ISO/IEC 17343:2004 specifies signalling interworking between "QSIG" and the Session Initiation Protocol (SIP) in support of basic services within a corporate telecommunication network (CN). "QSIG" is a signalling protocol that operates between Private Integrated Services eXchanges (PINX) within a Private Integrated Services Network (PISN). A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards, in particular ISO/IEC 11572 (call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the support of supplementary services) and a number of Standards specifying individual supplementary services. SIP is an application layer protocol for establishing, terminating and modifying multimedia sessions. It is typically carried over the Internet Protocol (IP) (IETF RFC 791 and IETF RFC 2460). Telephone calls are considered as a type of multimedia session where just audio is exchanged. SIP is defined in IETF RFC 3261.