  The significant growth in the number of WiFi-enabled devices as well as the
increase in the traffic conveyed through wireless local area networks (WLANs)
necessitate the adoption of new network control mechanisms. Specifically, dense
deployment of access points, client mobility, and emerging QoS demands bring
about challenges that cannot be effectively addressed by distributed
mechanisms. Recent studies show that software-defined WLANs (SDWLANs) simplify
network control, improve QoS provisioning, and lower the deployment cost of new
network control mechanisms. In this paper, we present an overview of SDWLAN
architectures and provide a qualitative comparison in terms of features such as
programmability and virtualization. In addition, we classify and investigate
the two important classes of centralized network control mechanisms: (i)
association control (AsC) and (ii) channel assignment (ChA). We study the basic
ideas employed by these mechanisms, and in particular, we focus on the metrics
utilized and the problem formulation techniques proposed. We present a
comparison of these mechanisms and identify open research problems.
