Question: What happens during PS to CS domain change from S1 mode due to SRVCC handover of a PDN connection for emergency bearer services that used the "null integrity protection algorithm" EIA0 in S1 mode?
Option 1: The MS and network perform key derivation using a new encryption algorithm.
Option 2: The MS and network skip key derivation but use a default encryption key.
Option 3: The MS and network do not perform key derivation.
Option 4: The MS and network renegotiate the integrity protection algorithm.
Explanation: When PS to CS domain change from S1 mode occurs due to SRVCC handover of a PDN connection for emergency bearer services that used EIA0 in S1 mode, neither the Mobile Station (MS) nor the network performs key derivation. This is because EIA0 is the "null integrity protection algorithm", implying no actual encryption or integrity protection was applied, thus eliminating the need for new key derivation during handover.