All system elements are 3G, except for the radio interface, which is 2G. This applies when a 3G subscriber 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 existing 2G BSS are replaced by 3G technology, while the VLR/SGSN is already 3G).
3G AKA is executed. The 2G BSS is transparent for 3G authentication parameters but not capable of handling ciphering and integrity protection keys CK and IK. Therefore the 3G VLR/SGSN and the 3G ICC have to compute Kc from CK, IK with conversion function c3 and send it to the BSS and to the ME. Despite a 2G radio access network is involved, 3G security context is established.
No service with a 3G single mode ME.
The USIM receives parameters RAND and AUTN and calculates RES, CK and IK. If service no. 27 is available, Kc is generated by conversion function c3 and additionally included in the response. The keys CK and IK are not needed in this security scenario and can be discarded by the ME. If the USIM does not support service no. 27, network access is not possible.