Project 25 Documents & Standards Reference
System Interfaces Described
Dividing the LMR network at defined interfaces lets manufacturers develop interoperable products specific to their areas of expertise. This frees users to buy products that meet their specific needs.
For example, a manufacturer might build fixed-location radios (identified as base station or fixed station), in which case the product must meet the requirements of two P25 interfaces (both the CAI and the FSSI described in the next bullet list). In addition, that manufacturer might offer non-standardized, value-added features as an extension to their P25 fixed radio offering, such as operation over large temperature spans (ideal for radios on mountaintop locations in isolated areas). Such a feature might be very important to a particular user. In another scenario, a manufacturer might highlight pricing considerations by offering a functionally limited fixed radio that still satisfies all of the mandatory P25 interface requirements. Again, this may be appealing to users for some applications.
The goal is to provide users the ability to choose from various manufacturers’ offerings to build out their P25 systems. P25-compliant equipment offerings must satisfy all mandatory and, as applicable, optional requirements. (These requirements are noted as Mandatory and Standard Option features in the P25 SoR. See User Needs.) This way, users can be confident that their P25 systems will work across the interface with other P25-compliant equipment regardless of manufacturer.
Following is a description of each P25 interface.
Enables wireless communication (voice and data) directly between P25 mobile and portable subscriber units (i.e., P25 radios) and between P25 mobile and portable subscriber units via a fixed/base station (and many times another radio, called a repeater, which is a component of the P25 equipment infrastructure).
o Phase 1 refers to P25 requirements and standards for a CAI based on FDMA using a 12.5 kHz channel. Phase 1 defines the necessary technologies to provide for channel reduction from 25 kHz to 12.5 kHz.
o Phase 2 refers to P25 requirements and standards for a CAI based on TDMA using a 6.25 kHz equivalent channel, two slots in a 12.5 kHz channel. Phase 2 defines an additional 50-percent reduction in channel size to 6.25 kHz equivalency. Implementation of Phase 2 necessitates that a P25 system be configured and deployed as a trunked radio system with backward compatibility to Phase 1 FDMA conventional mode. Compatibility with analog and Phase 1 FDMA trunked radio systems is optional.
Enables interconnection via a direct wireline of a P25 radio with laptops, terminals, or other data peripherals. A data peripheral connected to a vehicular mobile radio is referred to as a MDP.
Enables voice and control information to be transferred between an RFSS or console subsystem and a fixed/base station.
Enables voice and control information to be transferred between an RFSS and a console subsystem (a console is equipment that a dispatcher or a supervisor uses to oversee and control mission-critical voice communications among field personnel).
Enables administrators to comprehensively control and monitor P25 functional elements via RFSS connectivity.
Enables data to be communicated to/from external computers, data networks, data sources, etc., via RFSS connectivity.
Enables field personnel to make connections through the PSTN via RFSS connectivity by using their radios rather than, for example, using cellular telephones.
Enables different manufacturers’ RFSS equipment to interoperate, and includes support for roaming of P25 radio subscribers among different jurisdictions, agencies, cities, etc.
Key Fill Device — Mobile Radio Interface (KFR-MR)
(Future work item) Enables manual P25 encryption rekeying between a key fill device and a P25 radio to provision the radio with encryption keys or other key management-related information.
Inter-Key Management Facility Interface (IKI)
(Future work item) Once adopted, will enable the interconnection of P25 Key Management Facilities (KMFs) for secure exchange of P25 encryption keys in a P25 system. The IKI will employ open, standards-based cryptographic techniques for key protection and exchange based on P25 user requirements.
