Inter-RF Subsystem Interface (ISSI) Gateway

The Inter-RF Subsystem Interface (ISSI) Gateway provides TIA-standardized network-level communication between Project 25 (P25) radio systems regardless of manufacturer, allows authorized radios to roam between P25 systems, and prevents ID overlaps by assigning temporary unit and group IDs to roaming users.

ISSI Gateway Overview

The ISSI Gateway is based on the Inter-RF Subsystem Interface standard, issued by the Telecommunications Industry Association (TIA) responsible for publishing the P25 Land Mobile Radio (LMR) standards. TIA document TIA-102.BACA defines the interface that provides for interoperability between RF Subsystems (RFSS) regardless of the system manufacturer. L3Harris’ ISSI Gateway is a software application that resides on standard off-the-shelf hardware and can be added to any VIDA® network though a simple Ethernet connection into the IP-based VIDA infrastructure.

Inter-System Communication

In most scenarios, systems which need to interoperate are not managed by a common administrator nor installed at the same time. This means there is no coordination of the system databases and most likely there are overlaps between User and Group IDs. The ISSI Gateway solves this issue. When a group call is made on System 1 and the ISSI passes it to System 2, System 2 can assign a temporary working group identifier used for all subsequent calls made on that group. If desired, the group calls from System 1 can instead be directly affiliated with any existing group within System 2 such that a dispatcher patch is not required to initiate interoperable communications.

Unit Roaming Using ISSI

The ISSI Gateway allows P25 compliant radios to roam between systems, provided they are within the same frequency band and there is some coordination between the system administrators. When a radio from System 1 roams into the coverage area of System 2, the ISSI provides communication of the radio’s ability to roam and other user attributes such as user call priority, I-Call capability, etc. System 2 will register the group and unit, assign them temporary IDs, and allow the radio to communicate on the system. System 2 passes all of the radio’s transmissions back to System 1 via the ISSI Gateway. As mentioned, the roaming user’s group calls can be directly affiliated with a permanent group on System 2 if desired.