1> S1 Interface protocol which is the interface between E-EUTRAN and Core Network.
2> S1 Interface is a point - to - point logical interface which is based on IP Network.
3> S1 Interface includes both protocol for Control plane signalling and Data Plane.
S1 - Control Plane Interface
1> The interface between eNodeB and MME and responsible for control plane signalling.
2> It is also known as S1 - Application Part ( S1-AP / S1 - MME ) Protocol.
3> It uses S1-AP protocol and SCTP protocol as transport layer.
S1 - Data Plane Interface
1> The interface between eNodeB and S-GW is responsible for data transfer.
2> It is also known as S1 - User Part ( S1-U ) or GTP - U.
3> It uses GTP-U protocol and used UDP as transport layer protocol.
3GPP Specs to refer for S1 Interface
36.410 :- S1 General aspects and principles
36.411 :- S1 Layer 1
36.412 :- S1 Signalling transport
36.413 :- S1 Application protocol
36.414 :- S1 Data transport
S1 : Network Entity Interface View
1> S1 - Interfaces exist between E-EUTRAN(eNodeB) and Core Network (MME / S-GW)
2> eNB is responsible for MME selection and MME selects the S-GW and notifies to eNB.
3> Multiple S1-AP / S1-MME interface exist i.e A eNB can have multiple S1-AP to MMEs and A eNB can have multiple S1-U to S-GWs.
4> Refer the upper image to see the interface's possible between multiple eNB, MME, S-GW.
S1 : Protocol Interface View
1> Protocol exist for both Control plane (eNB - MME) and User plane (eNB - S-GW).
2> Refer the following image to see the protocol interface.
3> S1 - AP uses SCTP as Transport layer and S1-U uses UDP as transport layer.
4> The standard receiving port for S1-AP over SCTP is 36412 and for S1-U/GTP-U over UDP is 2152.
5> L1 represents Physical layer and L2 represents Data link layer same as OSI model.