Disaggregating the Broadband Network Gateway

The Broadband Network Gateway or BNG is an essential network element that controls subscriber access to a range of services and applications, including the internet. It’s role and requirements were introduced in Broadband Forum specification TR-101 and encompasses subscriber authentication, IP address allocation, bandwidth management and accounting [1]. Subsequent specifications expanded its scope and functionality from aggregating copper and fiber-based wireline access to include fixed wireless access and enable hybrid access [2], making the BNG an increasingly capable, versatile and strategic platform for broadband providers.

As the industry enters the digital era of 5G and the cloud, the Disaggregated Broadband Network Gateway (DBNG) marks the next major milestone in BNG evolution. The new BNG architecture is being specified by the Broadband Forum and separates control plane and user plane functions by means of open and standardized interworking protocols. This will drastically simplify BNG deployment and operation for multi-access broadband aggregation, while enabling the evolution to 5G fixed-mobile convergence. The Broadband Forum issued MR-459 [3] to inform the public about the progress and DBNG standardization work thus far in WT-459.

Nokia is a key contributor and supporter of the disaggregated BNG architecture standard, and this blog reviews the rationale and key objectives that drove its inception.

Why disaggregate the BNG?
Disaggregation is an important architecture concept that has already been applied in several domains to decouple functions to provide greater flexibility and scalability. The 5G Core architecture defined by the 3GPP working group is a prime example of disaggregating network functions. This functional separation allows to virtualize appropriate network functions and apply cloud and web scale technologies and architectures to enable the range of services and applications that users desire, whether they be consumer, residential or enterprise/industrial.  

Control User Plane Separation or CUPS is another great example of functional disaggregation for building a more agile, scalable and cost-efficient network. Control and user plane functions typically have different performance and scaling requirements: User plane functions scale is often driven by the bandwidth required per subscriber, while control plane functions scale as a factor of connected user devices and subscriber sessions and their compute processing requirements.

However, if both control and user plane functions are collocated on the same physical piece of equipment it is impossible to optimally address these individual needs. As a result, either user plane or control plane resources are often over- or underutilized, limiting scale or adding cost. For example, in large networks with high subscriber densities, it is ideal to locate BNG systems closer to the subscribers to reduce backhaul costs and latency, especially when content storage 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.

Figure 1 Benefits of a disaggregated BNG with control and user plane separation

BNG with control
A disaggregated BNG with CUPS addresses this issue by separating the subscriber management control plane and user plane. This simplifies operations and enables independent location, scaling and life cycle management for Control Plane Functions (CPFs) and User Plane Functions (UPFs). A single “cloud-native” control plane that can scale out as a needed in a centralized datacenter can then control and manage multiple user plane instances, that may be distributed in locations closer to subscribers to optimize throughput, latency and delivery cost. Cloud native control plane can further separate subscriber state and control processing to allow for flexible and more cost-effective redundancy models (e.g., N:K). A centralized control plane can provide more optimized, operationally simpler and dynamic IP address space (pool) management across a set of distributed user plane instances. The centralized control plane can also dynamically select the user-plane instance for hosting the subscriber session based on load, resources, supported services, and location of the user-plane instance. Control and user plane functions can be deployed on either physical or virtualized platforms to optimally scale capacity and minimize the impact of hardware and software upgrades (Figure 1).

Decoupling control plane functions also gives network operators more freedom to customize solutions and combine products from multiple vendors. Historically both operators and equipment vendors made many proprietary 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 such proprietary control plane extensions more easily in a cloud IT environment, while isolating user plane functions from these changes.

Multi-access edge and 5G core evolution
A disaggregated BNG offers substantial operational optimization and simplification compared to existing broadband deployments and at the same time unlocks new opportunities for broadband service innovation. By operating a heterogeneous user plane for wireline and wireless access under a common control plane, the DBNG can support the evolution to a multi-access edge and a universal 5G core with equally effective delivery for all service options: wireline only, wireless only or converged (hybrid access). 

Figure 2 High level disaggregated BNG architecture

BNG architecture
The Broadband Forum DBNG approach (Figure 2) accomplishes this goal by leveraging the same Packet Forwarding Control Protocol (PFCP) that the 3GPP working group defined for control user plane separation on mobile gateways in Technical Specification 29.244 v16.0.0 to control subscriber session state [3]. PFCP is complemented by a Control Packet Redirect Interface to enable the User Plane to forward RADIUS and DHCP control plane messages from the User Equipment to the Disaggregated BNG Control Plane (DBNG-CP), and a YANG based model-driven Management Interface to centrally manage the Disaggregated BNG User Plane (DBNG-UP). 

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 multi-vendor interoperability between disaggregated control and user plane functions, the chosen strategy offers a seamless path to fixed-mobile convergence for communication service providers and enables them to easily leverage their existing wireline broadband assets for delivering their 5G service experiences. 

A disaggregated BNG enables service providers to optimize service delivery cost and coverage by operating a common control plane across physical and/or virtualized user plane instances and  manage both existing wireline and new 5G wireless access aggregation. 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 consistent and blazing fast service experience anywhere.

There is still work left to do to standardize the DBNG state control interface, PFCP information exchange and information element (IE) extensions for fixed wireline access, but the direction is clear, and the strategy is sound. Nokia is fully committed to support the disaggregated BNG architecture with standard CUPS interfaces [5] and is actively collaborating with its industry partners to ensure multi-vendor interoperability