sequenceDiagram
participant UE
participant gNB
participant AMF
participant MME
participant SMF
participant SGW
participant UPF as PGW-U/UPF
gNB->>AMF: ① Handover Required(target=4G)
AMF->>SMF: ② 获取SM上下文+EPS承载映射
AMF->>MME: ③ Forward Relocation Request(N26)
MME->>SGW: ④ Create Session
MME-->>AMF: ⑤ Forward Relocation Response
AMF-->>gNB: ⑥ Handover Command
UE->>eNB: ⑦ 切换到4G
eNB->>MME: ⑧ Handover Notify
Note over UE,UPF: 用户面路径切换完成
图：5G 到 4G 切换流程概览（基于 TS 23.502 4.11.1.2.1）