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.