# A Software-Defined Networking Framework to Provide Dynamic QoS Management in IEEE 802.11 Networks
## 1 Introduction  
Wireless networks have experimented an exponential growth in the last few years. Currently, wireless is the most used technology by devices to access to the Internet or to communicate with one another or throughout an access point (AP). Wireless networks are also being used in other environments, which are very sensitive in terms of Quality of Service (QoS), such as Vehicular Ad-Hoc Networks (VANET) or Wireless Sensor Networks (WSN). In addition, it is expected that low-cost Single Board Computer (SBC) devices like Raspberry-pi or Odroid produce a further expansion of wireless networks, especially for real-time applications in IoT environments, which are also QoS aware. As a consequence, nowadays, the QoS in wireless networks is no longer an option but a requirement. One of the methods to provide QoS in multiple access networks is by giving more channel access opportunities to nodes involved in QoS-aware sessions, using the Binary Exponential Backoff mechanism. In fact, this is the background idea of the IEEE 802.11e in its contention-based medium access function, called _Enhanced Distributed Channel Access_ (EDCA) [1], which allows traffic differentiation for the stations in the network. EDCA operation is based on the idea of four AccessCategories (AC): Voice, Video, Best Effort, and Background. Access Categories map directly from Ethernet-level class of service (CoS) priority levels, defined in IEEE 802.1p. IEEE 802.11e provides long-term QoS in terms of application requirements. Different backoff values are defined for each access category, according by the physical layer supported by IEEE 802.11e. However, these values are fixed and cannot be dynamically modified to maintain the QoS requirements against network changes or to response to dynamic application needs. In this regard, we can highlight one of the contributions of this work. We propose DEDCA (_Dynamic Enhanced Distributed Channel Access_) mechanism for 802.11 networks. This mechanism offers an efficient solution for the dynamic management of the QoS requirements of terminals. In our proposal, independently of the kind of traffic, the configuration of the backoff values is not fixed over the time, but they depend on the punctual QoS requirements of the applications. Being able to respond to QoS requirements of applications in short-term intervals can be useful, especially in some scenarios such as VANET, WSN and IoT (Internet of Things). This new scenario of short-term QoS requirements could be solved by giving a light advantage to these particular stations assigning, only temporarily, more bandwidth to them at the expense of reducing the bandwidth of other nodes. This is the key idea of mechanism presented in this work. To perform this new QoS aware access method, a mathematical model to orchestrate who can--and who cannot-- temporarily use an advantageous backoff procedure would be very valuable. Unfortunately, the mathematical analysis proposed in the literature to model 802.11 networks (most of them based on Markov chains) are too complex to be used for this purpose. In this paper it is proposed an easy-to-use mathematical model, which allows the development of new QoS policies able to adapt to the network dynamics in real time. The mathematical model, another contribution of this work, relies only on the calculation of a scalar value called _gain_. This solution is computationally much simpler than most of the solutions proposed in the literature, and consequently can be useful even for low-performance devices like embedded systems. The mathematical model is in fact an extension of a preliminary study that was developed for PLC networks (_Power Line Communications_) [2]. Here, the model has been extended to enable that all the nodes can configure different levels of QoS independently, by the modification of its QoS related configuration parameters. As far as we know, there is not any other mathematical model in the literature with these characteristics.