Software-Defined Networking (SDN) is a new software-based network architecture that can provide significant benefits to 5G networks. SDN technology utilizes software-defined centralized network management and dynamic network configuration programming to give scalability and flexibility for 5G network provisioning and end-to-end network performance and monitoring.
SDN architecture is structured on three primary layers, the Data plane, which consists of the network elements, the Control plane which acts as the central Controller and the Application plane which consists of the different applications.
The basic SDN principle is based on the separation of the data plane from the control plane and the logical centralization of all the control functions.
SDN architecture
The SDN controller can provide a more centralized approach and an end-to-end view on the 5G network chain. The main concept is that each application communicates with the network directly via an interface to the central Controller without the need to communicate with each network element.
The Controller can monitor the network centrally, propose changes in the requirements, and implement any reconfiguration needed. In other words, it may act as a central control instance between the application and the network. For example, the SDN controller may suggest the optimum route across multiple domains by calculating the path with the shortest delay based on latency measurements on the network in case of URLLC critical services or by identifying the path with the sufficient capacity in case of eMBB services.
SDN can provide a programmable approach, where the features and functions of the network are not limited to the capabilities of the network elements but can be complemented by new processes developed on demand and per request by the network operator. This means that the Controller can implement via centralized decisions, the appropriate changes that result in an efficient automated manner and optimum 5G network utilization.
SDN Interfaces
The interface between the control plane and the data plane is known as the Southbound Interface (SBI). The aim of SBI is to allow implementation for standard protocols such as OpenFlow or Netconf so that the Controller may interact with the NEs. The SDN controller must interact with network elements in a multivendor environment seamlessly. That is why open standard protocols are preferred.
The interface between the application plane and control plane is known as the Northbound Interface (NBI). NBIs main aim is to allow the establishment of Application Programming Interfaces (APIs) so that applications can be developed independently from the physical network.
The APIs are configured in a specific programming language which is usually the object-oriented Python. Vendors either publish the Python commands for the operation of the APIs and typically offer a Python console which can be activated by the router or provide Python scripts that can be stored in the memory and used when required.
Typically, the user would like to have remote access to the APIs. For this reason, the Representational State Transfer (REST API) is used to access an API remotely via HTTP.
Northbound & Southbound protocols
In the Northbound direction, the usual practice is to communicate via REST protocol, even though in some cases programming languages such as Java may also be used.
For Southbound communication between the Controller and the elements, there are practically many options available, from an SSH connection with CLI commands to open standard protocols such as NETCONF, BGP, or OpFlex & OpenFlow.
NETCONF protocol is used to access the CLI of a router or switch remotely. It is used to centralize the configuration management and to synchronize the local configuration of a switch/router with a central configuration database on the server. The CLI commands are usually in standard encoding formats, such as XML and YANG. YANG is a data modeling language developed based on the IETF Standard RFC6020. YANG was designed to develop data models mainly for NetConf but also on other models.
OpenFlow protocol is used by OpenStack, a joint project of companies, software developers, and cloud specialists to develop a platform for setting up public and private clouds. The OpenFlow protocol is based on several data structures such as the flow and group tables and the OpenFlow channel. It is used to route the traffic over the tunnel connection and learn the MAC addresses of the elements.
Another Southbound interface is OpFlex, developed by Cisco, which is also used to provide communication between the SDN controller and the elements. However, it is mainly designed to centralize not only the control functions like OpenFlow but the actual policies, by offloading some of the controller functions towards the devices and providing greater flexibility and resiliency.
SDN Standardization concepts
SDN standardization principles are currently being worked on not only via the typical SDOs such as IT, IEEE, ETSI, etc. but by specific consortiums comprised by members of the industry, such as MNOs, telecom vendors and software programming companies. Software-defined networking principle is to focus on Open Source software mainly driven by the Open Source Initiative (OSI).
Open Networking Foundation (ONF) is working towards the evolution of SDN technology by performing various PoCs and has also developed the OpenFlow standard and the SDN Controller ONOS. The Linux Foundation is also working towards harmonizing the OpenSource networking under the label of Linux Foundation Networking (LFN). LNF has been working on the OpenDaylight platform focusing on an SDN controller, where the user it is possible to develop new and different components.
SDN applications
SDN can give a competitive edge to 5G network operators by combining NFV and the cloud, to support a flexible and scalable 5G network. With SDN and NFV, it is possible to use virtual networks whose topology is independent of the hardware network on which they are based. In an NFV system, the Virtual Network Functions (VNFs) are software-based functions which take over the role of the network element and are controlled by an element manager network system.
Together with the use of Network Slicing and MPLS Segment routing, the 5G Network Operator will be able to route 5G services in an automated and scalable way, providing high Quality of Experience to the 5G end user and a flexible and cost-effective end-to-end software-configurable 5G network.
