Enhanced MAC protocol to support multimedia traffic in cognitive wireless mesh networks

With the advanced physical layer techniques such as multiple-input and multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM), transmission real-time 2D/3D contents and applications becomes more and more necessary in wireless networks for the amazing growing in demand of customers. However, the low efficiency of medium access control (MAC) protocol degrades the performance of real-time traffic greatly in multihop, wireless and mobile environment. Focusing on supporting real-time multimedia traffic in cognitive wireless mesh networks (WMNs), an enhanced MAC protocol is proposed. And the contribution of this paper is twofold: (1) An efficient carrier sense multiple access with collision avoidance (CSMA/CA) compatible time division multiple access (TDMA)-like MAC protocol called T-MAC is proposed, which aims to improve the system performance by allocating more channel access time in centralized manner and decreasing overhead. (2) An optimal adaptive scheduling scheme is proposed to support real-time multimedia applications and guarantee QoS for different priority traffic, which aims to find the optimized schedule among all possible sequences of concurrent transmissions by minimizing the occupied resources. Detailed simulation results and comparison with IEEE 802.11e MAC scheme show that the proposed T-MAC can effectively improve quality of service (QoS) for multimedia traffic in terms of throughput, end-to-end delay and packet loss rate, which also manifests that T-MAC is an efficient multimedia applications transmission scheme for mobile terminals and MAPs in cognitive WMNs.     

一种基于WiMAX mesh网络的区分服务调度方案

IEEE 802.16支持多种不同类型的调度服务,并将服务质量支持机制引入媒体接入控制层,却没有规定相应的调度算法。在IEEE 802.16定义的mesh模式下,针对不同类型服务,提出了一种区分服务的调度方案,该方案采用集中式和分布式混合调度。仿真结果表明:该方案下系统平均时延和用户满意度均有所改善。     

Differentiation, QoS guarantee, and optimization for real-time traffic over one-hop ad hoc networks

Nodes having a self-centrically broadcasting nature of communication form a wireless ad hoc network. Many issues are involved to provide quality of service (QoS) for ad hoc networks, including routing, medium access, resource reservation, mobility management, etc. Previous work mostly focuses on QoS routing with an assumption that the medium access control (MAC) layer can support QoS very well. However, contention-based MAC protocols are adopted in most ad hoc networks since there is no centralized control. QoS support in contention-based MAC layer is a very challenging issue. Carefully designed distributed medium access techniques must be used as foundations for most ad hoc networks. In this paper, we study and enhance distributed medium access techniques for real-time transmissions in the IEEE 802.11 single-hop ad hoc wireless networks. In the IEEE 802.11 MAC, error control adopts positive acknowledgement and retransmission to improve transmission reliability in the wireless medium (WM). However, for real-time multimedia traffic with sensitive delay requirements, retransmitted frames may be too late to be useful due to the fact that the delay of competing the WM is unpredictable. In this paper, we address several MAC issues and QoS issues for delay-sensitive real-time traffic. First, a priority scheme is proposed to differentiate the delay sensitive real-time traffic from the best-effort traffic. In the proposed priority scheme, retransmission is not used for the real-time traffic, and a smaller backoff window size is adopted. Second, we propose several schemes to guarantee QoS requirements. The first scheme is to guarantee frame-dropping probability for the real-time traffic. The second scheme is to guarantee throughput and delay. The last scheme is to guarantee throughput, delay, and frame-dropping probability simultaneously. Finally, we propose adaptive window backoff schemes to optimize throughput with and without QoS constraints.     

Slot allocation algorithms in centralized scheduling scheme for IEEE 802.16 based wireless mesh networks

In IEEE 802.16 based wireless mesh networks (WMNs), TDMA (Time Division Multiple Access) is employed as the channel access method and only TDD (Time Division Duplex) is supported and there are no clearly separate downlink and uplink subframes in the physical frame structure. As the uplink and downlink traffic has different characteristics in that the uplink traffic decentralizes in each MSS (Mesh Subscriber Station) and the downlink traffic centralizes in the MBS (Mesh Base Station), different scheduling methods should be taken in the uplink and downlink. This paper presents a uniform slot allocation algorithm which is suitable for both uplinks and downlinks. To achieve higher spatial reuse and greater throughput and to avoid switching frequently between receiving and transmitting within two adjacent time slots when a relay node forwards traffic, different link selection criteria are taken into account when allocating slots for uplinks and downlinks. A combined uplink and downlink slot allocation algorithm is proposed for further improving the spatial reuse and network throughput. The proposed algorithms are evaluated by extensive simulations and the results show that it has good performance in terms of spatial reuse and network throughput. To the best of the authors’ knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.     

Enhancing multi-hop communication over multi-radio multi-channel wireless mesh networks: A cross-layer approach

The multi-channel multi-radio technology represents a straightforward approach to expand the capacity of wireless mesh networks (WMNs) in broadband wireless access scenarios. However, the effective leveraging of this technology in WMNs requires (i) enhanced MAC protocols, to coordinate the access to multiple channels with a limited number of radio interfaces, and (ii) efficient channel allocation schemes, to mitigate the impact of co-channel interference. The design of channel assignment schemes and MAC protocols is strictly interrelated, so that joint design should be considered to optimize the mesh network performance. In this paper, a channel assignment and fast MAC architecture (CAFMA) is proposed, which exploits the benefits provided by the multi-channel multi-radio technology to (i) enhance the performance of multi-hop communications, (ii) maximize the resource utilization, and (iii) support differentiation of traffic classes with different quality of service (QoS) requirements. CAFMA is designed with a cross-layer approach and includes (1) a novel MAC scheme, which provides multi-channel coordination and fast data relaying over multi-hop topologies, and (2) a distributed channel allocation scheme, which works in cooperation with the routing protocol. Simulation results confirm the effectiveness of CAFMA when compared with other single-layer and cross-layer solutions for multi-radio multi-channel WMNs.     

On the performances of IEEE 802.16(d) mesh CDS-mode networks using Single-Switched-Beam Antennas

The IEEE 802.16(d) mesh coordinated distributed scheduling (CDS) mode is a novel technology for future fixed wireless backbone networks and designed for the use of omnidirectional antennas. The use of Single-Switched-Beam Antennas (SSBAs) may have great potential to increase network capacity due to the antenna directivity. However, a network designed for omnidirectional antennas usually cannot operate well or achieve good performance with the presence of antenna directivity.In this paper, we review the designs of the IEEE 802.16 mesh CDS-mode network, study the issues of this network with the use of Single-Switched-Beam Antennas (SSBAs), and propose a complete solution to solve these issues. The performances of our proposed scheme is evaluated using simulations. The simulation results show that our proposed scheme can effectively solve the issues of using SSBAs in the IEEE 802.16 mesh CDS-mode network and greatly increase its network capacity.     

DTSMA: Distributed time-spread multiple access for wireless mesh networks with IEEE 802.16d MAC protocol

This paper studies the use of IEEE 802.16d mesh MAC protocol for multi-hop networking in an indoor domestic environment. The mesh network is expected to support time-sensitive audio–video applications with stringent QoS requirement. In the literature, time-spread multiple access (TSMA) is a promising technology to provide a minimum throughput guarantee in a multi-hop mesh network with dynamic topology. However, existing TSMA schemes require the number of nodes in the entire network and their global maximum node degree, be known a priori to a central controller. The requirement is not practical. In view of this problem, this paper proposes a distributed time-spread multiple access (DTSMA) scheme. The proposed DTSMA has the following main contributions: (a) A method for each node to determine locally its polynomial coefficients without a priori global knowledge of node number and maximum node degree, and (b) A method to distribute to neighbours the locally determined polynomial coefficients, and to resolve collision between two sets of identical polynomial coefficients from two neighbouring nodes. The proposed DTSMA has been evaluated through extensive simulations to confirm that it can indeed preserve the capability of providing a minimum throughput guarantee in the absence of the a priori global knowledge. In benchmark against the de facto distributed coordinated scheduling (DCS) in the original IEEE 802.16d mesh MAC protocol under various domestic wireless channel conditions, DTSMA outperforms in terms of packet delivery ratio and average end-to-end packet delay which are important metrics for time-sensitive audio–video applications. Simulation results also show that DTSMA outperforms TSMA in terms of average end-to-end packet delay and average delay jitter when the severity of propagation impairment is high.     

How to Effectively Use Multiple Channels in Wireless Mesh Networks

Operating on a frequency band occupying several nonoverlapping channels, IEEE 802.11 is now widely used in wireless mesh networks (WMNs). Many multichannel MAC protocols are proposed to improve the spatial reuse in the network under the assumption that the transmissions on nonoverlapping channels do not interfere with each other. Some joint routing and channel assignment algorithms are also designed to increase the network throughput based on the premise that we can switch between different channels freely. Although simulations show that great improvements on network throughput can be observed in both cases, two fundamental questions remain: 1) Can we really use multiple nonoverlapping channels freely in WMNs? 2) If we can, what will be the cost when we switch channels dynamically and frequently? In this paper, by conducting extensive experiments on our testbed, we attempt to answer these questions. We find that in spite of interference between both overlapping and nonoverlapping channels, we can still use multiple channels in mesh networks under certain conditions but with care. We also show that the channel switching cost is actually very significant in WMNs. We recommend not to switch the channels too frequently when designing the channel assignment algorithms, and those channel assignment algorithms selecting one channel for each packet are not really beneficial.     

Design and implementation of MobiSEC: A complete security architecture for wireless mesh networks

Wireless mesh networks (WMNs) have emerged recently as a technology for next-generation wireless networking. They consist of mesh routers and clients, where mesh routers are almost static and form the backbone of WMNs. WMNs provide network access for both mesh and conventional clients.In this paper we propose MobiSEC, a complete security architecture that provides both access control for mesh users and routers as well as a key distribution scheme that supports layer-2 encryption to ensure security and data confidentiality of all communications that occur in the WMN.MobiSEC extends the IEEE 802.11i standard exploiting the routing capabilities of mesh routers; after connecting to the access network as generic wireless clients, new mesh routers authenticate to a central server and obtain a temporary key that is used both to prove their credentials to neighbor nodes and to encrypt all the traffic transmitted on the wireless backbone links.A key feature in the design of MobiSEC is its independence from the underlying wireless technology used by network nodes to form the backbone. Furthermore, MobiSEC allows seamless mobility of both mesh clients and routers.MobiSEC has been implemented and integrated in MobiMESH, a WMN implementation that provides a complete framework for testing and analyzing the behavior of a mesh network in real-life environments. Moreover, extensive simulations have been performed in large-scale network scenarios using Network Simulator.Numerical results show that our proposed architecture considerably increases the WMN security, with a negligible impact on the network performance, thus representing an effective solution for wireless mesh networking.     

Load-balanced AP association in multi-hop wireless mesh networks

Wireless mesh networks (WMNs) provide high-bandwidth wireless access, which makes it capable for multimedia services. A user in a WMN may be covered by multiple APs, while it should be associated with only one to access the Internet. Conventional IEEE 802.11 user-AP association mechanism for WLANs employs signal strength as the sole metric. However, this may lead to network congestion and performance degradation in multi-hop networks such as WMNs, especially for multimedia services that require a large bandwidth and a low latency. Thus AP association in WMNs becomes an important research issue. In this paper we propose a novel AP association approach LBAA, taking AP’s load-balancing, WMN’s multi-hop characteristic, and user’s RSSI into consideration. We first propose a centralized algorithm and then extend it to a distributed one, with the latter one more practical and convenient for deployment. Network throughput and max–min user fairness are improved by LBAA. Performance evaluation demonstrating the benefits of our approach is given through a series of experiments in terms of collision probability, access bandwidth, end-to-end throughput, and average RSSI.     

On-Demand Medium Access in Multihop Wireless Networks with Multiple Beam Smart Antennas

The paper presents a detailed discussion of various issues involved in designing a medium access control (MAC) protocol for multihop wireless networks with nodes employing multiple beam smart antennas. Multiple beam smart antennas can form several beams simultaneously and can initiate concurrent transmissions or receptions .in them, thereby increasing the throughput of the bottleneck nodes. Traditional on-demand MAC protocols for omnidirectional and single beam directional antennas based on the IEEE 802.11 distributed coordination function (DCF) mechanism cannot take advantage of this unique capability of multiple beam antennas as they do not facilitate concurrent transmissions or receptions by a node. This paper introduces a novel protocol, hybrid MAC (HMAC), which enables concurrent packet reception (CPR) and concurrent packet transmission (CPT) at a node equipped with multiple beam antennas and is backward compatible with IEEE 802.11 DCF. Simulation results show the superior performance of HMAC in most ad hoc scenarios. Moreover, in some sample topologies, the throughput of HMAC is close to the theoretical maximum. The paper also presents a wireless mesh network architecture with heterogeneous antenna technologies and illustrates the advantages of employing multiple beam smart antennas and HMAC in such networks.     

一种基于邻接节点信息的合作式Ad Hoc网络协议

在802.11协议中,DCF（Distributed Coordination Function）机制是节点共享无线信道进行数据传输的基本接入方式,为了解决无线网络中隐藏节点问题,使用RTS/CTS机制减少冲突,然而当网络节点数增加时,节点传输的冲突次数亦增加,从而使网络性能明显下降。因此,需要设计新的MAC协议,以适应当前Ad Hoc网络应用的快速发展。在IEEE 802.11的分布式协调功能访问机制（DCF）基础上,本文设计新的节点合作式的网络协议（C-MAC）。C-MAC节点通过控制帧获得本节点2跳内的邻接节点信息,并且根据邻接节点的信息设计调度算法,使节点以轮询的合作方式传输数据,有效地避免冲突。仿真实验表明,在改变节点速率、帧长度、网络节点数等参数情况下,分别以吞吐量、单帧传输时间和公平性为指标,对DCF和C MAC协议进行性能比较。在节点传输速率为11Mbps时,C MAC协议吞吐量比标准DCF最多可增加50%。     

Application-aware scheduling for VoIP in Wireless Mesh Networks

Wireless Mesh Networks (WMNs) are seen as a means to provide last mile connections in Next Generation Networks (NGNs). Because of their auto-configuration capabilities and the low deployment cost WMNs are considered to be an efficient solution for the support of multiple voice, video and data services in NGNs. This paper looks at the optimal provision of resources in WMNs for Voice over IP (VoIP) traffic, which has strict performance requirements in terms of delay, jitter and packet loss. In WMNs, because of the challenges introduced by wireless multi-hop transmissions and limited resources, providing performance quality for VoIP comparable to the voice quality in the traditional circuit-switched networks is a major challenge.This paper analyses different scheduling mechanisms for TDMA-based access control in mesh networks as specified in the IEEE 802.16-2004 WiMAX standard. The performance of the VoIP applications when different scheduling mechanisms are deployed is analysed on a variety of topologies using ns-2 simulation and mathematical analysis. The paper concludes that on-demand scheduling of VoIP traffic – typically deployed in 802.11-based WMNs – is not able to provide the required VoIP quality in realistic mesh WiMAX network scenarios and is therefore not optimal from a network operator’s point of view. Instead, it is shown, that continuous scheduling is much better suited to serve VoIP traffic. The paper then proposes a new VoIP-aware resource coordination scheme and shows, through simulation, that the new scheme is scalable and provides good quality for VoIP service in a wide range of network scenarios. The results shown in the paper prove that the new scheme is resilient to increasing hop count, increasing number of simultaneous VoIP sessions and the background traffic load in the network. Compared to other resource coordination schemes the VoIP-aware scheduler significantly increases the number of supported calls.     

一种无线网络中有效的MAC层信道竞争方案

在研究IEEE802．11DCF的基础上．提出了一种简单有效的无线局域网MAC层冲突解决机制（PBCR）．主要通过均匀分布站点的退避计数器值来达到减少冲突的目的．建模的计算结果和仿真的统计结果都表明PBCR在性能上，包括吞吐量，公平因子以及发送延迟等方面比DCF有显著的提高．     

无线网状网的QoS研究

作为下一代无线通信网络的关键技术,无线网状网能够融合异构网络,满足多类型的业务需求,因此必须提供一定的服务质量(QoS)保证.对目前各种QoS体系结构进行了分析,讨论了无线网状网的QoS体系结构.针对无线网状网网络层以下各层的QoS问题,对近年来国内外在功率控制、无线环境感知、支持QoS的MAC协议、QoS路由以及跨层QoS设计等方向所取得的研究成果进行了全面的概括总结和比较分析.最后对未来的研究发展趋势提出了自己的观点.     

无线Mesh网络中MAC协议的研究分析

无线Mesh网络由于其低花销,普遍存在的特性正成为一种最有前途的宽带接入网.由于WNNs自身的多跳特性,使得标准IEEE802.11MAC协议不适合WMNs的要求.首先对WMNs做了简要介绍,分析了WMNs MAC协议设计面临的问题,对当前较受关注的MAC协议进行综述分析比较,为下一步对MAC协议的研究提出了展望.     

OLSR-aware channel access scheduling in wireless mesh networks

Wireless mesh networks (WMNs) have emerged as a key technology having various advantages, especially in providing cost-effective coverage and connectivity solutions in both rural and urban areas. WMNs are typically deployed as backbone networks, usually employing spatial TDMA (STDMA)-based access schemes which are suitable for the high traffic demands of WMNs. This paper aims to achieve higher utilization of the network capacity and thereby aims to increase the application layer throughput of STDMA-based WMNs. The central idea is to use optimized link state routing (OLSR)-specific routing layer information in link layer channel access schedule formation. This paper proposes two STDMA-based channel access scheduling schemes (one distributed, one centralized) that exploit OLSR-specific information to improve the application layer throughput without introducing any additional messaging overhead. To justify the contribution of using OLSR-specific information to the throughput, the proposed schemes are compared against one another and against their non-OLSR-aware versions via extensive ns-2 simulations. Our simulation results verify that utilizing OLSR-specific information significantly improves the overall network performance both in distributed and in centralized schemes. The simulation results further show that OLSR-aware scheduling algorithms attain higher end-to-end throughput although their non-OLSR-aware counterparts achieve higher concurrency in slot allocations.     

基于IEEE 802.15.6的无线体域网自适应MAC协议

针对无线体域网MAC协议自适应性不高且能效低的问题,提出一种基于IEEE 802.15.6的无线体域网自适应MAC协议（A-MAC）。对IEEE 802.15.6的超帧结构进行改进,竞争接入阶段和非竞争接入阶段的长度根据产生各优先级数据的节点所占的比例进行调整。竞争接入阶段又被划分为三个子阶段,子阶段的长度根据数据优先级情况进行动态调整。所有节点在竞争接入阶段按照信道接入策略竞争接入信道。最后仿真结果表明,在吞吐量、能耗和网络时延方面,使用A-MAC协议的网络性能明显优于使用IEEE 802.15.6 MAC和CA-MAC协议的网络性能。     

Backpressure scheduling in IEEE 802.11 wireless mesh networks: Gap between theory and practice

Achieving efficient bandwidth utilization in wireless networks requires solving two important problems: (1) which packets to send (i.e., packet scheduling) and (2) which links to concurrently activate (i.e., link scheduling). To address these scheduling problems, many algorithms have been proposed and their throughput optimality and stability are proven in theory. One of the most well-known scheduling algorithms is backpressure scheduling which performs both link and packet scheduling assuming a TDMA (Time Division Multiple Access) MAC (Medium Access Control) layer. However, there has been limited work on realizing backpressure scheduling with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) MAC layer (e.g., IEEE 802.11). In IEEE 802.11 networks, it is expected that the throughput optimality will not be achieved. In this paper, we investigate the extent of this throughput gap between theoretical TDMA-based backpressure scheduling and an approximation of it for IEEE 802.11 WMNs (Wireless Mesh Networks). Through extensive testbed measurements, we verify that there is indeed a non-negligible throughput gap. We present two main reasons behind this gap: Control inaccuracy that results from approximation of link scheduling and information inaccuracy that results from late or incorrect information, for instance, about queue lengths or network topology. Our results show that losses by MAC-layer collisions and backoff, which mainly occur due to control inaccuracy plays a major role for the throughput gap. On the other hand, while losses by queue drops, typically due to information inaccuracy, do occur, their effect can be tolerated. Nevertheless, both types of inaccuracies need to be mitigated in order to improve throughput.     

一种检测无线网络违规行为的改进退避算法

无线媒体访问控制（ＭＡＣ）协议通常使用分布式竞争机制来共享无线信道,但在动态和开放的网络环境中,部分违规节点会有意识地抢占信道以获取更多的信道资源．为此,通过对ＩＥＥＥ８０２．１１协议的ＤＣＦ 机制进行修改,提出一种改进的退避算法,可实现对网络中违规行为节点的有效检测,并通过惩罚机制加以纠正．仿真结果表明,该方法能够更有效地检测出无线网络中的违规行为,提高整个网络的吞吐量．