Foreword This European Standard (Telecommunications series) has been produced by ECMA on behalf of its members and those of the European Telecommunications Standards Institute (ETSI)@ and is now submitted for the ETSI standards One-step Approval Procedure. The present document is one of a series of standards defining services and signalling protocols applicable to Private Integrated Services Networks (PISN). The series uses the Integrated Services Digital Network (ISDN) concepts as developed by ITU-T and conforms to the framework of standards for Open Systems Interconnection (OSI) as defined by ISO/IEC. The present document specifies the signalling protocol for use at the Q reference point in support of the Call Identification and Call Linkage Additional Network Feature. The protocol defined in the present document forms part of the PSS1 protocol (informally known as QSIG). The present document endorses an International Standard@ ISO/IEC 21889.