Principles

TS 23.501 lists the following as the key principles for the architecture:

• Separate the user plane (UP) functions from the control plane (CP) functions to allow independent scalability, evolution, and flexible deployments (e.g., centralized location or distributed [remote] location).

• Modularize the function design (e.g., to enable flexible and efficient network slicing).

• Wherever applicable, define procedures (i.e., the set of interactions between network functions) as services so that their reuse is possible.

• Enable each network function (NF) and its network function services (NFS) to interact with other NF and its NFS directly or indirectly via a service communication proxy, if required. The architecture does not preclude the use of another intermediate function to help route control plane messages.

• Minimize dependencies between the access network (AN) and the core network (CN). The architecture is defined with a converged core network with a common AN–CN interface that integrates different access types (e.g., 3GPP access and non-3GPP access).

• Support a unified authentication framework.

• Support stateless NFs. A stateless NF separates an NF into a processing module and a data store module containing state information. This design results in a more agile NF in terms of scalability, resilience, and ease of deployment [KABL17].

• Support capability exposure. As mentioned in Chapter 2, this refers to the ability to provide relevant information about network capabilities to third parties.

• Support concurrent access to local and centralized services. To support low-latency services and access to local data networks, UP functions can be deployed close to the access network.

• Support roaming with both home-routed traffic as well as local breakout traffic in the visited public land mobile network (PLMN).

Roaming

The final item in the preceding list references some important concepts. A PLMN, often called a carrier, is a telecommunications network that provides mobile cellular services. A home PLMN for a given mobile phone subscriber is the PLMN that is contracted to provide cellular service to the subscriber. Roaming is the ability for a user to function in a serving network different from the home network, called the visited network. Roaming can occur internationally, in which case mobile subscribers get coverage and at least basic services similar to their domestic package from a network operator in another country. For 5G, any user device should work in any other country. With 4G and earlier, there are multiple technologies in use, and a cell phone from one country may not be able to operate on a network in another country. National roaming refers to the ability to move from one mobile operator to another in the same country.

Both the home and visited PLMN use the same network architecture, which defines protocols, services, and interfaces between the home PLMN and the visited PLMN. The 3GPP specifications support two models of operation: home routed and local breakout (LBO). In the home-routed roaming model, the subscriber’s data traffic is serviced by the subscriber’s home network, which gives the home network operator more control over the user’s traffic and is preferable when the relationship between the two operators is not totally trustworthy. In the LBO model, the subscriber’s data is serviced by the visited network; this model delivers more efficient routing in terms of bandwidth and latency. In the case of LBO, the home network owner loses control of the customer and has no role in delivering services to that user. The LBO model is used when there is a trusted relationship between the two operators.

An architecture that includes roaming adds complexity but no change to the basic network services and functionality. Therefore, this section focuses on the non-roaming case.

Architecture Diagrams

TS 23.501 contains a number of architecture diagrams from several different points of view and at varying levels of detail. All of the diagrams depict the architecture in terms of a number of interconnected network functions (NFs). An NF is a processing function in a network that has defined functional behavior and interfaces. A network function can be implemented either as a network element on dedicated hardware, as a software instance running on dedicated hardware, or as a virtualized function instantiated on an appropriate platform.

The interconnection between NFs is represented in two ways:

• Service-based representation: NFs within the control plane enable other authorized NFs to access their services. This representation illustrates how a set of services is provided/exposed by a given network interface. This interface defines how one network function within the control plane allows other network functions that have been authorized to access its services. This representation also includes point-to-point reference points where necessary.

• Reference point representation: This representation uses labeled point-to-point links to show the interaction that exists between two NFs or between an NF and an external functional module or network. The reference point representation is beneficial when showing message sequence charts. It shows the relationships between NFs that are used in the message sequence charts.

There are several advantages to this form of architectural representation. The modular structure provides a framework for developing detailed specifications for each NF. The service-based interfaces and reference points provide a framework for developing detailed specifications of the interaction between NFs in terms of data formats, protocols, and service calls. In addition, the detailed interface specifications promote interoperability between different hardware/software providers. Finally, this type of architecture definition provides a way of ensuring that all 5G functional and service requirements are satisfied.

Service-Based System Architecture

Figure 3.6, based on TS 23.501, depicts the overall non-roaming 5G service-based architecture (SBA) for the core network. The functional components of the control plane of the network are network functions (NFs) that offer their services to any other applicable NFs via a common framework of interfaces accessible to all NFs. Network repository functions (NRFs) allow every NF to discover the services offered by other NFs present in the network; network exposure functions (NEFs) expose capability information and services of the 5G core network NFs to external entities. This model aims to maximize the modularity, reusability, and self-containment of network functions and to foster the ability to grow flexibly while taking advantage of network functions virtualization and software-defined networking.

The figure includes the following NFs and other modules:

• Authentication server function (AUSF): Performs authentication between UE and the network.

• Access and mobility management function (AMF): Receives all connection- and session-related information from the user equipment (UE) (N1/N2) but is responsible only for handling connection, registration, reachability, and mobility management tasks. All messages related to session management are forwarded to the session management function (SMF).

• Network exposure function (NEF): Provides an interface for outside applications to communicate with the 5G network to obtain network-related information in the following categories:

  • Monitoring capability: Allows an external entity to request or subscribe to UE-related events of interest. The monitored events include a UE’s roaming status, UE loss of connectivity, UE reachability, and location-related events.

  • Provisioning capability: Allows an external entity to provide information about expected UE behavior to the 5G system (e.g., predicted UE movement, communication characteristics).

  • Policy/charging capability: Handles QoS and charging policy for the UE, based on a request from an external party.

  • Analytics reporting capability: Allows an external party to fetch or subscribe/unsubscribe to analytics information generated by the 5G system.

• Network repository function (NRF): Allows NFs to register their functionality and to discover the services offered by other NFs present in the network.

• Network slice selection function (NSSF): Selects the set of network slice instances to accommodate the service request from a UE. When a UE requests registration with the network, AMF sends a network slice selection request to NSSF with information on the preferred network slice selection. The NSSF responds with a message that includes a list of appropriate network slice instances for the UE.

• Network slice-specific authentication and authorization (NSSF): Performs authentication and authorization specific to a slice.

• Policy control function (PCF): Provides functionalities for the control and management of policy rules, including rules for QoS enforcement, charging, and traffic routing. PCF enables end-to-end QoS enforcement with QoS parameters (e.g., maximum bit rate, guaranteed bit rate, priority level) at the appropriate granularity (e.g., per UE, per flow, per protocol data unit [PDU] session).

• Session management function (SMF): Responsible for PDU session establishment, modification, and release between a UE and a data network. A PDU session, or simply a session, is an association between the UE and a data network that provides a PDU connectivity service. A PDU connectivity service is a service that provides for the exchange of PDUs between a UE and a data network.

• Unified data management (UDM): Responsible for access authorization and subscription management. UDM works with the AMF and AUSF as follows: The AMF provides UE authentication, authorization, and mobility management services. The AUSF stores data for authentication of UEs, and the UDM stores UE subscription data.

• User plane function (UPF): Handles the user plane path of PDU sessions. This function is described subsequently.

• Application function (AF): Provides session-related information to the PCF so that the SMF can ultimately use this information for session management. The AF interacts with application services that require dynamic policy control. The AF extracts session-related information (e.g., QoS requirements) from application signaling and provides it to the PCF in support of its rule generation. An example is the IP multimedia subsystem (IMS), which may interface with the PCRF to request QoS support for VoIP calls.

• User equipment (UE): Allows a user access to network services. An example is a mobile phone. For the purpose of 3GPP specifications, the interface between the UE and the network is the radio interface.

• (Radio) Access Network ((R)AN): Provides access to a 5G core network. This includes the 5G RAN and other wireless and wired access networks.

• Data network (DN): Allows UE to be logically connected by a session. It may be the Internet, a corporate intranet, or an internal services function within the mobile network operator’s core (including content distribution networks).

• Service communication proxy (SCP): Allows NFs and NFSs to communicate directly or indirectly. The SCP enables multiple NFs to communicate with each other and with user plane entities in a highly distributed multi-access edge compute cloud environment. This provides routing control, resiliency, and observability to the core network.

The ovals on NFs in Figure 3.6 indicate service interfaces that can be accessed by other NFs. Each interface is identified by a label consisting of an uppercase N followed by the abbreviation of the NF in lowercase. For example, the network slice selection function has a service interface labeled Nnssf.

It is informative to compare Figure 3.6 with Figure 2.13, which shows the ITU-T Y.3102 (Framework of the IMT-2020 Network, May 2018) representation of the core network, which provides a somewhat different functional breakdown. This can be considered an earlier version of the core network architecture that has been superseded by the current 3GPP architecture.

Reference Point Representation

Figure 3.7, based on TS 23.501, depicts the overall non-roaming 5G architecture using the reference point representation, showing how the NFs interact with each other.

Note that there are fewer interconnections depicted in Figure 3.7 than in Figure 3.6. Within the control plane, the interconnections in Figure 3.6 indicate which NFs can access the services of which other NFs. The interconnections of Figure 3.7 indicate which NFs communicate with each other directly, without going through an intermediate NF. The term directly does not mean that there is a physical point-to-point link between NFs connected on the diagram. Rather, it means that there is a protocol for the exchange of messages between the connected entities that is not relayed through another NF. Each such link is labeled with a reference point expressed as an uppercase N followed by a number. For example, the logical connection between the session management function and the policy control function is labeled reference point N7.

In Figure 3.7, two reference points loop back to the same function: N9 and N14. The N9 reference point is an interface between two distinct UPFs used for forwarding packets. The N14 reference point is between two AMFs, one acting as a source AMF for a data transfer and the other acting as a destination AMF.

User Plane Function

User plane functions handle the user plane path of PDU sessions. 3GPP specifications support deployments with a single UPF or multiple UPFs for a given PDU session. UPF selection is performed by SMF. UPF functions include:

• Packet routing and forwarding.

• Anchor point for intra-/inter-RAT mobility (when applicable). Anchor points are transit nodes in the network used for forwarding PDUs along a session from a UE to the destination.

• External PDU session point of interconnect to data network.

• Packet inspection (e.g., application detection based on a service data flow [SDF] template). An SDF provides end-to-end packet flow between an end user and an application; this is discussed in Chapter 9, “Core Network Functionality, QoS, and Network Slicing.”

• User plane part of policy rule enforcement (e.g., gating, redirection, traffic steering).

• Traffic usage reporting.

• QoS handling for the user plane, such as uplink/downlink rate enforcement.

• Uplink traffic verification (SDFs to QoS flow mapping). A QoS flow is the lowest level of granularity for defining end-to-end QoS policies. A QoS flow may contain multiple SDFs; this is discussed in Chapter 9.

• Transport-level packet marking in the uplink and downlink.

• Downlink packet buffering and downlink data notification triggering.

• Sending and forwarding of one or more end markers to the source NG-RAN node.

RAN Functional Areas

Figure 3.9 illustrates the following key functional areas in the NG-RAN:

• Inter-cell radio resource management: Allows the UE to detect neighbor cells, query about the best serving cell, and support the network during handover decisions by providing measurement feedback.

• Radio bearer control (RBC): Consists of the procedure for configuration (such as security), establishment, and maintenance of the radio bearer (RB) on both the uplink and downlink with different quality of service (QoS). The term radio bearer refers to an information transmission path of defined capacity, delay, bit error rate, and other parameters.

• Connection mobility control (CMC): Functions both in UE idle mode and connected mode. In idle mode, UE is switched on but does not have an established connection. In connected mode, UE is switched on and has an established connection. In idle mode, CMC performs cell selection and reselection. The connected mode involves handover procedures triggered on the basis of the outcome of CMC algorithms.

• Radio admission control (RAC): Decides whether a new radio bearer admission request is admitted or rejected. The objective is to optimize radio resource usage while maintaining the QoS of existing user connections. Note that RAC decides on admission or rejection for a new radio bearer, while RBC takes care of bearer maintenance and bearer release operations.

• Measurement configuration and provision: Consists of provisioning the configuration of the UE for radio resource management procedures such as cell selection and reselection and for requesting measurement reports to improve scheduling.

• Dynamic resource allocation (scheduler): Consists of scheduling RF resources according to their availability on the uplink and downlink for multiple pieces of UE, according to the QoS profiles of a radio bearer.

Access and Mobility Management

On the core network side, the NG-RAN nodes interact with three functions: the access and mobility management, session management, and user plane functions.

The AMF provides UE authentication, authorization, and mobility management services. The two main functions shown in Figure 3.9 for AMF are NAS security and idle state mobility handling.

The non-access stratum (NAS) is the highest protocol layer of the control plane between UE and the access and mobility management function (AMF) in the core network. The main functions of the protocols that are part of the NAS are the support of mobility of the UE and the support of session management procedures to establish and maintain IP connectivity between the UE and user plane function (UPF). It is used to maintain continuous communications with the UE as it moves. In contrast, the access stratum is responsible for carrying information just over the wireless portion of a connection. NAS security involves IP header compression, encryption, and integrity protection of data based on the NAS security keys derived during the registration and authentication procedure.

Idle state mobility handling deals with cell selection and reselection while the UE is in idle mode, as well as reachability determination.

Session Management Function

The two main functions depicted in Figure 3.9 for SMF are UE IP address allocation and PDU session control.

UE IP address allocation assigns an IP address to the UE at the time of session establishment. This ensures the ability to route data packets within the 5G system and also supports data reception and forwarding to outside networks and provides interconnectivity to external packet data networks (PDNs).

In cooperation with the UPF, the SMF establishes, maintains, and releases a PDU session for user data transfer, which is defined as an association between the UE and a data network that provides PDU connectivity.

User Plane Function

The two main functions depicted in Figure 3.9 for UPF are UE IP mobility anchoring and PDU handling.

UE mobility handling deals with ensuring that there is no data loss when there is a connection transfer due to handover that involves changing anchor points.

Once a session is established, the UPF has a responsibility for PDU handling. This includes the basic functions of packet routing, forwarding, and QoS handling.