Gn/Gp interface, is the interface between the SGSNs and the GGSNs.
'Gn interface' is the interface between the SGSNs and the GGSNs when they exist within the same mobile network.
In case of roaming scenario, the user is traveling to another mobile network. In this case, the SGSNs and the GGSNs will be in different mobile networks. In this case, the interface between the SGSNs and the GGSNs, will be 'Gp interface'.
The protocol stack for Gn/Gp interface shows that it uses TCP/IP for packet routing between different nodes.
While, it uses the GTP protocol 'GPRS Tunneling Protocol' in the application layer.
- GTP-C refers to 'GPRS Tunneling Protocol for the control plane', and it tunnels signalling messages between the SGSNs and the GGSN.
- GTP-U refers to 'GPRS Tunneling Protocol for the user plane', and it tunnels user data between the SGSNs and the GGSNs.
Th Control Plane is were the nodes exchange signaling message in order to initiate, modify or terminate the data session of the user.
While the user Plane is where the nodes exchange user data sent and received between the UE and the external network while the data session is active.
For more understanding for the difference between Control plane, and User Plane, please check this post.