The picture shows main network elements, interfaces and the protocols used in LTE.
The main interfaces in LTE are Uu, S1-MME, X2, S1-U, S11 and S5.
LTE Uu : -
This is the air interface between UE and eNB. LTE layer 1 is dealt with later. RRC is the protocol that is used for communication between UE and eNB. Above RRC there is a NAS layer in UE. This NAS layer terminates at MME and eNB shall silently pass the NAS messages to MME.
LTE S1-MME: -
eNB and MME communicate using this IP interface. S1-AP is application layer interface. The transport protocols used here is SCTP. (Stream control transmission protocol).
LTE X2: -
This interface is used by a eNB to communicate to other eNB. This again is a IP interface with SCTP as transport. X2-AP is the application protocol used by eNB’s to communicate.
LTE S11: -
An IP interface between MME and SGW! GTPv2 is the protocols used at the application layer. GTPv2 runs on UDP transport. This interface must and should run GTPv2.
LTE S5: -
This is the interface between SGW and PGW. This again is an IP interface and has two variants. S5 can be a GTP interface or PMIP interface. PMIP variant is used to support non-trusted 3GPP network access.
LTE S1-U: -
User plane interface between eNB and SGW! GTP-U v1 is the application protocol that encapsulates the UE payload. GTP-U runs on UDP.
All the above IP interfaces can be of IPv4 or IPv6. Few interfaces can be of IPv4 and
few can be of IPv6. From the specification side there are no restrictions.