SDN Based Wireless Networks – A Review

As the virtualization technologies (related to compute and networking resources) are gaining wider acceptance, the telco operators are becoming even more open to adopt architecture based on SDN & NFV. While these technologies have a direct impact on the network architecture, they also explode the network efficiency. Initially the focus of SDN market was more towards the carrier networks and data centers, providing them enhanced agility and responsiveness to meet the requirements of cloud computing. Value-addition in automating various processes and simplifying the data center architecture inspired people to extend the SDN concept to other areas as well.

How SDN fits in the Wireless World?

Majority of the end-user demand today is from the mobile devices using WiFi for connecting to the network. SDN provides scalable network control functions, flexibly adjusts the resource allocations and handles the changing traffic patterns of the wireless networks.

One of the prime strategies of SDN is to centralize the network intelligence ñ providing comprehensive visibility of a network. This can be used for automated dynamic routing to decide the optimal paths. The control path is the centralized decision making entity. The data path which is responsible for the actual forwarding of data packets is still distributed throughout the network.

Recently a lot of research is being oriented towards the usage of SDN concepts in 5G. SDN is expected to ensure that packet flows will be intelligently optimized to handle the real-time traffic demands of the 5G networks. In a mobile environment where end users constantly change their locations and their bandwidth demands are varied, path management based on individual service is very important. Apart from this, SDN enabled path/flow management can further assist the mobile operators in the multi-tenancy cases by maintaining traffic isolations.

Another applicability of SDN in wireless networks is towards orchestration and management of these networks. Such centralized control leads to more efficient resource management and better utilization of scarce spectrum.

Industry Trends

The year 2015 was more about virtualization wherein the industry showed significant progress in the area of NFV. Even SDN started picking up fast with the network operators moving towards the open source controllers such as the Open Network Operating System (ONOS), OpenDaylight.

WLAN vendors are moving towards acquiring the OpenFlow conformance certification for their WiFi equipments, from ONF . This certification will give customers the confidence that the vendorís product will work in a SDN enabled network as per the specific OpenFlow specification. ONF also has a mobile Wireless study group which is defining the use cases for the mobile operators and vendors.

The benefits of SDN have been realized throughout the wireless network from access to backhaul to the core.The Next Generation Mobile Networks (NGMN) Alliance recently defined the use cases and business requirements for 5G mobile networking. The 5G architecture is hugely based on a native SDN/NFV architecture ñ use cases ranging from management functions for orchestration of 5G systems to network slicing (via NBI interface in SDN).IEEE 802.1 recently started working on 802.1CF (network reference model) ñ the focus is to define interfaces with the SDN. For industrial usage, the interaction between 802.1 CF and other SDN standards is very crucial and is worked on between the OmniRAN Task Group and ONF, IETF. The focus is to come up with a generic model of IEEE 802 access network- this will be a heterogeneous network including multiple network interfaces, multiple access technologies to enable shared network control and active use of SDN principles in these networks.

Fitting SDN Controllers in a Network

The context of the SDN controller in a network is shown below. SDN architecture requires both northbound and SouthBound interfaces for facilitating interactions between networking devices, SDN controllers and SDN applications.

SouthBound Interfaces in SDN enabled Networks

On the SouthBound interface, OpenFlow standard is the most widely used standard. This requires an OpenFlow agent on the network element (viz. AP), capable of receiving traffic handling settings from the controller. Apart from OpenFlow, other options that available are BGP, NETCONF XMPP etc. Based on the service level information for all flows, the centralized controller can apply the traffic engineering principles to ensure that each flow is serviced as per its SLA. Additionally, centralized routing allows a pre-computation of fail-over paths to handle a possible link failure or total failure of the network element.

Opendaylight provides multiple SouthBound plugins which are frequently used in wireless. One of these is CAPWAP plugin which can be used for managing CAPWAP compliant WTP devices. Another is SNMP plugin which is used to manage and control the devices running SNMP agents. We can realize NMS application running over a SDN controller to provide fault detection and correction, bulk operations on the managed elements etc. The ODL notifications in this case can be correlated to analyse the faults and accordingly suggest corrections via the SDN controller.

NorthBound Interfaces in SDN Enabled Networks

On the NorthBound interface, no formal standard exists between the controller and the applications or services using it. The year 2016 has seen the industry inclining towards the NorthBound interface. This interface is used to provide programmability of the network ñ to provide an open API into the network element thus replacing the manual proprietary interface. This can help in automating configuration and policy management of these network elements and also enable the network to handle application requirements more dynamically. The controller basically exposes open NorthBound APIs which can be used by any proprietary application to manage the network. The commonly used SDN controller, ODL, supports bi-directional REST APIs in the NorthBound. RESTCONF is another protocol which works on the NorthBound interface to get/edit data, calls RPC and handles notifications.

HSC's expertise in SDN

HSC has been working with wireless OEMs for the last few years, to develop SDN applications on Centralized Network Controller which controls the topology / scheduling / routing decisions in a backhaul network.

HSCís SDN experience includes active work on the SouthBound interface, where focus has been on OpenFlow ñ the objective is to enhance Opendaylight controller code with proprietary centralized path selection based on the topology information of the network and the stats reported by the managed devices, and then use OpenFlow to communicate the flows to these devices.

HSC is also working on RESTCONF based NorthBound interface (using YANG based modelling for RPCs), which is used by a NorthBound application for SLA enforcement.