Lin Luo

Available bandwidth estimation and admission control for QoS routing in wireless mesh networks

This article presents an integrated admission control and routing mechanism for multi-rate wireless mesh networks. Admission control depends on precise estimates of available bandwidth at involved nodes and the bandwidth consumption required by a new flow. Estimating these parameters in wireless networks is challenging due to the shared and open nature of the wireless channel. Existing available bandwidth estimation techniques do not accurately consider interference from neighboring nodes and flow bandwidth requirement estimates or act overly conservative, restricting opportunities for parallel transmission due to spatial reuse. We propose the DCSPT method for available bandwidth estimation, based on dual carrier sensing with parallel transmission awareness. We also introduce a packet probing-based available bandwidth estimation method, suitable for legacy device implementations, and verify it experimentally. These techniques are integrated in an admission control mechanism designed for a hop-by-hop routing protocol (LUNAR), enabling alternate route identification when shortest paths are congested. Our protocol uses temporal accounting to enable bandwidth estimation across links using different bit-rates. Simulation results demonstrate that our admission control mechanism can effectively control the traffic load while considering parallel transmission opportunities, leading to cumulative system throughput improvements up to 80% compared to more conservative approaches. We further show that additional gains in system throughput come without significant cost in terms of packet delivery ratio or end-to-end delay and discuss our implementation experience on the ORBIT wireless research testbed.

A QoS routing and admission control scheme for 802.11 ad hoc networks

This paper presents an admission control mechanism for multirate wireless ad hoc networks. Admission control depends on precise estimates of bandwidth available in the network and the bandwidth required by a new flow. Estimating these parameters in wireless ad hoc networks is challenging due to the shared and open nature of the wireless channel. Available bandwidth can only be determined by also considering interference at neighboring nodes. Also, due to self-interference of flows the required bandwidth of a flow varies for each link of a route. The proposed admission control mechanisms is integrated with a hop-by-hop ad hoc routing protocol, thus enabling it to identify alternate routes if the shortest path is congested. Each node measures available channel bandwidth through passive monitoring of the channel. The mechanism improves estimation accuracy by using a formula that considers possible spatial reuse from parallel transmissions. The protocol also uses temporal accounting to enable bandwidth estimation across links using different bit-rates. Simulation results support that the admission control mechanism can effectively control the traffic load and that considering parallel transmission leads to improved bandwidth estimation accuracy. The admission control mechanism can admit more traffic while maintaining QoS.

Channel Assignment, Stream Control, Scheduling and Routing in Multi-Radio MIMO Wireless Mesh Networks

Multi-hop wireless mesh networks are designed as a cost-effective solution for last-mile broadband Internet access. Equipping mesh nodes with multiple radios further improves network capacity by using multiple radios to transmit simultaneously on orthogonal channels. In addition, multiple input multiple output (MIMO) antennas have emerged as a physical layer breakthrough for increasing throughput and suppressing interference from neighboring links. These wireless technologies are viewed as the key components in improving the performance of next-generation wireless networks. However, to obtain the full benefits of these technologies, the networking protocols should exploit their capabilities in a systematic way due to their interdependence. In this paper, we provide the first formal study on cross-layer optimization in multi-radio, multi-channel wireless mesh networks with MIMO links. We formulate a framework where data routing at the network layer, link scheduling and channel assignment at the MAC layer, and MIMO stream control at the physical layer can be jointly optimized for maximizing network throughput subject to fairness constraint among mesh nodes. We then present an algorithm in which routing is established on a longer-time scale for system stability, while scheduling, channel assignment and stream control are jointly determined for opportunistic bandwidth access and sharing in time, frequency and space dimensions based on instantaneous channel conditions and traffic dynamics.

End-to-End Performance Aware Association in Wireless Municipal Mesh Networks

In wireless municipal mesh (muni mesh) networks, a client station needs to associate with a mesh access point (MAP) in order to access the network. The end-to-end performance of a station depends on the access link quality between the station and the associated MAP as well as the multi-hop path quality from the associated MAP to the Internet gateway. However conventional association mechanisms used for single-hop WLANs assume that the wireless access link is the bottleneck and do not take the backhaul conditions into account, which may result in poor performance in muni mesh networks. In this paper, we propose a new joint MAP association mechanism for muni mesh networks to achieve optimal end-to-end communication performance from the station to the gateway. A station associates to one of its nearby MAPs by jointly considering the transmission capability of the access link and the backhual path. In addition, we design a new metric, called CAETT, to measure the access link quality. Different from previously proposed link metrics, CAETT takes into account the impact of 802.11 MAC layer contention on bandwidth sharing among the stations with multi-rate capability. It yields more accurate link throughput estimation. Furthermore, in order to reduce the association time, we develop an analytical model and propose a hybrid measurement/estimation method to enable a station to quickly determine the CAETT. We evaluate the performance of our system through simulations and demonstrate that the proposed joint association mechanism with the CAETT metric can greatly improve the end-to-end performance for the stations.

End-to-end performance aware association mechanism for wireless municipal mesh networks

In a wireless municipal mesh (muni mesh) network, a client station (STA) needs to associate with a mesh access point (MAP) for network access. Previous association mechanisms assume high-speed backhaul and only the access link being the bottleneck. This assumption holds for most WLANs, but in wireless mesh networks traffic could be bottlenecked either by the access link or by the bandwidth-limited wireless backhaul. In this paper, we propose a new joint MAP association mechanism for wireless muni mesh networks to improve STAs end-to-end communication performance. A STA makes its association decision by jointly considering the quality of the access link between the STA and the associated MAP as well as the cost of the multi-hop path from the associated MAP to the Internet gateway. In addition, we design two new metrics, Contention Aware Expected Transmission Time (CAETT) and Load Aware Expected Transmission Time (LAETT), to measure the access link quality. The main strength of CAETT is incorporating the impact of 802.11 MAC layer contentions on the bandwidth sharing of multi-rate stations. LAETT further captures the real traffic load on the shared medium. In order to reduce the association delay, we use an analytical model to derive a hybrid measurement/estimation method to enable a station to quickly determine the cost of CAETT and LAETT. We conduct extensive simulations to evaluate the performance of the proposed joint MAP association mechanism. Especially within the joint association framework, we investigate the impact of various combinations of access link metrics (RSSI, PB, CAETT, LAETT) and backhaul routing metrics (hopcount, ETT, RALA) on the system performance. We show that the joint association mechanism can significantly improve the network performance in terms of throughput and delay by up to 100%. In particular, the joint association mechanism with LAETT as the access link metric and RALA as the routing metric outperforms other schemes and metrics.

MIMO-Aware Spectrum Access and Scheduling in Multi-hop Multi-channel Wireless Networks

Software defined radios (SDRs) are a promising technology to enable dynamic channel access and sharing. Multiple Input Multiple Output (MIMO) is another radio technology breakthrough for increasing wireless throughput. To obtain the full benefits brought by SDR and MIMO technologies in wireless mesh networks (WMNs), the higher layer mechanisms should exploit their capabilities in a systematic way. In this paper, we propose a Stream Controlled Multiple Access (SCMA) scheme for multi-hop multi-channel WMNs, which is responsible for scheduling links and assigning channels for data transmission and controlling MIMO operation mode in a cross-layer approach. It enables efficient spectrum access and sharing in temporal, frequency, and spatial dimensions, and adapts to varying network conditions. The evaluation results show that the proposed SCMA scheme greatly improve the network performance compared to the traditional schemes.

Association, routing and scheduling algorithms for enhancing throughput and fairness in wireless mesh networks

Wireless mesh networks (WMNs) have emerged as a promising step towards the goal of ubiquitous broadband wireless access due to the ease of deployment and its low cost. Current research on WMNs aims at a number of challenges, including capacity limitation and poor fairness. In this thesis we carefully design association, routing and scheduling algorithms to enhance throughput and fairness in WMNs. The association mechanism specified by the IEEE 802.11 standard is based on the received signal strength. Employing this mechanism in WMNs may only achieve low throughput and low user transmission rates. We develop a new association framework in order to provide optimal association and network performance in WMNs. In this framework, we first propose two new access link metrics that are aware of channel condition, channel access contention as well as AP load. We then extend association mechanisms based on such metrics in a cross-layer manner taking into account information from the routing layer, in order to fit it in the operation of WMNs. We evaluate the performance of our system through simulations, and show that WMNs that use the proposed association mechanism can achieve up to 100% improvement in throughput and delay. Contention-based MAC protocols such as 802.11 greatly limit the throughput and fairness of WMNs. Significantly higher throughput and fairness are achievable if bandwidth is carefully allocated and transmissions are scheduled. To study the performance limits of WMNs, we first optimally allocate bandwidth to each data flow, jointly computing the user-router association and backbone routing solutions, such that network throughput can be maximized while certain fairness is achieved. We then focus on the integral association, single-path routing case and investigate the optimal performance of a WMN on a given tree topology. We also develop an efficient scheduling algorithm to coordinate channel access and to enforce the allocated bandwidth. Our evaluation shows that association and routing have a great impact on bandwidth allocation, namely constructing a good topology can improve throughput while enhancing fairness. Finally, multiple channel and Multiple-Input-Multiple-Output (MIMO) are two technologies being introduced into WMNs to mitigate interference and increase network capacity. Higher layer protocols need to be aware of these techniques in order to fully leverage their benefits, which makes cross-layer approach desirable. We first formulate a cross-layer optimization framework for maximizing an aggregate utility, which jointly allocates link bandwidth for data flows, and determines channel assignment and MIMO stream selection. We then present an efficient MIMO-aware scheduling algorithm called stream controlled multiple access (SCMA). SCMA determines a baseline schedule in the channel assignment stage where a set of non-interfering links are scheduled on each channel. The second stage of SCMA, link pairing, takes advantage of the performance gain of MIMO stream control. SCMA also incorporates a congestion control scheme at traffic sources to prevent the network from being overloaded. Simulation results show that the MIMO-aware scheduling algorithm leads to about 50%∼100% higher throughput while preserving fairness than the MIMO-oblivious algorithm. It achieves close-to-the-optimal performance in certain scenarios.