ME and BSS are 2G, the rest is 3G. This applies when a 3G subscriber with a 2G ME roams into a 2G radio access network, which is connected to a 3G VLR/SGSN (e.g. when in the start phase of a 3G network not yet all of the existing 2G BSS is replaced by 3G technology, while the VLR/SGSN is already 3G).
Upon request from a 3G VLR/SGSN, the 3G HLR/AuC delivers quintets. The VLR/SGSN, as it does not know what type of ME it is communicating with, forwards RAND and AUTN. The 2G ME simply ignores AUTN, therefore the UICC only receives RAND and responds with SRES for 2G AKA. After determination that 2G AKA is to be executed, the 3G VLR/SGSN generates Kc from CK/IK (conversion function c3) and RES from XRES (conversion function c2). It then also performs 2G AKA. In the UICC only the SIM application is active.