Rate adaptation techniques have been extensively studied for traditional wireless LANs as way to adjust the data transmission bandwidth as a function of the channel quality. However, existing solutions which are mostly based on statistics collection inherently experience a large delay in response to the wireless channel fluctuations, which is unsuitable for the rapid changing of the channel conditions in vehicular networks. In this paper, we propose an efficient self-adaptive model-tree-based rate adaptation scheme termed MTRA that can predict the packet error rate (PER) and adapt the data rate in real time. We also present a detailed methodology for the PER model tree building and update. We have performed comprehensive experimentations using ns-2 simulations which demonstrate that MTRA can achieve much better performance than the traditional rate adaptation approaches under various scenarios in vehicular networks.

In this paper, we propose a new open-loop rate/link adaptation algorithm (ARFHT) for the emerging high-speed IEEE 802.11n WLANs. ARFHT extends the legacy rate adaptation algorithms for SISO WLANs to make it applicable in the context of MIMO-based 802.11n WLANs. It adapts the MIMO mode in terms of spatial multiplexing and spatial diversity, the two fundamental characteristics of the 802.11n MIMO PHY. It also modifies the link estimation and probing behavior of legacy SISO algorithms. The combined adaptation to the appropriate MIMO mode as well as the appropriate modulation coding scheme selection achieves high channel utilization. In this paper, we provide the intuition and the design details of the ARFHT algorithm. A comprehensive simulation study using ns-2 will demonstrate that ARFHT achieves excellent throughput performance in most scenarios and is highly responsive to varying link conditions, with minimum overhead.

Overlay applications have used various tools to measure path properties in order to construct efficient overlay networks. Typical examples include delay measurement, connectivity measurement and residual bandwidth measurement. While different tools have different measurement targets, it is difficult to design general criterion for selecting paths to measure. In this paper, we study path properties and figure out some simple rules for path selection in measurements. We first examine the relationship between path delay and residual bandwidth. Our measurement results indicate that short paths often have high residual bandwidth. We then impose high preference on short paths in traceroute measurements. The results show that traceroute measurements with path preference achieve comparable inference efficiency and much lower overhead as compared to traditional traceroute measurements. Our study suggests that we can preferentially select short paths to measure. This can fulfill various requirements on path properties.

In IEEE 802.11 wireless local area networks (WLANs), TCP suffers unfairness between uplink and downlink flows due to its asymmetric reactions towards data and ACK losses at the AP (Access Point), and the AP's inability to distinguish itself from other contending stations accessing the medium. In this paper, we propose a novel dual queue management (DQM) scheme with ECN (Explicit Congestion Notification) at the AP's downlink buffer to improve TCP performance in infrastructure WLANs. Our approach maintains two queues for TCP ACK and data respectively, with their total length controlled by a PI (Proportional Integral) controller to prevent congestion. ACK/data packets are marked/dequeued dependent on uplink/downlink time usage of the wireless channel. We also propose an opportunistic scheduling mechanism of the two queues exploiting "multi-rate capability" and "multi-user diversity" for more efficient link utilization. We evaluate the proposed approach in ns-2 and our simulation results demonstrate that this design with few states can significantly improve TCP congestion control, fairness performance and link utilization in WLANs.

Wireless local area networks (WLANs) have become Increasingly popular due to the availability of affordable devices providing multirate capabilities. Under time- varying wireless channels, a device needs to tune its transmission rate dynamically for more efficient utilization of the physical link. Hence, rate adaptation mechanism, which is intentionally unspecified by the IEEE 802.11 standards, is critical to the system performance. In this paper, we propose a practical rate adaptation algorithm, Smart Sender, which utilizes both statistics and the received signal strength indicator (RSSI) of ACK packets to determine the transmission rate that maximizes the throughput. We implement our algorithm in commercial WLAN products and carry out extensive experiments for performance evaluation. The results demonstrate that using both statistics and RSSI of ACKs greatly improves system throughput and responsiveness under various wireless environments.

In this paper we address the unfairness problem between uplink and downlink TCP (Transmission Control Protocol) flows in the IEEE 802.11 infrastructure WLANs (c). It has been shown that TCP flows exhibit resource allocation (e.g., bandwidth) inequalities mainly due to two reasons: TCP's asymmetric reactions towards data and ACK (Acknowledgement) losses when the downlink buffer overflows at the AP (Access Point); and the AP's inability to distinguish itself from other contending stations when accessing the medium with the 802.11 MAC (Medium Access Control) layer based on the DCF (Distributed Coordination Function). To overcome these problems, we propose an AQM (Active Queue Management) approach (i.e., V2PI AQM) implemented at the AP, which utilizes two virtual queues, the TCP data and the TCP ACK queue, with their lengths controlled by PI (Proportional Integral) controllers. As a result, data losses can be reduced since the AP's downlink buffer is no longer overwhelmed by ACK packets destined to the uplink stations. In addition, when a discrepancy is observed by the AP monitoring the traffic intensity on the wireless link and the lengths of both queues, it uses an AIMD (Additive Increase Multiplicative Decrease) approach to adjust its contention window size, which compensates for the unfairness induced by the DCF MAC. We demonstrate using ns- 2 simulations that fair bandwidth sharing between uplink and downlink TCP flows can be achieved by AP's buffer management in conjunction with contention window adaptation.

To construct an efficient overlay network, the information of underlay is important. However, the inference of an underlay topology is not easy. We consider using end-to-end measurement tools such as traceroute to infer the underlay topology among a group of hosts. Since pair-wise traceroutes among hosts take a long time and generate much network traffic, Max-Delta has been proposed to infer a highly accurate topology with a low number of traceroutes. However, Max-Delta relies on a central server to collect traceroute results and to select paths for hosts to measure. It is hence not scalable to large groups. In this paper, we investigate a distributed version of Max-Delta scheme in order to support scalable inference. In our scheme, each host joins an overlay tree before conducting traceroutes. A host then independently selects paths to traceroute and exchanges traceroute results with others through the overlay tree. As a result, each host can maintain a partially discovered topology. We have studied two key issues in the scheme, i.e., how to construct a low-diameter overlay tree and how to reduce bandwidth consumption in measurements. As compared to Max-Delta, our scheme is fully distributed and scalable. In the scheme, each host computes its own traceroute targets, and the computational loads are distributed to all the hosts instead of a single server. Furthermore, each host only exchanges data with a few other hosts and does not need to set up connections with all the other hosts. Simulation results show that the constructed tree has a low diameter and can support quick data exchange among hosts, and that the use of a lookup table for routers can significantly reduce bandwidth consumption in data exchange.

Application-layer multicast (ALM, or overlay multicast) has been proposed to overcome limitations in IP multicast. While much measurement work (such as delay or connectivity measurement) has been conducted to build efficient ALM trees, several interesting questions therein are left unclear: (1) What are the measurement costs of the different measurement methods? (2) What is the major improvement to an ALM tree by using a certain measurement method? (3) To achieve the target tree performance, what is the most cost-efficient measurement method? In this paper, we study three representative measurement methods, i.e., delay measurement, connectivity measurement and available bandwidth measurement. We select six typical ALM protocols each adopting at least one of the measurement methods and evaluate their performance on Internet-like topologies. Our study shows that delay and connectivity measurements can effectively reduce end-to-end delay in overlay trees with low measurement costs. Only using delay measurement may lead to a tree consuming much network resource. The use of connectivity and bandwidth measurements can build a tree with low resource consumption.

In this paper the authors address the unfairness problem between uplink and downlink TCP flows in the IEEE 802.11 infrastructure WLANs. It has been shown that TCP flows exhibit resource allocation (e.g., bandwidth) inequalities mainly due to two reasons: TCP's asymmetric reactions towards data and ACK losses when the downlink buffer overflows at the AP; and the AP's inability to distinguish itself from other contending stations when accessing the medium with the 802.11 DCF MAC. To overcome this problem, we propose an AQM (active queue management) approach (V2PI AQM) to be implemented at the AP, which utilizes two virtual queues, namely the data queue and the ACK queue, with their lengths controlled by the PI controllers. As a result, data losses can be reduced since the AP's downlink buffer is no longer overwhelmed by the ACK packets destined to the uplink stations. In addition, when a discrepancy is observed by the AP monitoring the traffic intensity on the wireless link and the lengths of both queues, it uses an AIMD (additive increase multiplicative decrease) approach to adjust its contention window size, which compensates for the unfairness induced by the DCF MAC. We demonstrate using ns-2 simulations that fair bandwidth sharing between uplink and downlink TCP flows can be achieved by the AP's buffer management in conjunction with contention window adaptation.

With the rapid development in wireless communication technologies, the IEEE 802.11 WLANs are experiencing a huge popularity and widespread deployment. Designed with traditional layered architecture, current WLANs adopt functional layer partitioning and aim at optimization at individual layers. However, in a highly dynamic and media sharing wireless environment, the capacity enhancements at individual physical layers may not necessarily benefit, and sometimes even degrade the system performance with multiple users. It has been shown that in a multiuser setting, one can increase the throughput substantially if partial knowledge of the channels at the receiver sides is known. The challenge is to make good matching of the instantaneous channel conditions of multiple users with the bandwidth and time allocation to each user. In this paper, we address the issue of cross layer design in the proposed "weighted fair scheduling based on adaptive rate control" (WFS-ARC) framework, where the PHY layer knowledge is shared with the MAC and LLC layer in order to provide efficient resource allocation. We evaluate the WFS-ARC approach in ns-2 and the simulation results demonstrate that our design can significantly improve the system throughput.

Current peer-to-peer (P2P) streaming systems often assume that nodes are cooperative to upload and download data. However, in the open environment of the Internet, this is not true and there exist malicious nodes in the system. In this paper, we study malicious actions that can be detected through peer- based monitoring. We require each node to monitor the data received and to periodically send out monitoring messages about its neighbors to some trustworthy nodes. To efficiently store and search the messages among multiple trustworthy nodes, we organize the trustworthy nodes into a threaded binary tree. The trustworthy nodes also dynamically redistribute the monitoring messages among them to achieve load balancing. Our simulation results show that this scheme can efficiently detect malicious nodes with high accuracy, and that the dynamic redistribution method can achieve good load balancing among the trustworthy nodes.

Wireless local area networks (WLANs) have become increasingly popular due to the recent availability of affordable devices providing multirate capabilities. However, because of the time-varying characteristics of a wireless channel, no single rate can be optimal under all scenarios, and the device needs to tune its transmission rate dynamically. Therefore, rate adaptation is a critical component of its performance. In addition, the IEEE 802.11 standard does not specify an algorithm for automatic rate selection; it is intentionally left open to the vendors. In this paper, we propose a practical rate adaptation algorithm, which utilizes both statistics and RSSI of ACK to decide the transmission rate that maximizes the throughput. We implement our algorithm in commercial products and carry out extensive experiments for performance evaluation. The results demonstrate that using both statistics and RSSI of ACK greatly improves system throughput and responsiveness under various wireless environments.

IEEE 802.11 is currently the most popular standard for Wireless LANs. The Distributed Coordination Function (DCF) defines the primary medium access control of 802.11, which uses the CSMA/CA mechanism and Binary Exponential Backoff (BEB) for Contention Window (CW) adjustment when collisions occur. In this paper, we propose a new CW adjustment scheme, CW Idle-Slots-based Control (WISC), using a control-theoretic approach. Specifically, we design and implement a PD (Proportional and Derivative) controller at each contending station, that dynamically adjusts CW based on a locally available channel state, i.e., the average number of consecutive idle slots between two transmissions, such that the channel state converges to the optimal value. Simulation results demonstrate that the new scheme outperforms the standard BEB in terms of both throughput and fairness, especially at high contention levels.

IEEE 802.11 Wireless Local Area Networks (WLANs) are widely deployed nowadays. With traditional layered architecture, current WLAN adopts functional layer partitioning and aims at optimization in individual layers. However, in a highly dynamic and media sharing wireless environment, the capacity enhancement at physical layers may not necessarily benefit the overall system performance. Moreover, in a multiuser setting, throughput can be increased substantially if partial knowledge of the channel is known. In this paper, we address the issue of cross layer design in the proposed "Weighted Fair Scheduling based on Adaptive Rate Control" (WFS-ARC) framework, where the PHY layer knowledge is shared with the MAC/LLC layer. It can make good matching of instantaneous channel conditions of multiple users with resource allocation to each user.