上图的I-SMF是在R16提出的，目前还在讨论阶段。就是为了解决大区制下一个SMF不能跨区管理UPF的问题。所以，这种场景下的用户会话保持会非常麻烦。等待R16的规范冻结吧。
Functionality of I-SMF includes:
- Session management (e.g. support of Nsmf_PDUSession_CreateSMContext, Nsmf_PDUSession_UpdateSMContext, Nsmf_PDUSession_ReleaseSMContext service operations).
- N11 termination, incl. termination of SM parts of NAS messages and N2.
- UPF selection and control.
- Handling of buffering and downlink data notifications in case N3 connection is inactive.
上面提到，I-SMF的功能是：N11的终结点和NAS SM消息的终结点。但UE地址还是A-SMF分配的。因此，在45互操作时，N26传递的应该是I-SMF的信息。
Anchor SMF (A-SMF): the SMF serving the PDU Session when a single SMF is used as defined for non-roaming and LBO scenarios in Rel-15. A PDU Session always involves an A-SMF. An A-SMF is never changed for a PDU Session.
Intermediate SMF (I-SMF): the SMF that controls UPF(s) that the A-SMF cannot control. An I-SMF is inserted, changed or removed as needed. An I-SMF is used only in specific deployments.
这个在国内应该今后是一个必选方案。省间漫游就是I-SMF，国际漫游就是V-SMF。功能差不多。
23726：Study on Enhancing Topology of SMF and UPF in 5G Networks。或者叫ETSUN：-SM context can be transferred between different SMFs (I-SMF <->I-SMF + I-SMF <->A-SMF) during mobility during UE mobility procedure。
作者: admin 时间: 2019-9-22 15:30
具体的I-SMF插入流程在23726的6.1.2.5.2 Mobility Registration with insertion, removal and change of I-SMF的第7步：
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