APCO Project 25
Digital Land Mobile Radio
Copyright 2019, Faulkner Information Services. All Rights Reserved.
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
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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
- 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
|Category||Completed Revisions||Ongoing Revisions|
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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
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
to implement interoperable and seamless joint
communication in both routine and emergency circumstances.
- Ensure competition in system life cycle
procurements 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’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)
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
- 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
- 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 #||Description||TIA Document #|
25 System & Standard Definition
Operational Description For Conventional Channels
Measurement & Methods
Control Channel Formats
Control Channel Messages
Voice Requirements & Definitions
Control Protocol Specification
Mgmt Interface Def
Mgmt Interface Conformance
Down Test & Procedures
|35.0||Nationwide non-emergency number||APCO
|36.0||Universal standards for CAD (computer-aided
dispatch) and CAD-to-CAD exchanges
|37.0||Certification programs for public safety
|38.0||Wireless 9-1-1 Phase II services implementation||APCO
|39.0||Public safety 800Mhz interference||APCO
|40.0 RETAINS||Communications centers’ staffing||APCO
|42.0||Standards to achieve system interoperability and
create a common operating picture
|43.0||Broadband developments governing the role of PSAP.||APCO
Figure 1 shows one end of a municipal Project 25 system interface.
Figure 1. Project 25 System Interfaces
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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
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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