Question: What is the role of the GGSN when interworking with an ISP/PDN at the Gi reference point?
Option 1: The GGSN only relays PPP frames to the PDN without terminating the PPP connection.
Option 2: The GGSN terminates the PPP connection towards the MS but does not tunnel PPP frames to the PDN.
Option 3: The GGSN behaves like a LAC towards the external network and tunnels PPP frames to the PDN using a protocol other than L2TP.
Option 4: The GGSN either terminates the PPP connection towards the MS or further relays PPP frames to the PDN, possibly tunneling them in e.g. L2TP.
Explanation: According to the PDN Interworking Model, the GGSN has a dual role when interworking with an ISP/PDN at the Gi reference point. It can either terminate the PPP connection towards the MS or further relay PPP frames to the PDN, possibly tunneling them in e.g. L2TP. This flexibility allows for different scenarios and network configurations, making Option 4 the correct answer.