The present document defines the transport of the Inter-System Interface (ISI) using PSS1 as transport layer. It specifies: - the PSS1 signalling used for transport of ISI APDUs; and - the general protocol mechanism, called ISI Mediation Function which coordinates the communication between TETRA systems. The ISI Mediation Function applies to any TETRA Switching and Management Infrastructure (SwMI) which supports the ISI.