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MEC多接入边缘计算及关键技术

4.2.1 5G MEC System Architecture

The 5G system architecture specified by 3GPP and described in [10] has been designed to cater for a wide set of use cases ranging from a massive amount of simple IoT devices to the other extreme of high bit rate, high reliability mission critical services. Supporting all the use cases with the same and common architecture has required significant changes in design philosophies both for the RAN and the core network. One significant architectural change was made to the communications between the core network functions that until now have relied on a point-to-point paradigm. In the 5G system specification there are two options available for the architecture; one with the traditional reference point and interface approach and the other where the core network functions interact with each other using a Service Based Architecture (SBA). In this white paper the emphasis is on the SBA option of the 5G system architecture.

With the SBA, there are functions that consume services and those that produce services. Any network function can offer one or more services. The framework provides the necessary functionality to authenticate the consumer and to authorize its service requests. The framework supports flexible procedures to efficiently expose and consume services. For simple service or information requests, a request-response model can be used. For any long-lived processes, the framework also supports a subscribe-notify model. The API framework defined by ETSI ISG MEC is aligned with these principles and in fact does exactly the same for MEC applications as the SBA does for network functions and their services. The functionality needed for efficient use of the services includes registration, service discovery, availability notifications, de-registration and authentication and authorization. All this functionality is the same in both the SBA and the MEC API frameworks.

In the figure below the 3GPP 5G system with its SBA is shown on the left, while the MEC system architecture is on the right. In the remainder of this white paper the focus is on describing how to deploy the MEC system in a 5G network environment in an integrated manner where some of the functional entities of MEC (blue boxes in MEC system part) interact with the network functions of the 5G core network.

Figure 1. 5G Service-Based Architecture and a generic MEC system architecture The network functions and the services they produce are registered in a Network Resource Function (NRF) while in MEC the services produced by the MEC applications are registered in the service registry of the MEC platform. Service registration is part of the Application Enablement functionality [4]. To use the service, if authorized, a network function can directly interact with the network function that produces the service. The list of available services can be discovered from the NRF. Some of the services are accessible only via the NEF, which is also available to untrusted entities that are external to the domain, to access the service. In other words, the NEF acts as a centralized point for service exposure and also has a key role in authorizing all access requests originating from outside of the system. In addition to AF, NEF and NRF, there are a number of other functions that are worth introducing. The procedures related to authentication are served by the Authentication Server Function (AUSF).

One of the key concepts in 5G is Network Slicing that allows the allocation of the required features and resources from the available network functions to different services or to tenants that are using the services. The Network Slice Selection Function (NSSF) is the function that assists in the selection of suitable network slice instances for users, and in the allocation of the necessary Access Management Functions (AMF). A MEC application, i.e. an application hosted in the distributed cloud of a MEC system can belong to one or more network slices that have been configured in the 5G core network.

Policies and rules in the 5G system are handled by the PCF. The PCF is also the function whose services an AF, such as a MEC platform, requests in order to impact the traffic steering rules. The PCF can be accessed either directly, or via the NEF, depending whether the AF is considered trusted or not, and in the case of traffic steering, whether the corresponding PDU session is known at the time of the request.

The Unified Data Management (UDM) function is responsible for many services related to users and subscriptions. It generates the 3GPP AKA authentication credentials, handles user identification related information, manages access authorization (e.g. roaming restrictions), registers the user serving NFs (serving AMF, Session Management Function (SMF)), supports service continuity by keeping record of SMF/Data Network Name (DNN) assignments, supports Lawful Interception (LI) procedures in outbound roaming by acting as a contact point and performs subscription management procedures.

The User Plane Function (UPF) has a key role in an integrated MEC deployment in a 5G network. UPFs can be seen as a distributed and configurable data plane from the MEC system perspective. The control of that data plane, i.e. the traffic rules configuration, now follows the NEF-PCF-SMF route. Consequently, in some specific deployments the local UPF may even be part of the MEC implementation.

The following figure shows how the MEC system is deployed in an integrated manner in 5G network.