This post is an extract from the approach provided by Larry Peterson Oguz Sunay and Bruce Davie. I highly recommend you to go and read their online book. I did this extract to have a direct way to get into the all 5G acronyms, and also because this reading was one of the best I found online, I wanted to share it. This book brings a great overview for network professionals about 5G and all comments from the Authors brings a good understanding of the reasoning behind this standard and its definitions.
As of 3G, the generational designation corresponds to a standard defined by the 3rd Generation Partnership Project (3GPP). Even though its name has “3G” in it, the 3GPP continues to define the standards for 4G and 5G, each of which corresponds to a sequence of releases of the standard. Complicating the terminology, 4G was on a multi-release evolutionary path referred to as Long Term Evolution (LTE). 5G is on a similar evolutionary path, with several expected releases over its lifetime.
The understanding 4G is the first step to understanding 5G, as several aspects of the latter can be explained as bringing a new degree-of-freedom to the former.
This approach to describing 5G is grounded in two principles. The first is to explain the sequence of design decisions that lead to a solution. The second is aggressively disaggregate the system. Building a disaggregated, virtualized, and software-defined 5G access network is the direction the industry is headed. Breaking the 5G network down into its elemental components is also the best way to explain how 5G works.
The 3GPP terminology often changes with each generation (e.g., a base station is called eNB in 4G and gNB in 5G). We address situations like this by using generic terminology (e.g., base station), and referencing the 3GPP-specific counterpart only when the distinction is helpful.
The cellular network provides wireless connectivity to user equipments (UE): Mobile phones, drones, cars.
We have two main subsystems: the Radio Access Network (RAN) and the Mobile Core. The RAN manages the radio spectrum, making sure it is used efficiently and meets the quality-of-service requirements of every user. It corresponds to a distributed collection of base stations. As noted above, in 4G these are named eNodeB (eNB), which is short for evolved Node B. In 5G they are known as gNB. (The g stands for “next Generation”.)
The Mobile Core is a bundle of functionalities:
- Provides Internet (IP) connectivity for both data and voice services.
- Ensures this connectivity fulfills the promised QoS requirements.
- Tracks user mobility to ensure uninterrupted service.
- Tracks subscriber usage for billing and charging.
Note that Mobile Core is another example of a generic term. In 4G this is called the Evolved Packet Core (EPC) and in 5G it is called the Next Generation Core (NG-Core).
Even though the word “Core” is in its name, from an Internet perspective, the Mobile Core is still part of the access network, effectively providing a bridge between the RAN in some geographic area and the greater IP-based Internet. 3GPP provides significant flexibility in how the Mobile Core is geographically deployed.
The backhaul network is an implementation choice and not prescribed by the 3GPP standard.
Base station has an analog component (antenna) and a digital component (applications). The second is that the Mobile Core is partitioned into a Control Plane and User Plane, which is similar to the control/data plane split that someone familiar with the Internet would recognize. (3GPP also recently introduced a corresponding acronym—CUPS, Control and User Plane Separation—to denote this idea.)
4G Mobile Core
The 4G Mobile Core, which 3GPP officially refers to as the Evolved Packet Core (EPC), consists of five main components, the first three of which run in the Control Plane (CP) and the second two of which run in the User Plane (UP).
- MME (Mobility Management Entity): Tracks and manages the movement of UEs throughout the RAN. This includes recording when the UE is not active.
- HSS (Home Subscriber Server): A database that contains all subscriber-related information.
- PCRF (Policy & Charging Rules Function): Tracks and manages policy rules and records billing data on subscriber traffic.
- SGW (Serving Gateway): Forwards IP packets to and from the RAN. Anchors the Mobile Core end of the bearer service to a (potentially mobile) UE, and so is involved in handovers from one base station to another.
- PGW (Packet Gateway): Essentially an IP router, connecting the Mobile Core to the external Internet. Supports additional access-related functions, including policy enforcement, traffic shaping, and charging.
Although specified as distinct components, in practice the SGW (RAN-facing) and PGW (Internet-facing) are often combined in a single device, commonly referred to as an S/PGW.
5G Mobile Core
The 5G Mobile Core, which 3GPP calls the NG-Core, adopts a similar concept to microservices. However, even that 3GPP shows this level of disaggregation, it is really just defining a set of functional blocks and not an implementation.
The following organizes the set of functional blocks into three groups. The first group runs in the Control Plane (CP) and has a counterpart in the EPC.
- AMF (Core Access and Mobility Management Function): Responsible for connection and reachability management, mobility management, access authentication and authorization, and location services. Manages the mobility-related aspects of the EPC’s MME.
- SMF (Session Management Function): Manages each UE session, including IP address allocation, selection of associated UP function, control aspects of QoS, and control aspects of UP routing. Roughly corresponds to part of the EPC’s MME and the control-related aspects of the EPC’s PGW.
- PCF (Policy Control Function): Manages the policy rules that other CP functions then enforce. Roughly corresponds to the EPC’s PCRF.
- UDM (Unified Data Management): Manages user identity, including the generation of authentication credentials. Includes part of the functionality in the EPC’s HSS.
- AUSF (Authentication Server Function): Essentially an authentication server. Includes part of the functionality in the EPC’s HSS.
The second group also runs in the Control Plane (CP) but does not have a direct counterpart in the EPC:
- SDSF (Structured Data Storage Network Function): A “helper” service used to store structured data. Could be implemented by an “SQL Database” in a microservices-based system.
- UDSF (Unstructured Data Storage Network Function): A “helper” service used to store unstructured data. Could be implemented by a “Key/Value Store” in a microservices-based system.
- NEF (Network Exposure Function): A means to expose select capabilities to third-party services, including translation between internal and external representations for data. Could be implemented by an “API Server” in a microservices-based system.
- NRF (NF Repository Function): A means to discover available services. Could be implemented by a “Discovery Service” in a microservices-based system.
- NSSF (Network Slicing Selector Function): A means to select a Network Slice to serve a given UE. Network slices are essentially a way to partition network resources in order to differentiate service given to different users. It is a key feature of 5G that we discuss in depth in a later chapter.
The third group includes the one component that runs in the User Plane (UP):
- UPF (User Plane Function): Forwards traffic between RAN and the Internet, corresponding to the S/PGW combination in EPC. In addition to packet forwarding, it is responsible for policy enforcement, lawful intercept, traffic usage reporting, and QoS policing.
The second control plane group of instances is 3GPP’s way of pointing to a cloud native solution as the desired end-state for the Mobile Core. Of particular note, introducing distinct storage services means that all the other services can be stateless, and hence, more readily scalable.
5G 3GPP model adopts an idea that’s common in microservice-based systems, namely, to show a message bus interconnecting all the components rather than including a full set of pairwise connections. This also suggests a well-understood implementation strategy.
Some Nokia Terms
Nokia CMG bring UPF and SMF functions. And CMM is AMF