In the previous chapter we had an introduction to SBA and the interfaces used.
In this chapter we shall see the functionality of all the Network Functions
All the functions mentioned here are taken from TS 23.501
There are 2 types of Architecture:
1. Service Based Architecture
2. Point to Point interface Architecture.
AMF: Access and Mobility Management function
– Termination of RAN CP interface (N2).
– Termination of NAS (N1), NAS ciphering and integrity protection.
– Registration management.
– Connection management.
– Reachability management.
– Mobility Management.
– Lawful intercept (for AMF events and interface to LI System).
– Provide transport for SM messages between UE and SMF.
– Transparent proxy for routing SM messages.
– Access Authentication.
– Access Authorization.
– Provide transport for SMS messages between UE and SMSF.
– EPS Bearer ID allocation for interworking with EPS.
AMF will have 4 services:
Namf_Communication
Nams_EventExposure
Nams_MT
Namf_Location
For additional information check the TS 23.502
SMF: Session Management function
– Configures traffic steering at UPF to route traffic to proper destination.
– Termination of interfaces towards Policy control functions.
– Lawful intercept (for SM events and interface to LI System).
– Charging data collection and support of charging interfaces.
– Control and coordination of charging data collection at UPF.
– Termination of SM parts of NAS messages.
– Downlink Data Notification.
– Initiator of AN specific SM information, sent via AMF over N2 to AN.
– Determine SSC mode of a session.
– UE IP address allocation & management (including optional Authorization).
– SMF is responsible for setup of the connectivity for UE towards Data Networks.
– It will handle establishment, modification and release of sessions.
– SMF interacts with Policy Control Function to retrieve policies for a UE and then configures in UPF.
UPF: User plane function
– Anchor point for Intra-/Inter-RAT mobility (when applicable).
– External PDU Session point of interconnect to Data Network.
– User Plane part of policy rule enforcement, e.g. Gating, Redirection, Traffic steering).
– Lawful intercept (UP collection).
– Traffic usage reporting.
– QoS handling for user plane, e.g. UL/DL rate enforcement, Reflective QoS marking in DL.
– Uplink Traffic verification (SDF to QoS Flow mapping).
These are equivalent of GWs in 4G.
– UPF process and forwards user data and is controlled by SMF
– Buffering of downlink data for Idlw mode UE
– UPF will resembles SGW and PGW in some aspects.
PCF: Policy Control Function
– Supports unified policy framework to govern network behaviour.
– Provides policy rules to Control Plane function(s) to enforce them.
– Accesses subscription information relevant for policy decisions in a Unified Data Repository (UDR).
– PCF can interact with UE via NAS(AMF) to update discovery and selection policies for non 3GPP networks.
– Equivalent to a PCRF in 4G.
NEF: Network Exposure Function
– Secure provision of information from external application to 3GPP network:
It provides a means for the Application Functions to securely provide information to 3GPP network, e.g. Expected UE Behaviour.
– Translation of internal-external information:
It translates between information exchanged with the AF and information exchanged with the internal network function. For example, it translates between an AF-Service-Identifier and internal 5G Core information such as DNN, S-NSSAI.
– The Network Exposure Function receives information from other network functions.
NRF: Network Repository Function
– NRF is a repo of the profiles of NFs that are available in the network.
– Supports service discovery function. Receive NF Discovery Request from NF instance, and provides the information of the discovered NF instances (be discovered) to the NF instance.
– Maintains the NF profile of available NF instances and their supported services.
NF profile of NF instance maintained in an NRF includes the following information:
– NF instance ID
– NF type
– PLMN ID
– Network Slice related Identifier(s) e.g. S-NSSAI, NSI ID
– FQDN or IP address of NF
– NF capacity information
– NF Specific Service authorization information
– Names of supported services
– Endpoint Address(es) of instance(s) of each supported service
– Identification of stored data/information
UDM: Unified Data Management
– UDM is a front end for UDR (Unified Data Repository)
– UDM uses subscription data to perform access auth, registration management.
– UDM keeps track of which AMF is serving the user
– UDM is always located in Home PLMN
– Generation of 3GPP AKA Authentication Credentials.
– User Identification Handling (e.g. storage and management of SUPI for each subscriber in the 5G system).
– Support of de-concealment of privacy-protected subscription identifier (SUCI).
– Access authorization based on subscription data (e.g. roaming restrictions).
– MT -SMS delivery support.
– Lawful Intercept Functionality (especially in outbound roaming case where UDM is the only point of contact for LI).
– Subscription management.
– SMS management.
Similar to an HSS in 4G
AUSF: Authentication Server Function
– Termination of N2 and N3 interfaces to 5G Core Network for control – plane and user-plane respectively.
– Relaying uplink and downlink control-plane NAS (N1) signalling between the UE and AMF.
– Handling of N2 signalling from SMF (relayed by AMF) related to PDU Sessions and QoS.
– Establishment of IPsec Security Association (IPsec SA) to support PDU Session traffic.
– N3 user-plane packet marking in the uplink.
– Local mobility anchor within untrusted non-3GPP access networks using MOBIKE per IETF RFC 4555 [57].
– Supporting AMF selection.
AF: Application Function
The Application Function (AF) interacts with the 3GPP Core Network in order to provide services, for example to support the following:
– Application influence on traffic routing (see clause 5.6.7),
– Accessing Network Exposure Function (see clause 5.20),
– Interacting with the Policy framework for policy control (see clause 5.14),
UDR: Unified Data Repository
The Unified Data Repository (UDR) supports the following functionality:
– Storage and retrieval of subscription data by the UDM.
– Storage and retrieval of policy data by the PCF.
– Storage and retrieval of structured data for exposure.
– Application data (including Packet Flow Descriptions (PFDs) for application detection, AF request information for multiple UEs), by the NEF.
– It is a repository for all subscription related information.
– It offers services to other network functions like UDM, PCF.
UDSF
The UDSF is an optional function that supports the following functionality:
– Storage and retrieval of information as unstructured data by any NF.
SMSF
The SMSF supports the following functionality to support SMS over NAS:
– SMS management subscription data checking and conducting SMS delivery accordingly.
– SM-RP/SM-CP with the UE (see TS 24.011 [6]).
– Relay the SM from UE toward SMS-GMSC/IWMSC/SMS-Router.
– Relay the SM from SMS-GMSC/IWMSC/SMS-Router toward the UE.
– SMS related CDR.
– Lawful Interception.
NSSF: Network Slice Selection Function
– Selecting the set of Network Slice instances serving the UE,
– Determining the Allowed NSSAI and, if needed, the mapping to the Subscribed S-NSSAIs,
– Determining the Configured NSSAI and, if needed, the mapping to the Subscribed S-NSSAIs,
– Determining the AMF Set to be used to serve the UE, or, based on configuration, a list of candidate AMF(s), possibly by querying the NRF.
AMF many be dedicated to one or multiple network slices .
Example:
1. UE sends requested S-NSSAI in the registration message
2. gNB selects AMF or sends to default AMF
3. Selected AMF processes registration request
4. AMF requests UDM for subscription information
5. UDM responds with subscription information and AMF will check if it can service the UE for that specific NS. If it can service the UE then it will send registration accept message to UE.
if not then AMF1 will perform below additional steps:
6. AMF1 will query NSSF for other AMF that can service the UE.
7. NSSF will respond with the AMF set.
8a. AMF1 will send NAS request to gNB to send re-request to AMF2
8b. or AMF1 can send the request directly to AMF2.
8c. AMF2 will process registration request
9. AMF2 sends registration accept message to UE.
Note:
AMF and SMF now serve these functions of MME
UPF is similar to SGW & PGW functionality from EPC
PCF has part of the PCRF functionality from EPC
AUSF has part of HSS from EPC
UDM has part of HSS from EPC
Below image shows the mapping of network functions