This blog post includes the details related to Network Functions as shown in System Service Based Architecture. For your convenience, the picture of 5G System Service Based Architecture is described below once again.
Access & Mobility Management Function (AMF): It is a control plane function in 5G core network. The main functions and responsibilities of AMF are:
Next Generation Application Protocol (NGAP): AMF is also responsible for handling Next Generation Application Protocol (NGAP) signalling which is transferred between the AMF and 5G RAN node ( which means between an AMF and Base Station). NGAP specifications are described in 3GPP TS 38.413 and is similar to S1AP in case of 4G LTE ( S1 AP is between MME and Base Station).
NGAP Signalling procedures: Different categories of NGAP signalling procedures include:
5G Globally Unique Temporary Identifier (5G-GUTI) : AMF is responsible for allocating a 5G Globally Unique Temporary Identifier (5G-GUTI), which is a concatenation of the Globally Unique AMF Identifer (GUAMI) and 5G Temporary Mobile Subscription Identifer (5G-TMSI). The 5G-GUTI provides greater privacy than IMSI because it is a temporary identity which the AMF can re-assign at any time. 3GPP hase specified mapping rules between 5G-GUTI and 4G-GUTI.
Selecting an Appropriate Authentication Server Function (AUSF). AMF is responsible for selecting an appropriate Authentication Server Function (AUSF) during the registeration procedure. The AUSF allows the UE to authenticate itself with 5G Core Network for example to verify that the subscriber is genuine and authorised to access the network. The AMF may be configured to use a specific AUSF or the AMF may use the Network Function repository Function (NRF) to discover suitable AUSF within the 5G Core Network
Selecting an Appropriate Unified Data Management (UDM). AMF is responsible for selecting an appropriate Unified Data Management (UDM) function during the registration procedure. The UDM manages the user's subscription information. The AMF may be configured to use specific UDM or the AMF may use the Network Function Repository Function (NRF) to discover a UDM which manages the user's subscription.
Selecting Appropriate Policy Control Function: AMF is responsible for selecting an appropriate Policy Control Function (PCF) for the UE during the registration procedure. The PCF provides the AMF with an 'Access and Mobility Policy' for the UE. This may include a specification of allowed or forbidden Tracking Areas. The AMF may be configured to use a specific PCF or the AMF may use the Network Function Repository Function (NRF) to discover a PCF which can provide the relevant UE information.
Selecting an Appropriate Session Management Function (SMF): AMF is responsible for selecting an appropriate Session Management Function during PDU session establishment. AMF can apply a range of criteria during selection procedure
Data Network Name (DNN) : DNN is the 5G equivalent of 4G Access Point Name (APN). It refers to the data network to which PDU Session provides connectivity.
Subscription Information: AMF retrieves subscription information from the Unified Data Management (UDM) function during the Registration procedure. This can include information regarding the set of subscribed DNN.
Single Network Slice Selection Assistance Information (S-NSSAI). An S-NSSAI identifiesa Network Slice. It comprises a Slice/Service Type (SST) and a Slice Differentiator (SD). The SST defines the expected network behavior As an example
Support for Short Message Service (SMS): AMF provides support for Short Message Service (SMS). Mobile terminated SMS are received from SMS Function (SMSF) and are packaged within a NAS message before being transferred to the UE. A signalling Radio Bearer (SRB) is used to transfer the NAS message across the air interface. The AMF receives mobile originated SMS within uplink NAS messages and forwards them to the SMSF. The SMSF is an optional Network Function