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About standard

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). P25 is an open architecture, user-driven suite of system standards that define digital radio communications system architectures capable of serving the needs of Public Safety and Government organizations. 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 open system standards define the interfaces, operation and capabilities of any P25 compliant radio system. In other words, a P25 radio is any radio that conforms to the P25 standard in the way it functions or operates. P25 compliant radios can communicate in analog mode with legacy radios and in either digital or analog mode with other P25 radios. The P25 standard exists in the public domain, allowing any manufacturer to produce a P25 compatible radio product. 

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.

P25 radios operate very similar to conventional analog FM radios. In fact, P25 radios will operate in conventional analog mode, making them backwards compatible with existing analog radio systems. When the P25 radio operates in digital mode, the carrier is moved to four specific frequency offsets that represent four different two-bit combinations. This is a modified 4 level FSK used in analog radio systems. 

In analog mode, the P25 radio will operate exactly the same as conventional analog systems, with the capability for CTCSS, DCS, pre-emphasis and de-emphasis, wideband or narrowband operation and other standard analog features. 

In P25 digital mode, the P25 transmitter will convert all analog audio to packets of digital information by using an IMBE™ vocoder, then de-vocode the digital information back to analog audio in the receiver. Error correction coding is added to the digital voice information as well as other digital information. Analog CTCSS and DCS are replaced by digital NAC codes (as well as TGID, Source and Destination codes for selective calling). Encryption information can be added to protect the voice information, and other digital information can also be transmitted such as a user defined low-speed data word or an emergency bit. 

P25 systems use the Common Air Interface (CAI). This interface standard specifies the type and content of signals transmitted by P25 compliant radios. A P25 radio using the CAI should be able to communicate with any other P25 radio using the CAI, regardless of manufacturer. 

Current P25 radios are designed to use 12.5 kHz wide channels, allowing two conversations to take place where only one used to fit (on a 25 kHz channel). In Phase 2, P25 radios will use 6.25 kHz channels, allowing four times as many conversations compared to analog. P25 radios must also be able to operate in analog mode on 25 kHz or 12.5 kHz channels. This backward compatibility allows P25 users to gradually transition to digital while continuing to use older equipment. 

P25 transmissions may be protected by digital encryption. The P25 standards specify the use of the Advanced Encryption Standard (AES) algorithm, U.S. Data Encryption Standard (DES) algorithm, and other encryption algorithms. There is an additional specifi cation for overthe-air rekeying (OTAR) to update encryption keys in the radios using the radio network. 

P25 channels that carry voice or data operate at 9600 bits per second (bps). These voice or data channels are protected by a substantial amount of forward error correction, which helps receivers to compensate for poor RF conditions and improves useable range.P25 supports data transmission, either piggybacked with voice (low speed data), or in several other modes up to the full traffi c channel rate of 9600 bps.

P25 Compliant Systems for Mission Critical Communication

Motorola's implementation of the Project 25 digital trunking standard is marketed as ASTRO 25. Whether it is a scheduled event or an unexpected disaster, ASTRO  25 radio systems connect you to all your resources, internal and external, delivering the information you need to coordinate a safe and effective response.  

Renown for resiliency under extreme situations, ASTRO 25 systems can provide you the foundation for reliable communication today while preparing you for tomorrow. Because it is a modern IP platform defined largely by software, you gain the agility to expand and manage your system to meet your organization's growing needs. Easily add capacity and enhance functionality with easy software updates.

Expand beyond voice to include critical data communication. Quickly connect with others via P25 compliant interoperability.  Protect your voice and data information to comply with industry security requirements. And know that when your personnel need it most, their radio will always be a relied upon lifeline they can count on.


  • Interoperability
    Radio equipment that is compatible with P25 standards will allow users from different agencies or areas to communicate directly with each other. This will allow agencies on the federal state/provincial or local level (or any other agency) to communicate more effectively with each other when required (emergencies, law enforcement, etc.)
  • Multiple Vendors
    The P25 open standard will allow competing products from multiple vendors to be interoperable. This will allow customers of the P25 product to benefi t from multiple manufacturing sources (decreased costs, open bidding, non-proprietary systems).
  • Backwards Compatibility
    A basic requirement for Phase 1 P25 digital radio equipment is backward compatibility with standard analog FM radios. This supportsan 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 P25 standards, agencies are assured that their investment in the latest technology has a clear migration path for the future.
  • Encryption Capability
    The P25 standard includes a requirement for protecting digital communications (voice and data) with encryption capability. The encryption used in P25 is optional, allowing the user to select either clear (un-encrypted) or secure (encrypted) digital communication methods. The encryption keys also have the option of being re-keyed by digital data over the RF links. This is referred to as Over The Air Re-keying (OTAR). This capability allows the radio systems manager to change encryption keys without having the subscribers physically bring the radios back to a service shop.
  • Spectrum Effi ciency
    P25 maximizes spectrum effi ciency by narrowing bandwidths.The RF spectrum is a fi nite resource used by every country in the world. Spectrum effi ciency frees up more channels for radio system use.
  • Improved Audio Quality
    With more than one quarter of the channel capacity used for error correction, P25 digital signals have greatly improved voice quality over standard analog signals, especially at low or noisy RF carrier levels. The IMBE™ voice coder converts voice information into digital data and then the data is protected using error correction codes. The error correction is able to correct for small errors in the received signal. Since the audio is digitally encoded the background noise typically present in analog systems is also removed
  • Enhanced Functionality
    P25 radio systems use 2400 bits per second for signaling capabilities. This allows a vast array of additional functions and features to be standard in any P25 radio system. The signaling capabilities include selective calling (Source and Destination ID), talk groups (TGID), network (repeater) access codes (NAC) and emergency fl ags all as standard P25 digital features. P25 signaling also allows for Manufacturers ID’s which will allow different manufacturers to customize radio capabilities, Low Speed Data for user applications, encryption keys and algorithms for secure transmission and many other standard signaling formats.