APCO Project 25 (P25) Trunked Radio Systems (TRSs) because they follow the open APCO’s Project 25 Standard for Public Safety TRSs. Multiple vendors make and sell Project 25 systems and compliant radios. Audio on these systems is exclusively digital using the APCO-25 Common Air Interface (P25 CAI) standard.

There are several “Subsystem” features defined as part of the Project 25 Standard to enhance interconnectivity & interoperability and allow equipment from various manufacturers to work together.

These standards include
  • P25 Common Air Interface (P25 CAI)
    • Over-the-air modulation (digital audio)
  • P25 Inter-RF Subsystem Interface (ISSI)
    • The Inter-RF Subsystem Interface (ISSI) standard allows P25 systems from different manufacturers to be directly interconnected at the controller level, allowing seamless cross-system intercommunication, and system-to-system roaming for same-band systems.
  • P25 Console Subsystem Interface (CSSI)
    • The Console Subsystem Interface (CSSI) allows dispatch consoles from different manufacturers to be connected to the controller/core of other manufacturers’ systems. For example, a Zetron console could be connected to a Motorola system, or a Harris console could be connected to a Tait system.