Question: In the context of UMTS and combined UMTS/GSM networks, what is the primary trigger for incrementing the 'Attempted SGSN-Initiated PDP context modifications procedures' measurement counter?
Option 1: Receipt of a 'Modify PDP Context Accept' message from the User Equipment (UE)
Option 2: Transmission of a 'Modify PDP Context Request' message to the Serving GPRS Support Node (SGSN) from the Gateway GPRS Support Node (GGSN)
Option 3: Establishment of a new Radio Access Bearer (RAB) for an existing PDP context
Option 4: Transmission of a 'Modify PDP Context Request' message to the Mobile Station (MS)
Explanation: The correct answer is Option 4 because the measurement counter for 'Attempted SGSN-Initiated PDP context modifications procedures' increments upon the transmission of a 'Modify PDP Context Request' message to the Mobile Station (MS), directly reflecting an attempt by the SGSN to modify a PDP context, fitting the measurement's definition.