Project 25: the Quest for Interoperable Radios by Dan Hawkins SEARCH

Number 6 May 2007 Project 25: The Quest for Interoperable Radios By Dan Hawkins SEARCH Project 25. Few terms have been more The Origin munications interoperability. closely associated with the quest for Project 25 began in 1989 as a joint effort 1. The standards initiative was begun communications interoperability among of the Association of Public-Safety Com- and driven by public safety agencies public safety agencies. But what exactly munications Officials–International, Inc. and organizations. is Project 25? Where did it come from (APCO) and the National Association and where is it headed? What is its value of State Telecommunications Directors 2. It proceeded with both a vision of to public safety agencies and technology (NASTD) to ensure a future with an forthcoming technological change managers? open, standards-based alternative for and the need for graceful migration digital radio systems. With support of between technologies used by public This Issue Brief attempts to answer those other public safety organizations, federal safety agencies. questions, providing background and agency radio users, and industry, the ef- 3. Competition founded on open stan- current information for decision-makers fort has progressed for many years. dards would produce the best technol- who may be considering use of radios ogy, at the best prices for public safety and radio systems built around standards Today, Project 25 provides those stan- agencies. that have arisen from the project. dards and is being extended to eliminate the vendor lock-in that adopters of pro- Many individuals representing agencies A Short History of prietary trunked systems have faced. at all levels of government and through- Project 25 out the land mobile radio industry have The Name contributed to P25 over the years. Many The late 1980s were the beginning of Project 25 received its name following continue to contribute to this day. Some significant technological change in the of the biggest contributors were public realm of police, fire, and other public APCO’s tradition of numbering its broad initiatives that affect the public safety safety agencies that adopted the technol- safety radio systems. After many years ogy early, in effect serving as testers while of analog (FM or frequency modulated) communications world. Project 25 or P25, as it is also commonly known, is the standards and their implementations radio technologies predominating, and were honed. after several years of early trunked radio association’s best-known project. Today, systems evolution, leaders in the public “P25” has come to be synonymous with “public safety digital radio standards.” Early on, public safety participants pro- safety communications community saw vided specifications in the form the P25 digital technologies emerging. They iden- The Work Statement of Requirements (SOR) docu- tified a need—and an opportunity—to ment. This became the base document Three key aspects of Project 25 make it escape proprietary systems by setting for future standards. Today, portions of future standards. particularly important for improved com- these specifications have been codified The CAI was a major step because it de- Trunking 101: How Trunked by the American National Standards fined how P25 radios would communicate and Conventional Radio Institute (ANSI) through efforts of the with one another at the most basic level. Systems Differ Telecommunications Industry Associa- Where the analog world of conventional tion (TIA) and the Electronic Industries FM radio is both well standardized and Trunked radio systems Alliance (EIA). Formally, P25 specifica- devoid of proprietary techniques that dynamically assign tions are defined in the ANSI/TIA/EIA hinder compatibility between manufactur- radio frequencies 102 suite of standards. ers, digital radio technology is not. Project and physical 25 undertook a major effort to assure that channels Interface Standards intellectual property rights of inventors and manufacturers did not impede devel- to users. Project 25 defined a general system opment of and competition in providing From the model for public safety radio communica- public safety digital radio. The CAI, there- perspective tions with eight open interfaces. These fore, is key for technological interoperabil- interfaces connected components, or of users, ity between P25 systems. subsystems, of radio systems that were talkgroups Other Interfaces are selected But P25 is much more than just the CAI. It on their radio goes well beyond to define more than 30 to steer transmissions to services provided across its CAI and seven the intended audience. other interfaces. A full explanation of the The user does not know P25 system model is beyond the scope of which frequency or physical this article, but recent work on three key becoming increasingly complex year after interfaces offers promise for future interop- radio channel that a given erability of digital radio systems. transmission will be made year. Beyond just transmitters, receivers, and dispatch consoles, today’s advanced • Inter-RF Subsystem Interface (ISSI): upon. By contrast, users of radio systems may have data, telephone, Defines how different P25 radio net- conventional radio systems and network management subsystems, works can connect with one another—a select fixed frequencies or as well as connections with other radio key issue of communications interoper- physical channels when they systems. Between each subsystem is some ability. type of interface. transmit, anticipating that a • Console Subsystem Interface (CSSI): Defines how the radio frequency (RF) predetermined set of users The Common Air Interface will be on that channel. components of a P25 system and con- The first set of standards developed soles, as are commonly used by public through Project 25 dealt with the Com- safety dispatchers, connect with one mon Air Interface or CAI—the point The primary value of another. of connection between radio transmit- trunked radio is channel ters and receivers. Simply put, the CAI • Fixed Station Interface (FSI): Defines efficiency: More users can defines the technical form and function how components of a P25 radio system that are fixed in place—as contrasted share a fixed number of of the digital signal that goes over the to those that are mobile or portable in channels without regularly airwaves. It specifies how audio is digi- tized and encoded on inherently analog operation—connect with other com- interfering with one another. radio waves. The CAI specifies the data ponents of the system. Dispatcher con- The number of simultaneous rate of the digital signal stream and how soles are typically used to access fixed transmissions that can that stream is split into frames variously RF stations, so the CSSI and FSI are interdependent in most applications. occur is essentially the same controlling communications, carrying the between conventional and audio payload, and passing forward error correction information to help receivers These three interfaces are crucial to trunked systems. recover missing bits and pieces. interoperability between digital radio 2 Project 25: The Quest for Interoperable Radios systems using P25 standards. As of early Singularly, Project 25 provides the means sufficient to replace all existing systems, 2007 they are still under development to standardize digital voice radio systems meaning that requirements to use digital and the timeframe for availability of for public safety. Early on, it provided technology could actually reduce interop- equipment meeting a full complement a common system model to guide the erability. of standards is unknown. Each interface development of standards, assurances provides multiple services, eventu- for inventors and manufacturers that On the other hand, since P25-compli- ally involving baseline and increasingly intellectual property rights would be ant radios are backwardly compatible, complex standards. By comparison, the protected, and the guarantee that fair newly purchased radios can be used on CAI is incorporated in equipment widely licensing of component technologies legacy systems. There is a cost premium available today. would provide competition. These steps, today for P25-capable radios, but many alone, provided a nucleus around which consider this the cost of cross-systems How Does P25 Improve standards for emerging technology arose, compatibility and expect it to disappear promoting interoperability. over time. Some agencies have purchased Interoperability? P25-capable radios to operate solely on P25 has matured during a period of great Backward Compatibility analog systems, expecting their neighbors need and demand for communication in- If standards for future systems were all to transition to digital or even to make teroperability among first responders. Its that P25 meant to interoperability, that the transition themselves at some point. value for interoperability is twofold: First, alone would be significant. But techno- harnessing the naturally disruptive trend logical change doesn’t occur in a vacuum. Federal funding requirements for the of technological change and, second, Analog radio uses will continue in public use of P25 are simply advisory today. requiring backward compatibility. Let’s safety applications for the foreseeable For example, consider the following look at these two effects on interoperabil- future. P25 established early on that the Department of Homeland Security grant ity individually. first and subsequent phases of technolo- program statement:

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