Software Defined Networking

Description

Software Defined Networking (SDN) is an architectural framework that fundamentally separates the network's control logic (the control plane) from the underlying routers and switches that forward traffic (the data plane). In a traditional network, each device runs proprietary software that integrates both control and forwarding functions. SDN extracts the control plane from these distributed devices and consolidates it into a logically centralized software-based controller. This controller maintains a global view of the network and dictates the forwarding behavior of the data plane devices via a standardized southbound interface, most commonly the OpenFlow protocol.

In the 3GPP ecosystem, SDN principles are applied to create more flexible, programmable, and efficient mobile networks. The architecture typically consists of three layers: The Application Layer (where network apps for orchestration, slicing, or policy reside), the Control Layer (the SDN Controller), and the Infrastructure Layer (the data plane network elements). The SDN Controller communicates northbound with applications via RESTful APIs, allowing applications to program the network based on high-level policies. It communicates southbound with data plane elements (e.g., switches, routers, or in a mobile context, transport network nodes and potentially RAN/Core Network functions) to install flow rules. These rules specify how traffic matching certain criteria (like source IP, destination IP, protocol) should be treated—forwarded, dropped, or modified.

For 5G, SDN is a cornerstone technology that works in tandem with Network Function Virtualization (NFV). While NFV virtualizes network functions (like AMF, SMF), SDN provides the programmable connectivity between these virtualized functions. This combination enables key 5G capabilities like network slicing, where multiple logical, end-to-end networks with different characteristics are created on shared physical infrastructure. The SDN controller can dynamically establish and adjust the virtual links and bandwidth for each slice. Furthermore, SDN enables service chaining—steering user plane traffic through a specific sequence of VNFs (e.g., firewall, optimizer). This programmability allows operators to automate network provisioning, optimize traffic flows in real-time, and rapidly introduce new services.

Purpose & Motivation

The purpose of adopting SDN in mobile networks is to overcome the limitations of traditional, hardware-centric, and vertically integrated network architectures. Legacy networks are often rigid, complex to manage, and slow to adapt to new service requirements due to manual configuration of distributed devices. SDN addresses these issues by introducing centralization, abstraction, and programmability. It solves the problem of operational complexity and high costs associated with managing thousands of network devices individually.

Historically, mobile network evolution (from 2G to 4G) led to increased complexity in the transport and core networks. The advent of 5G, with its diverse requirements for Enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), and Massive IoT (mIoT), demanded unprecedented network agility. SDN was identified as a key enabler to meet these demands. It allows operators to dynamically allocate resources, optimize traffic paths for low latency, and create isolated virtual networks (slices) tailored for specific services—all through software, without touching physical hardware.

Moreover, SDN facilitates innovation and reduces vendor lock-in. By standardizing the interfaces between the control and data planes, operators can mix hardware from different vendors and write their own control applications. This openness, combined with NFV, transforms the network into a flexible platform where new services can be deployed rapidly, scaling up or down elastically based on demand. Thus, SDN's primary purpose is to make the network a programmable entity that can be easily adapted to support the business and technical goals of 5G and beyond.

Key Features

• Decoupling of control plane (software-based controller) from data plane (forwarding elements)
• Logically centralized network intelligence and global topology view
• Programmability of the network through open APIs (Northbound and Southbound)
• Dynamic, flow-based traffic management and steering
• Foundation for automated network provisioning and orchestration
• Essential enabler for end-to-end network slicing and service chaining

Evolution Across Releases

Defining Specifications