Effect of 802.11 Adaptive Exponential Backoffs on the Fluidity of Downlink Flows in Mesh Networks

Efficient multihop traffic management is a need for successful Wireless Mesh Networks (WMNs) deployment. Using an analogy with fluid mechanism, we classify a flow as laminar if the packets flow smoothly from the Wired Access Point (WAP) over the mesh network, and as turbulent otherwise. We identify a particular but frequent collision scenario, which sets the flow to be turbulent, resulting in a strongly reduced downlink end-to-end throughput. We show that the exponential backoff mechanism in an 802.11WMN is responsible for this problem and suggest a modification of the current exponential backoff policy of 802.11 for WMNs. We support these findings both with simulations and real measurements on a testbed infrastructure

Promiting Fluidity in the Flow of Packets of 802.11 Wireless Mesh Networks

Wireless Mesh Networks (WMNs) are based on packet forwarding and therefore require efficient multi-hop protocols for their deployment. Toward this objective, we study the flow of packets through the network and using an analogy with fluid physics we classify them as being either laminar in the case of a smooth propagation or turbulent otherwise. Following this terminology, we present the tendency of current 802.11 to generate turbulent flows, i.e. to queue packets at the intermediate nodes for a non-deterministic time. However numerous applications such as VoIP, TCP and streaming are very delay sensitive and therefore laminar behavior is desirable. We model existing 802.11 multi-hop networks and identify the exponential backoff policy as a main parameter in the transition between laminar and turbulent behavior

Model validation through experimental testbed: the fluid flow behavior example

This testbed practice paper presents our efforts to validate an analytical model for fluid flow behavior in wireless mesh networks with an experimental evaluation. We have developed a fluid model for multihop communication in wireless mesh networks and analyzed it with simulations. Now, we describe our efforts to reproduce the modeled and simulated network with an indoorWiFi mesh network and to measure flow parameters that allow us to verify that the underlying assumptions and the flow behavior can be matched in real networks. Our experiences emphasize the need to gap the bridge between simulations and experimental validation as well as the lack of tools to efficiently validate results. These findings are particularly true in wireless mesh networks where interference is beyond the control of the experiment and where nodes are distributed such that an easy coordination and monitoring of the nodes is not possible

Bringing Stability to Wireless Mesh Networks

Wireless mesh networks were designed as a mean to rapidly deliver large-scale communication capabilities without the support of any prior infrastructure. Among the different properties of mesh networks, the self-organizing feature is particularly interesting for developing countries or for emergency situations. However, these benefits also bring new challenges. For example, the scheduling decision needs to be performed in a distributed manner at each node of the network. Toward this goal, most of the current mesh deployments are based on the IEEE 802.11 protocol, even if it was not designed for multi-hop communications. The main goals of this thesis are (i) to understand and model the behavior of IEEE 802.11-based mesh networks and more specifically the root causes that lead to congestion and network instability; (ii) to develop an experimental infrastructure in order to validate with measurements both the problems and the solutions discussed in this thesis; (iii) to build efficient hop-by-hop scheduling schemes that provide congestion control and inter-flow fairness in a practical way and that are backward-compatible with the current protocol; and (iv) to explain the non-monotonic relation between the end-to-end throughput and the source rate and to introduce a model to derive the rationale behind this artifact. First, we propose a Markovian model and we introduce the notion of stealing effect to explain the root causes behind the 3-hop stability boundary, where linear networks up to 3 hops are stable, and larger topologies are intrinsically unstable. We validate our analytical results both through simulations and through measurements on a small testbed deployment. Second, to support the experimental research presented in this thesis, we design and deploy a large-scale mesh network testbed on the EPFL campus. We plan our architecture to be as flexible as possible in order to support a wide range of other research areas such as IEEE 802.11 indoor localization and opportunistic routing. Third, we introduce EZ-flow, a novel hop-by-hop congestion-control mechanism that operates at the Medium Access Control layer. EZ-flow is fully backward-compatible with the existing IEEE 802.11 deployments and it works without any form of message passing. To perform its task EZ-flow takes advantage of the broadcast nature of the wireless medium in order to passively derive the queue size at the next-hop node. This information is then used by each node to adapt accordingly its channel access probability, through the contention window parameter of IEEE 802.11. After detailing the different components of EZ-flow, we analyze its performance analytically, through simulations and real measurements. Fourth, we show that hop-by-hop congestion-control can be efficiently performed at the network layer in order to not abuse the contention mechanism of IEEE 802.11. Additionally, we introduce a complete framework that jointly achieves congestion-control and fairness without requiring a prior knowledge of the network capacity region. To achieve the fairness part, we propose the Explore & Enhance algorithm that finds a fair and achievable rate allocation vector that maximizes a desired function of utility. We show experimentally that this algorithm reaches its objective by alternating between exploration phases (to discover the capacity region) and enhancement phases (to improve the utility through a gradient ascent). Finally, we note that, as opposed to wired networks, the multi-hop wireless capacity is usually unknown and time-varying. Therefore, we study how the end-to-end throughput evolves as a function of the source rate when operating both below and above the network capacity. We note that this evolution follows a non-monotonic curve and we explain, through an analytical model and simulations, the rationale behind the different transition points of this curve. Following our analysis, we show that no end-to-end congestion control can be throughput-optimal if it operates directly over IEEE 802.11. Hence, this supports the methodology of performing congestion control in a hop-by-hop manner. After validating experimentally the non-monotonicity, we compare through simulations different state-of-the-art scheduling schemes and we highlight the important tradeoff that exists in congestion-control schemes between efficiency (i.e., throughput-optimality) and robustness (i.e., no throughput collapse when the sources attempt to operate at a rate above the network capacity)