P25 Phase 2, each physical base station provides two voice channels. With TDMA, two independent conversations share the same channel.

Another benefit of P25 Phase 2 is longer battery life. Because of TDMA, the transmitter is only working half the time during a transmission, therefore increasing the battery life of the portable. In addition, Phase 2, only available for trunked P25 operation, is backwards compatible with Phase 1 radios, allowing a new Phase 2 radio to operate with users of P25 Phase 1 equipment and to operate on Phase 1 network equipment.

Customers can choose from multiple P25 system configurations, including direct mode, repeated, single site, multi-site, voting, multicast and simulcast systems. Optional features create cost-effective system applications and geographic coverage based on unique user requirements. Backward and forward compatibility considerations are built into the P25 standards, easing future system expansions and migration to new technology upgrades.

P25’s standardised interfaces

In a recent social media poll asking what the most significant advantage of open standards is in the critical communications industry, more than half (53%) of poll respondents said “interoperability” was the most considerable advantage of open standards. While other considerations including freedom of vendor choice, lower prices and avoiding technology obsolescence are essential, interoperability is a massive issue for communications managers around the globe. Communication during disasters is rarely easy and often crosses different agencies and companies, involving several emergency networks. Interoperable communications systems are the key to keeping first responders safe and for the most effective and efficient response to incidents requiring mutual aid.

One way to address interoperability challenges is by purchasing standards-based equipment. The TIA TR-8 Working Group that oversees P25 standards updates the technology, with recent enhancements to security, for example, and ensures it continually addresses first responders’ needs.

The P25 standard includes several interfaces to help ensure interoperability. The Common Air Interface (CAI) standardises the point of connection between radio transmitters and receivers. Simply put, the CAI defines the technical form and function of the digital signal that goes over the airwaves and how P25 radios would communicate with one another at the most basic level. For example, the CAI allows radios from Vendor A to operate on P25 infrastructure from Vendor B.

Some other interfaces include the following:

• The Inter-RF Subsystem Interface (ISSI) defines how different P25 radio networks can connect with one another — a key issue of communications interoperability. This effectively allows the full P25 system of Vendor A to be connected to the full P25 network of Vendor B, for example.
• The Console Subsystem Interface (CSSI) allows the radio frequency (RF) components of a P25 system and command and control consoles to connect with one another.
• The Fixed Station Interface (FSI) defines how components of a P25 radio system that are fixed in place connect with other components of the system. Dispatcher consoles are typically used to access fixed RF stations, so the CSSI and FSI are interdependent in most applications.
   

The P25 Compliance Assessment Program (CAP), overseen by the US Department of Homeland Security (DHS), ensures interoperability between the equipment from the various P25 manufacturers. The program allows suppliers to publicly attest to their products’ compliance through P25 CAP testing at DHS-recognised laboratories. As proof, suppliers are required to submit test reports and other documents to confirm the testing. All the documents and lists of compliant equipment are housed on the P25 CAP website at https://www.dhs.gov/science-and-technology/p25-cap. This interoperability transparency means that clients can verify that features offered by manufacturers are CAP-compliant and not proprietary.

Also, as public safety agencies adopt broadband services to enhance their data connectivity, P25 systems can interoperate with commercial 4G/5G services. The 3GPP-compliant Inter Working Function (IWF) with authentication anchored on the respective core networks will support P25 and LTE interoperability.

P25 interoperability in mining environments

Beyond public safety agencies, many mining companies also chose P25 for similar interoperability needs. As contractors move between various mine sites, they can use a single P25 radio to operate on networks from multiple manufacturers. In addition, multi-protocol radios that operate on, say, P25 and DMR enable these contractors to move between networks based on varying technologies.

*Sandra Wendelken is the Strategy and Insights Manager at Tait Communications, with several decades of experience in the LMR and wireless industry. Before joining Tait in 2023, she was a senior research analyst at market research firm IDC from 2020 to 2023 covering B2B and consumer wireless technology. She was also editor of Radio Resource International and Mission Critical Communications magazines from 2006 to 2020, where she closely tracked the US and global mission-critical communications industries.