Project 25 (P25) or APCO-25 refer to a suite of standards for digital radio communications for use by federal, state/province and local public safety agencies in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies. In this regard, P25 fills the same role as the European Tetra protocol, although not interoperable with it.
Suite of Standards Overview
Public safety radios were upgraded from analog to digital decades ago owing to the several limitations that analogs posed and also because technological advancements provided wireless, shared systems that in turn provided enhanced communication services critical to first responders. Therefore, in the late 1970s a functional specification called Project 16 (P16) was developed for Public Safety trunked systems by the Association of Public Safety Communications Officials International (APCO) to include definitions for Public Safety radio communications analog with voice, and radio channel trunking using the newly-allocated 800 MHz frequency band. .
NTIS-produced two-way radio audio. Sequence segments (1) original audio, (2) recovered analog FM with noisy signal, (3) recovered digital audio. — 199 KB
Varying user protocols and different public safety frequency bands, made it difficult for P16 to achieve interoperability and widespread acceptance among the public safety agencies. However, the lessons learned during the disasters that the United States have faced in the past decade, have forced agencies to assess their requirements during a disaster when the basic infrastructure is in a state of failure. To meet the growing demands of public safety digital radio communications, the FCC at the directive of the Congress initiated an inquiry in 1988, to receive recommendations from users and manufacturers to improve the communication systems in existence. . Based on the recommendations, in October of 1989 APCO Project 25 came into existence in a coalition with APCO, National Association of State Telecommunications Directors (NASTD), National Telecommunications and Information Administration (NTIA), the National Communications System (NCS), National Security Agency (NSA) and the US Department of Defense (DoD), to find solutions that best serve the needs of public safety management. .. In addition, a steering committee consisting of representatives from the above mentioned agencies along with FPIC, DHS Coast Guard and the Department of Commerce's the National Institute of Standards Technologies (NIST), Office of Law Enforcement Standards was established to decide the priorities and scope of technical development of P25. .
Interoperable emergency communication is integral to initial response, public health, safety of communities, national security and economic stability. Of all the problems experienced during disaster events, one of the most serious problems is communication due to lack of appropriate and efficient means to collect, process and transmit important and timely information. In some cases, radio communication systems are incompatible and inoperable not just within a jurisdiction but within departments or agencies within the same community.  Non-operability occurs due to use of outdated equipment, limited availability of radio frequencies, isolated or independent planning, lack of coordination and cooperation between agencies, community priorities competing for resources, funding and ownership and control of communications systems. [www.ncjrs.gov/pdffiles1/nij/204348.pdf]. Recognizing and understanding this need, Project 25 (P25) was initiated collaboratively by public safety agencies and manufacturers to address the issue with emergency communication systems. P25 is collaborative project to ensure that two-way radios are interoperable. The goal of P25 is to enable public safety responders to communicate with each other and, thus, achieve enhanced coordination, timely response, and efficient and effective use of communications equipment. P25 was established to address the need for common digital public safety radio communications standards for First Responders and Homeland Security/Emergency Response professionals. TIA TR-8 facilitates such work through its role as an ANSI-accredited Standards Development Organization (SDO).
|“||Project 25 (P25) is a set of standards produced through the joint efforts of the Association of Public Safety Communications Officials International (APCO), the National Association of State Telecommunications Directors (NASTD), selected Federal Agencies and the National Communications System (NCS), and standardized under the Telecommunications Industry Association (TIA)... The P25 suite of standards involves digital Land Mobile Radio (LMR) services for local, state/provincial and national (federal) public safety organizations and agencies...
P25 is applicable to LMR equipment authorized or licensed, in the U.S., under the National Telecommunications and Information Administration (NTIA) or Federal Communications Commission (FCC) rules and regulations.
Although developed primarily for North American public safety services, P25 technology and products are not limited to public safety alone and have also been selected and deployed in other private system application, worldwide.
P25 equipment has also been selected for a railroad system, including rolling stock, personnel, and transportation vehicles.
P25-compliant systems are being increasingly adopted and deployed. Radios can communicate in analog mode with legacy radios, and in either digital or analog mode with other P25 radios. Additionally, the deployment of P25-compliant systems will allow for a high degree of equipment interoperability and compatibility.
P25 standards use the Improved Multiband Excitation (IMBE) vocoders which were designed by DVSI to encode/decode the analog audio signals.
P25 may be used in "talk around" mode without any intervening equipment between two radios, in conventional mode where two radios communicate through a repeater or base station without trunking or in a trunked mode where traffic is automatically assigned to one or more voice channels by a Repeater or Base Station.
The protocol supports the use of DES encryption (56 bit), 2-key Triple-DES encryption (112 bits), 3-key Triple-DES encryption (168-bits), AES encryption at up to 256 bits keylength, RC4 (40 bits, sold by Motorola as Advanced Digital Privacy), or no encryption.
P25 Open Interfaces
P25's Suite of Standards specify eight open interfaces between the various components of a land mobile radio system. These interfaces are:
- Common Air Interface (CAI) standard specifies the type and content of signals transmitted by compliant radios. One radio using CAI should be able to communicate with any other CAI radio, regardless of manufacturer
- Subscriber Data Peripheral Interface standard specifies the port through which mobiles and portables can connect to laptops or data networks
- Fixed Station Interface standard specifies a set of mandatory messages supporting digital voice, data, encryption and telephone interconnect necessary for communication between a Fixed Station and P25 RF Subsystem
- Console Subsystem Interface standard specifies the basic messaging to interface a console subsystem to a P25 RF Subsystem
- Network Management Interface standard specifies a single network management scheme which will allow all network elements of the RF subsystem to be managed
- Data Network Interface standard specifies the RF Subsystem's connections to computers, data networks, or external data sources
- Telephone Interconnect Interface standard specifies the interface to Public Switched Telephone Network (PSTN) supporting both analog and ISDN telephone interfaces.
- Inter RF Subsystem Interface (ISSI) standard specifies the interface between RF subsystems which will allow them to be connected into wide area networks
P25-compliant technology is being deployed in several phases:
- Phase 1
Phase 1 radio systems operate in 12.5 kHz analog, digital or mixed mode. Phase 1 radios use Continuous 4 level FM (C4FM) modulation for digital transmissions at 4800 baud and 2 bits per symbol, yielding 9600 bits per second total channel throughput. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviationat symbol time as C4FM while using only 6.25 kHz of bandwidth.
Vendors are currently shipping Phase 1 P25-compliant systems. These systems involve standardized service and facility specifications, ensuring that any manufacturers' compliant subscriber radio has access to the services described in such specifications. Abilities include backward compatibility and interoperability with other systems, across system boundaries, and regardless of system infrastructure. In addition, the P25 suite of standards provides an open interface to the radio frequency (RF) subsystem to facilitate interlinking of different vendors' systems.
- Phase 2
To improve spectrum utilization, Phase 2 is currently under development with concurrent work being done on 2-slot TDMA and FDMA (CQPSK) modulation schemes. Phase II will use the AMBE vocoder to reduce the needed bitrate so that one channel will only require 4800 bits per second.
Significant attention is also paid to interoperability with legacy equipment, interfacing between repeaters and other subsystems, roaming capacity and spectral efficiency/channel reuse. In addition, Phase 2 work involves console interfacing between repeaters and other subsystems, and man-machine interfaces for console operators that would facilitate centralized training, equipment transitions and personnel movement.
Conventional Project 25 Implementation
Conventional P25 systems don't support CTCSS tone or DCS code for access. Instead they use what is called a Network Access Code, or NAC. This is a 12 bit code that prefixes every packet of data sent (including voice packets).
The NAC is a feature similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only break squelch when receiving the correct NAC. NAC's are programmed as a 3 digit hexadecimal code that is broadcast along with the digital signal being transmitted.
Since the NAC is 3 digit hexadecimal number (12 bits), it gives 4096 possible NAC's for programming, which far exceeds all its analog counterparts combined. It should be noted that 3 of the possible NAC's have special meaning:
- 0x293 ($293) - the default NAC
- 0xf7e ($F7E) - a receiver set for this NAC will unsquelch on any NAC received
- 0xf7f ($F7F) - a repeater receiver set for this NAC will allow all incoming signals and the repeater transmitter will retransmit the received NAC.