Question: What is the primary purpose of the network-side nonce FRESH in the context of 3G security?
Option 1: To authenticate the user's identity
Option 2: To encrypt signalling messages between the user and the network
Option 3: To protect against replay attacks by the network on the user
Option 4: To protect the network against replay of signalling messages by the user
Explanation: The FRESH parameter is used to prevent replay attacks by the user, ensuring that the network receives authentic and non-replayed MAC-Is. This mechanism provides an additional layer of security in the 3G network.