Tehuang Liu

Interference-aware QoS routing for multi-rate multi-radio multi-channel IEEE 802.11 wireless mesh networks

QoS routing in multi-channel wireless mesh networks (WMNs) with contention-based MAC protocols is a very challenging problem. In this paper, we propose an on-demand bandwidth-constrained routing protocol for multi-radio multi-rate multi-channel WMNs with the IEEE 802.11 DCF MAC protocol. The routing protocol is based on a distributed threshold-triggered bandwidth estimation scheme, implemented at each node for estimating the free-to-use bandwidth on each associated channel. According to the free-to-use bandwidth at each node, the call admission control, which is integrated into the routing protocol, predicts the residual bandwidth of a path with the consideration of inter-flow and intra-flow interference. To select the most efficient path among all feasible ones, we propose a routing metric which strikes a balance between the cost and the bandwidth of the path. The simulation results show that our routing protocol can successfully discover paths that meet the end-to-end bandwidth requirements of flows, protect existing flows from QoS violations, exploit the capacity gain due to multiple channels, and incurs low message overhead.

Location-Dependent Throughput and Delay in Wireless Mesh Networks

In this paper, we model the location-dependent throughput and delay in wireless mesh networks. We analyze the packet-arrival and the packet-departure rates for the forwarding queues at relaying nodes and then derive the per-user throughputs and packet delays experienced by nodes at different hop-count distances from the gateway. Based on this analytical model, we propose two network design strategies to provide fair resource sharing and minimize the end-to-end delay in wireless mesh networks. We then conduct simulations to validate the correctness of our analytical model and evaluate the performance of the proposed network design strategies. The results show that our analytical framework can accurately model the location-dependent throughput and delay in wireless mesh networks and that our proposed strategies effectively and efficiently provide fair throughputs for nodes and reduce the end-to-end packet delay. This paper not only provides a framework for studying the performance of wireless mesh networks but also gives insights into the network design strategy for wireless mesh networks.

Utility-Based Resource Allocation for Layer-Encoded IPTV Multicast in IEEE 802.16 (WiMAX) Wireless Networks

In this paper, we propose a utility-based resource allocation scheme for layer-encoded IPTV multicast streaming service over IEEE 802.16 WiMAX networks. Unlike existing utility-based schemes, this mechanism is designed for wireless networks which support adaptive modulation and coding. Each video stream (or program) is encoded into different layers. Then, our mechanism adjusts the number of each users received layers dynamically according to its channel condition and the available network bandwidth, so as to maximize total utility. We prove that this problem is NP-hard, and show that our scheme is bounded in performance to the optimal solution and can run in polynomial time. The simulation results show that this scheme can allocate resource flexibly according to the utility function of each program, the popularity of each program (i.e., the number of users receiving each program), and the amount of total available resource in the network. The result also shows that the fairness of the system can be guaranteed.

Capacity-Aware Routing in Multi-Channel Multi-Rate Wireless Mesh Networks

Employing multiple channels in wireless multi-hop networks is regarded as an effective approach to increasing network capacity. However, existing routing protocols may not be able to properly utilize the advantages of multiple channels in such networks. In this paper, we focus on IEEE 802.11-based wireless mesh networks with stationary nodes, such as wireless backhaul networks and community wireless networks. We propose a new path metric called Bottleneck Link Capacity (BLC) which accounts for the link quality, the interference among links, and the traffic load on the links. Then, we develop a routing protocol called Capacity-Aware Routing (CAR) which makes use of BLC as the routing metric. Finally, we evaluate the performance of BLC via simulations. The results show that our path metric outperforms others in terms of system throughput and end-to-end delay.

Adaptive Downlink/Uplink Bandwidth Allocation in IEEE 802.16 (WiMAX) Wireless Networks: A Cross-Layer Approach

In this paper, we study adaptive bandwidth allocation for uplink and downlink channels in time division duplex (TDD)-based IEEE 802.16 (WiMAX) wireless networks. In a TDD system, uplink and downlink transmissions share the same frequency at different time intervals. The TDD framing can be adaptive in the sense that the downlink to uplink bandwidth ratio can vary with time. For WiMAX channels, we consider the impact of improper bandwidth ratio on the performance of TCP and propose an adaptive bandwidth allocation scheme (ABAS) which adjusts the bandwidth ratio according to the current traffic profile. Our scheme also cooperates with the scheduler to throttle the TCP source when acknowledgements are infrequent. The performance of our mechanism is validated via ns-2 simulations. The results show that our scheme outperforms static allocation in terms of higher aggregate throughput.

Multicast Routing in Multi-Radio Multi-Channel Wireless Mesh Networks

In this paper, we tackle the multicast problem with the consideration of the interference between multicast trees in multi-radio multi-channel wireless mesh networks (MR-MC WMNs). We consider a dynamic traffic model, i.e., multicast session requests arrive dynamically without any prior knowledge of future requests. Each node in the network acts as a Transit Access Point (TAP), and has one or multiple radios tuned to non-overlapping channels. We prove that in MRMC WMNs, the minimum cost multicast tree (MCMT) problem, i.e., finding the multicast tree with minimum transmission cost, is NP-hard. We then formulate the problem by an Integer Linear Programming (ILP) model to solve it optimally, and propose a polynomial-time near-optimal algorithm, called Wireless Closest Terminal Branching (WCTB), for the MCMT problem. To alleviate the interference between multicast trees (sessions), we present a polynomial-time algorithm that computes the minimum interference minimum cost path in MR-MC WMNs, and integrate it into WCTB without altering the performance bound of WCTB on the tree cost. The experimental study shows that the tree cost produced by WCTB is very close to the optimal and that the proposed algorithm for interference alleviation is effective. To the best of our knowledge, this is the first paper that studies the MCMT problem in MR-MC WMNs.

Adaptive downlink and uplink channel split ratio determination for TCP-based best effort traffic in TDD-based WiMAX networks

In this paper, we study the determination of downlink (DL) and uplink (UL) channel split ratio for Time Division Duplex (TDD)-based IEEE 802.16 (WiMAX) wireless networks. In a TDD system, uplink and downlink transmissions share the same frequency at different time intervals. The TDD framing in WiMAX is adaptive in the sense that the downlink to uplink bandwidth ratio may vary with time. In this work, we focus on TCP based traffic and explore the impact of improper bandwidth allocation to DL and UL channels on the performance of TCP. We then propose an adaptive split ratio (ASR) scheme which adjusts the bandwidth ratio of DL to UL adaptively according to the current traffic profile, wireless interference, and transport layer parameters, so as to maximize the aggregate throughput of TCP based traffic. Our scheme can also cooperate with the base station (BS) scheduler to throttle the TCP source when acknowledgements (ACKs) are transmitted infrequently. The performance of the proposed ASR scheme is validated via ns-2 simulations. The results show that our scheme outperforms static allocation (such as the default value specified in the WiMAX standard and other possible settings in existing access networks) in terms of higher aggregate throughput and better adaptivity to network dynamics.

Interplay of Network Topology and Channel Assignment in Multi-Radio Multi-Rate Multi-Channel Wireless Mesh Networks

Employing multiple channels can effectively improve the network capacity in wireless mesh networks (WMNs). In multi-radio multi-channel WMNs, the channel assignment problem is to assign each radio a channel such that the network capacity is maximized. Since whether two nodes can communicate with each other depends on the channels they use, different channel assignments may lead to different network topologies. Most existing channel assignment algorithms are based on specific network topologies, such as trees, A-connected graphs, etc. This pre-determined network topology is then used as the input of the channel assignment algorithm. In this paper, we study the importance of the input network topology of channel assignment algorithms on the network performance, and propose an algorithm for constructing efficient input network topologies for existing channel assignment algorithms to increase the capacity gain due to multiple channels. The simulation results show that the proposed algorithm improves the network capacity dramatically without the need to modify existing channel assignment algorithms.

Location-Dependent Network Performance and Design Strategies for Wireless Mesh Networks

In this paper, we model the location-dependent throughput and delay in wireless mesh networks. We analyze packet arrival rates and packet departure rates for the forwarding queues at relaying nodes and then derive the throughput and packet delay experienced by nodes at different hop count distances to the gateway. Based on this model, we further analyze how network design strategies affect the throughput and delay of each node. We then conduct simulations to validate our analytical model and evaluate the performance of different network design strategies. This paper not only provides a framework for studying the location-dependent throughput and delay in wireless mesh networks but also gives insights into the network design strategy for wireless mesh networks.