•Interface between different MMEs
•Used during inter-MME tracking area updates (TAU) and handovers
•Inter-MME TAU: The new MME can contact the old MME the user had been registered before to retrieve data about identity (IMSI), security information (security context, authentication vectors) and active SAE bearers (PDN gateways to contact, QoS, etc.)
•Obviously S10 is a pure signaling interface, no user data runs on it.
•Interface between the MME and the HSS
•The MME uses it to retrieve subscription information from HSS (handover/tracking area restrictions, external PDN allowed, QoS, etc.) during attaches and updates
•The HSS can during these procedures also store the user’s current MME address in its database.
•Interface between MME and a Serving GW
•A single MME can handle multiple Serving GW each one with its own S11 interface
•Used to coordinate the establishment of SAE bearers within the EPC
•SAE bearer setup can be started by the MME (default SAE bearer) or by the PDN Gateway.
•Interface between Serving GW and PDN GW
•S5: If Serving GW and PDN GW belong to the same network (non-roaming case)
•S8:If this is not the case (roaming case)
•S8 = S5 + inter-operator security functions
•Mainly used to transfer user packet data between PDN GW and Serving GW
•Signaling on S5/S8 is used to setup the associated bearer resources
•S5/S8 can be implemented either by reuse of the GTP protocol from 2G/3Gor by using Mobile IPv6 with some IETF enhancements.