S5 interface is the interface between the SGW and the PGW that exist in the same network (PLMN).

S5 Interface works in both the Control Plane and the User Plane.

It uses GTP-C protocol in the Control Plane, which creates a GTP tunnel between the SGW and the PGW in order to exchange signaling messages between each other.

And It uses GTP-U protocol in the User Plane, which creates a GTP Tunnel between the SGW and the PGW in order to exchange user plane data between each other.