Question: When does the Mobile Station (MS) start ciphering in A/Gb mode during a routing area update if ciphering was previously used and authentication is omitted?
Option 1: Immediately after sending the Routing Area Update Request message
Option 2: Upon receiving a ciphered Routing Area Update Accept message from the SGSN (Serving GPRS Support Node)
Option 3: Before sending the Authentication and Ciphering Response message
Option 4: Simultaneously with the SGSN when the routing area update is acknowledged
Explanation: In A/Gb mode, during a routing area update where ciphering was previously used and authentication is skipped, the MS resumes ciphering upon receiving confirmation that the network also intends to maintain the secure connection. This confirmation comes in the form of a ciphered Routing Area Update Accept message from the SGSN, signaling to the MS that it should resume ciphering for the continued exchange.