Connection Management is used to establish and release the signalling connection between UE and AMF.
CM mainly deals with establishing and releasing a NAS signalling connection between UE and AMF over N1.
There are 2 CM states to reflect NAS signalling connection of the UE with AMF.
1. CM-IDLE state
UE in CM-IDLE state, it has no NAS signalling connection with AMF over N1.
Then UE will perform cell selection and PLMN selection.
UE is both in CM-IDLE state and in RM-REGISTERED state will perform below steps:
1. Respond to paging by performing a Service Request procedure.
When UE state in AMF is CM-IDLE and RM-REGISTERED, the AMF shall,
perform a network triggered service request procedure what it has signalling or mobile terminated data to be sent to UE.
2. CM-CONNECTED state
A UE in CM-CONNECTED state has NAS signalling connection with the AMF over N1.
UE in CM-CONNECTED state, will enter CM-IDLE state whenever the AN signalling connection is released.
AMF in CM-CONNECTED state, will enter CM-IDLE state the logical NGAP signalling connection and the N3 user plane connection for this UE are released upon completion of the AN Release procedure
UE in RRC Inactive state will follow :
1. UE readability is managed by the RAN.
2. UE paging is managed by RAN.
3. UE monitor for paging with UE.
CM-CONNECTED with RRC Inactive state
In 5G NR there is a new state for UE i.e RRC Inactive state defined in TS 38.306.
The “RRC Inactive Assistance Information” includes:
-> UE specific DRX values;
-> UE specific extended idle mode DRX values;
-> The Registration Area provided to the UE;
-> Periodic Registration Update timer;
UE in CM-CONNECTED with RRC Inactive state, UE may resume RRC Connection to:
-> Uplink data pending;
-> Mobile initiated NAS signalling procedure;
-> As a response to RAN paging;
-> Notifying the network that it has left the RAN Notification Area;
-> Upon periodic RAN Notification Area Update timer expiration.
Source TS 23.501