Disaggregating the Broadband Network Gateway with Control User Plane Separation
Network operators around the world are making big investments in fiber upgrades to expand broadband coverage and capacity. To capitalize on faster access and avoid bandwidth constraints upstream, they must also architect the Broadband Network Gateways (BNG) at the edge of the network for bigger scale.
The BNG is an essential network element that controls subscriber access to broadband content and applications on the internet, in private clouds and content delivery networks. It’s role and requirements are defined in Broadband Forum specification TR-101 and include subscriber authentication, IP address allocation, bandwidth management and usage accounting . Subsequent specifications expanded its scope to include both wireline and fixed-wireless access aggregation, making the BNG an increasingly capable, versatile and strategic platform for broadband service providers.
As the industry enters the digital era of 5G and cloud-based ICT, the Disaggregated Broadband Network Gateway (DBNG) marks the next major milestone in BNG evolution. The DBNG architecture is specified in Broadband Forum TR-459  and separates control plane and user plane functions by means of open and standardized interworking protocols. This approach can simplify and converge BNG deployment for wireline and 4G/5G fixed-wireless access.
Nokia led the specification and implementation of the BNG CUPS architecture standard. This blog discusses the rationale and key objectives that drove its inception and implementation.
Why disaggregate BNG control and user plane functions?
Traditionally, BNGs are deployed as integrated systems with both user and control plane functions residing on a routing appliance. While this deployment model scales for many deployments, hosting control and user plane functions on the same physical equipment does not always address their individual scaling needs optimally. User plane functions primarily scale with session bandwidth requirements, while control plane functions are more compute and storage intensive and scale with the number of connected user devices and active sessions.
When co-located, either user plane or control plane resources are often over- or underutilized, which may limit scale or add cost. Centralized BNGs are easier to manage, but in large networks with high subscriber densities it is better to locate BNG systems closer to the subscribers to reduce backhaul costs and latency, especially when content caches and compute resources are distributed as well. But as the BNG footprint grows, the subscriber management control points also proliferate, increasing operational complexity and cost.
A disaggregated BNG with control and user plane separation (CUPS) solves this dilemma. CUPS allows for independent location, scaling and life cycle management of Control Plane Functions (CPFs) and User Plane Functions (UPFs). CPFs can be virtualized and scale on compute and storage servers in a datacenter for best operational efficiency, while UPFs can be distributed in locations closer to subscribers for optimal performance (Figure 1).
Figure 1 Benefits of a Distributed BNG with CUPS
Separating and centralizing subscriber state and control processing also allows for flexible and more cost-effective redundancy models (e.g., N:M) and more optimized, operationally simpler and dynamic IP address space (pool) management. CUPS also simplifies maintenance because control plane functions (software lifecycle) and user plane functions (hardware dependent lifecycle) typically evolve at different speeds and can then be upgraded independently of each other.
Decoupling and virtualizing control plane functions also gives network operators more freedom to customize and automate broadband service delivery. Historically both operators and equipment vendors made many deployment-specific extensions to BNG control plane functions through vendor-specific RADIUS and DHCP attributes and custom developed code that ultimately added significant cost and inertia to change. A disaggregated BNG combined with a standardized CUPS interface allows to develop control plane extensions more easily in a cloud-native IT environment, while isolating user plane functions from these changes.
Evolving to a multi-access edge with 5G core interworking
Besides more flexibility to scale and optimize wireline broadband deployments, a disaggregated BNG with CUPS also facilitates the evolution to a multi-access edge with converged wireline and fixed-wireless access convergence under a common 5G service-based architecture.
The Broadband Forum TR-459 specification accomplishes this goal by leveraging the same Packet Forwarding Control Protocol (PFCP) that the 3GPP working group defined for control and user pane separation on mobile gateways to control subscriber session state on the DBNG . PFCP is complemented by a Control Packet Redirect Interface to enable the disaggregated User Plane to forward subscriber signaling (e.g., DHCP and PPPoE) to the BNG Control Plane. Leveraging a proven protocol specification such as PFCP significantly reduces the cost and time to market required to develop, test and integrate product solutions.
Besides assuring interoperability between BNG control and user plane functions, the chosen strategy offers a clear evolution path for converged operators to seamlessly expand broadband services over 5G FWA.
Figure 2 Nokia Multi-Access Gateway with CUPS
The Nokia Multi-Access Gateway product solution  implements the TR-459 CUPS recommendations for disaggregated BNGs and Fixed-Wireless Access (FWA). The Multi-Access Gateway controller (MAG-c) implements the virtualized control plane functions and can be hosted in regional data centers, while user plane functions are located on physical (7750 SR) or virtualized service routers at the broadband edge (Figure 2). A demonstration of its capabilities is available at .
The standards are in place and the solutions are available. In May this year, the Broadband Forum published the second issue of the TR-459 BNG CUPS specification with further improvements in resiliency, scalability and deployment times. The Nokia Multi-Access Gateway implements the disaggregated BNG architecture with CUPS and we are already seeing the first large scale commercial deployments.
A disaggregated BNG with Control User Plane Separation enables service providers to optimize service delivery cost and coverage by operating a common control plane across physical and/or virtualized user plane instances for wireline and 5G fixed-wireless access.
Users will have the freedom to connect to their desired services and applications using any broadband access technology available in their area to enjoy a fast and seamless broadband experience anywhere.
References and further reading
 Migration to Ethernet-Based Broadband Aggregation. Broadband Forum TR-101.
 Disaggregated BNG. Broadband Forum TR-459.
 Interface between the Control Plane and the User Plane Nodes. 3GPP TS 29.244.
 Broadband Network Gateway evolution with Control and User Plane Separation. Nokia Application Note.
 Demonstration of Nokia Multi-Access Gateway with CUPS for wireline and fixed-wireless access in 5G SA mode. Add url.
For more information about our broadband edge solutions, please visit https://www.nokia.com/networks/broadband-edge.
This blog was originally published on October 21, 2019, and last updated on August 22, 2023.