The commercial pressures of 5G, including limited choice and price competition in the network equipment market, are motivating many operators to explore new network solutions and different equipment suppliers.
Open RAN is one approach that promises a more efficient, flexible radio network architecture by using open interfaces to create vendor interoperability between the radio and baseband unit (BBU). The technology also stands to increase vendor competition by unlocking the market for new players.
4G EPC
The LTE EPC comprises several network elements listed below.
The Mobility management Entity (MME) serves the UE providing mobility and session management.
The S-GW transports user traffic between the 4G RAN and the P-GW.
The P-GW transports user traffic between the S-GW and external data networks, for example the internet or IMS. The P-GW also assigns UE IP addresses and provides QoS enforcement for user traffic.
The HSS stores customer profile data and generates authentication vectors used by the MME for subscriber authentication.
An auxiliary node, the Policy and Charging Rules Function (PCRF) provides quality of service information to the P-GW for specific traffic types, including charging rules, flow control rules and traffic priority.
TRANSITION TO THE 5G CORE
These same functions need to be performed in the 5G core, together with new capabilities. However, the architecture is different. Some functions have been split into multiple separate components, while some functions have been combined. The 5G network is also designed to provide a clear delineation between control and user plane functions, enabling an implementation using Network Functions Virtualization (NFV) and Software-Defined Networking (SDN).
The Access and Mobility Management Function (AMF) serves part of the role of the 4G MME, that of mobility management. The AMF maintains a NAS signaling connection with the UE and manages the UE registration procedure. The AMF is also responsible for paging.
Subscriber authentication, during registration or re-registration with 5G, is managed by the Authentication Server Function (AUSF), which obtains authentication vectors from the UDM.
The 5GC Session Management Function (SMF) provides the session management functionality of the 4G MME and additionally combines some control plane functions of the S-GW and P-GW. The SMF allocates IP addresses to the UE.
The User Plane Function (UPF) combines the user traffic transport functions previously performed by the S-GW and P-GW in the 4G EPC. The UPF anchors the UE IP addresses. The overall uplink traffic flow is from UE to gNB to UPF and the reverse for downlink traffic. The UPF provides the QoS enforcement function.
As shown in the diagram, the MME functions have essentially been distributed among the AMF, the SMF and the AUSF.
The two gateways in the 4G EPC, S-GW and P-GW, have both been combined and split into control plane and user plane functions and absorbed into the SMF and the UPF.
When a UE is in idle mode, downlink traffic is buffered at the UPF. The UPF signals the SMF which then signals to the AMF to start the paging procedure.
Management of Subscriber, Policy, and Other Data Information about subscribers, application-specific data and policy data can be stored in the UDR. The Unified Data Management (UDM) interfaces with NFs such as AMF and SMF so that relevant data become available to AMF and SMF. The Unified Data Repository (UDR) can store structured data that can be exposed to an NF. In contrast, the Unstructured Data Storage Function (UDSF) is intended for storage and retrieval of unstructured data by a suitable network function. The UDM generates authentication vectors when requested by the AUSF.
POLICY
Policy control in 5G is provided by the Policy Control Function (PCF), essentially the same as the 4G PCRF but with added capabilities. In 5G, the AMF interfaces with the PCF for retrieval of access and mobility policies. The PCF maintains its traditional diameter protocol-based interface to an Application Function (AF), but is also enhanced to allow for resource reservation requests using an HTTP/XML-based interface from other services. This opens the 5G network QoS capabilities to other non-operator services, providing subscribers with QoS-controlled services from internet providers; for example, a movie subscription service.
NSSF FOR NETWORK SLICING
Network slicing is a new capability of the 5G network. It allows an operator to set up multiple virtual slices of the RAN, core and transport networks to meet specific service requirements. The Network Slice Selection Function (NSSF) maintains a list of the network slice instances defined by the operator, including their definitions, such as required resources. A UE may be configured with a list of subscribed network slices and can request these during the registration procedure. The AMF will authorize the use of the network slices using subscription information from the UDM or by a query to the NSSF.
N3IWF
The Non-3GPP access Inter-Working Function (N3IWF) is similar to the ePDG in the 4G EPC, which is a secure gateway for the UE access non-trusted networks, such as Wi-Fi and the public internet. One important enhancement is the transport of NAS messages from the UE over non-trusted networks and AMF selection. A 5G UE can perform a network registration from networks other than the 5G RAN and when connected on, for example, 5G RAN and Wi-Fi will have two, distinct active NAS connections with the AMF.
With growing operator interest in Open RAN, incumbent RAN vendors – if they are to compete – must acknowledge the openness trend, whether they are prepared to embrace the principles of open interfaces and greater interoperability or not.
To conclude, this article is written about Open RAN, how it works, what the options are and why it’s gaining momentum.
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