All system elements are 3G, except for the BSS and the HLR/AuC, which are 2G. It is possible to keep a 3G subscription in a 2G HLR/AuC, however on request by a 3G VLR/SGSN this can only deliver 2G triplets RAND, RES and Kc. The 3G VLR/SGSN is backward compatible and behaves like a 2G VLR/SGSN: Between the VLR/SGSN and the USIM 2G AKA is executed, i.e. using RAND in the request and SRES in the response. No service with a 3G single mode ME.
To handle 2G AKA, the USIM must be capable to accept a request with RAND and return a response with SRES and Kc. The support of the virtual 2G mode is indicated by service no. 38 in the USIM Service Table. Since a 2G BSS is involved, service no. 27 is also necessary. In case the USIM does not support services no. 27 and no. 38, network access is not possible.