Network Function | Access & Mobility Management Function (AMF) in 5G Core Network | 5G System (5GS)

​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:

1. Registration Management
2. Reachability Management
3. Connection Management
4. Mobility Management
   

• Registration Management allows a UE to register and de-register with the 5G system. A UE must complete the registration procedure to receive authorization to use 5G services.  
  Registration moves the UE from RM-Deregistered state to the RM-Registered state. Registration creates a UE Context within the network. ​
• Connection Management establishes and release the control plane signalling connections between the UE and AMF (across N1 interface). N1 signalling connection moves the UE from CM-Idle to CM-Connected.  
  
• Reachability Managment ensures that a UE is always reachable, i.e., it is possible to page the UE when there is a requirement to establish a mobile terminated connection. Paging a UE which is in the CM-Idle state triggers the UE to initiate the NAS Service request procedure and subsequently establish an N1 signalling connection before moving into the CM-Connected state.
• Mobility Management is used to maintain knowledged of UE's location within the network. The UE is required to complete periodic registration updates after it has completed initial registration. These periodic updates act as keep-alive to verify that UE remains on the system, and has not moved out of coverage or become unavailable due to any other reason (for example UE stopped working, battery died). UE is also required to complete updates due to mobility. These updates are triggered if UE moves outside the current registration area (outside of tracking area or list of tracking areas within which the UE is currently registered)
  

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: 

1. PDU Session Management
2. UE Context Managment
3. UE Mobility Management
4. Paging Procedures
5. Transport of NAS messages
6. Interface Management
7. Configuration Transfer
8. Warning Message Transmission

• PDU Session Management procedures are used to setup, modify and release resources at the Base station and UE. SMF is responsible for PDU Session Management so these procedures are completed after the AMF has been instructed by the SMF
• UE Mobility Management procedures are used to support handover procedures. Xn based handover procedures use the NGAP Path Switch Signalling procedure. N2 based handover procedures use the NGAP Handover Required, Handover Request, Handover Command and Handover Notify signalling procedures.
• Transport of NAS messages. NGAP is used to transfer NAS messages between the BTS and AMF, while a signalling radio bearer (SRB) is used to transfer NAS messages between the UE and BTS. The combination of these hops provides the end-to-end transfer of NAS messages across the N1 interface between UE and AMF.  The combination of NAS and NGAP signalling is shown in Figure below.
  

• Interface Management procedure. These procedures are used to setup the NG connection between the Base Station and AMF. Both N1 and N2 interface use the NG connection between the Base Station and AMF. Interface Management procedures also allow both RAN and AMF configuration  updates to be provided.  As an example RAN configuration update allows the Base Station to inform the AMF of any changes to the supported PLMN or Tracking Areas.
• Configuration Transfer procedure. This procedure allow the Base Station and AMF to exchange information related to Self Optimizing Networks . As an example, procedures can be used to support Automatic Neighbor Relations (ANR). The end to end transfer of information could be between a pair of Base Stations but the information has to be sent via the AMF if an Xn interface does not exist between the Base Stations.
  

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  

• Value 1 refers to enhanced Mobile Broadband (eMBB),  
• ​Value 2 refers to Ultra Reliable Low Latency Communication (URLLC)

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

References

1. ​​3GPP TS 23.501
2. 3GPP TS 23.502
3. 3GPP TS 24.501
4. ​3GPP TS 38.413
5. 5G NR in Bullets by Chris Johnson
   

​​​