For the mobile users (MUs) installed with IEEE 802.11 wireless adapters, the default best-signal-strength-based access point (AP) selection is limited since it does not consider fairness and multi-rate issues. In this paper, we propose a distributed AP selection algorithm for newly joining MU and already associated MUs to be adaptive to their joining order and the dynamic wireless environment. We also prove the stability of proposed algorithm and analyze its complexity. Finally, with extensive evaluations, we show that proposed scheme can greatly improve the fairness without significantly sacrificing the efficiency.

In this research, we first investigate the cross-layer interaction between TCP and routing protocols in the IEEE 802.11 ad hoc network. On-demand ad hoc routing protocols respond to network events such as channel noise, mobility, and congestion in the same manner, which, in association with TCP, deteriorates the quality of an existing end-to-end connection. The poor end-to-end connectivity deteriorates TCP's performance in turn. Based on the well-known TCP-friendly equation, we conduct a quantitative study on the TCP operation range using static routing and long-lived TCP flows and show that the additive-increase, multiplicative-decrease (AIMD) behavior of the TCP window mechanism is aggressive for a typical multihop IEEE 802.11 network with a low-bandwidth-delay product. Then, to address these problems, we propose two complementary mechanisms, that is, the TCP fractional window increment (FeW) scheme and the Route-failure notification using BUIk-losS Trigger (ROBUST) policy. The TCP FeW scheme is a preventive solution used to reduce the congestion-driven wireless link loss. The ROBUST policy is a corrective solution that enables on-demand routing protocols to suppress overreactions induced by the aggressive TCP behavior. It is shown by computer simulation that these two mechanisms result in a significant improvement of TCP throughput without modifying the basic TCP window or the wireless MAC mechanisms.

In this paper, we address access point (AP) selection for mobile nodes (MNs) in IEEE 802.11 wireless mesh network (WMN). Unlike the hotspot-based AP model, the service capacity of mesh AP in WMN is limited by the wireless connection to gateway AP and varies with the hop distance to the gateway AP. Using the multi-rate throughput model of (1), we formulate the problem of AP selection in WMN as a nonlinear optimiza- tion problem where the tradeoff between total throughput and fairness is achieved by applying a weighting parameter λ .W e evaluate the tradeoff over several WMN topologies, and show that the AP selection result from the proposed formulation can result in a reasonable tradeoff.

Analyzing TCP operation over 802.11 multihop ad hoc networks involves a cross-layer study. In this work, we investigate the effect of congestion and MAC contention on the interaction between TCP and on-demand ad hoc routing protocol in the 802.11 ad hoc networks. Our study reveals several problems stemming from lack of coordination and sharing in such networks. It is observed that TCP induces the over-reaction of routing protocol and hurts the quality of end-to-end connection. So, one of the critical sources of lowering TCP throughput lies in the TCP window mechanism itself. To fix this problem, we propose a fractional window increment (FeW) scheme for TCP to prevent the over-reaction of the on-demand routing protocol by limiting TCP's aggressiveness. The proposed scheme is applicable to a wide range of transport protocols using the basic TCP mechanism, and the protocol behavior is analytically tractable. Our simulation results demonstrate that the proposed scheme can dramatically improve TCP performance and network stability in a variety of 802.11 multihop networks. For example, in some chain-like topologies, the proposed scheme outperforms basic TCP by over 90%, and recent related variants of TCP (ADTCP, LRED) by over 70%.

The design of a TCP-friendly layered video multicast scheme with thin layer partitioning is studied in this research. It was shown in Nahm et al. (2003) that the cumulative layered multicast with static layers can achieve TCP-friendly network adaptation with good fairness via the usage of thin layers. However, if the layer is thinner than the scale of network fluctuation, the convergence to the target throughput may not be attainable even in the steady state. In this work, we propose a simple nonlinear preprocessing, called the minmax averaging operation, in the basic loss rate estimation algorithm, to help a TCP-friendly throughput estimation robust to network fluctuation. It is demonstrated via computer simulation that the stability of the network adaptation performance has been improved with the usage of the proposed scheme.

We propose TCP-friendly layered multicast with explicit congestion notification (TFLME) protocol, designed for video streaming to multitude of heterogeneous receivers using IP multicast service. In the design of TFLME protocol, we introduce a congestion control strategy consisting of passive measurement of network congestion and non-integer exponential partition of video layers. The strategy is directly applicable to the layered video format in the standard track like MPEG4-FGS to achieve TCP-friendliness and fairness in presence of TCP-friendly competing traffic, while minimizing the ill effects of the coarse granularity of layer partition commonly found in the typical layered multicast. TFLME protocol utilizes explicit congestion notification (ECN) driven by random early detection (RED) algorithm, which provides advantage in achieving TCP- friendliness with less packet loss. The effectiveness of the proposed strategy and the performance of TFLME protocol are evaluated in the simulation with NS-2 simulator.

A layered video multicast framework for differentiated service (DS) networks, which provides various levels of QoS guarantee for heterogeneous users, is examined in this research. The proposed system consists of three key components: extended active queue management, hierarchical priority marking, and receiver-driven layered multicast with ECN (RLME). Particularly, we introduce RLME that effectively utilizes advanced features of active queues such as random early drop (RED) and early congestion notification (ECN) in DS networks. The usage of ECN combined with the early response to congestion of RLME reduces actual packet losses significantly, and the priority support from DS networks minimizes the packet loss effect on video quality. The network performance and video quality from simulation results show that the proposed system successfully achieves stable and controllable video QoS guarantee for heterogeneous video clients.