The E-UTRANⁱ consists of eNBs, providing the E-UTRA user plane (PDCP/RLC/MACⁱ/PHY) and control plane (RRCⁱ) protocol terminations towards the UEⁱ. The eNBs are interconnected with each other by means of the X2ⁱ interface. The eNBs are also connected by means of the S1ⁱ interface to the EPCⁱ (Evolved Packetⁱ Core), more specifically to the MMEⁱ (Mobilityⁱ Management Entity) by means of the S1-MME and to the Serving Gateway (S-GWⁱ) by means of the S1-Uⁱ. The S1 interface supports a many-to-many relation between MMEs / Serving Gateways and eNBs.

X2-Inter eNB interface
Handoverⁱ coordination without involving the EPC
During HO, Source eNB can use the X2 interface to forward downlink packets still buffered or arriving from the serving gateway to the target eNB.
This will avoid loss of a huge amount of packets during inter-eNB handover.
S1-MME
Control interface between eNB and MME
S1flex allows 1 eNB to connect to several MME
MME and UE will exchange non-access stratum signaling via eNB through this interface.
E.g.: if a UE performs a tracking area update the TRACKING AREA UPDATE REQUEST message will be sent from UE to eNB and the eNB will forward the message via S1-MME to the MME.
S1-U
User plane interface between eNB and serving gateway.
It is a pure user data interface (U=User plane).
S1flex-U also supported: a single eNB can connect to several Serving GWs.
Which Serving GW a user’s SAEⁱ bearer will have to use is signaled from the MME of this user.