Why the 5G core converges wireless with wireline

A toolkit lets integrated operators run their mobile and fixed-line operations on a common 5G core.

Prior to 5G, the 3^rd Generation Partnership Project (3GPP) focused entirely on mobile networks. With 5G, 3GPP has laid out specifications for a new and more cost-efficient 5^th generation network core whereby all aspects of the control and user plane can run on either virtualized or cloud-native software. Embodied in this work was a desire to embrace access from networks not specified by 3GPP.

Operators hesitate to operate two separate and distinct network cores: one for mobile users and one for fixed access. Instead, operators can opt to use the 5G core (5GC) as the common core, where the control plane and user plane can span across mobile and fixed networks (including fixed wireless access). The evolution of network infrastructure combined with the architectural features of the 5GC results in a near “turn-key” evolution to virtualization.

The new converged network does away with needless duplication of functions such as authentication, subscriber databases, charging, and network-management tools. It also allows for a common subscriber-management system. This new 5G core is enhanced with procedures specific to wireline access; at the same time the broadband access domain is updated to connect to the new 5G core with interfaces that are similar to the ones used by radio access nodes. Properly done, it should also be relatively straightforward to connect different access types and customer-premises equipment (CPE) upgrades within this environment. In turn, operators can extend 5GC reach with fixed-mobile convergence, where services take advantage of multiple access links. The converged architecture holds out the promise of users being able to get the best Quality of Experience (QoE) possible by leveraging the best of each world, or even both worlds simultaneously. Users can also bundle wireless and wireline bandwidth to create one logical link that provides increased speed and better Quality of Service (QoS) and reliability.

Network transformation of this sort, along with more cost-efficient production of services, holds out the promise of much better returns on 5G network investment. But how do operators go about a smooth transformation to a converged network without disrupting existing services?

Network transformation toolkit
The industry has for some time examined the question of what an optimal fixed-mobile convergence (FMC) architecture should look like. Taking the lead on how to adapt to and integrate non-3GPP wireline access for 5GC like 3GPP, Broadband Forum prefers to talk about wireless-wireline convergence (WWC), rather than FMC because “fixed” and “mobile” define the ways end users consume services, rather than types of network access.

Working in close collaboration with 3GPP, the Broadband Forum has developed a “transformation toolkit, a high-level guide aimed at architects and network planners. The design principles that underpin the toolkit, informed by input from operators that participate in both Broadband Forum and 3GPP, help explain why the FMC schematic looks the way it does (see Figure 1).

For a start, there is acknowledgement that no two operators will have the same 5G starting point. As such, the toolkit supports multiple FMC deployment scenarios, accommodating different types of residential gateway (RG) and interfacing models with 5GC (see “Five ways to deploy FMC”). One scenario uses an interworking function aimed at operators wanting to use legacy equipment and customers who want to upgrade their CPE at their own pace when initially deploying 5GC FMC.

Five ways to deploy FMC Fixed wireless access (5G-RG): The 5G Residential Gateway (RG) is connected over the NG-RAN or LTE RAN. EPC-5GC interworking is used if the RAN is LTE based. Multi-access (5G-RG): The 5G-RG is connected over both the wireline access network (direct mode) and the NG or LTE RANs. The 5G-RG may be connected over one of the accesses at once (active standby) or may use both accesses in ‘steering’ (flow-based load spreading) or ‘splitting’ (per packet-based load spreading). Integration in ‘direct mode’ (5G-RG): The RG is connected over the wireline access network. An Access Gateway Function (AGF) mediates between the wireline access network (aggregated at layer 2) and the 5G core network, based on N2 control plane and N3 user plane interfaces. The 5G-RG is able to register and interact directly with the 5G core network based on the N1 control interface. For this reason, the AGF is said to integrate the session in “direct mode.” Integration in ‘adaptive mode’ (FN-RG): The fixed network RG is connected over the wireline access network, and the AGF mediates layer 2 or layer 2.5 traffic with the 5G core network based on N2 control plane and N3 user plane interfaces. However, FN-RG does not support native 5G signalling , so the AGF acts as an end point of N1 control plane on behalf of the FN-RG. The AGF is said to integrate the session in ‘adaptive mode’. This can be done for PPPoE access, IPoE access, or handed off from a BNG using L2TP. Interworking (FN-RG): The session is managed by a BNG (broadband network gateway). Services that are based on the 5G core network are passed to the 5G core via a Fixed Mobile Interworking Function (FMIF). The FMIF supports N2 control plane and N3 user plane interfaces to the 5G core network. Since the FNRG does not support native 5G signalling, the FMIF acts as an end point of N1 control plane on behalf of the FN-RG. Note: There is a sixth deployment scenario, “co-existence,” but this is not strictly FMC as the sessions here are not part of 5GC. The purpose of co-existence is to make available non-5GC services in a converged service provider network, where subscriber sessions are managed by the BNG or specialized platforms.

Another deployment possibility is hybrid wireless/wireline access. Here, the RG has both a wireline and wireless interface. Having both interfaces lets customers self-install in advance of the deployment of fixed facilities, a model increasingly attractive to operators challenged by the current world situation. It also provides the opportunity for higher bandwidth aggregation and enhanced resiliency, important capabilities with the current evolution to work from home. See “5GC support for the Connected Home.”

5GC support for the connected home 5GC has been extended to support TR-069/369 for CPE management. This is a significant development. TR-069, Broadband Forum’s well-known CPE WAN management protocol, already has over one billion deployments globally.The new TR-369 User Services Platform (USP) standard, designed and built by service providers and vendors within Broadband Forum, is an evolution of TR-069 and allows introduction of next-generation devices and related IP services. These include wireless meshes, smart-home automation, customer self-care and Internet of Things.By mapping TR-069/369 onto 5GC, it permits a significant suite of service capabilities to be easily ported to the converged core. In turn, it strengthens the business case further for FMC transformation. RG data models for TR-069/TR-369 have been updated for 5G-RG with the latest update of TR-181.

Other design aspects of the toolkit include elimination of as many “dependencies” in the network transformation process as possible. This is in the form of eliminating any tight coupling between the actions of any two actors in the evolution process such as between the operator and the subscriber. FMC must be compatible with existing fixed-access nodes and even legacy CPEs. On the other hand, we expect fixed access elements to continue to evolve (more speed, access disaggregation, distribution, etc.) and that cannot require a 5G core change every time an operator modifies a fixed-access element. Any step taken towards FMC transformation — populating the United Data Management (UDM) with line-ID based identifiers for subscribers to be transitioned, for example, or deploying the preferred interworking solution — won’t require extensive effort.

Broadband Forum emphasizes that its proposed extensions to 5GC specifications laid out by 3GPP — to accommodate wireline access — have been kept to a minimum. This is viewed as a truly transformative embrace of the 5G architecture. The network transformation toolkit can ease both implementation and reduce operational complexity. The goal is a single operational support system (OSS) and business support system (BSS), which can apply the same network policies — regardless of access bearer — on traffic management and quality of service (QoS).

3GPP Coordination
Broadband Forum output on 5GC architecture and common interfaces parallels with 3GPP release cycles. The collaborative effort of the two organizations has already produced firm results.

3GPP Release 16 (R16), completed in July 2020, incorporates specification work related to WWC. A partnership set up by Broadband Forum and 3GPP at the urging of network operators — and which also works with CableLabs — 5G WWC made two recommendations that are now aligned with 3GPP Release 16: TR-470 Wireless Wireline Convergence Architecture for 5G and TR 456 Wireline Access Gateway Function Requirements. Both feed into the Forum’s network transformation toolkit.

The Wireline Access Gateway Function (AGF) brings wireline and mobile broadband services together under the same OSS umbrella. As Figure 2 shows, the AGF sits between wireline access nodes and the 5G packet core. As far as the wireline access is concerned, the AGF can be deployed in an AN or corresponding to where Broadband Network Gateway (BNG) is situated in the network. From the 5GC’s point of view, the AGF appears like a 5G base station (gNodeB).

With an AGF in place that can support a current generation Fixed Network-Residential Gateway (FN-RG) CPE as well as a 5G-Residential Gateway (5G-RG) operators can use the converged 5GC to deliver broadband services over copper or fiber. This will require a new generation of wireline CPE (known as the 5G-RG) that can natively support 5G signalling protocols, capabilities, and integrates the 5G QoS model to the home. See “5GC support for the Connected Home.” They can be wireline or both wireline and wireless, in the case of hybrid access. A 5G-RG is an RG enhanced with 5G functionalities, similar to a 5G terminal/handset. This includes support for mobile signalling (“Non-access stratum (NAS)”), session multiplexing (“PDU sessions”) and 5G QoS. The Broadband Forum has defined the specifications for the 5G-RG (TR-124 updates) and discussions are ongoing to kick off an open source initiative related to 5G-RG.

Market movement is nonetheless starting to happen in the AGF space. Earlier this year Metaswitch (now owned by Microsoft) undertook testing of its 5G WWC compliant AGF — in conjunction with its cloud native “5G Fusion Core” offering at Vodafone’s labs in the UK.

“We have been very active in supporting the WWC activity in the Broadband Forum,” remarked Gavin Young, Head of Fixed Access Centre of Excellence at Vodafone. He added that the trial “more than justifies our faith in the value and the practicality of broadband convergence.”

Transport infrastructure
The converged network is arguably not solely defined by 5GC. Underlying transport networks, essentially the plumbing beneath 3GPP 5G New Radio (NR), need careful design if they are to ensure that legacy fronthaul and backhaul infrastructure can deliver on various 5G uses cases without compromising 5G NR performance including when 5G NR is based on Open RAN architecture.

Delivering enhanced mobile broadband is a basic 5G requirement, but operators are looking for more. They are seeking ultra-reliable low latency and network slicing, for example, where there is separation of services in the mobile network. This necessarily leads to FMC in the transport network, as operators, if they are to keep TCO under control, will want to leverage existing fixed-line capabilities – VPNs and VLANs in the case of network slicing – to support 5G performance requirements.

With these challenges in mind, Broadband Forum has developed a new 5G Transport Architecture and Requirements specification, which introduces the use of new transport and routing technologies. It then applies them to the 5G ‘split’ cloud RAN architecture, where baseband unit functionality is split into two functional units: a centralised unit and a distributed unit.

Automated coordination between radio-access networks (RANs), mobile core networks, and the underlying transport network lie at the heart of the Broadband Forum’s proposed 5G architecture.

5G promises to transform network operators from connectivity providers to platform providers. FMC shifts the paradigm from access-defined to service-defined networking. Operators and technology providers have completed the first step on this journey by jointly developing the 5G convergence standards. We now expect to see deployments in the leading tier-1 operators.

David Allan, Ericsson and Work Area Director for Wireless-Wireline Convergence at Broadband Forum.

Greg Dalle, Juniper Networks and 5G Project Stream Lead at Broadband Forum.