P25 was established to address the need for common digital public safety
radio communications standards for First Responders and Homeland Security/Emergency
Response professionals.
TIA TR-8 facilitates such work through its role as an ANSI-accredited
Standards Development Organization (SDO).
Project
25 (P25) is a set of standards produced through the
joint efforts of the Association of Public Safety Communications
Officials International (APCO), the National Association
of State Telecommunications Directors (NASTD), selected
Federal Agencies and the National Communications System
(NCS), and standardized under the Telecommunications Industry
Association (TIA)... The P25 suite of standards involves
digital Land Mobile Radio (LMR) services for local, state/provincial
and national (federal) public safety organizations and agencies...
P25 is applicable to LMR equipment authorized
or licensed, in the U.S., under the National Telecommunications
and Information Administration (NTIA) or Federal Communications
Commission (FCC) rules and regulations.
Although developed primarily for North American public
safety services, P25 technology and products are not limited
to public safety alone and have also been selected and deployed
in other private system application, worldwide.
P25 equipment has also
been selected for a railroad system, including rolling stock, personnel,
and transportation vehicles.
P25-compliant systems are being increasingly adopted and deployed. Radios
can communicate in
analog mode with legacy radios, and in either
digital or
analog mode with other P25 radios. Additionally, the deployment
of P25-compliant systems will allow for a high degree of equipment interoperability
and compatibility.P25 standards use the Improved Multiband Excitation
(IMBE) vocoders which were designed by DVSI to encode/decode the analog
audio signals.
P25 may be used in "talk around" mode without any intervening equipment
between two radios, in conventional mode where two radios communicate
through a repeater or base station without trunking or in a trunked
mode where traffic is automatically assigned to one or more voice channels
by a
Repeater
or Base Station.
The protocol supports the use of DES encryption (56 bit), 2-key Triple-DES
encryption (112 bits), 3-key Triple-DES encryption (168-bits), AES encryption
at up to 256 bits keylength, RC4 (40 bits, sold by Motorola as Advanced
Digital Privacy), or no encryption.
The protocol also supports the ACCORDION 1.3, BATON, FIREFLY, MAYFLY
and SAVILLE Type 1 ciphers.
P25 Open Interfaces: P25's Suite of Standards specify eight open interfaces between
the various components of a land mobile radio system. These interfaces
are:
Common Air Interface (CAI)
standard specifies the type and content of signals transmitted by
compliant radios. One radio using CAI should be able to communicate
with any other CAI radio, regardless of manufacturer
Subscriber Data Peripheral
Interface standard specifies the port through which mobiles
and portables can connect to laptops or data networks
Fixed Station Interface
standard specifies a set of mandatory messages supporting digital
voice, data, encryption and telephone interconnect necessary for
communication between a Fixed Station and P25 RF Subsystem
Console Subsystem Interface
standard specifies the basic messaging to interface a console subsystem
to a P25 RF Subsystem
Network Management Interface
standard specifies a single network management scheme which will
allow all network elements of the RF subsystem to be managed
Data Network Interface
standard specifies the RF Subsystem's connections to computers,
data networks, or external data sources
Telephone Interconnect Interface
standard specifies the interface to Public Switched Telephone Network
(PSTN) supporting both analog and ISDN telephone interfaces.
Inter RF Subsystem Interface
(ISSI) standard specifies the interface between RF subsystems which
will allow them to be connected into wide area networks
P25-compliant technology is being deployed in several phases:
Phase 1
Phase 1 radio systems operate in 12.5
kHz analog, digital or mixed mode. Phase 1 radios use Continuous 4 level
FM (C4FM) modulation for digital transmissions at 4800 baud and 2 bits
per symbol, yielding 9600 bits per second total channel throughput.
Receivers designed for the C4FM standard can also demodulate the "Compatible
quadrature phase shift keying" (CQPSK) standard, as the parameters of
the CQPSK signal were chosen to yield the same signal deviation at symbol
time as C4FM while using only 6.25 kHz of bandwidth.
Vendors are
currently shipping Phase 1 P25-compliant systems. These
systems involve standardized service and facility specifications,
ensuring that any manufacturers' compliant subscriber radio
has access to the services described in such specifications.
Abilities include backward compatibility and interoperability
with other systems, across system boundaries, and regardless
of system infrastructure. In addition, the P25 suite of
standards provides an open interface to the radio frequency
(RF) subsystem to facilitate interlinking of different vendors'
systems.
Phase 2
To improve spectrum utilization,
Phase 2 is currently under development with concurrent work
being done on 2-slot TDMA and FDMA (CQPSK) modulation schemes.
Phase II will use the
AMBE vocoder to reduce the needed bitrate so that one
channel will only require 4800 bits per second.
A
hand-held Project 25 radio used in US systems.
Significant attention is also paid to interoperability with legacy
equipment, interfacing between repeaters and other subsystems, roaming
capacity and spectral efficiency/channel reuse. In addition, Phase 2
work involves console interfacing between repeaters and other subsystems,
and man-machine interfaces for console operators that would facilitate
centralized training, equipment transitions and personnel movement.
Adoption:
Adoption of these standards has been slowed by budget problems in the
US; however, funding for communications upgrades from the Department
of Homeland Security usually requires migrating to APCO-25. It is also
being used in other countries world wide including Australia, Singapore
and Russia, although the Terrestrial Trunked Radio (TETRA) systems are
more popular in Europe. And while Terrestrial Trunked Radio systems
are sometimes cheaper - handsets are often not suited to more rugged
environments. Both P25 and TETRA can offer varying degrees of functionality,
depending on available radio spectrum, terrain and project budget.
While interoperability is a major goal of P25, many P25 features present
interoperability challenges. In theory, all P25 compliant equipment
is interoperable. In practice, interoperable communications isn't achievable
without effective governance, standardized operating procedures, effective
training and exercises, and inter-jurisdictional coordination. The difficulties
inherent in developing P25 networks using features such as digital voice,
encryption, or trunking sometimes result in feature-backlash and organizational
retreat to minimal "feature-free" P25 implementations which fulfill
the letter of any APCO-25 migration requirement without realizing the
benefits thereof. Additionally, while not a technical issue per se,
frictions often result from the unwieldy bureaucratic inter-agency processes
that tend to develop to coordinate interoperability decisions.
