Question: In 3GPP security, what is the primary purpose of including the 'DIRECTION' parameter (1 bit) when generating a keystream for encrypting transmissions between a UE (User Equipment) and an RNC (Radio Network Controller)?
Option 1: To indicate the type of cryptographic algorithm used for encryption.
Option 2: To differentiate between control plane and user plane data.
Option 3: To signal the use of asymmetric bearer services.
Option 4: To prevent the reuse of keystreams for simultaneous uplink and downlink transmissions.
Explanation: The inclusion of 'DIRECTION' ensures that even if the same cipher key is used for both uplink and downlink channels, the keystream generated will be different for each direction, preventing keystream reuse and enhancing security, especially when uplink and downlink channels are multiplexed onto a single physical layer frame.