Question: In a UE originating CS multimedia call fallback to speech scenario, what triggers the measurement of falling back to the less preferred service (speech)?
Option 1: Receiving a Call Setup message from the UE with an 'Information transfer capability' set to speech.
Option 2: Sending a Call Proceeding message to the UE with the preferred service's BCIE (Bearer Capability Information Element) for CS multimedia.
Option 3: Sending a Call Proceeding message to the UE with the less preferred service’s BCIE, which is speech, indicating 'Information transfer capability’ set to speech or alternate speech/facsimile group 3 - starting with speech.
Option 4: Establishing the call as CS multimedia after receiving an Alerting message from the MSC Server Function.
Explanation: This option correctly identifies the trigger for measuring the fallback to speech. The act of sending a Call Proceeding message with the BCIE set for speech (the less preferred service) signifies the network's decision to fall back from the preferred CS multimedia service, due to possibly insufficient resources or compatibility issues, thus initiating the measurement as defined in the performance management context.