In this chapter we shall have a look at the functions of 5G Core Network elements
1. AMF: Access and Mobility Function
AMF is similar to MME in 4G
AMF interacts with NG RAN using N2 interface
AMF interact with UE over N1 interface for authentication, registration, handovers, paging.
One UE can connect with only one AMF at a time
It will relay all session management signalling between SMF and UE.
AMF is connected to UE with N1 interface
Relay messages between UE and PCF
AMF includes security functionality for authentication, authorization of UE
It will derives keys for integrity and ciphering
2. UE Registration States:
UE needs to register with the network to receive network services.
Once the UE registers, it will update its registration with the network as:
1. Periodically in order to remain reachable
2. upon mobility
3. When UE needs to update its capabilities.
There are 2 Registration Management (RM) states.
1. RM-DEREGISTERED
2. RM-REGISTERED
1. RM-DEREGISTERED
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When the UE is not registered with the network it will be in this state.
AMF will not hold the UE context.
UE will remain in this state if it receives Registration reject upon initial registration
If UE wants to connect to network, then it will try to register with the selected PLMN using initial registration procedure. TS 23.502 [3] clause 4.2.2.2
2. RM-REGISTERED
In this state UE will be registered with the network.
In RM-Registered state UE shall perform below tasks:
It will perform Mobility Registration Update procedure if the current TAI of the serving cell is not in the list of TAIs.
Performs periodic registration update procedure
Performed Mobility Registration update procedure
For more details refer TS 23.501
3. UE Connection Management States:
Connection Management (CM) is used to establish and release control plane signaling connection between UE and AMF.
It means, when UE is in CM-IDLE uUE has no NAS signaling connection established with AMF over N1 interface.
CM has 2 states: CM-IDLE, CM-CONNECTED
CM-IDLE state
Ue in CM-IDLE state has no NAS signaling with AMF.
There will be no AN signaling connection, N2 connection and N3 connection for UE
If UE is in CM-IDLE state and in RM-REGISTERED state, then UE shall perform below tasks:
1. Respond to paging by performing a service request procedure
2. Perform a service request procedure when the UE has uplink signaling or user data to be sent.
When the UE state in the AMF is RM-REGISTERED, UE information required for initiating communication with the UE shall be stored.
CM-CONNECTED state
UE in CM-CONNECTED state has a NAS signalling connection with the AMF over N1.
UE in CM-CONNECTED state shall perform below tasks:
1. Enter CM-IDLE state whenever the AN signalling connection is released.
For more details refer TS 23.501
Services Provided by AMF
There are 4 services that are provided my AMF
1. Namf_Communication:
This service enables NF like SMF, PCF to communicate with UE and NG-RAN via AMF.
Allows one AMF to fetch UE contexts from other AMF.
2. Namf_EventExposure:
Allows other NF to subscribe to events such as UE rechability, mobility changes.
3. Namf_MT:
Allows other NFs to ensure UE is reachable
4. Namf_Location
Allows NF to get location of UE.
For more information follow table 5.2.2.1 of 3GPP TS 23.502