Let's talk about LTE Interfaces. In LTE, every interface between two network nodes, is having an interface name, that represents the protocols used in the communication between those nodes.

Below graph shows LTE Interfaces that exist between different nodes,

In order to understand LTE Interfaces, we need first to understand the difference between the control plane, and the user plane.

• The control plane is where the nodes exchange signaling messages.
• While the user plane, is where the nodes exchange user data, sent and received between the UE, and the PDN.

The above graph shows the path of both the control plane, and the user plane between different nodes in LTE network.

LTE Interfaces in the Control Plane

S1-MME Interface

• First the eNodeB connects to the MME through the S1-MME interface.
• The eNodeb communicates with the MME through the S1-MME interface, using S1-AP protocol, which refers to 'S1 Application Protocol'.
• S1AP protocol supports the transfer of the signaling between the eNodeB and the MME.

S6a Interface

• Then the MME connects to the HSS through the S6a interface.
• The MME communicates with the HSS using the Diameter protocol.

S11 Interface

• From the other side, the MME connects to the SGW using S11 interface.
• S11 Interface uses GTP-C protocol. GTP-C refers to 'GPRS tunneling Protocol for control plane'.
• GTP-C protocol supports the exchange of signaling messages between nodes, by creating a GTP tunnel between the nodes.
• The signaling messages will move between the nodes into this GTP tunnel.

S5/S8 Interface

• Then the SGW connects to the PGW through the S5/S8 interface.
• S5/S8 interface also uses GTP-C protocol in communication between the nodes.
• The interface between the SGW and the PGW that exist in the same PLMN is named the 'S5' Interface, while the interface between the SGW and the PGW in different PLMNs is named the 'S8' interface.

LTE Interfaces in the User Plane

S1-U Interface

• First the eNodeB connects directly to the SGW thorough S1-U interface.
• S1-U refers to 'S1 user plane interface', and it uses GTP-U protocol.
• GTP-U referes to 'GPRS tunneling protocol' for user plane, and it creates a GTP tunnel between the nodes, so that user plane data will move into this tunnel created between the nodes.

S5/S8 Interface

• Then the SGW connects to the PGW through the S5/S8 interface.
• S5/S8 interface in the user plane uses also GTP-U protocol to exchange user data.
• The interface between the SGW and the PGW that exist in the same PLMN is named the 'S5' Interface, while the interface between the SGW and the PGW in different PLMNs is named the 'S8' interface.

SGi Interface

• The PGW connects to the PDN through the SGi interface, and it uses  IP (Internet Protocol) to exchange the packets with the PDN.