The Inter-RF Subsystem Interface (ISSI) supports selected services and
functions that span more than a single Radio Frequency Subsystem (RFSS) to
provide an extended service coverage area. To satisfy these needs, the ISSI
defines a means to track and locate desired subscriber units (SUs) belonging to a
given P25 system, and a means to route service support to these SUs. This
interface is available for Project 25 (P25) systems at the G reference point
between RFSSs (see Figure 1 [102BACC-A]).