APCO Project 25
And Its PROJECT 25 (P25) STANDARDS
FOR PUBLIC SAFETY
DIGITAL RADIO

DEFINITION OF PROCESS AND STANDARDS

While the Project and its process are known as APCO Project 25, the standards that evolve from that process are known as simply Project 25 or P25 standards in recognition of the fact that those standards represent the work and active participation of multiple associations, and numerous Federal, state, city, county agencies as well as participants other countries. The Final standards documents are known as TIA – P25 standards in recognition of the joint effort between our partners in industry that serves public safety and the Telecommunications Industry Association (TIA) that develops those standards.

PROVEN INTEROPERABILITY

The final work on the APCO P25 Common Air Interface documents was approved for ballot in August 1995 at the APCO International Conference and Exposition in Detroit, Michigan.

At this event, Motorola Inc., Stanilite Pacific Ltd, E.F. Johnson Co., BK Radio Inc., and Transcrypt International participated in a history-making interoperability demonstration of APCO 25-compliant equipment utilizing a Stanilite base station and portables from BK Radio, Motorola, E.F. Johnson, Transcrypt International on the exhibition floor. Feedback from the thousands of attendees who examined the systems indicates that the test was an overwhelming success. Even those who doubted the concept of multi-vendor participation were impressed and convinced of the viability of APCO 25 as a voluntary standard. Since then progress has continued on a full suite of Phase 1, 12.5 kHz FDMA standards and Phase 2, 6.25 kHz TDMA standards.

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APCO INTERNATIONAL'S COMMITMENT TO ADVANCED TECHNOLOGY

As public safety agencies evaluate their two-way radio needs for the future, many are reaching some similar conclusions:

• radio spectrum is becoming more congested and regulators are requiring public safety to do more with less.
• the demand for data transmission is more pronounced and often unachievable in narrow-band systems
• systems need increased functionality
• secure communication is a growing necessity
• Improved digital voice quality is essential over more of the coverage area.

Of course, upgrading a communications network is a major undertaking in terms of time, energy and expense. A number of possible solutions are available, including the digital technologies that are now available. They offer the potential to address many or all major concerns, and they can provide a true platform for the future.

However, the decision to go digital is only the first step. There are several different digital technologies on the market, making the selection difficult. Each one has its own set of features thatmay or may not suit public safety organizations.

The Association of Public-Safety Communications Officials - International, Inc. (APCO International) is committed to making the selection process easier through APCO 25, an industry-wide effort to set the recommended voluntary standards of uniform digital two-way radio technology for public safety organizations. By working together with APCO International, public safety agencies can take this opportunity to move technology along a common path that benefits the greatest number of users.

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WHAT IS APCO Project 25?

APCO Project 25, its Steering Committee, User Needs Committee and partners in the Telecommunications Industry Association bring together representatives from many local, state and federal government agencies who evaluate basic technologies in advanced land mobile radio. The objective is to find solutions that best serve the needs of the public safety marketplace. In addition, the committee has encouraged the participation of numerous international public safety organizations and equipment suppliers, making this a truly worldwide recommended standard-setting initiative.

Specifically, APCO 25 is co-chaired by APCO International and the National Association of State Telecommunications Directors (NASTD). The steering committee, which makes the decisions on technology attributes, consists of APCO International and NASTD representatives, along with federal representatives from the FPIC, DHS Coast Guard and the Department of Commerce's the National Institute of Standards Technologies (NIST), Office of Law Enforcement Standards. The Steering Committee is supported by several subcommittees that research specialized areas. A list of participating agencies is attached to this document.

OPEN REVIEW PROCESS

At the core of the APCO Project 25 process, is the full understanding that our partners in TIA are recognized by the American National Standards Institute’s (ANSI) as a Standards Development Organization (SDO) This That APCO Project 25 and TIA teams has have been conducting a fully open process to seek out new two-way digital radio technologies from a large number of industry-leading private sector developers. A setAll of these standards are based on a comprehensive Users Statement of Requirements (SoR) that serves as an evolving articulation of the user’s needs and requirements.

From its inception in 1989 the APCO Project 25 Steering Committee sought to establish open system standards so multiple vendors could make competing products that are compatible. Some of the important technologies and Intellectual Property Rights (IPR's) considered have been privately owned or proprietary. However, APCO 25 only selected technologies which were covered by IPR when in their opinion they best fit the needs of the public safety community and the owners of the IPR offered them in accordance with the Project 25 Memorandum of Understanding at fair and reasonable terms and conditions. A list of participating companies is attached to this document.

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THE BENEFITS OF Project 25 Standards

Every aspect of Project 25 is designed to benefit public safety professionals who seek a new level of performance, efficiency, capabilities, and quality in two-way radio communications. Four key objectives guided the steering committee through this open process:

• provide enhanced functionality with equipment and capabilities focused on public safety needs.
• improve spectrum efficiency.
• ensure competition among multiple vendors through Open Systems Architecture.
• allow effective, efficient, and reliable intra-agency and inter-agency communications.

By adhering to these objectives, APCO 25 makes it easier for users to make the most informed decision possible when planning to convert existing analog system to digital systems. Each vendor's system will begin on a level playing field determined by an agreed upon base line set of standards. This allows users to more accurately compare the direct features and benefits of both entire systems and individual radio products. This will make bidding processes more competitive among prospective vendors. Plus, users have the opportunity to mix and match equipment among Project 25-compliant suppliers since their equipment will follow all basic standards.

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ORDERLY MIGRATION TO DIGITAL

APCO 25 is committed to protecting the public safety community's current investment in equipment by public safety agencies. A basic requirement for new digital radio equipment is backward compatibility with standard analog FM radios. This supports an orderly migration into mixed analog and digital systems, enabling users to gradually trade out radios and infrastructure equipment. By selecting products and systems that comply with Project 25 recommended standards, agencies are assured that the investment in the latest technology has a clear migration path for the future.

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FEATURES OF THE TIA - PROJECT 25 (P25) STANDARDS

The CQPSK modulator consists of a table look-up, the two outputs of which (I and Q) are Nyquist filtered and then amplitude modulated, in phase and quadrature phase, before summing. The information bits are processed by the look-up table to yield a 5-level I signal and a 5-level Q signal. The I and Q signals are filtered with the Nyquist Raised Cosine Filter previously described. The I signal is then multiplied by the carrier and the Q signal is multiplied by the carrier after it has been delayed by 90 degrees. The modulated I and Q carriers are then summed together to yield the modulator output.

12.5kHz Bandwidth (C4FM) Modulation

The C4FM modulator consists of a Nyquist Raised Cosine Filter, cascaded with a shaping filter, cascaded with a frequency modulator.

Addressing

A very large number of radio addresses are provided, both for individual radios and talk-groups.

Aggregate Bit Rate

The aggregate bit rate is 9600 bits/s. In the case of data transmission, data packets basically consist of a header, containing overhead information, followed by data. In the case of digitized voice transmission, after the transmission of a header containing error protected overheadinformation, 2400 bits/s is devoted to periodically repeating the overhead information needed to allow for late entry (or the missed reception of the header).

C4FM Frequency Modulator

The deviation is +1.8 kHz for dibit 01, +0.6 kHz for dibit 00, -0.6 kHz for dibit 10, and -1.8 kHz for dibit 11.

C4FM Nyquist Filter

The dibits of information (i.e., 4800 symbols) are filtered with a raised cosine filter which satisfies the Nyquist criterion minimizing inter-symbol interference. The group delay of the filter is flat over the pass-band for |f| < 2880 Hz. The magnitude response of the filter is approximately 1 for |f| < 1920 Hz, 0.5 + 0.5 cos (2(f/1920) for 1920 Hz < |f| < 2880 Hz, and 0 for |f| > 2880 Hz.

C4FM Shaping Filter

The shaping filter has a flat group delay over the band-pass for |f| < 2880Hz. The magnitude response of the filter for |f| < 2880 Hz is sin(pif/4800)/(pif/4800).

Data Packet Data Blocks

Confirmed Data Blocks contain a 7-bit serial number (to allow for selective transmission), 9 bits of error detection on the entire block, and 14 octets of data. Unconfirmed Data Blocks contain 12 octets of data (note: a 32-bit error detection code is appended to the end of all data packets).

Data Packet Error Correction

Header Blocks normally use a rate 1/2 trellis coder for error correction. Unconfirmed data packets normally use a rate 1/2 trellis coder, while confirmed data packets normally use a rate 3/4 trellis coder. Interleaving is applied over Data Blocks.

Data Packet Header

The Header Block used for communications between a radio and a packet data node (i.e., switch) contains 10 octets of address and control information, followed by 2 octets of error detectioncoding. Information contained in the header includes: the identity of the Service Access Point to which the data is being directed, a manufacturers identification (to allow for non-standard functions), a logical link identifier to identify the sending radio of an inbound packet to a node and the receiving radio of a packet outbound from a node, the number of blocks to follow in the packet, the number of pad octets to fill out the last block, the sequence number of the packet, and the Fragment sequence number. For packet data communications directly between two radios, essentially two 12-octet Header Blocks are used at the beginning of each packet in order to convey the necessary address and control information. The header is preceded by 48 bits of synchronization and 64 bits of network identifier (to prevent confusion between radios in different networks).

Data Packet Structure

Data messages are divided into Fragments of less than 512 octets. Fragments are, in turn, divided into Blocks of M octets, where M=12 for unconfirmed messages and M=16 for confirmed messages (except Header Blocks are 12 octets in length). Note: a Fragment, preceded by header information, is defined as a Data Packet.

Demodulator

The demodulator is capable of receiving both the C4FM and the CQPSK signals. It consists of a frequency modulation detector, followed by an Integrate and Dump Filter and then a stochastic gradient clock recovery device. The Integrate and Dump Filter has a flat group delay over the band-pass for |f| < 2880 Hz. The magnitude response of the Integrate and Dump Filter, for |f| < 2880Hz, is approximately sin (pif/4800)/(pif/4800).

Digitized Voice Coder Method

After evaluating several candidates, Project 25 selected IMBE (Improved MultiBand Excitation)vocoder, operating at 4400 bits/s. An additional 2800 bits/s of forward error correction is added, for error correction of the digitized voice. IMBE has also been selected by INMARSAT, for use in digital voice maritime satellite communications.

Digitized Voice Frame Structure

The Header Word, transmitted at the start of every transmission, contains 120 bits of information and 528 bits of error correction. Voice Frames are 180 ms in length and pairs of Voice frames compose a 360 ms Superframe. The first of a pair of voice frames transmits, in addition to digitized voice and its error corrective coding, 72 bits of link control information, 168 bits of error corrective coding on the link control information, and 16 bits of embedded low-speed data signaling with 16 bits of error corrective coding. The second of the pair of voice frames transmits an additional 16 bits of low-speed data signaling with 16 bits of error corrective coding, 96 bits of encipherment information, and 144 bits of error protective coding on the encipherment information.

Digitized Voice Header Word

The header is preceded by 48 bits of synchronization signal and a 64-bit network identifier (to prevent confusion between radios in different networks). The 120 bits of information in the Header Word consists of 72 bits for the encipherment initialization vector, 8 bits for a manufacturer identifier, 8 bits to identify the encipherment algorithm, 16 bits to identify which encipherment key variable is being employed (in systems with multiple encipherment key variables), and 16 bits for the talkgroup address. Including error corrective coding, status symbols (22 bits) not described in this brief contribution, and preceding signaling, the header requires 82.5 ms for transmission.

Digitized Voice Encryption Information

The 96 bits of encipherment information consist of the three encipherment-related fields in the header: the 72-bit encipherment initialization vector, the 8-bit encipherment algorithm identifier, and the 16-bit encipherment key variable identifier. The encipherment information is protected by 144 bits of error corrective coding. Note: the encipherment process does not change during a transmission. The repeating of this information every 360 ms is primarily for late-entry by receiving radios.

Digitized Voice Link Control Information

As previously stated, 72 bits of link control information is contained in the first of each pair of Voice Frames. If the addressee is a talk-group, the link control information consists of 1 octet describing the type of information (e.g., talk-group type of transmission), 1 octet containing the manufacturer identifier, 1 bit indicating whether or not the transmission is of an emergency nature, 15 bits reserved for future use, 2 octets for the talk-group address, and 3 octets for the transmitting radio's identifier. If the addressee is another radio, the last 7 of the 9 octets consist of : 1 octet reserved for future use, 3 octets for the destination radio's identifier, and 3 octets for the transmitting radio's identifier. This information is protected with 168 bits of error corrective coding.

Encryption

Information required for decoding to take place (including the encoding initialization vector) is transmitted at the beginning of all transmissions, and is embedded in the signaling overhead throughout all digitized voice transmissions. This allows for the use of multiple encoding algorithms and "key variables". An adequate number of bits have been assigned to allow for even the highest (i.e., most secure) levels of encoding.

Error Protection

To provide for the maximum possible coverage (i.e., range of operation), a high degree of forward error correction and interleaving has been provided for. The mobile environment is subject to severe Rayleigh fading and the Project 25 techniques have been designed to operate in bit error rate environments of up to 7 percent.

Flexible Modulation Method

A pair of modulation methods have been selected that utilize a common receiver. The first, which utilizes a constant-envelope 4-level Frequency Modulation (FM) variant, can utilize simple, high-efficiency amplifiers and has emissions that fit within a 12.5 kHz bandwidth. This method will be fielded in most equipment initially. The second, which utilizes a CQPSK variant with amplitude components, requires the use of highly linear or linearized amplifiers and has emissions that fit within a 6.25 kHz bandwidth. (The receiver, common to both, has a 6.25 kHz bandwidth.)

Low-Speed Data With Digitized Voice

An 88.9 bit/s low-speed data channel is provided in the digitized voice frame structure. No application is currently defined. One application under discussion is to use the low-speed data channel for the transmission of accurate geographic location information.

Talk-Group Operation

Unlike normal telephone operation, all members of a talkgroup can receive the transmissions of all other members of that talk-group. Members of a talk-group do not have to receive the beginning of a digitized voice transmission (i.e., late entry is provided for).

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HISTORY AND CURRENT STATUS OF APCO 25

In October of 1989 APCO Project 25 came to being in a meeting jointly sponsored by APCO, NASTD, National Telecommunications and Information Administration (NTIA), the National Communications System (NCS) and the US Department of Defense (DoD). In January of 1993, APCO Project 25 moved forward by adopting a proposed system architecture through six interface standards that would determine the future of digital technology for use in the United States public safety markets. The first of these interfaces to be defined was the critically important Common Air Interface (CAI). CAI created the standards that ensure basic radio-to-radio digital interoperability and compatibility. This means that in an APCO Project 25-compliant system, mobile and portable equipment from any manufacturer are able to intercommunicate and have been able to do so since the mid-1990s.

In November 1993, the APCO Project 25 Steering Committee accepted the TIA-recommended Common Air Interface Standard document, which specifies

• the FDMA access method
• QPSK-C modulation
• 9.6 data rate
• the DVSI baseline vocoder, using a 12.5 KHZ channel^[1]

The other five interfaces that were defined are

• data port
• data host
• inter-connect
• network management
• inter-system interfaces

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CONTINUED MIGRATION PATH FORWARD:

Since the mid-1990's the APCO Project 25 process has been on a continuing and unrelenting path forward, expanding the standards base and greatly enhancing the potential for standardized product that fit today and tomorrow's needs in a clear and concise manner while ensure public safety users always had backward compatibility to analog FM from Phase 1 and backward compatibility from Phase 2 to Phase 1.

Project 25 standards are unique in a significant number of ways that benefit the public safety community and the citizens they serve. For example consider the following:

• Project 25 standards are based on the public safety communities needs as they define them.
• Project 25 is open to all users and product providers.
• The Project 25 process takes the operational and technical requirements of the end users and in cooperation with industry creates a vast array of standards proposals within the APIC process.
• Those standards proposals are refined and modified as necessary in the TIA-TR8 process so that they can be balloted as TIA - ANSI standards.
• Project 25 Standards allow system manufacturers to scale their systems from the very smallest of systems to the largest and most complex systems.
• P25 standards were created to ensure the end user had RF coverage that was equivalent to or nearly equivalent to the old analog FM.
• These standards include trunked system and conventional systems.
• Inherent within the standards at all levels is unit-to-unit or car-to-car talk around capability.
• A comprehensive suite of standards either exists or is final development to ensure those who require it can have access to a full suite of encryption standards including Over-The-Air-Rekeying (OTAR) and Over-The-Air-Reprogramming (OTAP)
• P25 compliant portables and mobiles will function within any other manufacturer system all other things being equal.
• The Project 25 Inter-Sub-System-Interface will allow roaming units from one agency to access another agency system on a transparent basis with preauthorization and through an authorization process without pre-authorization.
• Through proper design and implementation methods the ISSI can be used to interconnect both dissimilar Project 25 systems and dissimilar system. For example the ISSI can be used to interconnect to remote P25 system via satellite.
• As the ISSI rolls out, it will be or is now capable of transporting both voice and system data.
• Project 25 standards were specifically created to ensure a smooth transition between scarcely populated geographical areas and dense urban areas.
• Both Project 25 Phase 1 FDMA standards and the more recent Phase 2 TDMA Standards will function well in both a rural and urban environment. However, in large urban areas where spectrum is a scarce commodity, the Phase 2 standards will provide both superior coverage and improved spectrum efficiency than may otherwise be available.
• Project 25 standards based systems and technologies are offered by and built by manufacturers world-wide.
• Project 25 is in the process of implementing a rigorous compliance assessment program.
• Project 25 compliant products must meet compliance assessment tests, conformance test and interoperability tests.
• APCO, the Project 25 Steering Committee and the Project 25 Technology Interest Group (PTIG) are working to ensure the Project 25 logo only appears on Project 25 Compliant Products.
• Project 25 standards have been created to provide interoperability when it is necessary and to allow individual manufacturer to enhance their products with added value features and functions as long as they do not interfere with interoperability and are not advertized as P25 compliant features or functions.
• Our Project 25 Standards are created in an iterative process that is based on a full public private partnership between the users of the technology, the industry that provides that technology and an ANSI certified standards body that brings the vision into focus.

Many US cities, Counties and States either require or recommend P25 technology for their public safety agencies. In addition many Federal agencies use P25 based technology to fulfill their needs and in fact many of the Federal Governments grant programs recommend P25 technology as the preferred solution to interoperability in the United States of America. However, neither the Project 25 Steering Committee, or our users nor partners in industry have ever recommended that the use of P25 equipment be required as has taken place in other parts of the world using other standardized platforms.

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YOUR ROLE IN THE FUTURE OF DIGITAL TECHNOLOGY

APCO and the APCO Project 25 Steering Committee strongly believe that the public safety community should consider all available options as they build out the system to meet their current and future needs. While we believe in the majority of cases a P25 solution will be a community's best solution, we also believe that is and should be a local decision. We also believe that by specifying and purchasing digital radios and systems that are fully Project 25-compliant, you will help all public safety agencies ensure interoperability. Of equal importance is the need for the Public Safety Community to participate in the Project 25 process to better communicate public safety's needs to manufacturers who can respond with a family of ever changing digital technology set to meet all your current and future needs.

APCO International stands behind you with a strong, unified voice for performance, interoperability, and value in recommending the best in digital two-way radio communications for the public safety user of today and tomorrow.

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ASSOCIATION OF PUBLIC-SAFETY COMMUNICATIONS OFFICIALS - INTERNATIONAL, INC.

APCO 25 PARTICIPANTS - PUBLIC SECTOR

(Note: the following lists do not indicate endorsement of P25, but are a reference to those entities that have provided input on the development of the standard.)

APCO 25 PARTICIPANTS - PRIVATE SECTOR

APCO 25 DOCUMENTS

¹Since 1993 a number of improvements and changes have been made to the vocoder Return to footnote reference