APCO Project 25 (P25) Radio Frequently Asked Questions

— September 19, 2013 — Leave a comment

APCO P25 LogoQ. What are the benefits of moving to a P25 digital radio solution?

Digital radio provides key benefits such as:

  • Improved audio quality and clarity – especially at low received signal levels near the edge of coverage
  • Enhanced user features
  • More capacity – from the same number of frequencies (spectral efficiency)
  • Greater coverage – than analog, with a sharp drop at full range
  • Secure end-to-end encryption – compared to analog scrambling/encryption techniques, digital encryption has no degradation in voice quality.

P25 is a standard specified by public safety users for public safety users, and as such:

  • It has the objectives of enabling multi-vendor sourcing and interoperability
  • It specifies and provides backwards compatibility to analog and conventional systems
  • It is a proven technology used operationally by hundreds of public safety agencies around the world.

Q. What is P25?

APCO Project 25 (P25) is a digital radio standard that has been specified, in conjunction with the public safety community, to meet the needs of the public safety community, such as interoperability. Technology compliant with the P25 open standard is implemented in several phases:

P25 Phase 1:

  • The P25 Common Air Interface (CAI) on P25 Phase 1 uses a technology known as Frequency Division Multiple Access (FDMA), where the channels are divided according to frequency
  • The channel bandwidths for voice and traffic channels are 12.5kHz
  • It is backwards compatible with analog.

P25 Phase 2:

  • This phase specifies requirements and standards for P25 systems with a CAI incorporating Time Division Multiple Access (TDMA) technology and providing a 6.25kHz-equivalent channel bandwidth for greater spectral efficiency
  • TDMA divides the 12.5kHz into two time slots, effectively giving a 6.25kHz-channel bandwidth
  • The control channel in a P25 Phase 2 system is still a 12.5kHz FDMA channel
  • Data channels, to deliver functionality such as OTAR, are still 12.5kHz FDMA channels
  • It is backwards compliant with P25 Phase 1
  • This is currently only available for trunked networks.

Q. Why are people planning on moving to Phase 2 and what are the benefits?

There are a number of reasons why people may plan to move to P25 Phase 2, including:

Meeting regulatory compliance

  • Specifically, to meet the FCC mandate in the USA where systems in 700 MHz must move to 6.25kHz by January 1, 2017.

Increasing capacity

  • With P25 Phase 2 they can achieve extra spectral efficiency (6.25kHz equivalence in a 12.5kHz channel), which may be a concern for organizations wishing to prepare for additional business-as-usual capacity or incident headroom.

Future-proofing their investment

  • With P25 Phase 2 they will be prepared for future regulatory changes and future growth.
  • A Tait P25 Phase 2-capable solution is software upgradable to Phase 2 operation, meaning they can upgrade their system from P25 Phase 1 to meet their future requirements when needed.

Emergency communications

  • Talker pre-emption (interrupt conversations in an emergency) means that, in an emergency, a transmitting subscriber unit can be signaled to stop transmitting in order to re-allocate the channel – this is done with SACCH signaling.

Possible future benefits associated with P25 Phase 2 (Phase 2 TDMA) include:

  • Ability to dynamically adjust up-link power level
  • Ability to indicate to the speaker when they’re out of range whilst they’re speaking.

Q. Do I have to go to Phase 2 and what does this mean for Phase 1?

No, you don’t need to move to Phase 2 unless you are in the USA using the 700MHz band, in which case you must be Phase 2 operational by January 1, 2017.

If you have capacity issues with your existing trunked system and are unable to obtain additional channels, then installing Phase 2 is a potential solution. As indicated above, Phase 2 can bring the benefit of extra spectral efficiency (6.25kHz equivalence) in trunked networks.

There are likely to be some future value-added features that can only be implemented on a TDMA system (i.e. duplex, pre-emptive emergency and simultaneous voice and data). However, the value created by these is likely to be insufficient to justify a complete system replacement.

Phase 2 does not obsolete Phase 1; it is simply an extension to the standard, much in the same way that other capabilities, such as Inter-RF Subsystem Interface (ISSI) and Console Subsystem Interface (CSSI), have been added. You can still operate in Phase 1, and the Phase 2 standard requires backwards compatibility to Phase 1.

Phase 1 equipment is not obsolete, and the Phase 1 standards are the only standards for conventional digital systems. So if you are a conventional user, Phase 2 will only be a consideration if you are looking at replacing your existing system with a trunked system.

If you choose to operate a network that contains both Phase 2 and Phase 1 terminals, then the efficiency (and capacity) of the network will change depending on the number of talk-groups with terminals of mixed capabilities (i.e. Phase 1 and Phase 2).

  • For example, if a talk-group includes Phase 1 and Phase 2 terminals, it will always use a full physical channel. A Phase 2-only talk-group would use only half a physical channel.
  • A mixed network is always less efficient, but careful migration planning can minimize this inefficiency.

Q. If Phase 1 provides nearly all the communications services that most Public Safety users need, why was Phase 2 developed?

Phase 2 was developed to address the capacity issues that agencies in major metro areas were facing, as well as to enable better use of a limited spectrum (spectral efficiency). Phase 2 at 6.25kHz channel width provided the opportunity for more calls using less spectrum (even though the control channel is still 12.5kHz), so it should generate savings and/or opportunities for extra capacity without extra cost.

Q. If I install a Phase 2-upgradable system, what will my coverage be like when moving to Phase 2, and will I need to put in extra sites? Am I likely to suffer from any interference issues in moving to Phase 2?

Phase 2-upgradable systems should be deployed taking into account both Phase 1 and Phase 2 figures, and using the lowest performer in terms of each of the coverage figures (especially for simulcast). Coverage for each system will be different, as each manufacturer’s base stations will act differently.


For more information on different types of P25 radio networks, download our complimentary whitepaper Navigating the P25 technology maze: which system is right for me?.

What-is-P25-600x600To learn more, you can also visit the Introduction to P25 page at the Tait Radio Academy to watch free informational videos on P25. Be sure to register for the Academy to keep track of your course progress and ask and answer questions in the forum.

And when you’re ready, visit our P25 Best Practice site for free guides with information on everything from choosing your P25 system to managing and getting the most from your network.p25 Best Practice 5 guides - Manage your P25 system on top

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