Recommendation ITU-T Q.3931.1 describes the core concepts of testing to establish a performance benchmark for an initial release of a PSTN/ISDN emulation subsystem (PES).The same tests can be used also for legacy PSTN/ISDN networks or for interworking tests between a PSTN/ISDN emulation subsystem and legacy PSTN and ISDN.The metrics measured and reported are for performance of this subsystem under a communications application load.This Recommendation covers Part 1 – Core Concepts,the first of the four performance aspects.