Question: Where is ciphering performed if a radio bearer is using a non-transparent RLC mode?
Option 1: In the MAC sub-layer
Option 2: In the PDCP sub-layer
Option 3: In the IP layer
Option 4: In the RLC sub-layer
Explanation: According to the rules, if a radio bearer is using a non-transparent RLC mode (AM or UM), ciphering is performed in the RLC sub-layer. This ensures that data confidentiality and integrity are maintained at the Radio Link Control layer.