5G EPS Fallback三径信令全链路解析：N26接口与运营商成本博弈

gNB收到QoS Flow建立请求（IMS语音）
    │
    ├── gNB是否配置支持EPS Fallback？ ──否──→ 拒绝QoS Flow，流程终止
    │
    ├── AMF是否指示"Redirection for EPS Fallback possible"？
    │       │
    │       ├── 否 → EPS Fallback不执行
    │       │
    │       └── 是或未指示 → 继续判断
    │
    ├── N26接口是否可用？
    │       │
    │       ├── 是 → 可选择HO或Redirection（有N26）
    │       │
    │       └── 否 → 仅Redirection（无N26）
    │
    ├── UE是否支持E-UTRA能力？
    │
    └── 周边是否有合适的LTE邻区？──测量报告

sequenceDiagram
    participant UE
    participant gNB as NG-RAN(gNB)
    participant AMF
    participant SMF as SMF+PGW-C
    participant UPF as UPF+PGW-U
    participant MME
    participant SGW
    participant eNB as E-UTRAN(eNB)

    Note over UE,gNB: Step 1: UE在5G发起IMS语音(MO/MT)
    UE->>gNB: SIP INVITE (IMS信令 via 5G PDU Session)

    Note over gNB,SMF: Step 2: 网络发起PDU Session Modification
    SMF->>AMF: Nsmf_PDUSession_UpdateSMContext<br/>(QoS Flow for IMS Voice)
    AMF->>gNB: N2 PDU Session Request<br/>(QoS Flow Setup for Voice)

    Note over gNB: Step 3-4: gNB决策EPS Fallback
    gNB->>gNB: 判断: VoNR不支持<br/>评估UE能力/N26/邻区

    gNB->>AMF: PDU Session Modification Response<br/>(Reject + "Fallback for IMS voice ongoing")
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Response<br/>(Cause: Fallback ongoing)
    Note over SMF: SMF+PGW-C保持PCC规则<br/>报告EPS Fallback事件给PCF

    Note over gNB,SMF: Step 5: gNB发起5GS→EPS HO
    gNB->>AMF: Handover Required<br/>(Target eNB ID, Source→Target Container,<br/>Inter-system HO indication)

    Note over AMF,SMF: Step 2a-2c: AMF获取EPS承载上下文
    AMF->>SMF: Nsmf_PDUSession_ContextRequest<br/>(EBI list, Target MME capability)
    SMF->>UPF: N4 Session Modification<br/>(建立CN Tunnel for EPS bearer)
    UPF-->>SMF: N4 Session Modification Response
    SMF-->>AMF: Nsmf_PDUSession_ContextResponse<br/>(Mapped EPS PDN Connection Context)

    Note over AMF,MME: Step 3: AMF→MME转发重定位
    AMF->>MME: Forward Relocation Request<br/>(EPS Bearer Contexts, MM Context,<br/>SGW selection indication)

    Note over MME,SGW: Step 4-9: EPC侧资源准备
    MME->>SGW: Create Session Request
    SGW-->>MME: Create Session Response
    MME->>eNB: Handover Request<br/>(E-RAB Setup List)
    eNB-->>MME: Handover Request Acknowledge<br/>(HO Command)

    MME-->>AMF: Forward Relocation Response<br/>(Target→Source Container)

    Note over AMF,SMF: Step 10a-10c: 数据转发隧道建立
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext<br/>(Data Forwarding Info)
    SMF->>UPF: N4 Session Modification<br/>(Data Forwarding Tunnel)
    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response

    AMF->>gNB: Handover Command<br/>(Transparent Container, Data Forwarding Info)

    Note over UE,eNB: Step 11: 执行切换
    gNB->>UE: RRC Reconfiguration<br/>(HO Command → target LTE)
    UE->>eNB: RRC Connection Reconfiguration Complete
    eNB->>MME: Handover Notify

    Note over MME,SMF: Step 12-16: 路径切换
    MME->>SGW: Modify Bearer Request
    SGW->>SMF: Modify Bearer Request<br/>(via S5/S8)
    SMF->>UPF: N4 Session Modification<br/>(DL UP path切换到E-UTRAN)
    SMF-->>SGW: Modify Bearer Response

    Note over UE: Step 18: UE发起TAU
    UE->>MME: Tracking Area Update

    Note over SMF: Step 19: SMF+PGW-C重建专用承载
    SMF->>SGW: Create Dedicated Bearer<br/>(IMS Voice QCI=1)
    Note over UE: 语音呼叫在VoLTE网络继续建立

sequenceDiagram
    participant UE
    participant gNB as NG-RAN(gNB)
    participant AMF
    participant SMF as SMF+PGW-C
    participant MME
    participant eNB as E-UTRAN(eNB)

    Note over UE,gNB: Step 1-4: 同HO-based方案
    UE->>gNB: IMS Voice Request
    gNB->>AMF: Reject + "Fallback ongoing"
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext

    Note over gNB,UE: Step 5: RRC Release with Redirection
    gNB->>UE: RRC Release<br/>(RedirectedCarrierInfo: E-UTRA ARFCN)

    Note over UE: UE重选到LTE小区
    UE->>eNB: RRC Connection Request
    eNB-->>UE: RRC Connection Setup

    Note over UE,MME: Step 6a: TAU (有N26)
    UE->>MME: Tracking Area Update Request<br/>(Active Flag, Mapped GUTI from 5G-GUTI)

    Note over AMF,MME: AMF通过N26传递上下文
    MME->>AMF: Context Request (via N26)
    AMF-->>MME: Context Response<br/>(MM Context + EPS Bearer Contexts)

    MME-->>UE: TAU Accept

    Note over SMF: Step 7: SMF+PGW-C重建IMS语音专用承载
    SMF->>MME: 触发Dedicated Bearer Setup<br/>(QCI=1 for IMS Voice)

sequenceDiagram
    participant UE
    participant gNB as NG-RAN(gNB)
    participant AMF
    participant HSS_UDM as HSS+UDM
    participant MME
    participant SMF as SMF+PGW-C
    participant eNB as E-UTRAN(eNB)

    Note over UE,gNB: Step 1-4: 同上

    gNB->>UE: RRC Release<br/>(Redirect to E-UTRA)

    UE->>eNB: RRC Connection Request
    eNB-->>UE: RRC Connection Setup

    Note over UE,MME: Step 6b: Attach with "handover"
    UE->>MME: Attach Request + PDN Connectivity<br/>(Request Type = "handover",<br/>指示从5GC移动过来)

    Note over MME,HSS_UDM: MME通过HSS+UDM获取SMF+PGW-C地址
    MME->>HSS_UDM: Update Location Request
    HSS_UDM-->>MME: Update Location Answer<br/>(SMF+PGW-C FQDN + APN/DNN)

    Note over MME,SMF: MME通过DNS解析SMF+PGW-C的S5/S8地址
    MME->>SMF: Create Session Request<br/>(via S5/S8, "handover" indication)

    Note over SMF: SMF+PGW-C关联5G PDU Session与EPS PDN Connection
    SMF-->>MME: Create Session Response<br/>(IP地址保持)

    MME-->>UE: Attach Accept + PDN Connectivity Accept

    Note over SMF: SMF+PGW-C重建IMS专用承载

graph LR
    subgraph "5GC QoS参数"
        A[5QI] --> B[QFI]
        C[ARP Priority/Preemption]
        D[GFBR/MFBR]
        E[PDB]
    end

    subgraph "SMF+PGW-C映射逻辑"
        F[5QI → QCI映射表]
        G[QFI → EPS Bearer ID]
        H[ARP → ARP]
        I[GFBR/MFBR → GBR/MBR]
    end

    subgraph "EPC QoS参数"
        J[QCI]
        K[EPS Bearer ID]
        L[ARP]
        M[GBR/MBR]
    end

    A --> F --> J
    B --> G --> K
    C --> H --> L
    D --> I --> M

stateDiagram-v2
    [*] --> 5G_Registered: UE在5G注册
    5G_Registered --> IMS_Voice_Requested: SIP INVITE
    IMS_Voice_Requested --> QoS_Flow_Setup: PDU Session Modification
    QoS_Flow_Setup --> Fallback_Decision: gNB判断
    Fallback_Decision --> HO_Preparation: HO-based
    Fallback_Decision --> RRC_Redirection: Redirection-based
    HO_Preparation --> Forward_Relocation: N26交互
    Forward_Relocation --> HO_Execution: Handover Command
    RRC_Redirection --> LTE_Camping: UE重选LTE
    HO_Execution --> LTE_Connected: 切换完成
    LTE_Camping --> TAU_Procedure: TAU/Attach
    TAU_Procedure --> Bearer_Setup: 重建语音承载
    Bearer_Setup --> Voice_Established: VoLTE通话
    Voice_Established --> Fast_Return: 通话结束
    Fast_Return --> 5G_Registered: 返回5G

graph TB
    subgraph "部署成本对比"
        A[方案A: HO-based + N26]
        B[方案B: Redirection + N26]
        C[方案C: Redirection 无N26]
    end

    subgraph "成本要素"
        D[MME软件升级支持N26]
        E[AMF软件升级支持N26]
        F[SMF+PGW-C合设部署]
        G[N26链路运维]
        H[用户体验损失]
    end

    A --> D
    A --> E
    A --> F
    A --> G
    B --> D
    B --> E
    B --> F
    B --> G
    C --> F
    C --> H

    subgraph "收益"
        I[语音无缝体验]
        J[语音基本可用]
        K[语音可用但有中断]
    end

    A --> I
    B --> J
    C --> K

stateDiagram-v2
    state "5G CM-CONNECTED" as 5G_Conn
    state "EPS Fallback决策中" as FB_Decision
    state "HO Preparation" as HO_Prep
    state "RRC Release (Redirect)" as RRC_Rel
    state "LTE小区搜索" as LTE_Search
    state "HO Execution" as HO_Exec
    state "LTE ECM-CONNECTED" as LTE_Conn
    state "TAU/Attach Procedure" as TAU_Proc
    state "VoLTE通话中" as VoLTE
    state "VoLTE通话结束" as VoLTE_End
    state "Fast Return" as FR
    state "5G CM-IDLE" as 5G_Idle

    [*] --> 5G_Conn
    5G_Conn --> FB_Decision: IMS Voice QoS Flow<br/>被gNB拒绝
    FB_Decision --> HO_Prep: gNB选择HO-based
    FB_Decision --> RRC_Rel: gNB选择Redirection
    HO_Prep --> HO_Exec: Forward Relocation<br/>Response收到
    HO_Exec --> LTE_Conn: RRC HO Complete
    RRC_Rel --> LTE_Search: RRC Release<br/>with redirect
    LTE_Search --> TAU_Proc: 驻留LTE小区
    TAU_Proc --> LTE_Conn: TAU/Attach完成
    LTE_Conn --> VoLTE: 专用承载建立<br/>QCI=1
    VoLTE --> VoLTE_End: SIP BYE
    VoLTE_End --> FR: Fast Return策略触发
    FR --> 5G_Conn: HO/Redirect/Reselection
    FR --> 5G_Idle: 重选到5G