Project 25


Project 25 is a suite of standards for interoperable Land Mobile Radio systems designed primarily for public safety users. The standards allow analog conventional, digital conventional, digital trunked, or mixed-mode systems. P25 was originally developed for public safety users in the United States but has also gained acceptance for security, public service, and some commercial applications worldwide. P25 radios are a replacement for analog UHF radios, adding the ability to transfer data as well as voice for more natural implementations of encryption and text messaging. P25 radios are commonly implemented by dispatch organizations, such as police, fire, ambulance and emergency rescue service, using vehicle-mounted radios combined with repeaters and handheld walkie-talkie use.
Starting around 2012, products became available with the newer Phase II modulation protocol. The older protocol known as P25 became P25 Phase I. P25 Phase II products use the more advanced AMBE2+ vocoder, which allows audio to pass through a more compressed bitstream and provides two TDMA voice channels in the same RF bandwidth, while Phase I can provide only one voice channel. However, P25 Phase II infrastructure can provide a "dynamic transcoder" feature that translates between Phase I and Phase II as needed. In addition to this, Phase II radios are backwards compatible with Phase I modulation and analog FM modulation, per the standard. The European Union has created the Terrestrial Trunked Radio and Digital Mobile Radio protocol standards, which fill a similar role to Project 25.

Suite of standards overview

History

Public safety radios have been upgraded from analog FM to digital since the 1990s because of an increased use of data on radio systems for such features as GPS location, trunking, text messaging, metering, and encryption with different levels of security.
Various user protocols and different public safety radio spectrum made it difficult for Public Safety agencies to achieve interoperability and widespread acceptance. However, lessons learned during disasters the United States faced in the past decades have forced agencies to assess their requirements during a disaster when basic infrastructure has failed. To meet the growing demands of public safety digital radio communication, the United States Federal Communications Commission at the direction of the United States Congress initiated a 1988 inquiry for recommendations from users and manufacturers to improve existing communication systems. Based on the recommendations, to find solutions that best serve the needs of public safety management, in October 1989 APCO Project 25 came into existence in a coalition with:
A steering committee consisting of representatives from the above-mentioned agencies along with FPIC, Coast Guard and the Department of Commerce's National Institute of Standards and Technology, Office of Law Enforcement Standards was established to decide the priorities and scope of technical development of P25.

Introduction

Interoperable emergency communication is integral to initial response, public health, community safety, national security and economic stability. Of all the problems experienced during disaster events, one of the most serious is poor communication due to lack of appropriate and efficient means to collect, process, and transmit important information in a timely fashion. In some cases, radio communication systems are incompatible and inoperable not just within a jurisdiction but within departments or agencies in 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. Recognizing and understanding this need, Project 25 was initiated collaboratively by public safety agencies and manufacturers to address the issue with emergency communication systems. P25 is a 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. The Telecommunications Industry Association's engineering committee facilitates such work through its role as an ANSI-accredited standards development organization and has published the P25 suite of standards as the TIA-102 series of documents, which now include 49 separate parts on Land Mobile Radio and TDMA implementations of the technology for public safety.
P25-compliant systems are being increasingly adopted and deployed throughout the United States, as well as other countries. 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 proprietary Improved Multi-Band Excitation and Advanced Multi-Band Excitation voice codecs which were designed by Digital Voice Systems, Inc. to encode/decode the analog audio signals. It is rumored that the licensing cost for the voice-codecs that are used in P25 standard devices is the main reason that the cost of P25 compatible devices is so high.
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 Data Encryption Standard encryption, 2-key Triple-DES encryption, three-key Triple-DES encryption, Advanced Encryption Standard encryption at up to 256 bits keylength, RC4, or no encryption.
The RC4 Advanced Digital Privacy can withstand casual attackers. It is supposed to offer 40-bit security, where an attacker must test the 2 to the power of 40 possible keys to find the right one. This level of encryption offers no real protection and there is software that allows you to find the key.
The protocol also supports the ACCORDION 1.3, BATON, Firefly, MAYFLY and SAVILLE Type 1 ciphers.

Standards development process

The P25 User Needs Working Group, which represents P25 users, identifies user needs for the P25 standards, which are communicated to the P25 Steering Committee. The P25 Steering Committee adds identified P25 user needs to the Statement of P25 User Needs document. The TIA TR-8 Engineering Committee and its subcommittees, which represents manufacturers in the P25 industry, is then expected to develop TIA-102 standards that satisfy identified P25 user needs.
Once developed, TIA-102 standards may also subsequently be adopted by the P25 Steering Committee as P25 standards, and adopted by ANSI as American National Standards; however, TIA-102 standards do not automatically become P25 standards, and some TIA-102 standards have never been adopted by ANSI. The TIA-102 standards, P25 standards, and associated ANSI standards have not been adopted by ISO as de jure international standards; however, P25 systems have been deployed in 83 countries, so they nonetheless serve as one set of de facto international standards alongside other international Land Mobile Radio standards such as TETRA and DMR.

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 – 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 supporting both analog and ISDN telephone interfaces.
  • Inter RF Subsystem Interface – standard specifies the interface between RF subsystems which will allow them to be connected into wide area networks
  • Key Fill Interface - standard messaging protocol for bidirectional update of encryption keys via transfer of unencrypted and encrypted key variables from a Key Fill Device to the equipment containing the encryption service
  • Inter-KMF-Interface - interface for encrypted interoperability between radios managed by different Key Management Facilities
  • KFD-KMF Interface - interface between KFD and KMF for radios managed by different KMF