QSIG is a signalling protocol that operates between Private Integrated services Network eXchanges (PINX) within a Private Integrated Services Network (PISN). A PISN provides circuit-switched basic services and supplementary services to its users.
SIP is an application layer protocol for establishing, terminating and modifying multimedia sessions. It is typically carried over IP.
ISO/IEC 23915:2005 specifies signalling interworking for call diversion during the establishment of calls between a PISN employing QSIG and a corporate IP network employing SIP. It covers both the impact on SIP of call diversion in the QSIG network and the impact on QSIG of request retargeting in the SIP network.
ISO/IEC 23915:2005 is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and a corporate IP network employing SIP.