Question: What is the approach specified by GSM/UMTS specifications for protocols between the Service Centre (SC) and the Mobile Switching Centre (MSC) below the transfer layer?
Option 1: A mandatory protocol stack is defined to ensure interoperability.
Option 2: A set of recommended proprietary protocols are outlined for consistency.
Option 3: The choice of protocol is left entirely to international telecommunications standards.
Option 4: No specific protocol is mandated; instead, it's subject to agreement between SC and PLMN operators.
Explanation: This answer reflects the flexibility allowed by GSM/UMTS specifications for interconnections below the transfer layer. By not mandating a specific protocol, SCs and PLMN operators can negotiate and implement protocols that best suit their needs, supporting customization and innovation while ensuring basic compatibility requirements are met through operator agreements.