APCO Project 25 Digital Land Mobile Radio

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APCO Project 25
Digital Land Mobile Radio

by Faulkner Staff

Docid: 00011180

Publication Date: 1908

Report Type: TUTORIAL


The Association of Police Communications Officers (APCO) began consideration
of a standard for digital land mobile radio in 1989, hoping to
standardize the conversion of public safety communications equipment to digital.
To some extent, the use of incompatible communications systems on a local,
state, and national basis – as well as between
different agencies – continues despite the
adoption of APCO Project 25. The government’s push toward interoperable first
responders’ communication systems based on existing broadband networks, it is
hoped, may break the logjam, save money, and create
more resilient, reliable emergency networks. Many federal agencies have adopted
the standard, and in 2016, the Project 25 Technology Interest Group released a
white paper on improvements to the standard so that it can be used in
firefighting operations. This tutorial examines APCO Project 25 in greater

Report Contents:

Executive Summary

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The Association of Police Communications Officers (APCO)’s
Project 25 – or P25 – is a joint initiative between members of the telecom
industry and various public-safety organizations to develop interoperability
standards for the two-way radios used by emergency personnel such as

Per APCO, the need for such standards is due to a number of factors, among them:

  • Congestion on (public) radio spectrum.
  • Requirements for voice and data functionality.
  • Minimum standards for voice quality, across a coverage area, to
    perform tasks.
  • Need for communications security.
  • Equipment interoperability – and compliance assessment – with
    other equipment that is based on ANSI / TIA (American National Standards
    Institute / Telecommunications Industry Association) standard processes.
  • Improved support for maintaining public safety and public

Although P25 has been around since 1989, the events of September 11, 2001 did shine
a new light on the
shortcomings of various US public-safety networks. Of the aforementioned items,
likely the most critical is interoperability. For example, after the 9/11 attacks, the commanding officer of the
Port Authority Police Department (PAPD) on the scene ordered the World Trade Center complex evacuated, but the
deputy fire safety director of the New York City Fire Department on duty in the South Tower did not receive the order
because his radio was not interoperable with the PAPD.

Sadly, budgetary issues and a stalled standards-making process have slowed
progress toward this integration goal.

In particular, standards development is a process that is largely the captive of vendors
that stand to benefit from defending territory in the niche
markets that were established using their own proprietary technology.
These standards cover both voice and data
transmissions, and provide for interoperability between and among agencies
and different levels of government. An added benefit is that competition has
arisen among suppliers, although licenses and long-term contracts do play a

Each year, the Project 25 technology-interest group pushes for further
adoption and modification of the standard.

The latest of these updates was published in 2018, and includes
provisions as noted in Table 1.

1 details the APCO Project 25 published documents and the TIA Document

Table 1. June 2018 Project 25 Revisions
Category Completed Revisions Ongoing Revisions
Air Interfaces
  • Conventional Interoperability Test
  • Proposals to Clarify and Improve
    Performance Modeling Methods
  • Performance Verification
  • Requirements for Battery-Powered,
    Portable Land Mobile Radio Applications in Class I, II, and
    III, Division 1, Hazardous (Classified) Locations.
  • Trunking Control Channel Messages
Wireline Interfaces
  • ISSI/CSSI Interoperability Test
    Procedures for Trunked Voice Operation
  • ISSI Recommended Compliance Assessment
    Tests Bulletin
  • Security Services Overview Document
  • Tier 2 Location Service Specification
  • TCP/UDP Port Number Assignments
  • Mobile Data Peripheral Interface
  • CMS Specification for Packet Data
  • Packet Data Logical Link Control
  • Packet Data Host Network Interface


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Project 25 has its origins in APCO’s Project 16, which set functional and performance recommendations
and user-defined features. Project 16 did not, however, tell manufacturers how to
build the recommendations into their products, and thus is not a
technical standard. APCO 16, however, became a de facto standard that is still
referenced in all analog trunked radio system procurement. This
progress led to
the development of non-interoperable, proprietary systems by major
manufacturers such as EF Johnson, GE, and Motorola.

APCO started P25 in 1989 with help from the federal government, law
enforcement agencies, and several telecommunications industry
associations, including the Telecommunications Industry Association
(TIA). Despite being around for almost 30 years, the standards and
equipment that use them continue to evolve. In N2010, the TIA
completed a standard to allow for TDMA trunked public safety
equipment for first-responders to meet certain FCC regulations
for spectrum use while doubling the voice capacity of current
transmission stations.

Benefits of P25

According to the Project 25 Technology Interest Group, P25 has four primary

  • Allow effective, efficient, and reliable intra- and inter-agency communications
    for organizations
    to implement interoperable and seamless joint
    communication in both routine and emergency circumstances.
  • Ensure competition in system life cycle
    to allow agencies to choose from multiple vendors and
    products, ultimately saving money and gaining the freedom to
    select from the widest range of equipment and features.
  • Provide user-friendly equipment for users to
    take full advantage of a radio’s lifesaving capabilities on
    the job, even under adverse conditions with minimal training.
  • Improve radio spectrum efficiency for networks to have
    capacity for handling calls and allowing room for growth, even
    in areas where spectrum is crowded and it is difficult for
    agencies to obtain licenses for additional radio frequencies.

Other P25 benefits include:

  • Interoperability – Allows emergency personnel
    to communicate with each other at all times. It is frequency
    agnostic and allows one system to support multiple frequency
    bands. It is also designed to work with analog equipment, so
    municipalities do not have to spend money on upgrading equipment
    right away. It allows analog radios to communicate with both
    analog and digital radios. Municipalities can also use equipment
    from multiple vendors without interoperability concerns.
  • Scalability – Covers more area with fewer transmission towers. The P25
    standard supports emergency personnel that operate in both
    densely populated and rural areas. In markets that have low
    population density, emergency personnel can communicate directly
    with each other by selecting the appropriate channel in their
    radios without needing to set up a signal repeater. Areas with a
    higher population density might not be able to support a direct
    user-to-user connection because there would be too many people
    using one channel. P25 addresses this by trunking the signal so
    users can access a collection of signals seamlessly. This sort
    of configuration is also ideal for smaller municipalities that
    want to team up to form a regional communications network.
  • Voice / Data Support – Supports voice and data transmissions.
  • Secure Transmission – Features to allow agencies to secure information,
    including equipment authentication and encrypted transmissions.

APCO Standard

APCO’s Project 25 offers a user-driven, user-controlled effort to establish a standard
for digital land mobile (or trunked) radio systems in North America. The
project is not driven by manufacturers, although many are signatories to the
agreement. The goal of the project was to develop a suite of standards
focused on the six system interfaces: the data, common air, intersystem, host
data, interconnect, and network management interfaces. The standards had to
ensure interoperability, backward compatibility, and forward migration;
control systems cost; increase spectrum efficiency; simplify procurement
decisions; set levels of system performance; accelerate adoption of new
technology; and limit the number of technologies that manufacturers must
offer. The standards have been approved as the US Federal Government
Agencies’ standards, and they have been accepted by the ITU standards body.
The more common digital standard in much of Africa, the Middle East, and
Europe, however, is Terrestrial Trunked Radio [TETRA], although P25 is
interoperable with networks and equipment in Canada, South America, Eastern
Europe, Asia, and Australia.

Air Interface (CAI)

The steering
committee adopted the CAI standard document recommended by the TIA. The CAI
specifies FDMA access, QPSK-C modulation, a 9.6Kbps data rate, and the DVSI
vocoder, using a 12.5-KHz channel.

FDMA Channel Access. After reviewing Code Division Multiple Access (CDMA),
Time Division Multiple Access (TDMA), and Frequency Division Multiple Access
(FDMA) as methods for channel access, the FDMA was accepted by both the TIA and
the steering committee. The FDMA channel is split in half, which gives each
user access to 50% of the channel, 100% of the time.

Reasons for FDMA adoption include:

  • Early Equipment Availability – Doubling available
    channels in bands above 450 MHz.
  • Interoperability and Talk-Around Mode – Supports
    portable-to-portable and / or mobile-to-mobile intercommunications without the
    use of repeaters, a technology that can be necessary in tactical
  • Consolidation and Political Considerations – Does not require that small
    agencies consolidate within geographical radio coverage areas to meet channel
    loading requirements.
  • Graceful Migration – Migration path
    for FM equipment, negating the need for a changeout.
  • Interference – Unlike TDMA equipment, does not cause a phenomenon called audio
    rectification that causes interference (a buzzing noise) to be produced in
    audio instruments within a certain geographical area.

Data Rate. As part of the CAI, Project 25 Phase 2 adopted a data rate of


Spectrum efficiency is naturally affected by both the channel access
method and the data rate. Spectrum efficiency can be defined as the
number of communications links or talk paths that can operate
simultaneously within a given time period and how much information can
be passed along the path in the same period. Current analog spectrum
usage demands that users be separated by a number of miles to prevent

APCO Project 25 consists of many different standards that are available from
both APCO and the ITU. Some of the more important features of the standards

  • CQPSK Modulation – 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.
    Information bits are processed by the look-up table to yield a 5-level I
    signal and a 5-level Q signal.
  • C4FM Modulation – Consists of a Nyquist Raised Cosine Filter associated with a shaping filter and a frequency
  • Addressing – Provides a large number of individual and talk-group radio addresses.
  • Aggregate Bit Rate – Aggregate bit rate of12Kbps
    (Phase II) is supported.
  • C4FM Frequency Modulator – Deviation allowed for modulation 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 4,800 information symbols are filtered by a raised
    cone filter that satisfies the Nyquist criterion for minimizing inter-symbol
  • C4FM Shaping Filter – Group delay over the band-pass
    for the shaping filter of less than 2880Hz.
  • Data Packet Data Blocks – Contain a seven-bit serial number
    to allow for selective transmission, nine bits of error detection for the
    entire block, and 14 octets of data. Unconfirmed Data Blocks contain 12
    octets of data.
  • Data Packet Error Correction. – 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 use a rate 3/4 trellis coder. Interleaving is
    applied over Data Blocks.
  • Data Packet Header – Contains 10 octets of address and
    control information, followed by two octets of error detection coding.
    Information contained in the header include the identity of the Service
    Access Point to which the data are being sent, a manufacturer’s identity, a
    logical link identifier to identify the sending radio or 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.
  • Data Packet Structure – Data messages divided into fragments of less than 512
    octets. Fragments are divided into Blocks of 12 for unconfirmed messages and
    16 for confirmed messages.
  • Demodulator – Receives both the C4FM and the CQPSK signals. The demodulator
    consists of a frequency modulator detector followed by an integrate and dump
    filter, and then a stochastic gradient recovery device.
  • Digitized Vocoder – Improved Multiband Excitation vocoder operates at 4.4Kbps with
    2.8Kbps of forward error correction.
  • Digitized Voice Frame Structure – Transmitted at the beginning of every transmission
    is a header word that contains 120 bits of information and 528 bits of error
  • Digitized Voice Header Word – Precedes the header contains 48
    bits of synchronization signal and a 64-bit identifier.
  • Digitized Voice Encryption Information – Contained in the 96-bit
    encipherment information are the 72-bit encipherment initialization vector,
    the 8-bit encipherment algorithm ID, and the 16-bit encipherment key variable
    ID. Also included are 144 bits of error corrective coding.
  • Digitized Voice Link Control Information – Makeup of the link control
    information varies based on addressee.
  • Encryption. – All information for encoding and decoding is transmitted at the beginning of
    all transmissions and is embedded in the signal overhead throughout digitized
    voice transmissions. Thus, the encryption can change during a transmission
    without affecting service.
  • Error Protection – High degree of forward error correction and interleaving provide
    for a maximum range of operation in a bit-error environment of up to
  • Flexible Modulation Method – Includes two modulators that use a common receiver can be used.
  • Low-Speed Data With Digitized Voice – 88.9bps
    low-speed data channel provided in the voice frame structure. This could
    be used for geographic location information.

2 details the APCO Project 25 published documents and the TIA Document

Project 25 Documents
Project # Description TIA Document #
1.0 Project
25 System & Standard Definition
2.0 Common
Air Interface
3.0 CAI
Conformance Testing
4.0 CAI Reserved
5.0 CAI
Operational Description For Conventional Channels
6.0 Vocoder
7.0 Vocoder
MOS Test
8.0 Vocoder
Reference Test
9.0 Vocoder
Selection Process
10.0 Transceiver
Measurement & Methods
11.0 Transceiver
Performance Recs
12.0 Trunking
13.0 Trunking
Control Channel Formats
14.0 Trunking
Control Channel Messages
15.0 Link
Control Words
16.0 Conventional
Control Words
17.0 Trunking
18.0 ISSI
19.0 ISSI
Messages Definition
20.0 ISSI
21.0 Telephone
Voice Requirements & Definitions
22.0 Data
23.0 Packet
Data Specification
24.0 Circuit
Data Specification
25.0 Radio
Control Protocol Specification
26.0 Network
Mgmt Interface Def
27.0 Network
Mgmt Interface Conformance
28.0 Security
Services Overview
29.0 DES
Encryption Protocol
30.0 DES
Encryption Conformance
31.0 OTAR
32.0 OTAR
Operational Description
33.0 OTAR
Operational Conformance
34.0 Lock
Down Test & Procedures
Doc Only
35.0 Nationwide non-emergency number APCO
Doc Only
36.0 Universal standards for CAD (computer-aided
dispatch) and CAD-to-CAD exchanges
Doc Only
37.0 Certification programs for public safety
communications personnel
Doc Only
38.0 Wireless 9-1-1 Phase II services implementation APCO
Doc Only
39.0 Public safety 800Mhz interference APCO
Doc Only
40.0 RETAINS Communications centers’ staffing APCO
Doc Only
42.0 Standards to achieve system interoperability and
create a common operating picture
Doc Only
43.0 Broadband developments governing the role of PSAP. APCO
Doc Only

Figure 1 shows one end of a municipal Project 25 system interface.

Figure 1. Project 25 System Interfaces

Figure 1. Project 25 System Interfaces

Source: http://www.pscr.gov/

Current View

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Traditionally, communities across the US have been slow to introduce APCO Project
25 radios because of budget problems. The federal government has attempted to
help push along the roll out by requiring communities to upgrade their
first-responder communications networks to the technology when receiving any
kind of funding for communications upgrades from the US Department of Homeland


of the reasoning behind launching APCO Project 25 was that governments and
users of analog land mobile radio systems were tired of the lack of systems
interoperability, but even more importantly, they were tired of the limited
number of companies from which they could choose products. There were
essentially three producers of land mobile radio systems: Motorola, GE, and
EF Johnson. In reality, Motorola had patents on many of the technologies
being used, and GE and EF Johnson were the only vendors licensed to use those
technologies. In essence, Motorola had a monopoly.

quest to break that monopoly was only partially successful. Some of the
standards that were adopted by APCO Project 25 were proprietary to Motorola,
so that any company wishing to enter the market still had to license Motorola
technology. Motorola agreed with APCO to give companies a better deal on the
licensing, so more competitors entered the market for digital land mobile
radio business. Today, there are 40 P25 equipment manufacturers and
service providers in the market, but Motorola is still the leading
player in the US market .

APCO includes more than 28,000 members in all. The following vendors listed
in Figure 2 are among those registered with The Project 25 Technology Interest
Group as manufacturing APCO P25 equipment.

Figure 2. APCO Project 25 Vendors

Figure 2. APCO Project 25 Vendors

Source: The Project 25 Technology Interest Group


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APCO 25 is the only user defined public
safety standard, and it continues to be widely implemented across the
public safety sectors. This standard supports all operating modes,
including Simplex, Repeater, Trunked. The standard evolves with
improvements in technology. However, the shortcomings of APCO have

  • Standards have not
    been fully specified and published for all eight interfaces.
  • Public safety/first
    responder communications facilities can be APCO 25 compliant without being
    standards based, limiting interoperability.
  • Neither industry nor
    the federal government has rigorously articulated  the architecture,
    infrastructure and operating characteristics of an APCO 25 communication
  • There is no single,
    definitive compliance and validation auditing process for APCO 25 users and


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The Project 25 Technology
Interest Group notes that’s continued barriers to
interoperability including technical barriers such as competing
standards, frequency bands, and ID plans; operational barriers such as
disparate operating procedures and planning. The standard will continue
to evolve to address the needs of users and changing FCC/NTIA
requirements. With every year, improvements are resolving
interoperability issues, adding securing upgrades, improving testing and
performance, and creating additional P25 interfaces (such as one for LTE
broadband data networks).

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