Question: What type of server may the PDN Gateway interact with over the SGi interface to provide authorization and other network services?
Option 1: Home Subscriber Server (HSS)
Option 2: 3GPP AAA Proxy
Option 3: RADIUS or Diameter Server in an external PDN network
Option 4: Mobility Management Entity (MME)
Explanation: The correct answer is Option 3 because the PDN Gateway may interact with a AAA server over the SGi interface, which could be a RADIUS or Diameter Server in an external PDN network. This type of server provides authorization and other network services for UE access to the EPC.