7 Xn interface protocol structure
38.4203GPPNG-RANRelease 17TSXn general aspects and principles
7.1 Xn Control Plane
The control plane protocol stack of the Xn interface is shown on Figure 7.1-1. The transport network layer is built on IP transport. For the reliable transport of signalling messages, SCTP is added on top of IP. The application layer signalling protocol is referred to as XnAP (Xn Application Protocol).
Figure 7.1-1: Xn Interface Control Plane
7.2 Xn User Plane
The Xn user plane (Xn-U) interface is defined between two NG-RAN nodes. The Xn-U interface provides non-guaranteed delivery of user plane PDUs between two NG-RAN nodes.
The protocol stack for Xn-U is shown in Figure 7.2-1.
Figure 7.2-1: Xn-U protocol structure
The user plane packets conveyed by GTP-U may be PDCP PDUs (e.g. in case of dual connectivity), PDCP SDUs (e.g. in case of DRB level data forwarding), or SDAP SDUs (e.g. in PDU Session level data forwarding).
User plane protocol messages (as defined in TS 38.425 [7] and TS 38.415 [10]) are carried by container fields in the GTP-U extension header as specified in TS 29.281 [11]. A single GTP-U packet may carry a user plane packet and/or a user plane protocol message. The mapping between container fields and Xn user plane protocol procedures and functions is described in Table 7.2-1.
Table 7.2-1: Mapping between container fields and Xn user plane procedures / functions
|
Xn-U Function |
Container Type |
Xn UP Protocol Procedure |
|---|---|---|
|
Data transfer |
NR RAN Container, as per TS 29.281 [11] (Note 1) |
Transfer of Downlink User Data, TS 38.425 [7] |
|
PDU Session Container, as per TS 29.281 [11] (Note 2) |
Transfer of DL PDU Session Information, TS 38.415 [10] Transfer of UL PDU Session Information, TS 38.415 [10] |
|
|
No container (Note 3) |
NA |
|
|
Flow control |
NR RAN Container as per TS 29.281 [11] (Note 4) |
Downlink Data Delivery Status, TS 38.425 [7] Transfer of Downlink User Data, TS 38.425 [7] |
|
Fast retransmission |
NR RAN Container as per TS 29.281 [11] (Note 4) |
Downlink Data Delivery Status, TS 38.425 [7] Transfer of Downlink User Data, TS 38.425 [7] |
|
Assistance information |
NR RAN Container as per TS 29.281 [11] (Note 4) |
Transfer of Assistance Information, TS 38.425 [7] |
|
Note 1: optionally used in Dual Connectivity DL data transfer. Note 2: in case of PDU Session level forwarding only. Note 3: all other cases of data transfer when no other Xn-U functionality is required Note 4: optionally used in Dual Connectivity |
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