text: The high bandwidth demand of Internet applications has recently driven the need of increasing the residential download speed. A practical solution to the problem has been proposed aggregating the bandwidth of 802.11 Access Points (APs) backhauls in range via 802.11 connections. Since 802.11 devices are usually single-radio, the communication to multiple APs on different radio-channels requires the introduction of a time-division multiple access (TDMA) policy at the client station. Current investigation in this area supposes that there is a sufficient number of TCP flows to saturate the Asymmetric Digital Subscriber Line (ADSL) behind the APs. However, this may be not guaranteed according to the user traffic pattern. As a consequence, a TDMA policy introduces additional delays in the end-to-end transmissions that will cause degradation of the TCP throughput and an under-utilization of the AP backhauls. In this paper, we first perform an in-depth experimental analysis with a customized 802.11 driver of how the usage of multi-AP TDMA affects the observed Round-Trip-Time (RTT) of TCP flows. Then, we introduce a simple analytical model that accurately predicts the TCP RTT when accessing the wireless medium with a Multi-AP TDMA policy. Based on this model, we propose a resource allocation algorithm that runs locally at the station and it greatly reduces the observed TCP RTT with a very low computational cost. Our proposed scheme can improve up to 1:5 times the aggregate throughput observed by the station compared to state-of-the-art multi-AP TDMA allocations. We also show that the throughput performance of the algorithm is very close to the theoretical upper-bound in key simulation scenarios.
  @cite_4: The widespread availability of 802.11-based hardware has made it the premier choice of both researchers and practitioners for developing new wireless networks and applications. However, the ever increasing set of demands posed by these applications is stretching the 802.11 MAC protocol beyond its intended capabilities. For example, 802.11 provides no control over allocation of resources, and the default allocation policy is ill-suited for heterogeneous environments and multi-hop networks. Fairness problems are further exacerbated in multi-hop networks due to link asymmetry and hidden terminals. In this paper, we take a first step towards addressing these problems without replacing the MAC layer by presenting the design and the implementation of an Overlay MAC Layer (OML), that works on top of the 802.11 MAC layer. OML uses loosely-synchronized clocks to divide the time in to equal size slots, and employs a distributed algorithm to allocate these slots among competing nodes. We have implemented OML in both a simulator and on a wireless testbed using the Click modular router. Our evaluation shows that OML can not only provide better flexibility but also improve the fairness, throughput and predictability of 802.11 networks.
  @cite_9: Software-defined cognitive radio has recently made the jump from a purely research driven endeavor to one that is now being driven commercially. Such radios offer the promise of spectrum agility and re-configurability through flexibility at the MAC and physical layers. In the wireless domain, it has been shown that hybrid-MAC layer algorithms can lead to improved overall performance in varying network conditions. A hybrid MAC that uses CSMA in low-contention periods and switches to TDMA in high-contention periods can outperform CSMA or TDMA individually. However, such hybrid systems do not offer the flexibility and cognition required of dynamic spectrum networks. In this paper, we describe MultiMAC, a framework and experimental platform for evaluating algorithms that dynamically reconfigure MAC and physical layer properties. MultiMAC acts as a mediating MAC layer and dynamically reconfigures and or selects from a collection of alternative MAC layers. As a result of monitoring current network metrics, MultiMAC chooses the MAC layer capable of achieving the best performance while ensuring that incoming frames are decoded using the correct MAC layer algorithm. MultiMAC incorporates decision processes to select the appropriate MAC component based on per-node and per-flow statistics. This engine will allow intelligent reconfiguration of the MAC and physical layers in response to changes in external conditions and or requirements optimizing use of the available spectrum
  @cite_1: Exponential growth in the number of wireless devices that operate in the limited unlicensed frequency spectrum necessitates the next generation of radio devices to be reconfigurable and sensitive to changes in network conditions and spectrum availability. Most modern wireless devices offer increased software programmability and control over radio communication parameters. Since a large portion of the MAC protocol is implemented in software, with the firmware providing a set of functional primitives, it is possible to design and implement alternate MAC protocols in real testbeds equipped with commodity 802.11 devices. This paper describes FreeMAC, a reconfigurable MAC protocol development framework that enables the design and implementation of a general class of multi-channel MAC protocols on a typical Linux system. FreeMAC provides support for frequent channel switching and fine control over the timing of packet transmissions. We also propose a mechanism to reduce the latency in the scheduling of periodic operations of a software MAC protocol that have strict timing requirements. Results from our six node testbed indicate that using our approach, the scheduling latency of slot transitions in a TDMA-style MAC can be improved by up to an order of magnitude, with minimal overhead. FreeMAC also exports a number of radio configuration parameters as API functions to enable cross layer interactions among wireless networking protocols. As a proof of concept, we implement a simple multi-channel TDMA MAC on our testbed to demonstrate the utility of FreeMAC as a development framework.
  @cite_15: 
  @cite_12: 
  @cite_17: In IEEE 802.11-based wireless mesh networks a user is associated with an access point (AP) in order to communicate and be part of the overall network. The association mechanism specified by the IEEE 802.11 standard does not consider the channel conditions and the AP load in the association process. Employing the mechanism in its plain form in wireless mesh networks we may only achieve low throughput and low user transmission rates. In this paper, we propose an association mechanism that is aware of the uplink and downlink channel conditions. We introduce a metric that captures the channel conditions and the load of the APs in the network. The users use this metric in order to optimally associate with the available APs. We then extend the functionality of this mechanism in a cross-layer manner taking into account information from the routing layer. The novelty of the mechanism is that the routing QoS information of the back haul is available to the end users. This information can be combined with the uplink and downlink channel information for the purpose of supporting optimal end-to-end communication and providing high end-to-end throughput values. We evaluate the performance of our system through simulations and we show that 802.11-based mesh networks that use the proposed association mechanism are more capable in meeting the needs of QoS-sensitive applications.
summary: The need for 802.11 resource allocation schemes has been extensively studied in the literature @cite_17 @cite_15 @cite_12 . Many of the proposed schemes rely on either non-standard compliant features @cite_9 , or completely develop an entire new MAC protocol @cite_1 . Both strategies may be undesirable, and so we avoid them. Given that, the resource allocation scheme that more closely relates to ours is @cite_4 , that studied the problem of absence of application-specific 802.11 resource allocation schemes. As a solution, they designed and implemented an overlay MAC layer (OML) to divide the time into slots of equal size. Then, they used a distributed algorithm to allocate the slots across the competing nodes, where each competing node receives a number of slots proportional to its weight function. However, the authors let as an open issue the understanding of the increased delay for TCP flows in presence of the slotted mechanism @cite_4 .