Question: In a videophone session between two users, where one user initiates an upgrade from an audio session, who can be charged extra for the use of the video component according to 3GPP charging principles?
Option 1: Only the user who initiated the upgrade to videophone
Option 2: Both users, as the video component requires extra resources at both ends
Option 3: Neither user, as upgrades are not chargeable under 3GPP principles
Option 4: The network operator, as they are solely responsible for resource allocation costs
Explanation: According to the charging principles outlined, when a component (like video in this case) is added to a session and requires an increase in resources at both ends, it should be possible to charge all users who need this increase in resource. This ensures fairness in billing for the enhanced service utilization.