Question: In an Evolved Packet Core (EPC) network, when is a failed update location procedure counted towards the 'MM.UpdLocHssFail.Cause' measurement on the S interface?
Option 1: Upon sending an 'Update-Location-Request' message from MME to HSS.
Option 2: When a UE initiates a handover between two eNodeBs.
Option 3: On receipt of an 'Update-Location-Answer' message by MME from HSS, where the Experimental-Result AVP indicates an S error.
Option 4: After a successful authentication procedure for a UE.
Explanation: This is because the measurement is incremented based on the receipt of a specific error indication (Experimental-Result AVP indicating an S error) in the 'Update-Location-Answer' message from HSS to MME, as this signifies a failed update location procedure due to a particular cause.