ME, BSS and VLR/SGSN are 2G, ICC and HLR/AuC are 3G. This applies when a 3G subscriber with a 2G ME roams into a 2G network.
Upon request from a 2G VLR/SGSN, the 3G HLR/AuC must produce 2G triplets out of 3G quintets.
It therefore applies conversion function c2 to generate RES from XRES and conversion function c3 to generate Kc from CK, IK.
RAND is left unchanged and AUTN is discarded.
The 2G triplet is sent to the VLR/SGSN. The authentication and key agreement procedure is performed according to 2G specifications, i.e. using RAND in the request and SRES in the response. In the UICC only the SIM application is active.