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.
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 .
@cite_4 @cite_9 @cite_1 @cite_15 @cite_12 @cite_17
2041718516 2053770984 1999990623 2109679596
The fair allocation of resources among different nodes is one of the critical problems in wireless mesh networks. Existing solutions mainly focus on rate-limitation policies or distributed fair MAC schemes at the potential expense of total network utilization. This paper investigates a special starvation problem among TCP flows that are different hops away from the BS, as well as its recently proposed solution, the ‘Minimum Content Window’ policy based on IEEE 802.11e. It is found that the aggregate throughput degrades sharply because the effect of this policy on the TCP congestion mechanism has been overlooked. This paper proposes a priority-based congestion control by using ‘Cross-Layer Explicit Congestion Notification’. Analysis and simulation results demonstrate that our scheme can improve the fairness of TCP flows while the aggregate throughput is at least 20 higher than the ‘Minimum Content Window’ policy. Copyright © 2011 John Wiley & Sons, Ltd. The IEEE 802.11 standard specifies both radio and MAC protocol design. We observe that its CSMA protocol helps avoid much of co-channel interference by sharing radio resources in time at the potential expense of degraded network performance. Due to the coupling between the physical and MAC layers, conventional frequency allocation methods for typical cellular networks cannot be applied directly to the 802.11 networks. In this paper, by focusing on interactions among access points, we formulate the channel assignment problem for the 802.11 network, considering the traffic load at the MAC layer, and prove that the problem is NP-complete. In light of computational complexity, a heuristic algorithm is proposed and analyzed. The algorithm is then applied to two cellular settings with known optimal assignments for verification. For one of the settings, the proposed technique generates the optimal channel assignment. As for the second case of a large network, although only a suboptimal solution is obtained by the algorithm, it is shown to be excellent. Thus, as the 802.11 networks are widely deployed, the proposed method can serve as a valuable tool for frequency planning of networks with non-uniform coverage and load. 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. There has been extensive work on network architectures that support multi-path routing to improve performance in wireless mesh networks. However, previous work uses ad-hoc design principles that cannot guarantee any network-wide performance objectives such as conjointly maximizing resource utilization and improving fairness. In parallel, numerous theoretical results have addressed the issue of optimizing a combined metric of network utilization and fairness using techniques based on back-pressure scheduling, routing and flow control. However, the proposed theoretical algorithms are extremely difficult to implement in practice, especially in the presence of the 802.11 MAC and TCP. We propose Horizon, a novel system design for multi-path forwarding in wireless meshes, based on the theoretical results on back-pressure. Our design works with an unmodified TCP stack and on top of the existing 802.11 MAC. We modified the back-pressure approach to obtain a simple 802.11-compatible packet-forwarding heuristic and a novel, light-weight path estimator, while maintaining global optimality properties. We propose a delayed reordering algorithm that eliminates TCP timeouts while keeping TCP packet reordering to a minimum. We have evaluated our implementation on a 22-node testbed. We have shown that Horizon effectively utilizes available resources (disjoint paths). In contrast to previous work, our design not only avoids bottlenecks but also optimally load-balances traffic across them when needed, improving fairness among competing flows. To our knowledge, Horizon is the first practical wireless system based on back-pressure.