29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS
5.1 Key characteristics of ISUP/BICC based CS networks
This signalling interface to a PSTN is either based on BICC Capability Set 2 as specified in ITU-T Recommendations Q.1902.1 to Q.1902.6 , or on ISUP (see ITU-T Recommendations Q.761 to Q.764 ).
The interface towards a CS-PLMN may either be one of the interfaces mentioned in the paragraph above or a signalling interface based on BICC with 3GPP specific extensions, as specified for the 3GPP Nc interface in 3GPP TS 29.205 , and the IM-MGW may support the 3GPP Nb interface, as specified in 3GPP TS 29.414  and 3GPP TS 29.415 . If the 3GPP Nc interface is applied as signalling interface, the 3GPP Nb interface is used as user plane interface and the Nb UP Framing protocol is applied.
5.2 Key characteristics of IM CN subsystem
The IM CN subsystem uses SIP to manage IP multimedia sessions in a 3GPP environment, it also uses Ipv6, as defined in RFC 2460 , as the transport mechanism for both SIP session signalling and media transport. The 3GPP profile of SIP defining the usage of SIP within the IM CN subsystem is specified in 3GPP TS 24.229 . Example callflows are provided in 3GPP TS 24.228 .