5G Architecture

In the Evolution of Mobile technology , we discussed how 5G is being developed keeping in mind the various use cases. That is one of the reasons you will see why industry verticals like Automotive, Public Safety, Media Delivery, Rail Industry and many more are actively participating and influencing 3gpp and ETSI on the 5G standards. These industry verticals have been broadly classified by ITU under three 5G network services, namely, eMBB, URLLC, and mMTC. There have been many feasibility studies around these major 5G technology drivers and they have been captured under 3gpp technical requirements documents (TR) which formed the basis of the newly proposed 5G architecture.

 

While we touch upon the 5G architecture in this post , we will approach the architecture from a use case perspective and compare it to the 4G model in the upcoming blogs.

 

5G Core Architecture

 

In the monolithic 4G architecture, there was a single piece of network designed primarily for the smart phones. Towards the later part of the generation, new use cases like NBIoT, eMTC and not to forget the ever growing demand of speed due to proliferation of smartphones, started making waves. There was excitement around these varied nature of use cases but the 4G network could not provide anything more than a dumb pipe. There was no service or performance differentiation based on the type of use cases. Moreover, most of the 3gpp interfaces had different protocols running between them taking significant development cycles from the vendors release after release.

 

Most of the initial deployments were on custom hardware from the vendors. However with the increased computational power and efficient capacity utilization delivered by x86 servers, Network Function Virtualization (NFV) along with Software Defined Networking (SDN) gained a lot of momentum in the service provider community in 4G deployments. The year 2017 saw a lot of NFV based proof-of-concepts and trials move into the real world but the adoption has been on the slower side. The service providers who did make some progress on this front not only saw this as a financial benefit but also as a stepping stone to the 5G.   

 

5G is happening and there is a lot of interest and expectations around it. At the center of all the deliverables, was the network architecture and we must say that the 3gpp has done a good job so far.

 

On a high level, the main concepts introduced in 5G include :

 

  • Control & User Plane Separation (CUPS)
  • Network Slicing
  • Cloud Native and Microservices style architecture
  • Service Based Architecture (SBA)

 

On the core side, 5G is conceptualised as one single architecture but comes in two different flavors. From the figure below, you can see the two approaches, one is a Point-to-Point architecture and the other is a Service Based Architecture (SBA).

 

                                                  Fig 1: Point-to-Point Reference Architecture

 

                                                   Fig 2: Service Based Architecture (SBA)

 

The point-to-point architecture looks pretty much like the earlier generation models where discrete interfaces are defined between each of the nodes and mostly running different protocols as well. This model has its flaws from being closed and not being programmable. With 5G, service providers are looking for operational agility and programmability in the cloud environment especially because of the varied nature of use cases waiting for them. However, all the initial 5G deployments are expected to follow this architecture.

 

Cloud Native Architecture solves lot of these problems. It is far from just moving the workload into the cloud as was in the case of 4G based NFV deployments. The application itself will be developed by keeping the cloud characteristics in mind. It follows a microservices style of architecture where each self-contained service is created and maintained independently utilising the container technologies like Docker which is easy to build, deploy and update. Each service will expose its capabilities and the others can interface with it using the APIs.  

 

Automation services like Auto Scaling, Auto Healing, Auto Configuration and others which are still being explored in the 4G deployments today, will become an integral part of the 5G architecture. Upgradation procedures of the VNFs is a challenge today. With a container based architecture, the traditional upgrade procedures vanish and it will be all about deleting the old version of the instance and bringing up the new one. Thanks to the CI/CD techniques and minimal start up times of the containers, each module/service can be upgraded independently. The upgrades should almost give us a zero down time now.

 

Looking at all the advantages of microservices, 3gpp has decided to go with a Service Based Architecture (SBA) for 5G as shown in Figure 2. All the Control Plane Network Functions are expected to be connected to a Message Bus and will expose their functionality over service based interfaces. The User Plane is expected to continue using GTPv1 with modifications. In this new model, it is expected that all the traditional 4G core control plane protocols like GTPv2, Diameter will make way for HTTP2.0 as a protocol on the service bus. Each Network Function Service will expose a set of REST APIs and the primitive model of request/response and subscribe/notify is set to be used for all interactions between the different services. QUIC, a protocol from Google, is being explored as the transport layer protocol replacing the TCP/TLS. A representation of how this SBA model is expected to work is shown below.

                                                                 Fig 3: Service Based Interaction

 

The major components of 5G core and high level description is mentioned below:

 

  • Access and Mobility Management Function (AMF) – Like the MME, it takes care of access control and mobility. UE registration happens via AMF. It interfaces with AUSF for authentication, UDM for subscription information, talks to NSSF to get slice information and SMF for PDU establishment.  The MM information from the UE is processed by AMF and the SM information is transparently passed to the SMF.
  • Session Management Function (SMF) – Plays a role similar to S/PGW control plane elements in 4G. Takes care of Session Establishment and maintains the tunnel between UPF and Access Node. It manages the UPF over a Sx like interface. It also interfaces with PCF for network wide policies.
  • User Plane Function (UPF) – Its a combination of SGW and PGW user planes in 4G. It interacts with the Access Node over a GTPU interface. It does QoS handling and Policy enforcement on the User plane part. They can be deployed anywhere in the network in various configurations.
  • Policy Control Function (PCF) – From the same family as PCRF in 4G, it takes care of network policies to manage network behaviour. It gets the subscription information from the UDM. It interfaces to both AMF to manage the mobility context and the SMF to manage the session contexts. It also plays a crucial role in providing a schema for network slicing and roaming. PCF triggers the UE route selection policy (URSP) which enables the UE to determine how a certain application should be handled in the context of an existing or new  PDU session. This can be compared to the TFT or flow descriptors in the PCRF response. It also plays an important role in setting the Traffic steering policy for steering the subscriber traffic to appropriate service functions. PCF can get slice specific analytics from NWDAF , an analytics service which provides network wide analytics.
  • Unified Data Management (UDM) – Like the HSS in 4G, it stores the subscriber data. It primarily interfaces with the AMF.
  • Authentication Server Function (AUSF) – An authentication server which will also handle the EAP requests from the non-3gpp world.

 

The Service Based Architecture (SBA) is modeled around microservices and REST APIs. One of the properties of microservices is statelessness. But any mobile network is characterized around user data/profiles which needs to be stateful. To accommodate the above requirements,a set of new nodes have been proposed as part of the 5G architecture.

 

  • NF Repository Function (NRF) – The SBA architecture has all the nodes running as services and each of them exposing their own functionalities and there could be many such nodes in the network serving different set of features. The service discovery plays a key role here. NRF acts like DNS helping in service discovery. Every service registers itself with NRF as it comes up. The SBA interaction in the above Figure will explain it more.
  • Network Slice Selection Function (NSSF) – Different use cases like IoT, eMBB, URLLC will be served by different set of nodes in the network based on the service and performance characteristics. AMF queries the NSSF based on the Slice details it gets from the UE during the PDU establishment.
  • Network Exposure Function (NEF) – An API gateway to the mobile network like the SCEF in 4G. Third party applications can use the APIs to get information like network events, statistics for analytics, provisioning capability, policies and charging services exposed by the mobile network. NEF also interfaces with UDR to store and retrieve information.
  • Unified Data Repository (UDR) – A central repository like UDR will help in storage and retrieval of subscription data, policy data and application data. NEF can reach out to UDR for specific information.

 

The 5G innovation is built around higher throughput, low latency, security and programmability of the network. These requirements are driven by the various industry verticals and their use cases and the Service Based Architecture (SBA) is expected to provide the framework for this. In the upcoming blog, we will see these requirements in the context of various use cases and how 5G provides a solution to these.

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