EPC Specific Interfaces

Interfaces:

S1-MME: This  is the interface between eNodeB and MME (Mobility Management Entity). As the MME handles control plane only, this interface manages all the control plane signalling between eNodeB and MME.

S1-U: This interface is between eNodeB and Serving GW (SGW). It handles all the userplane data. This interface works on GTPv1 protocol. 

S11:  This is the reference point between MME and SGW. This works on protocol GTPv2-C. The UDP Destination Port number for GTP-C request messages is 2123. It is the registered port number for GTPv2-C. This interface also handles control plane messages.

S5: This tunnel interface is between SGW and PDN-GW (PGW). This interface handles both Control plane and Data plane. S5 can be GTP based or PMIP based. If it is GTP based, the control plane messages use GTPv2-C where as the userplane uses GTPV1-U. The UDP Destination Port number for GTP-U is 2152. If it is PMIP based, the interface runs on PMIPv6.

 Gx:  This interface is between PCRF (Policy and Charging Rules Function) and PCEF (Policy and Charging Enforcement Function). Normally PCEF is colocated with PGW. This interface mainly provides transfer of all QoS policy and chargin rules from PCRF to PCEF.

Ref:  3GPP TS 23.401