20.1 User Plane

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

The X2 user plane interface (X2-U) is defined between eNBs. The X2-U interface provides non guaranteed delivery of user plane PDUs. The user plane protocol stack on the X2 interface is shown in Figure 20.1-1. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs.

The X2-U interface protocol stack is identical to the S1-U protocol stack.

For DC, if X2-U user data bearers are associated with E-RABs for which the split bearer option is configured, GTP-U conveys PDCP PDUs in uplink and downlink and a RAN Container containing flow control information. The RAN Container is carried in the "RAN Container" field of the GTP-U extension header.

Figure 20.1-1: X2 Interface User Plane (eNB-eNB)

20.1.1 Flow Control Functions

The Flow Control function is applied when an E-RAB is configured for the split bearer option and only for DL i.e. the flow control information is provided only by the SeNB to the MeNB for the MeNB to control the downlink user data flow to the SeNB. The flow control function is further described in TS 36.425 [61].