Interoperability with ISSI

The P25 Inter RF Sub-system Interface (ISSI) provides IP-based open standard system-level interoperability. Using the ISSI you can interconnect two or more P25-compliant trunked systems – even if they are from different vendors or operating in different frequency bands (e.g., UHF vs. 700 MHz); using different versions of P25 (Phase 1 or Phase 2); or all of the above. The basic requirement is that each network must install an ISSI interface.


Given the capabilities of the ISSI, it is easy to imagine some scenarios for its deployment:

  • Designing a very wide-area ‘system of systems’ for an organization by interconnecting multiple smaller P25 networks through the ISSI.
  • Agencies that wish to set up mutual-aid communications can interconnect their independent P25 systems (and consoles) through the ISSI and CSSI.
  • A public-safety agency can deploy a radio system from its preferred vendor rather than joining a statewide systems, while at the same time honoring its interoperability obligations by connecting to the statewide system via the ISSI.

For mobile or portable radios roaming across or between networks, the ISSI supports a variety of call and data services, including:

  • P25 addressing
  • Encrypted and clear voice calls (both individual and group)
  • Authentication of roaming radios
  • Mobility management
  • Call control
  • Push-to-talk management
  • P25 supplementary data service (e.g., call alert)
  • P25 packet data on the Data Network – Interface (including OTAR)
  • ISSI support of the P25 CSSI console interface
  • P25 conventional voice service (including mixed trunked/conventional voice service)

Management of P25 addressing lies at the heart of the ISSI. Radios have identities that can be tracked and controlled when they roam from one network to another. They have “home” networks that they normally belong to and “visited” networks to which they roam. Similarly, they have home consoles (also with IDS) that operate in their home network and which must communicate with consoles in visited networks, and they have a home talk group to which they belong and talk groups they wish to connect to when they visit a different network.


As a result, the trunking concepts of registration (for mobility management) and authentication (for security), as well as call privileges permitted to the visitor, must be carefully negotiated between the home and visited networks.

The planning effort for ISSI internetworking should not be underestimated. It is a highly complex undertaking which will be different for each group of agencies and users. Do not expect implementing ISSI to be a ‘plug-and-play’ technology exercise.


Useful guidance on the complexities of planning for ISSI may be found in:

Best practices for Planning and Implementation of P25 Inter-RF Subsystem Interface (ISSI) and Console Subsystem Interface (CSSI): Volume I (2019) and Volume II (2020).

Both these documents are available for free download through the P25 Technology Interest Group (PTIG) at

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