[5g NR] NAS connection management - cm status

 

Catalog

NAS Connection management

CM-IDLE state

CM-CONNECTED state

5GS CM State model

---

NAS Connection management

        NAS Connection management includes through N1 Interface in UE and AMF Between building and releasing NAS Function of signaling connection .NAS The signaling connection is used to realize UE And the core network NAS Signaling exchange . When UE Access 5G When the network , First, establish with the base station RRC Connect , When RRC After the connection is established ,UE The air interface connection with the base station is successfully established . And then UE And 5GC Of AMF establish NAS Connect , after NAS Layer message encoding and decoding and message processing ,UE In the end in 5GC Successfully registered , At this time UE And 5GC Successfully set up PDU conversation , You can send and receive business data .

        UE And AMF There are two types of connection states ：

                - CM-IDLE

                - CM-CONNECTED

        3GPP The protocol distinguishes between those used for 3GPP Access and non access 3GPP Access CM States are independent of each other , For us , Do not consider non 3GPP Access , Just care 3GPP Access . So let's see CM-IDLE and CM-CONNECTED The specific contents of the two states .

CM-IDLE state

         When UE And AMF Connection status is CM-IDLE when , Express UE stay N1 There is no such thing as AMF establish NAS Signaling connection , There is no access network signaling connection ,N2 The connection and N3 Connect . Whenever in the base station and AMF Between UE establish N2 When the connection ,AMF Will enter the UE Of CM-CONNECTED state .AMF Received initial N2 news （ for example N2 INITIAL UE MESSAGE）, Will start from CM-IDLE The switch to CM-CONNECTED state .
         If UE At the same time CM-IDLE State and RM-REGISTERED state , When UE When a paging message is received , You can respond to a page by executing a service requestor ; When UE To send uplink signaling or uplink user data , The service request process is also performed .
         When AMF Medium UE Status is RM-REGISTERED when ,AMF Will store origination and UE For communication UE Information . AMF have access to 5G-GUTI To retrieve the origination and UE Storage information required for communication .
         stay RRC Connection establishment phase ,UE Use 5G-S-TMSI As UE identification . When UE And the base station RRC After the connection is established successfully , initial NAS news （Registration Request, Service Request perhaps Deregistration Request） Transferred to the AMF,UE Will start from CM-IDLE State transition to CM-CONNECTED state .
 

CM-CONNECTED state

        UE be in CM-CONNECTED state , Express UE And AMF stay N1 Interface has NAS Signaling connection . NAS Signaling connection uses UE And NG-RAN Between RRC Connection and AN And AMF Between NGAP UE Association for 3GPP Access . UE be in CM-CONNECTED state , among NGAP UE Association is not bound to AN and AMF Between any TNLA. At the completion of NAS After the signaling process ,AMF Can decide to release and UE Of NAS Signaling connection .
        UE stay CM-CONNECTED State, , Happen when RRC Release when ,UE Get into CM-IDLE state . When AMF Medium UE CM Status as CM-CONNECTED when , Whenever it's time to UE Of NGAP Signaling connection and N3 When the user interface connection is released , Will be UE Set as CM-IDLE state .AMF Can be UE CM The state remains in CM-CONNECTED State of AMF in , until UE Log off from the core network .

         be in CM-CONNECTED State of UE Can be in RRC Inactive state , When UE be in RRC Inactive In the state of ：

        -  UE reachability from RAN Manage with the help of the core network ;

        -  UE paging from RAN management ;

        -  UE Use 5G-S-TMSI and RAN identifier To monitor paging .

5GS CM State model

 UE Medium CM State transition

         about UE Come on , When UE When the connection with the base station is established ,UE Get into CM-CONNECTED state , When the connection is released ,UE Get into CM-IDLE state .

 AMF Medium CM State transition

         about AMF Come on , When N2 When the context is established ,AMF in UE CM Status as CM-CONNECTION, When N2 When the context is released ,UE CM State transition to CM-IDLE. When UE Get into CM-IDLE Post state , The user side PDU The session will go into deactivation .