A basic requirement for Phase 1 P25 digital radio equipment is backwards compatibility with standard analog FM radios. This supports an orderly migration into mixed analog and digital systems, enabling users to gradually trade out radios and infrastructure equipment.
Agencies can invest in the latest P25 technology and operate it initially in analog mode with the assurance that there is a clear migration path to the future.
The P25 Common Air Interface (CAI)
Some of the earliest and most fundamental Telecommunication Industry Association decisions relate to the CAI standard. 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. The CAI also defines the ‘to’ and ‘from’ addresses, encryption, trunking and conventional control messages. The coverage that can be achieved by P25 is equivalent to that of analog equipment (the industry benchmark), but without sacrificing audio quality. The design of the CAI was carefully engineered to ensure that public safety users could get the benefits of superior digital clarity without needing to reach into their pockets to fund significantly more radio sites to match the coverage they already had with analog equipment.
To meet the goal of achieving the highest level of interoperability (Level 6), the P25 standards have been expanded to include a standard for interconnecting different radio systems. The Inter RF Subsystem Interface (ISSI), which has been available since 2009, ties together radio systems regardless of vendor or frequency band. Previously, public safety agencies were forced to rely on vendors to connect two radio networks.
The P25 Compliance Assessment Program (CAP)
The P25 CAP is a US Government-sponsored program that establishes a process for ensuring that equipment complies with P25 standards and is capable of interoperating across manufacturers. CAP certification is likely to become mandatory for any vendor intending to sell digital radio equipment as ‘P25 compliant’. It is the only guarantee that exists for P25 interoperability.
Tait Communications’ TELTEST laboratory has been officially recognized by the US Government as one of eight P25 CAP recognized laboratories. P25 compliant declared equipment is qualified as such if it:
- has been tested at a P25 CAP recognized laboratory,
- has been determined by the supplier to be compliant with all current requirements of the P25 CAP,
- possesses a Supplier’s Declaration of Compliance (SDoC) and,
- has a Summary Test Report (STR) that has been reviewed by the P25 CAP Laboratory Program Manager.
Typically, equipment is independently tested for P25 compliance at three separate laboratories with the consequence that competing vendors will test each other’s equipment and thereby keep the process honest. Each manufacturer’s declaration of compliance can be viewed on a government website, the Responder Knowledge Base.
Radio frequency use with P25
P25 can be used on a wide range of VHF, UHF and 800MHz frequencies, allowing existing analog channels to be upgraded gradually to P25 digital channels. A new block of spectrum does not need to be purchased. P25 channels are designed to be more spectrally efficient than traditional analog wideband channels. In a crowded RF environment, converting to P25 will allow more radio channels to operate in the same amount of RF spectrum.
In order to improve the efficiency of radio spectrum use, P25 has been rolled out in two phases:
- P25 Phase 1: 12.5 kHz channel spacing is twice as efficient as traditional analog wideband channels, which generally use 25 kHz. Phase 1 P25-compliant systems are backwards compatible and interoperable with legacy analog conventional FM systems. Thus old analog portable and mobile radios can work alongside new digital radios until such time as the old radios are replaced.
- P25 Phase 2: Phase 2 is the next version of P25 and is twice as spectrally efficient as Phase 1. It can fit two voice or data streams into a 12.5 kHz channel where Phase 1 allows only one. To achieve this, a new CAI needed to be designed, but modern P25 radios will support the Phase 1 CAI as well. As part of the standard, Phase 2 P25-compliant systems are backwards compatible and completely interoperable with P25 Phase 1 systems.
If you are deciding on a P25 system and want to know which P25 technology best suits your needs, download our complimentary whitepaper Navigating the P25 Maze: which system is right for me?.