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 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 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 , 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 - 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

 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: gy under its banner would be backwardly “When procuring equipment for A Digital Standard compatible. That is, radios built to P25 communication system development As mentioned, P25 was born from the standards would be technically capable and expansion, a standards based widespread trend toward digital radio of communicating with earlier analog approach should be used to begin technologies across most communica- radios, including within trunked systems. migration to multi-jurisdictional and tions markets. Analog transmission tech- The standard specifies that each vendor’s multi-disciplinary interoperability. nologies have been replaced with digital P25 equipment must be backwardly Specifically,all new voice systems in many commercial and common carrier compatible with its own analog trunked should be compatible with the ANSI/ systems, such as cellular telephone. De- technology. TIA/EIAA-102 Phase 1 (Project 25 or mand for greater capacity and features, P25) suite of standards. This recom- combined with limited radio frequency This principle of backward compatibility mendation is intended for govern- spectrum, has driven a transition to digi- will ensure that as P25 evolves, emerging ment owned or leased land mobile tal technologies in the public safety world P25-compatible radios likewise will be public safety radio equipment and as it has elsewhere. capable of communicating with earlier its purpose is to make sure that such P25 generations. equipment or systems are capable of Digital radios of any form—the first interoperating with other public safety responder’s portable radio, a cellular tele- When Is P25 Required? land mobile equipment or systems. It phone, or otherwise—are not inherently is not intended to apply to commer- more or less interoperable than analog The factors influencing interoperability cial services that offer other types of radios. Both use technologies that convert have led to much discussion and some interoperability solutions and does not the human voice and other consumable promotion of Project 25-compliant exclude any application if it demon- forms of information into forms unintel- radios to solve the nation’s interoperabil- strates that the system or equipment ligible to humans, then move it invisibly ity woes. Early enthusiasm for requiring being proposed will lead to enhanced over the airwaves to other radios where it as a condition of all federal funding interoperability.”1 the process is reversed. The difference for public safety radios has waned as it became apparent that, nationwide, some 1 FY 2006 Homeland Security Grant Program: is that digital techniques require more Program Guidance and Application Kit, (Washing- complex electronics and often involve analog systems will continue to be used ton, D.C.: U.S. Department of Homeland Security, proprietary standards. well into the future. Funding was far from December 2005) at p. K-1. Emphasis in original.

Project 25: The Quest for Interoperable Radios  One realm where P25 is required by fed- Narrowband Channels eral regulation is in the use of interoper- Most public safety radio users in the ability frequencies specifically designated United States are facing an FCC require- by the Federal Communications Commis- ment to cut the width of their channels sion (FCC) in public safety segments of in half over the next few years. P25 users the 700 MHz band. Anticipating broad have already met this “narrowbanding” use of digital technologies in this band mandate because their digital technol- and to standardize use, the FCC desig- ogy already operates within the narrower nated P25 as the only authorized mode of channel widths. It was designed that way. transmission on the interagency frequen- The FCC requirement can also be met cies. with most existing, FM analog radios without migrating to digital technologies Does P25 Guarantee This is not a P25 flaw, but rather an simply by reprogramming channels. Ana- Interoperability? unfortunate reality. Different frequency log users, however, face likely reductions No technology can guarantee that public bands offer differing advantages. The ad- in transmission range with channels half safety agencies have communications vantages of VHF for state highway patrol as wide, while evidence suggests that P25 interoperability with their cooperators in officers spread widely across a rural state signals, using no more bandwidth, have a emergency response. Other factors, such are different from those of 800 MHz for range comparable to that of traditional, as how the technology is used, whether police officers in the concrete and steel wider FM transmissions. common or compatible procedures are canyons of our major cities. As long as in use, and whether they are incorpo- different radio types are needed for such Shared Systems widely separated frequency bands, in- rated into a single incident management P25 has found favor in systems shared teroperability issues remain—regardless system, greatly affect how well agencies among multiple agencies, primarily in of P25 standardization. communicate with one another. trunked systems where each can have virtually private channels for intra-agency Even in the realm of technology, P25 is What Value Is P25 to My communications. Use of shared systems not a guarantor of technological interop- Agency? of any form provides the technological erability. Radios in different frequency compatibility between agencies needed The increased availability of funding bands using P25 digital standards are for interagency communications as well. today no more able to communicate for public safety radio systems has led to many new systems in recent years. directly with one another than if they Whether trunked or conventional, shared Agencies looking to take advantage of were using FM analog transmissions. The systems generally improve both intra- the efficiencies and capabilities of digital CAI provides a standard of technological and interagency communications. interoperability between radios operating technologies have turned to P25 as the public safety standard for voice radio sys- across a common network, or in a com- Embedded Data Capabilities mon band on separate systems by direct tems. Federal funding has promoted this Digital radio techniques are a natural and unit-to-unit transmissions. However, P25 for interoperability purposes, but P25 efficient tool for data transmission. In ef- user radios in different bands still must offers much in addition to technological fect, they turn analog audio and signaling be interconnected through a gateway of compatibility. information into data anyway, so status some form to communicate, just as they codes, text, and other forms of data can would be if using traditional FM analog Most commonly used in trunked radio be transmitted as easily as voice. In addi- or even proprietary systems. systems, P25 brings the natural spectrum efficiency of both digital and narrowband tion, data can be embedded in the digital stream independent of the encoded voice For example, responders from an agency radio techniques. P25 trunked systems signal. Useful information, such as the using a P25 system with conventional are largely still proprietary today, but transmitting unit’s identification number, repeaters in VHF-high band and those portable and mobile radios operating location, and other information useful from another agency using a P25 800 on them are technologically capable of for tracking the status of the radio user, MHz trunked system will have to rely on communicating with conventional (non- can be carried along with the voice signal their systems being connected together trunked) digital and analog users of the payload. through gateways just as they would if no same frequency band, offering additional P25 standards were in use. interoperability options.

 Project 25: The Quest for Interoperable Radios Project 25 foresaw the value in this for Are There Disadvantages? Trunked systems prevent this through an public safety, squeezing every bit out of automatic request-to-send/clear-to-send the digital data stream to help make the Despite these advantages, there are disad- handshake between user radios and a transmission more resilient to fading and vantages of P25 radios and systems. The central controller connected to fixed sites. interference, as well as making it suitable two most oft-cited disadvantages—cost Since trunked user radios talk to and for carrying embedded data. Many of the and conventional use interference—have through these fixed sites to communicate proprietary signaling techniques used more to do with the newness of the tech- with one another, the system regulates with analog radios can be implemented nology and effects of digital radio rather who talks when. Trunked radio users are as standard features in P25 systems. than with the standard suite itself. The trained to press the push-to-talk (PTT) third disadvantage, complexity, is unfor- button on their radios and wait for a par- tunately inherent in the use of more and ticular tone signaling that they are clear more advanced technologies to support to talk. If no channel is available, they Encryption has long been difficult to public safety response. implement well in analog systems. Being hear a different tone. naturally encoded, the P25 digital signal Disadvantage #1: Cost is at once less susceptible to interception As illustrated in Figure 1 on page 6, con- than an FM signal, but more important, As with any technology, early adopters ventional (nontrunked) system users can the digital payload can be truly encrypted have paid a premium for P25 radios com- easily interfere with one another. In this with no additional overhead on the ever- pared to more “mature” technologies. Use example, User 1 transmits while User 2 is limited communications channel. of P25 is limited largely to government listening to User 3. User 1 can’t hear User agencies in the United States, leading to 3, so is unaware that the transmission will Traditional encryption techniques on higher costs compared to less specialized interfere with another one in progress. analog channels have invariably brought technology. While P25 equipment will reduced range and management chal- drop in price as use spreads, it likely will Whether digital or analog, conventional lenges. Not only is an encrypted P25 always sell at something of a premium radio use suffers this sort of interference signal indistinguishable in terms of range over radio technologies in broader, more problem. Analog radio users quickly from its unencrypted counterpart, but general use. come to recognize the sound of colliding the standards effort sought strongly to transmissions and typically announce make encryption a more practical tool Disadvantage #2: Conventional that someone “walked on” another radio for public safety agencies. It brought the Use and Interference user. term “over the air re-keying” (OTAR) into P25 has found limited adoption in the mainstream to greatly reduce the conventional systems. This may be due In the digital world, however, this sort of need for technicians to physically touch to difficulty in justifying the added cost interference results in the receiver losing each and every user radio when a new without the need for and benefits of the signal altogether. That is, the stream encryption key is established to maintain trunked systems. It may also be because of digital bits flowing to the receiver sud- security—ideally fairly often. without the inherent channel access denly has another stream interspersed. controls that trunked systems impose, The result is that the receiver goes silent, The compromises in coverage and transmission collisions between conven- unable to extract an intelligent signal prohibitive management overhead of en- tional system users are more significant from the airwaves. Interference isn’t cryption in the analog world are ancient when using digital channels than when heard; both signals are considered noise history to P25 system managers. using analog channels. and discarded.

To explain this latter point, consider that This effect is not unique to Project 25 it is not uncommon for two field users digital radios. Most users of digital cel- of a conventional repeater to transmit lular telephones recognize the audible simultaneously, leading to a noticeable effect when a signal starts to go bad, for distortion of the input signals retransmit- example. By definition, there is no central ted by the repeater. It is equally common controller or control channel in a con- in direct, nonrepeated use of a chan- ventional radio system regulating which nel for users to “walk on” one another radio gets to transmit at a given time. unintentionally through simultaneous Direct digital transmissions between P25 attempts to talk. radios are susceptible to this particularly

Project 25: The Quest for Interoperable Radios  Figure 1: Interfering Users destructive form of radio interference, The complexity and richness of technol- the broad base they have established, reducing the value of digital techniques ogy made possible by P25 brings oppor- incrementally defining finer and finer in conventional radio use. tunities for various implementations that points necessary to meet P25 goals. don’t necessarily or automatically allow Disadvantage #3: Complexity interoperability. Incompatibilities have P25 Phase 1 Breeds Incompatibilities arisen between equipment and systems Efforts to establish a system model, P25 (ANSI/TIA/EIA-102) is a rich set of nominally built to the first phase of P25 define core and interface standards, and standards that can be interpreted and standards. provide the first technological specifica- implemented in different ways. This isn’t tions for digital radios are referred to as to say that it is vague, but rather that Today, there is no general conformance Project 25 Phase 1. During this period, it has many features and configuration testing program to guarantee that a specifications necessary for digital radios options. particular system, in all of its options and transmitting within the FCC’s currently complexity, meets a composite standard defined “narrowband” channel widths Encryption is a seemingly simple, but of compatibility. (12.5 kHz) have been established. Much often important, feature with many of the effort necessary to complete such configuration options that can lead to in- What Is the Current Status a far-reaching standards suite has been teroperability challenges. Under normal of P25? completed. Subsequent standards build operations, two agencies using otherwise upon the central ones. compatible systems may need to secure The status of P25 standards is constantly their communications from all others; yet changing as more and more of the com- Today, mobile and portable radios that in an emergency requiring joint opera- plex suite is fleshed out. Of the 90 or so can communicate with one another are tions, they need to share their secured standards eventually anticipated as part available from multiple manufactur- channels. This requires a good deal of of the suite, 34 have been established. ers. Though the feature sets of modern shared infrastructure, not to mention These are the central, most fundamental trunked radio systems are rich and careful preplanning. It doesn’t happen standards defining public safety digital complex, there have been successful dem- simply by virtue of having adopted P25. radio. Standards to follow will build upon onstrations of field radios from different

 Project 25: The Quest for Interoperable Radios manufacturers being used on systems gists expect that some day in the future, made by others. demands for more public safety chan- nels will require further reduction of Beyond the CAI, other key baseline stan- the channel widths, allowing more radio dards for interfaces have been approved communications into a limited amount in the past year, including for the key ISSI of spectrum. and FSI interfaces. These standards are pending publication by TIA, after which Project 25 Phase 2 standards setting has compliant equipment will begin to be proceeded quietly as Phase 1 standards available. The Project 25 Steering Com- are solidified. Participants anticipate that mittee established and met its goals for of Interoperability and Compatibility to federal spectrum regulators will again completion of further aspects of these in- work with the National Institute of Stan- require channel widths to be halved—or terface standards, as well as preliminary dards and Technology (NIST), a division their effective use doubled. P25 Phase 2 CSSI specifications by the end of 2006. of the U.S. Department of Commerce, to has proceeded down a technological path create a conformity assessment program. that would interleave two voice transmis- Though standards exist, there is no This program is intended to assure that sions within the space of a single, current guarantee that manufacturers will build P25 equipment purchased with federal channel. This time division multiple equipment that complies with the stan- grant funds conforms to the standards access (TDMA) technique effectively dou- dards. The market will determine when suite. bles the capacity of a channel, gaining ca- equipment meeting some or all of the pacity in exchange for greater complexity P25 standards is available. For example, As of the publication of this Issue Brief, and dependence on system infrastructure as more and more agencies specified several changes to existing P25 standards to synchronize channel sharing. Further P25-compliant radios in procurements are being considered to resolve identified technical details are beyond the scope of involving federal grant funds, the avail- issues. Since much of the capability of this article, but it should be noted that ability and compatibility of equipment digital radios is contained in software, it TDMA is already widely used in tele- from multiple manufacturers increased. is hoped that most changes to the stan- communications systems, including in Also, prices declined due to demand and dards can be implemented in equipment the most widespread cellular telephone competition. Likewise, radio system in- through routine upgrades or all together technology worldwide. frastructure built upon the ISSI, FSI, and without changing end-user equipment. CSSI will become available and competi- Once again backward compatibility will tively priced as the market demands. In the future, “P25 compliance” will be a be required, particularly compatibility more formal, tested measure of con- with Phase 1 technology. Most observers Conformity Assessment formance with the standards suite. The expect P25 Phase 1 technology to serve Program NIST Office of Law Enforcement Stan- many users indefinitely into the future. Vendors have used self-certification dards has called for increased practitio- Urban areas with high demand for radio to this point to identify equipment as ner and industry participation. spectrum will push the development of compliant with P25 standards. A product P25 Phase 2 standards and equipment is considered “P25 compliant” today if it manufacture. At this point, it is impos- What Will the Future sible to predict when P25 Phase 2 equip- uses the CAI and P25 voice encoder/de- Bring? coder (vocoder) that converts sound into ment will become available or even when a digital bit stream. Technology standards are never easy the standards will be settled. However, to develop. Efforts to do so generally TDMA is already used in some public Over the past few years, multivendor bring compromises and the persistent implementations have identified some reminder that technological progress issues of technical incompatibility. This marches on while the standards are being has led the Project 25 Steering Commit- set. Even while P25 Phase 1 standards are tee, in concert with TIA and the National still in development, Phase 2 efforts are Institute of Justice, to establish a process underway. to identify and address these issues. Concurrently, Congress directed the De- Project 25 Phase 2 partment of Homeland Security’s Office managers and technolo-

Project 25: The Quest for Interoperable Radios  safety radio systems with user radios also capable of using P25 Phase 1. Further Information Technical Assistance For further information on Project A Lasting Legacy 25, see the following web sites: Available The most lasting legacy of Project 25 APCO – The Association of Public- SEARCH is the technical will not be the digital radio technology Safety Communications Officials, assistance (TA) provider to the that bears its stamp of compatibility, but International, Inc.: http://www. U.S. Department of Justice Office of Community Oriented Policing rather the model of standards develop- apcointl.org/frequency/ ment that it built, led by public agencies Services (COPS) Interoperable project25/. Communications Technology that created an SOR for industry to ad- PTIG – The Project 25 Technology Program (ICTP). SEARCH is a dress. The P25 SOR broke new ground by Interest Group: national nonprofit organization establishing architectural and functional http://www.project25.org. that has provided more than 37 needs for advanced two-way radio capa- years of expert assistance to state bilities and standard interfaces between OLES – The Office of Law Enforce- and local criminal justice agencies public safety subsystem components. ment Standards at NIST: http:// on the use of information and This document alone set a benchmark www.eeel.nist.gov/oles/ identification technology. SEARCH for other public safety communications public_safety.html. has a long-standing program of standardization efforts that followed in CommTech – The Communica- providing direct, no-cost, tailored the intervening years. tions Technology Program at the TA to law enforcement and public National Institute of Justice: safety agencies in planning for, procuring, implementing, http://www.ojp.usdoj.gov/nij/ Through Project 25, the public safety and managing information community has matured in conceiving topics/commtech/welcome. technology. technology to serve its ever-growing html. demands for communications, just as the Areas of Assistance: hue and cry for greater interoperability • Effective governance structures has risen to new heights. The effort to development create technology standards, in and of itself, has served to raise the level of • Strategic planning discourse about just what communica- • infrastructure assessment and tions interoperability entails. Toward that development end and even more practically, Project 25 • Needs analysis and assessment has served well in the quest for interoper- ability. ■ • operational requirements development • Policy and procedure development This project was supported by Cooperative Agreement #2004-IN-WX-K002 by the U.S. Department • risk management of Justice Office of Community Oriented Policing Services. Points of view or opinions contained in this document are those of the author and do not necessarily represent the official position or policies of the U.S. Department of Justice. To apply for TA in these areas or review additional SEARCH Francis X. Aumand III TA focus areas, see http://www. Chairman search.org/services/ta/. Ronald P. Hawley Executive Director Kelly J. Harris Deputy Executive Director

7311 Greenhaven Drive, Suite 145 SEARCH Sacramento, CA 95831 The National Consortium for Justice 916.392.2550 Information and Statistics www.search.org

 Project 25: The Quest for Interoperable Radios