Question: In the context of 3GPP authentication and key agreement for User Equipment (UE), what is the primary purpose of concatenating SQN with AMF to obtain CDOUT in the test USIM and SS operations?
Option 1: To derive the Ciphering Key (Kc) for GSM ciphering.
Option 2: To calculate the Authentication Token (AUTN) for SS and AUC.
Option 3: To generate the input (CDOUT) for calculating XMAC/MAC in Step 4.
Option 4: To extract RES/XRES, CK, IK, and AK from XDOUT.
Explanation: The correct answer is Option 3 because concatenating SQN with AMF to obtain CDOUT prepares the input (CDOUT) used in conjunction with XDOUT to calculate XMAC (for test USIM) or MAC (for SS and AUC) in Step 4, highlighting CDOUT's role in generating a crucial authentication component (XMAC/MAC).