Realizing software-defined wireless networks (2016)

Fueled by the increasing popularity of wireless enabled mobile end-devices and the advent of the Internet-of-Things networks, the demand for wireless access technology is growing rapidly. Today, many hotels and cafés—and sometimes also entire cities—already offer free WiFi services. Moreover, several mobile operators operate massive WiFi HotSpot, as well as HotSpot 2.0, deployments for traffic offloading, i.e., reducing the stress on their cellular networks. Specifically, WiFi (also known as IEEE 802.11 or WLAN) stands as an attractive inexpensive wireless alternative to cellular access technologies due to its simplistic MAC layer design, reduced costs due to its prevalence in consumer off-the-shelf devices, and operation in the unlicensed spectrum. Supporting the ever increasing number of WiFi capable devices across residential, public, and enterprise networks is non-trivial. In particular, the (last) wireless hop is often critical for network performance, as it can contribute a non-negligible delay and may constitute a bandwidth bottleneck. Thus, future WiFi architectures are challenged by optimized medium utilization, mobility support, and network management. The latter challenge is integrating wired, cellular, and WiFi networks seamlessly. While today point solutions exist for some of the WiFi-specific network challenges, commodity off-the-shelf hardware is outside the purview of such ossified, expensive, and vertically integrated solutions. Software-Defined Networking (SDN) is a new paradigm which introduces programmability to overcome network ossification. The key idea of SDN is to decouple the control and data plane to consolidate and outsource the control over a set of network devices to a logically centralized software controller. This allows the control plane to evolve independently of the data plane, enabling faster innovations. SDN is also an enabler for a second paradigm shift in the Internet: Network Functions Virtualization (NFV). Modern networks include many middleboxes to provide a wide range of network functions to improve performance as well as security. NFV aims to virtualize these network functions, and replace dedicated hardware appliances by software applications running on generic compute resources. The resulting orchestration flexibilities can be used for a faster and cheaper service deployment. SDN can be exploited to steer flows through the appropriate network functions. Decoupling data plane and control plane operations, à la Software-Defined Networking, can greatly simplify network management and improve the overall performance and utilization of wired networks. However, SDN and NFV have not yet received as much attention in the context of wireless networks, due to fundamental differences between wireless and wired networks. First and foremost, wireless networks feature many peculiarities and knobs that often do not exist in wired networks. For example, wireless networks need interfaces for flexible resource allocation, client mobility, client-based load balancing, and fine/grained traffic engineering is paramount. Furthermore, today’s trend towards Bring-Your-Own-Device (BYOD), implies that the network has to accommodate a more diverse set of user device types of different generations. Moreover, today’s home networks, unlike enterprise networks, typically suffer from a non-existing dedicated control channel, rendering fine-grained centralized control challenging. But, applying the SDN and NFV concepts of softwarization and virtualization to WiFi networks has the potential to render WiFi ready for its future. However, little is known today on how to introduce and benefit from the concepts in WiFi networks. In this thesis, we present a Software-Defined Wireless Network (SDWN) approach which combines SDN and NFV with wireless access technology. With our SDWN approach we overcome the aforementioned challenges in WiFi networks. We make the following contributions: (i) we present an SDN framework, called Odin, that introduces our Light Virtual Access Point Abstraction (LVAP), which abstracts the complexities of the upper-802.11 MAC, and a control plane that allows the orchestration of WiFi and wired networks in unison, by leveraging OpenFlow for the Ethernet-based portion of the network. (ii) We present the design, implementation, and evaluation of OpenSDWN, a flexible, novel WiFi architecture based on a joint SDN and NFV approach. Specifically, OpenSDWN introduces WiFi datapath programmability to enable service differentiation and fine-grained transmission control, facilitating the prioritization of critical applications. OpenSDWN implements per-client virtual middleboxes, to render network functions more flexible and support mobility and seamless migration jointly with LVAPs. Moreover, control over the network is exposed through a participatory interface. (iii) We present the design and implementation of a novel WiFi-SDN control plane architecture called AeroFlux. It exploits locality in SDN control plane operations for scalability reasons. Specifically, AeroFlux is based on a 2-tiered control plane that handles frequent, localized events close to where they originate, i.e., close to the data plane, by so-called near-sighted controllers. Global events, which require a broader picture of the network state, are handled by the centralized part of the control plane. (iv) We present LegoFi, a modularized Software-Defined Wireless Network that follows the trend of NFV. I.e., realizing WiFi function blocks as virtualized and programmable wireless network functions (WVNFs), that are allocated (and composed) where and when they are most useful. Specifically, through WVNFs, we achieve a functional decomposition of the WiFi architecture, allowing to overcome inflexibilities found in today’s monolithic, vertically integrated and expensive WiFi architectures. Thus, SDWN allows us to overcome today’s ossified WiFi architectures by orchestrating and modularizing the WiFi building blocks. Moreover, it provides the necessary abstractions to introduce common features of enterprise networks to residential and hotspot deployments, i.e., WiFi networks based on off-the-shelf commodity hardware. The practicality of our approaches has been successfully demonstrated at several international conferences and is currently deployed and running in two WiFi access networks, i.e., one university enterprise and one larger (30 household) residential network. Moreover, it has gained commercial interest by network vendors and operators. Therefore, we believe that our SDWN approach constitutes a relevant step forward to modern and future-proof WiFi networks.