Question: In 3GPP security context, what does the 1-bit field "DIRECTION" indicate when its value is set to '1' in signaling messages between a User Equipment (UE) and a Radio Network Controller (RNC)?
Option 1: The type of cryptographic algorithm used for message encryption.
Option 2: The direction of transmission from the RNC to the UE.
Option 3: The integrity key expiration status.
Option 4: The simultaneous use of an integrity key for both uplink and downlink channels.
Explanation: According to the 3GPP security specifications, the "DIRECTION" field is used to indicate the transmission direction of signaling messages. A value of '1' specifically signifies that the message is being transmitted from the Radio Network Controller (RNC) to the User Equipment (UE), highlighting the directional flow of network-to-user communication.