SGi: This is the interface to external packet data networks such as (internet, corporate networks). It contains end-user’s IP point of presence. All user and control plane functions that use the Gi interface are handled above the end-user’s IP layer. All terminal mobility within 3GPP will be handled before (below) the SGi interface.
S1: This is the interface between RAN and Core Network, i.e., this interface is between eNodeBs , MME and SGWs.
In the user plane, this interface is based on Gateway Tunneling protocol (GTP ) user data tunneling (GTP-U) .
In the control plane, this interface is more similar to Radio Access Network Application part (RANAP), with some simplifications and changes due to different functional split and mobility within the overall Evolved Packet System (EPS).
S1-interface is split into a S1-CP (control plane ) and S1-UP (user plane). The signaling transport on S1-CP will be based on SCTP. Signaling protocol for S1 is called S1-AP (S1-Application Protocol).
S1 is a many to many interface.
S3: This is a control interface between the MME and 2G/3G SGSNs. The interface is based on Gn/GTP-C (SGSN-SGSN), possibly with some new functionality to support signaling free idle mode mobility between E-UTRAN and WCDMA/GSM.
S3 does not support packet forwarding.
S3 is a many to many interface and is backward compatible with Gn interface. From the functionality perspective S3 interface is similar to S10 interface between MMEs which is used for intra-LTE mobility between two MME pool areas.
S4: This is the interface used between P-GW and 2G/3G SGSNs.This interface is based on Gn/ GTP (SGSN-GGSN).The user plane interface is based on GTP-U similar to S1-UP interface ,while the control plane is based on GTP-C similar to S11.
S4 is a many-to-many interface and is backwards compatible with the Gn interface.
S11: This is the interface between the MME and SGW. The interface is based on Gn/GTP-Control (GTP-C) with additional functions for paging coordination and mobility.
S11 is a many to many interface.
X2: This is the interface between eNodeBs. The interface is mainly used to support active mode UE mobililty . This interface may also be used for multi-cell Radio Resource Management (RRM ) functions. The X2-CP interface consists of a signaling protocol called X2-AP on top of SCTP.
The X2-UP interface is based on GTP-U.
The X2-UP interface will be used to support loss-less mobility (packet forwarding).
The X2 interface is a many to many interface.
S10: This is a control interface between the MMEs which is very similar to S3 interface between the SGSN and MME.The interface is based on GTP-C with additional functionality.
S10 is a many to many interface.
S5/S8: This is the interface between the S-GW and P-GW. In principle, technically S5 and S8 is the same interface.
The only difference is that S8 is used when roaming between different operators occurs while S5 is network internal.
The S5/S8 interface can exist in two flavors , one based on GTP and other use the IETF specified Proxy Mobile IP (PMIP) for mobility control with additional mechanism to handle QoS.
The idea behind using PMIP flavor of S5/S8 mainly come from CDMA and WiMAX operators and vendors interested in interworking with LTE , GSM or UMTS or using the 3GPP EPS specified mechanism also for intra WiMAX or CDMA mobility.
In case of non-roaming scenario, the S-GW and P-GW functions can be performed in one physical node.
S5/S8 is a many-to-many interface.