Question: What happens to the GSM security context when a GSM subscriber undergoes an intersystem change to a GSM BSS controlled by another SGSN?
Option 1: The initial SGSN deletes the 64-bit GSM cipher key Kc.
Option 2: The new SGSN generates a new 64-bit GSM cipher key Kc.
Option 3: The ME stops applying the GSM cipher key Kc.
Option 4: The initial SGSN sends the 64-bit GSM cipher key Kc to the new SGSN, which stores and applies it.
Explanation: When a GSM subscriber undergoes an intersystem change to a GSM BSS controlled by another SGSN, the initial SGSN sends the 64-bit GSM cipher key Kc agreed during the latest GSM AKA procedure to the new SGSN. The new SGSN then stores and applies this key, becoming the new anchor point for the service. This ensures seamless security context transfer between SGSNs.