This Recommendation gives an overview of those in the Q.1100-Series for interworking between the international public switched telephone network/ISDN and the INMARSAT mobile satellite systems. It also contains definitions of the terminology used in these Recommendations.The interworking of all INMARSAT standard systems with the PSTN/ISDN is based on a common set of principles.However, due to the differences between the various INMARSAT standard signalling systems, the specific interworking procedures for each particular INMARSAT standard system is unique.