S5 - Interface between Serving GW and PDN GW. Functionality 1. User plane tunneling and tunnel management. 2. It is used for Serving GW relocation due to UE mobility or connections to a non-collocated PDN GW.
S6a - Interface between HSS and MME Functionality 1. Enables transfer of subscription and authentication data. Required for evolved system -AAA interface. Gx- Interface between VPCRF and PCEF in PDN GW. Functionality: Transfer of (QoS) policy and charging rules.
S8 - Interface between Serving GW in the VPLMN and the PDN GW in the HPLMN. Functionality Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN.