What is Project 25?

What is P25 or Project 25?

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- everything RF

Jul 17, 2023

P25 or Project 25 is a set of standards for interoperable digital public land mobile radio (LMR) systems (or P25 radio systems). P25 radio systems are used by public safety organizations such as the police & fire department, ambulance services, law enforcement, and other government agencies to ensure public safety & security during emergency situations and disaster events. These public safety organizations typically implement the P25 public radio network/P25 radio system by using vehicle-mounted radios combined with repeaters. They usually connect to this network using handheld walkie-talkies. 

P25 standards are open system standards that standardize the interfaces between the various components (e.g., P25 radios, repeaters, etc) of the digital LMR systems/P25 radio systems that emergency responders use.

Interoperable emergency communication is essential for initial response, public health & security, as well as national security and economic stability. But, during disaster events, interoperable emergency communication is often hampered due to several factors such as non-aligned spectrum planning, independent or isolated planning, lack of coordination & cooperation between agencies, and limited availability of radio frequencies. In some cases, incompatible and inoperable radio communication systems within a jurisdiction, and also within agencies or departments in the same community may occur which will lead to poor communication. Because of poor communication, it is difficult to collect, process, and transmit important information in a timely fashion during disaster events. Public safety agencies and manufacturers collaboratively initiated Project 25 (P25) to solve the issues (as mentioned above) with emergency communication systems.

To find solutions that best serve the needs of public safety management/responders, in October 1989, Project 25 was initiated by the Association of Public Safety Communications Officials International (APCO) in coalition with the National Association of State Telecommunications Directors (NASTD), National Telecommunications and Information Administration (NTIA), National Communications System (NCS), National Security Agency (NSA), and Department of Defense (DoD), and standardized under the Telecommunications Industry Association (TIA). TIA develops and maintains the TIA-102 series of standards for APCO P25; on behalf of APCO International.

Project 25, or P25 is a collaborative project for ensuring that the two-way digital radios are interoperable. The primary goal of the P25 radio system is to provide a reliable & interoperable emergency communication network for public safety responders to communicate with each other; thereby, achieving enhanced coordination, and timely response. P25 public safety radio network also supports direct mode communication when the local infrastructure is down/faulty, or users are out of network coverage.

P25 radios are designed to support both conventional and trunked radio systems. P25 technology is being deployed in several phases (e.g., Phase 1 P25 & Phase 2 P25), with future phases yet to be finalized. P25 radio systems offer benefits like interoperable radio systems, backwards compatibility, encryption capability, maximum spectrum efficiency, better audio quality, frequency-reuse, and price competition. The P25 radio systems are available with frequency bands: VHF (136 – 174 MHz), UHF (403 – 512 MHz, 806 – 870 MHz), and 700 MHz (746 – 806 MHz).

P25 radio technology is being deployed in several phases with future phases yet to be finalized.

Phase 1: Phase 1 radio systems are designed to operate in 12.5 kHz analog, digital or mixed mode and carry one voice call (one person talking) per transmission. In the P25 Phase 1 radio systems, the digital transmission utilizes Continuous 4 level FM (C4FM) modulation, at 4,800 baud and 2 bits per symbol, yielding 9,600 bits per second total channel throughput. Phase 1 P25 radios are backward compatible and interoperable with legacy systems (i.e., analog FM systems). P25 Phase 1 uses the IMBE (Improved Multi-Band Excitation) voice codec.

Phase 2: Phase 2 radio system is more spectrally efficient than the Phase 1 radio system. The Phase 2 radio systems use a 2-slot TDMA scheme, providing two effective channels per 12.5 KHz bandwidth. The Time Division Multiple Access or TDMA scheme allows two independent conversations to share the same channel. P25 Phase 2 radios are backwards compatible with Phase 1 radios and also allow to save battery life. Phase 2 uses CQPSK modulation.

P25 Phase 2 was developed for trunked P25 operation. Phase 1 is still used to support radio to radio and conventional P25 operation. A trunked radio system is a more advanced form of radio communication that optimizes the utilization of available frequencies. Unlike conventional systems, trunked systems dynamically allocate frequencies to users on-demand; based on immediate communication needs. Instead of assigning a dedicated frequency/channel to each user or group, the system dynamically assigns an available channel for each individual call or transmission. This allows for more efficient use of limited frequency resources, as channels are shared among multiple users. Trunked systems rely on control channels and computerized algorithms to manage frequency assignments, call setup, and resource allocation.

Phase 3: Phase 3 standards are under development. The implementation of Phase 3 will address the need for high-speed data for public-safety use. It can be used to transmit and receive voice, video, and high-speed data in widearea, multiple-agency networks. The TIA and European Telecommunications Standards Institute (ETSI) are working together on Phase 3, known as Project MESA (Mobility for Emergency and Safety Applications).

The P25 technology was originally developed primarily for North American public safety services but has also been adopted for public safety, security, public service, and commercial applications worldwide. The European Union has created the Terrestrial Trunked Radio (TETRA) digital standard which fills a similar role to P25, for use in Europe.

The P25 and TETRA are the two leading digital Land Mobile Radio standards in the world today. However, there are additional digital radio systems with enhanced spectral efficiency that have been submitted to the International Telecommunication Union's Radiocommunication Sector (ITU-R) Study Group 8 and its Working Party 8A.

Countries with P25 interoperable equipment or network (image from dvsinc.com, P25-training-guide)

P25 radio system architecture and working principle

P25 radios operate similarly to conventional analog FM radios. In fact, P25 radios can operate both in digital mode and analog mode. The analog mode of operation makes the P25 radios backwards compatible with existing analog radio systems. Typical P25 radio system architecture is shown in the following figure.

Figure: P25 radio system architecture (image from dvsinc.com, P25-training-guide)

In the P25 analog mode, the P25 radio will work similar to a conventional analog system, with the capability for DCS, CTCSS, pre-emphasis and de-emphasis, and other standard analog features.

In P25 digital mode, the P25 transmitter uses  Improved Multi-Band Excitation (IMBE™) vocoder (speech coder) to convert all analog audio input to packets of digital information,  then de-vocode the digital information back to analog audio (using speech decoder) in the receiver.The channel coder on the transmitter side will add error correction codes to the transmitted data and utilize data protection techniques (encryption) to ensure that the data (voice or control) arrives and is recovered correctly. Error correction and data protection improve the system performance by overcoming channel impairments such as noise, fading, and interference.   Error correction and data protection improve the system performance by overcoming channel impairments such as noise, fading, and interference. 

The P25 radio system modulator modulates the signal and sends the signal via the antenna. In the case of the P21 Phase 1 radio system, the digital transmission utilizes the C4FM modulation method to transmit information. P25 radio supports a data transmission rate of 9600 bps. On the receiver side, the demodulator demodulates the received signal and converts it back into the original audio with the help of a channel decoder, a speech decoder, and a digital-to-analog converter.

Key benefits of the P25 Public Safety Radio Network/P25 Radio Systems:

Open standards: P25 standards are open standards that enable interoperability among P25 products of multiple manufacturers. This means that different safety service departments / agencies and teams can communicate, even if they have purchased their P25 equipment from different vendors. Also, the products from multiple manufacturers ensure the availability of a wide range of products in the market and also provide choice and price competition. Note that P25 products (e.g., P25 radios, repeaters, network equipment, etc) from various manufacturers should compliant with P25 standards.

Backward Compatibility with existing equipment: The digital P25 radios (e.g., Phase 1 P25 radios) are backward compatible and include an analog mode of operation. The analog mode of operation allows the P25 radios to communicate with legacy radios that use analog FM technology. Additionally, the P25 radios can communicate with other P25 radios in either digital or analog mode. This will enable the user a gradual migration from the analog system to the digital radio system while continuing to use older equipment.

P25 backwards compatible (image from dvsinc.com, P25-training-guide)

Price Competition: P25 open standards allow the purchasing of P25 radio systems equipment from different vendors, which results in price competition at the time of a new network installation, as well as throughout the life of the system as it expands or parts get replaced.

Encryption Capability: One of the key requirements of the P25 standard is: protecting digital radio communications (voice and data) with encryption. By encrypting communications, a third party cannot be intercepted the communication; this can be very important for police and tactical teams. The encryption used in the P25 radio system is the user's option. It allows the user to select either un-encrypted (clear) or encrypted (secure) digital communication methods.

P25 maximizes spectrum efficiency: P25 maximizes spectrum efficiency by narrowing bandwidths. The spectrum efficiency frees up more channels for radio system use.

Improved audio quality: Digital radio system provides better audio quality as compared to the analog system. The analog system is usually suffered from background noise, hiss, and distortion on the radio channel.

Frequency re-use: P25 equipment is available in many different frequency bands, including VHF, UHF, 700, and 800 MHz frequency bands. P25 occupies the same bandwidth as a narrowband FM radio channel. This means that organizations and agencies can upgrade their existing equipment to P25 without needing to change their radio channels (i.e., frequency re-use).

Key Featured of P25 Radios:

  • Voice calls to groups 
  • Individual or unit-unit calling
  • Emergency calls to help somebody who needs help while in danger.
  • Talking party identification
  • Inhibit and uninhibit features help to disable a lost or stolen radio so it no longer functions
  • Call alert & status updates
  • Ability to carry IP data for advanced data applications, and support for GPS and location services, strengthens user safety by being able to immediately locate them in an emergency situation