Question: Which entity initiates the re-keying for KeNB, KRRC-enc, KRRC-int, KUP-int, and KUP-enc when an EPS AS security context different from the currently active one is to be activated?
Option 1: eNB (Evolved Node B)
Option 2: MME (Mobility Management Entity)
Option 3: UE (User Equipment)
Option 4: SGW (Serving and Packet Data Network Gateway)
Explanation: The correct answer is the MME, as it is responsible for initiating re-keying for these specific keys when there's a need to activate a different EPS AS security context. This ensures secure key management across the network, particularly during changes in security contexts. The eNB's role is more focused on initiating key refreshes due to PDCP COUNT wrap-around, while UE and SGW are not directly involved in initiating such re-keying procedures for these specific keys.