P25 ISSI

P25 ISSI refers to the Project 25 Inter-Subsystem Interface. It is a standard defining how different P25 (Project 25) radio systems, often from different manufacturers or serving different jurisdictions, can communicate with each other. This interoperability allows users on disparate P25 systems to communicate directly, enhancing coordination during emergencies and facilitating mutual aid. The ISSI specifies the protocols and procedures for voice, data, and control message exchange between these systems. Key aspects addressed by the ISSI include addressing, call setup, and security protocols to ensure secure and reliable communication across interconnected P25 networks.