5g core network function part-1 : amf

Access and Mobility Management Function

It is a control plane function in a 5G core network. AMF’s main function is as follows

1. Registration Management

When the device is turned on, the device has to try to connect to the network, and then due-process it has to verify and validate that it is connecting to a genuine network that is intended to connect. Similarly, the network has to validate the device has the right privilege and credentials to connect to the network and access 5G services. This is handled by registration management. It refers to the collection of different principles that supports the device registration or deregistration with the network and when the registration is successful, registration management establishes the user context. User Context is the combination where the core network understands the identity of devices, and the identity of subscriptions and establishes the necessary encryption parameters for further communication between devices and the core network. There are two states in registration management as follows:-

a) Registered

b) Deregistered

Initial when the device is powered on, it is in the de-registered state. And when the registration procedure is accepted, then it enters the registered state. When the device is in a registered state it still has to perform a registration update once in a while. Now the registration update happens on a few different occasions as follows:-
a) Periodically
b) Mobility
c) Update Capabilities

In some cases, registration acceptance is not successful and if it is detected, then the device goes back to de-registered. Finally, when the registration is successful then it means the core network knows what is the identity of the device that’s trying to communicate with the network, what are the capabilities of that device and what services it wants to access. And it also establishes the encryption parameters that both the device and the core network use to communicate with each and that makes the registration successful.

2. Connection Management

Connection management is all about establishing N1 connectivity, which is the interface between the device and AMF. It has two components:- first, the device connects to RAN using RRC signaling and second, the N2 is used to establish between AMF and gNB. When both the above components are up and running, it means the device can now access the AMF using the N1 signal. There are terms called NAS(Non-Access Stratum) and AS(Access Stratum). NAS refers to the connectivity between the device and the AMF. So in connection management, one aim is to establish a NAS-signaling device and AMF. AS refers to the connectivity of wireless access, which is the connectivity between the device and gNB.

When the device is in a CM-Idle state, it means that there is no signaling that is possible between the device and the AMF. It means NAS signaling is not yet established. So in the CM-Idle state, they do not support mobility. Mobility means the reselection of the cell again and again until the device finds the better cell to connect. And if NAS signaling is successful it goes under CM-Connected. It means the device has a signaling connection established with AMF which means it can communicate with the core network and start availing of 5G services.

AMF Services

Every network function produces some services and consumes some services. We will discuss the services which were consumed by other functions.

a) Namf_Communication
Provides the service operation for transporting N1 messages to UE.
Provides the service operation for initiating N2 messages towards the access network.
Allows network functions to subscribe and unsubscribe for notification of specific N1 messages from UE.
Allows network functions to subscribe and unsubscribe for notification of specific information from the access network.
Security Context management
UE information management and transfer (including its security context)

b) Namf_MT
Paging UE if UE is in Idle state and responds to other network functions after the UE enters CM-Connected State.
Respond to requester network function if UE is in connected mode.
Providing the terminating domain selection information for IMS voice to the consumer network functions.

c) Namf_Location
Allows network functions can request the current geodetic and optionally civic location of a target UE.
Allows network functions to be notified of event information related to emergency sessions.
Allows network functions to request network provider location information(NPLI) and/or local time zone corresponding to the location of the target UE.

d) Namf_EventExposure
Location changes
Time Zone changes
Access Type changes
Registration State changes
Connectivity State changes
UE loss of Communication
UE Reachability Status

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