5G EPS Fallback三径回降与N26互操作信令博弈

flowchart TB
    subgraph 触发条件
        A[UE发起IMS语音呼叫<br/>SIP INVITE]
        B[SMF收到Voice QoS Flow<br/>建立请求]
        C{NR是否支持VoNR?}
    end

    subgraph 决策路径
        D[N26接口是否存在?]
        E{gNB能力评估}
    end

    subgraph 三径回降
        F[路径1: HO-based<br/>with N26]
        G[路径2: Redirection-based<br/>with N26]
        H[路径3: Redirection-based<br/>without N26]
    end

    A --> B --> C
    C -->|不支持VoNR| D
    C -->|支持VoNR| Z[直接VoNR]
    D -->|存在| E
    D -->|不存在| H
    E -->|HO能力| F
    E -->|仅重定向| G

    style Z fill:#4CAF50,color:#fff
    style F fill:#2196F3,color:#fff
    style G fill:#FF9800,color:#fff
    style H fill:#F44336,color:#fff

PDU Session Resource Setup Response Transfer
├── PDU Session Resource Setup Response Item
│   ├── PDU Session ID
│   ├── PDU Session Resource Setup Response Transfer
│   │   ├── QoS Flow Per TNL Information  ← 成功的QoS Flow列表（可能为空）
│   │   └── Cause ← 取值 "Multiple DSCP QoS Flows Not Supported" 或其他
└── 

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant SMF
    participant PCF
    participant MME
    participant SGW
    participant PGW as PGW/C-UPF
    participant CSCF as P-CSCF

    Note over UE,CSCF: Phase 1: IMS语音触发
    UE->>CSCF: SIP INVITE (含SDP)
    CSCF->>PCF: AAR (Rx)
    PCF->>SMF: SM Policy Modification (5QI=1)
    SMF->>AMF: N1N2MessageTransfer<br/>(PDU Session Mod, QoS Flow)
    AMF->>gNB: PDU Session Resource Setup<br/>(含5QI=1 QoS Flow)

    Note over UE,CSCF: Phase 2: gNB拒绝 & HO准备
    gNB-->>AMF: PDU Session Resource Setup Response<br/>(REJECT + VoiceFallbackIndicator)
    AMF->>gNB: HANDOVER REQUIRED<br/>(Target=eNB ID)

    Note over UE,CSCF: Phase 3: N26上下文迁移
    gNB-->>AMF: HANDOVER REQUIRED
    AMF->>SMF: Nsmf_PDUSession_StatusNotify<br/>(Trigger=EPS Fallback HO)
    SMF->>PGW: SMF PGW-C隧道信息更新
    AMF->>MME: Forward Relocation Request (N26)
    Note right of MME: 包含: MM Context,<br/>PDN Connections,<br/>UE Security Capabilities
    MME->>SGW: Create Session Request
    SGW-->>MME: Create Session Response
    MME-->>AMF: Forward Relocation Response

    Note over UE,CSCF: Phase 4: HO执行
    AMF->>gNB: HANDOVER COMMAND
    gNB->>UE: RRC Reconfiguration<br/>(HO to eNB, mobilityControlInfo)
    UE->>UE: 切换到LTE
    UE->>MME: Tracking Area Update

    Note over UE,CSCF: Phase 5: VoLTE语音建立
    MME->>SGW: Modify Bearer Request
    SGW->>PGW: Modify Bearer Request
    PGW-->>CSCF: 通过IMS承载继续SIP信令
    Note over UE,CSCF: SIP 183 Session Progress<br/>→ VoLTE通话建立

flowchart LR
    subgraph "5GC侧"
        PS[PDU Session<br/>DNN=internet]
        QF1[QoS Flow 1<br/>5QI=9 Default]
        QF2[QoS Flow 2<br/>5QI=1 Voice]
        PS --> QF1
        PS --> QF2
    end

    subgraph "映射规则"
        MR1[Default QoS Flow →<br/>Default EPS Bearer<br/>EBI=5]
        MR2[Voice QoS Flow →<br/>Dedicated EPS Bearer<br/>EBI=6]
    end

    subgraph "EPC侧"
        PDN[PDN Connection<br/>APN=internet]
        EB1[EPS Bearer 5<br/>QCI=9 Default]
        EB2[EPS Bearer 6<br/>QCI=1 Voice]
        PDN --> EB1
        PDN --> EB2
    end

    QF1 --> MR1 --> EB1
    QF2 --> MR2 --> EB2

    style QF2 fill:#FF5722,color:#fff
    style EB2 fill:#FF5722,color:#fff

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant SMF
    participant MME
    participant SGW
    participant PGW as PGW/C-UPF

    Note over UE,PGW: Phase 1: 触发 & 重定向
    gNB->>AMF: PDU Session Resource Setup Response<br/>(REJECT + VoiceFallbackIndicator)
    AMF->>gNB: UE Context Modification Request<br/>(Redirection指示)
    gNB->>UE: RRC Release<br/>(redirectedCarrierInfo=LTE EARFCN)
    Note right of UE: UE搜索LTE频点<br/>驻留目标eNB

    Note over UE,PGW: Phase 2: N26上下文传递
    AMF->>MME: Forward Relocation Request (N26)
    MME->>SGW: Create Session Request
    SGW-->>MME: Create Session Response
    MME-->>AMF: Forward Relocation Response

    Note over UE,PGW: Phase 3: TAU & 语音建立
    UE->>MME: Tracking Area Update Request<br/>(含N26分配的GUTI)
    MME->>MME: 验证UE上下文<br/>（已通过N26获得）
    MME->>SGW: Modify Bearer Request
    SGW->>PGW: Modify Bearer Request
    Note over UE,PGW: VoLTE语音承载建立完成

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant MME
    participant SGW
    participant PGW as PGW/SMF
    participant HSS as UDM/HSS

    Note over UE,HSS: Phase 1: 重定向
    gNB->>UE: RRC Release<br/>(redirectedCarrierInfo=LTE EARFCN)
    Note right of UE: UE搜索LTE频点<br/>驻留目标eNB

    Note over UE,HSS: Phase 2: EPS Attach（无N26辅助）
    UE->>MME: Attach Request<br/>(PDN Connectivity Req<br/>Request Type=handover)
    Note right of MME: Request Type=handover<br/>指示这是EPS FB场景

    MME->>HSS: Update Location Request (diameter)
    HSS-->>MME: Update Location Answer<br/>(含订阅数据)

    Note over UE,HSS: Phase 3: PGW-C/SMF发现与会话恢复
    MME->>PGW: Create Session Request<br/>(PGW FQDN从HSS或DNS获取)
    Note right of PGW: SMF(PGW-C)查找<br/>已有PDU Session上下文
    PGW-->>MME: Create Session Response

    MME->>SGW: Create Session Request
    SGW-->>MME: Create Session Response

    Note over UE,HSS: Phase 4: 默认承载建立
    MME->>UE: Attach Accept + Activate Default EPS Bearer
    UE->>MME: Attach Complete

    Note over UE,HSS: Phase 5: 专用语音承载建立
    MME->>SGW: Create Bearer Request (QCI=1)
    SGW->>PGW: Create Bearer Request
    PGW-->>SGW: Create Bearer Response
    SGW-->>MME: Create Bearer Response
    MME->>UE: Activate Dedicated EPS Bearer Context Request
    UE-->>MME: Activate Dedicated EPS Bearer Context Accept

    Note over UE,HSS: VoLTE语音承载建立完成

stateDiagram-v2
    [*] --> IMS_Voice_Triggered: 收到gNB REJECT<br/>+VoiceFallbackIndicator

    IMS_Voice_Triggered --> Check_N26: 检查N26接口配置

    state Check_N26 {
        [*] --> N26_Supported: AMF配置支持N26
        [*] --> N26_Not_Supported: AMF未配置N26
    }

    N26_Supported --> HO_Preparation: gNB发送<br/>HANDOVER REQUIRED
    N26_Supported --> Redirection_N26: gNB发送<br/>RRC Release + Redirection

    N26_Not_Supported --> Redirection_NoN26: 仅重定向路径

    HO_Preparation --> N26_Forward_Relocation: N26消息交互

    state N26_Forward_Relocation {
        [*] --> Send_FWD_RELOC_REQ: AMF→MME
        Send_FWD_RELOC_REQ --> Wait_FWD_RELOC_RSP: 等待MME响应
        Wait_FWD_RELOC_RSP --> FWD_RELOC_CPLT: 收到Response
    }

    Redirection_N26 --> N26_Context_Transfer: UE RRC Release后
    Redirection_NoN26 --> UE_Attach_LTE: UE自主Attach

    N26_Forward_Relocation --> HO_Command: 发送HO COMMAND
    Redirection_N26 --> TAU_LTE: UE发起TAU
    Redirection_NoN26 --> Attach_Complete: MME完成Attach

    HO_Command --> VoLTE_Established: 语音承载建立
    TAU_LTE --> VoLTE_Established
    Attach_Complete --> VoLTE_Established

    VoLTE_Established --> [*]

flowchart TD
    subgraph "EPS Fallback前（5G）"
        UE1[UE] -->|NR Uu| gNB1[gNB]
        gNB1 -->|N3 GTP| UPF1[UPF/PGW-U]
        UPF1 -->|N6| DN1[IMS/Internet]
    end

    subgraph "EPS Fallback后（4G）"
        UE2[UE] -->|LTE Uu| eNB1[eNB]
        eNB1 -->|S1-U GTP| SGW1[S-GW]
        SGW1 -->|S5 GTP| PGW1[PGW-U/UPF]
        PGW1 -->|SGi| DN2[IMS/Internet]
    end

    UE1 -.->|回降| UE2
    UPF1 -.->|同一物理实体| PGW1

    style UPF1 fill:#E91E63,color:#fff
    style PGW1 fill:#E91E63,color:#fff

5GS Network Feature Support (部分)
├── IMS VoPS (1 bit): 5GC是否支持IMS语音 over PS
├── EMC (2 bits): 紧急呼叫支持指示
├── EMF (2 bits): 紧急呼叫回退指示
├── IWK_N26 (1 bit): N26互通是否支持
├── MCSI (1 bit): MCS指示
└── ...

现象：UE发出SIP INVITE后，被叫未振铃，UE回到Idle态
排查路径：
1. 检查gNB是否发送了VoiceFallbackIndicator → NGAP Response
2. 检查AMF是否收到HANDOVER REQUIRED → 无则gNB配置问题
3. 检查N26 Forward Relocation Request/Response → MME是否响应
4. 检查MME Create Session → SGW/PGW是否正常

现象：UE重定向到LTE后，Attach Request被MME拒绝
排查路径：
1. 检查HSS/UDM是否正确同步了UE订阅数据
2. 检查PGW-C/SMF是否可达（DNS解析PGW FQDN）
3. 检查Request Type=handover是否正确携带
4. 检查UE是否在Attach中携带了正确的DRX参数

现象：主叫SIP INVITE发出后，超过5秒才听到回铃音
根因：Redirection-based场景下，SIP 100 Trying/183丢失
解决方案：
1. 优先启用HO-based with N26（缩短中断时间）
2. 配置IMS SIP Timer B适当增大（>5s）
3. 确保PGW-U/UPF缓冲下行SIP消息