High-performance architectures for IP-based multihop 802.11 networks

The concept of a forwarding node, which receives packets from upstream nodes and then transmits these packets to downstream nodes, is a key element of any multihop network, wired or wireless. While high-speed IP router architectures have been extensively studied for wired networks, the concept of a "wireless IP router" has not been addressed so far. We examine the limitations of the IEEE 802.11 MAC protocol in supporting a low-latency and high-throughput IP datapath comprising multiple wireless LAN hops. We first propose a wireless IP forwarding architecture that uses MPLS with modifications to 802.11 MAC to significantly improve packet forwarding efficiency. We then study further enhancements to 802.11 MAC that improve system throughput by allowing a larger number of concurrent packet transmissions in multihop 802.11-based IP networks. With 802.11 poised to be the dominant technology for wireless LANs, we believe a combined approach to MAC, packet forwarding, and transport layer protocols is needed to make high-performance multihop 802.11 networks practically viable.     

Performance Analysis of IEEE 802.11 DCF in Imperfect Channels

IEEE 802.11 is the most important standard for wireless local area networks (WLANs). In IEEE 802.11, the fundamental medium access control (MAC) scheme is the distributed coordination function (DCF). To understand the performance of WLANs, it is important to analyze IEEE 802.11 DCF. Recently, several analytical models have been proposed to evaluate the performance of DCF under different incoming traffic conditions. However, to the best of the authors' knowledge, there is no accurate model that takes into account both the incoming traffic loads and the effect of imperfect wireless channels, in which unsuccessful packet delivery may occur due to bit transmission errors. In this paper, the authors address this issue and provide an analytical model to evaluate the performance of DCF in imperfect wireless channels. The authors consider the impact of different factors together, including the binary exponential backoff mechanism in DCF, various incoming traffic loads, distribution of incoming packet size, queueing system at the MAC layer, and the imperfect wireless channels, which has never been done before. Extensive simulation and analysis results show that the proposed analytical model can accurately predict the delay and throughput performance of IEEE 802.11 DCF under different channel and traffic conditions.     

Quality-of-service provisioning system for multimedia transmission in IEEE 802.11 wireless LANs

IEEE 802.11, the standard of wireless local area networks (WLANs), allows the coexistence of asynchronous and time-bounded traffic using the distributed coordination function (DCF) and point coordination function (PCF) modes of operations, respectively. In spite of its increasing popularity in real-world applications, the protocol suffers from the lack of any priority and access control policy to cope with various types of multimedia traffic, as well as user mobility. To expand support for applications with quality-of-service (QoS) requirements, the 802.11E task group was formed to enhance the original IEEE 802.11 medium access control (MAC) protocol. However, the problem of choosing the right set of MAC parameters and QoS mechanism to provide predictable QoS in IEEE 802.11 networks remains unsolved. In this paper, we propose a polling with nonpreemptive priority-based access control scheme for the IEEE 802.11 protocol. Under such a scheme, modifying the DCF access method in the contention period supports multiple levels of priorities such that user handoff calls can be supported in wireless LANs. The proposed transmit-permission policy and adaptive bandwidth allocation scheme derive sufficient conditions such that all the time-bounded traffic sources satisfy their time constraints to provide various QoS guarantees in the contention free period, while maintaining efficient bandwidth utilization at the same time. In addition, our proposed scheme is provably optimal for voice traffic in that it gives minimum average waiting time for voice packets. In addition to theoretical analysis, simulations are conducted to evaluate the performance of the proposed scheme. As it turns out, our design indeed provides a good performance in the IEEE 802.11 WLAN's environment, and can be easily incorporated into the hybrid coordination function (HCF) access scheme in the IEEE 802.11e standard.     

SoftMAC: Layer 2.5 Collaborative MAC for Multimedia Support in Multihop Wireless Networks

In this paper, we present the challenges in supporting multimedia, in particular, VoIP services over multihop wireless networks using commercial IEEE 802.11 MAC DCF hardware, and propose a novel software solution, called Layer 2.5 SoftMAC. Our proposed SoftMAC resides between the IEEE 802.11 MAC layer and the IP layer to coordinate the real-time (RT) multimedia and best-effort (BE) data packet transmission among neighboring nodes in a multihop wireless network. To effectively ensure acceptable VoIP services, channel busy time and collision rate need to be well controlled below appropriate levels. Targeted at this, our SoftMAC architecture employs three key mechanisms: 1) distributed admission control for regulating the load of RT traffic, 2) rate control for minimizing the impact of BT traffic on RT one, and 3) nonpreemptive priority queuing for providing high priority service to VoIP traffic. To evaluate the efficacy of these mechanisms, extensive simulations are conducted using the network simulator NS2. We also implement our proposed SoftMAC as a Windows network driver interlace specification (NDIS) driver and build a multihop wireless network testbed with 32 wireless nodes equipped with IEEE 802.11 a/b/g combo cards. Our evaluation and testing results demonstrate the effectiveness of our proposed software solution. Our proposed collaborative SoftMAC framework can also provide good support for A/V streaming in home networks where the network consists of hybrid WLAN (wireless LAN) and Ethernet     

A MAC-Layer QoS Provisioning Protocol for Cognitive Radio Networks

Due to the proliferation of diverse network devices with multimedia capabilities, there is an increasing need for Quality of Service (QoS) provisioning in wireless networks. The MAC layer protocol with enhanced distributed channel access (EDCA) in the IEEE 802.11-2007 is able to provide differentiated QoS for different traffic types in wireless networks through varying the Arbitration Inter-Frame Spaces (AIFS) and contention window sizes. However, the performance of high priority traffic can be seriously degraded in the presence of strong noise over the wireless channels. Schemes utilizing adaptive modulation and coding (AMC) technique have also been proposed for the provisioning of QoS. They can provide limited protection in the presence of noise but are ineffective in a high noise scenario. Although multiple non-overlapped channels exist in the 2.4 and 5?GHz spectrum, most IEEE 802.11-based multi-hop ad hoc networks today use only a single channel at anytime. As a result, these networks cannot fully exploit the aggregate bandwidth available in the radio spectrum provisioned by the standards. By identifying vacant channels through the use of cognitive radios technique, the noise problem can be mitigated by distributing network traffic across multiple vacant channels to reduce the node density per transmission channel. In this paper, we propose the MAC-Layer QoS Provisioning Protocol (MQPP) for 802.11-based cognitive radio networks (CRNs) which combines adaptive modulation and coding with dynamic spectrum access. Simulation results demonstrate that MQPP can achieve better performance in terms of lower delay and higher throughput.     

A Load-Balancing and Push-Out Scheme for Supporting QoS in MANETs

Currently, mobile ad hoc networks (MANETs) lack load-balancing capabilities, and thus, they fail to provide good performance especially in the case of a large volume of traffic. Ad hoc networks lack also service differentiation. However, in these wireless environments, where channel conditions are variable and bandwidth is scarce, the differentiated services developed for the Internet are suboptimal without lower layers' support. The IEEE 802.11 standard for Wireless LANs is the most widely used WLAN standard today. It has a mode of operation that can be used to provide service differentiation, but it has been shown to perform badly. In this paper, we present a simple but very effective method for support Quality of Service, by the use of load-balancing and push-out scheme. This approach offers to the mobile node: the ability to alleviate congestion by traffic distribution of excessive load, and to support priority of packets in the single MAC buffer. We evaluate the performance of our algorithm and compare it with the original IEEE 802.11b protocol. Simulation results show that this new approach reduces packet loss rate and increases throughput as well as provides service differentiation in the MAC layer.     

A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs

The admission control algorithm that can be performed at the MAC (Medium Access Control) layer in a real-time is proposed for the decision for accepting or rejecting the requests for adding traffic streams to an IEEE 802.11e wireless LAN (Local Area Network). In numerical examples, we apply the proposed admission control algorithm to VOIP (Voice Over Internet Protocol) traffic streams, and obtain the maximum numbers of VOIP traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs for various delay requirements.     

IEEE 802.11 Wireless LAN: can we use it for multimedia?

The IEEE P802.11 committee developed the 802.11 Wireless LAN standard to cover wireless networks for fixed, portable, and moving stations within a local area. This standard addresses the need for wireless connectivity to stations, equipment, or automatic machinery that requires rapid deployment and may be portable, handheld, or mounted on moving vehicles. It can function totally wireless or connected to a wired network. Most people familiar with the standard expect to use it in providing wireless networks for personal computers or stations connected to the global wired infrastructure through access points. Now that the 802.11 standard is finally here, it will energize the wireless LAN market and result in the proliferation of low cost wireless connectivity in the office and home. Study groups are working on higher rates at 2.4 GHz and at 5 GHz for future inclusion into the standard. These higher rates will make it even more practical to employ this standard for multimedia traffic     

无线Ad Hoc网络中基于业务感知的多信道MAC协议(英文)

Existing multi-channel Medium Access Control (MAC) protocols have been demonstrated to significantly increase wireless network performance compared to single channel MAC protocols. Traditionally, the channelization structure in IEEE 802.11 based wireless networks is pre-configured, and the entire available spectrum is divided into subchannels and equal channel widths. In contrast, this paper presents a Traffic-Aware Channelization MAC (TAC-MAC) protocol for wireless ad hoc networks, where each node is equipped with a single half duplex transceiver. TAC-MAC works in a distributed, fine-grai-ned manner, which dynamically divides variable-width subchannels and allocates subchannel width based on the Orthogonal Frequency Division Multiplexing (OFDM) technique according to the traffic demands of nodes. Simulations show that the TAC-MAC can significantly improve network throughput and reduce packet delay compared with both fixed-width multi-channel MAC and single channel 802.11 protocols, which illustrates a new paradigm for high-efficient multi-channel MAC design in wireless ad hoc networks.     

A survey of QoS enhancements for IEEE 802.11 wireless LAN

Quality‐of‐service (QoS) is a key problem of today's IP networks. Many frameworks (IntServ, DiffServ, MPLS etc.) have been proposed to provide service differentiation in the Internet. At the same time, the Internet is becoming more and more heterogeneous due to the recent explosion of wireless networks. In wireless environments, bandwidth is scarce and channel conditions are time‐varying and sometimes highly lossy. Many previous research works show that what works well in a wired network cannot be directly applied in the wireless environment. Although IEEE 802.11 wireless LAN (WLAN) is the most widely used IEEE 802.11 wireless LAN (WLAN) standard today, it cannot provide QoS support for the increasing number of multimedia applications. Thus, a large number of 802.11 QoS enhancement schemes have been proposed, each one focusing on a particular mode. This paper summarizes all these schemes and presents a survey of current research activities. First, we analyze the QoS limitations of IEEE 802.11 wireless MAC layers. Then, different QoS enhancement techniques proposed for 802.11 WLAN are described and classified along with their advantages/drawbacks. Finally, the upcoming IEEE 802.11e QoS enhancement standard is introduced and studied in detail. Copyright © 2004 John Wiley & Sons, Ltd.     

A traffic control system for IEEE 802.11 networks based on available bandwidth estimation

To support Quality of service (QoS)‐sensitive applications like real‐time video streaming in IEEE 802.11 networks, a MAC layer extension for QoS, IEEE 802.11e, has been recently ratified as a standard. This MAC layer solution, however, addresses only the issue of prioritized access to the wireless medium and leaves such issues as QoS guarantee and admission control to the traffic control systems at the higher layers. This paper presents an IP‐layer traffic control system for IEEE 802.11 networks based on available bandwidth estimation. We build an analytical model for estimating the available bandwidth by extending an existing throughput computation model, and implement a traffic control system that provides QoS guarantees and admission control by utilizing the estimated available bandwidth information. We have conducted extensive performance evaluation of the proposed scheme via both simulations and measurements in the real test‐bed. The experiment results show that our estimation model and traffic control system work accurately and effectively in various network load conditions without IEEE 802.11e. The presence of IEEE 802.11e will allow even more efficient QoS provision, as the proposed scheme and the MAC layer QoS support will complement each other. Copyright © 2006 John Wiley & Sons, Ltd.     

A Smart TCP Acknowledgment Approach for Multihop Wireless Networks

Reliable data transfer is one of the most difficult tasks to be accomplished in multihop wireless networks. Traditional transport protocols like TCP face severe performance degradation over multihop networks given the noisy nature of wireless media as well as unstable connectivity conditions in place. The success of TCP in wired networks motivates its extension to wireless networks. A crucial challenge faced by TCP over these networks is how to operate smoothly with the 802.11 wireless MAC protocol which also implements a retransmission mechanism at link level in addition to short RTS/CTS control frames for avoiding collisions. These features render TCP acknowledgments (ACK) transmission quite costly. Data and ACK packets cause similar medium access overheads despite the much smaller size of the ACKs. In this paper, we further evaluate our dynamic adaptive strategy for reducing ACK-induced overhead and consequent collisions. Our approach resembles the sender side's congestion control. The receiver is self-adaptive by delaying more ACKs under nonconstrained channels and less otherwise. This improves not only throughput but also power consumption. Simulation evaluations exhibit significant improvement in several scenarios     

Infrastructure-based MAC in wireless mobile ad-hoc networks

The IEEE 802.11 standard is the most popular Medium Access Control (MAC) protocol for wireless local area networks. However, in an ad-hoc environment, the Point Coordination Function (PCF), defined in the standard, cannot be readily used. This is due to the fact that there is no central authority to act as a Point Coordinator (PC). Peer-to-peer ad-hoc mode in the IEEE 802.11 standard only implements the Distributed Coordination Function (DCF). In this paper, an efficient and on-the-fly infrastructure is created using our proposed Mobile Point Coordinator (MPC) protocol. Based on this protocol, we also develop an efficient MAC protocol, namely MPC–MAC. Our MAC protocol extends the IEEE 802.11 standard for use in multi-hop wireless ad-hoc networks implementing both the DCF and PCF modes of operation. The goal, and also the challenge, is to achieve QoS delivery and priority access for real-time traffic in ad-hoc wireless environments while maintaining backward compatibility with the IEEE 802.11 standard. The performance of MPC–MAC is compared to the IEEE 802.11 DCF-based MAC without MPC. Simulation experiments show that in all cases the use of PCF benefits real-time packets by decreasing the average delay and the discard ratio. However, this may come at the expense of increasing the average delay for non-real-time data. On the other hand, the discard ratio for both real-time and non-real-time packets improves with the use of PCF. Therefore, our MPC–MAC outperforms the standard DCF IEEE 802.11 MAC protocol in multi-hop ad-hoc environments.     

Busy tone contention protocol: a new high‐throughput and energy‐efficient wireless local area network medium access control protocol using busy tone

Design of an efficient wireless medium access control (MAC) protocol is a challenging task due to the time‐varying characteristics of wireless communication channel and different delay requirements in diverse applications. To support variable number of active stations and varying network load conditions, random access MAC protocols are employed. Existing wireless local area network (WLAN) protocol (IEEE 802.11) is found to be inefficient at high data rates because of the overhead associated with the contention resolution mechanism employed. The new amendments of IEEE 802.11 that support multimedia traffic (IEEE 802.11e) are at the expense of reduced data traffic network efficiency. In this paper, we propose a random access MAC protocol called busy tone contention protocol (BTCP) that uses out‐of‐band signals for contention resolution in WLANs. A few variants of this protocol are also proposed to meet the challenges in WLAN environments and application requirements. The proposed BTCP isolate multimedia traffics from background data transmissions and gives high throughput irrespective of the number of contending stations in the network. As a result, in BTCP, admission control of multimedia flows becomes simple and well defined. Studies of the protocol, both analytically and through simulations under various network conditions, have shown to give better performance in comparison with the IEEE 802.11 distributed coordination function. Copyright © 2011 John Wiley & Sons, Ltd.     

On the feasibility of bandwidth estimation in wireless access networks

Bandwidth estimation refers to the measurement of a bandwidth-related metric over a network path, performed only by the end hosts, without access to the intermediate routers. The problem of fast bandwidth estimation has been extensively studied in the wired Internet. More recently, researchers have shown that tools developed for the wired Internet cannot be used in wireless networks, due to the different characteristics of these networks which invalidate many of the assumptions made for the wired Internet. This observation has led to new tools that take into account the different characteristics of wireless networks. These tools have only been evaluated in controlled environments, under controlled settings and interferers, and only for 802.11 CSMA-based WLANs. In addition, no tool has been reported so far in the literature for cellular networks. This paper presents the first study of the feasibility of fast bandwidth estimation in wireless networks “in the wild”, i.e., in deployed operational networks. We discuss the challenges associated with bandwidth measurements in operational networks in contrast to in controlled lab environments. We study the applicability of a state-of-the-art probe-based tool for 802.11 WLANs in a commercial 1 × EVDO network. In addition, we evaluate the accuracy of the tool in three different WLAN environments, i.e., lab, home, and hotspot. Our results show that bandwidth estimation using a probe-based tool can be challenging in certain WLAN environments, and practically infeasible in EVDO cellular networks, due to the short-scale dynamics in this type of networks.     

Toward an improvement of H.264 video transmission over IEEE 802.11e through a cross-layer architecture

The recently developed H.264 video standard achieves efficient encoding over a bandwidth ranging from a few kilobits per second to several megabits per second. Hence, transporting H.264 video is expected to be an important component of many wireless multimedia services, such as video conferencing, real-time network gaming, and TV broadcasting. However, due to wireless channel characteristics and lack of QoS support, the basic 802.11-based channel access procedure is merely sufficient to deliver non-real-time traffic. The delivery should be augmented by appropriate mechanisms to better consider different QoS requirements and ultimately adjust the medium access parameters to the video data content characteristics. In this article we address H.264 wireless video transmission over IEEE 802.11 WLAN by proposing a robust cross-layer architecture that leverages the inherent H.264 error resilience tools (i.e., data partitioning); and the existing QoS-based IEEE 802.11e MAC protocol possibilities. The performances of the proposed architecture are extensively investigated by simulations. Results obtained indicate that compared to 802.11 and 802.11e, our cross-layer architecture allows graceful video degradation while minimizing the mean packet loss and end-to-end delays.     

SDN-based wireless user management system

The degradation of end-to-end quality of service (QoS) on mobile users is becoming a common issue for IEEE 802.11 infrastructure-based networks in crowded areas. This research tackles the issue by employing an SDN framework on an integrated wireless/wired environment. Thereby, we present the development and implementation of a system that performs user management by analyzing the network load from the OpenFlow statistics, as well as the wireless information collected from the associated users. In order to analyse the behaviour of the proposed user migration algorithm, we evaluate the system under scenarios with different traffic load and user session duration. From the experiments, we observed that in several cases wireless users get a considerable QoS improvement at the application layer (up to 30% improvement in throughput and up to 20% in delay in our simulations) once the system is activated. Based on the results, we present an analysis on how and when user migration in multi-access point IEEE 802.11 networks can be most effective.     

Medium Access Control for Integrated Multimedia Wireless Access with the Use of a Video Packet Discard Scheme

A well designed Medium Access Control (MAC) protocol for wireless networks should provide an efficient mechanism to share the limited bandwidth resources, and satisfy the diverse and usually contradictory Quality of Service (QoS) requirements of each traffic class. In this paper a new MAC protocol for next generation wireless communications is presented and investigated. The protocol uses a combined Packet Discard/Forward Error Correction scheme in order to efficiently integrate MPEG-4 videoconference packet traffic with voice, SMS data and web packet traffic over a noisy wireless channel of high capacity. Our scheme achieves high aggregate channel throughput in all cases of traffic load, while preserving the Quality of Service (QoS) requirements of each traffic type, and is shown to clearly outperform DPRMA, another efficient MAC protocol proposed in the literature for multimedia traffic integration over wireless networks. Dr. Polychronis Koutsakis was born in Hania, Greece, in 1974. He received his 5-year Diploma in Electrical Engineering in 1997 from the University of Patras, Greece and his MSc and Ph.D. degrees in Electronic and Computer Engineering in 1999 and 2002, respectively, from the Technical University of Crete, Greece. He was a Visiting Lecturer at the Electronic and Computer Engineering Department of the same University for three years (2003–2006). He is currently an Assistant Professor at the Electrical and Computer Engineering Department of McMaster University, Canada. His research interests focus on the design, modeling and performance evaluation of computer communication networks, and especially on the design and evaluation of multiple access schemes for multimedia integration over wireless networks, on call admission control and traffic policing schemes for both wireless and wired networks, on multiple access control protocols for mobile satellite networks, wireless sensor networks and powerline networks, and on traffic modeling. Dr. Koutsakis has authored more than 45 peer-reviewed papers in the above mentioned areas, has served as a Guest Editor for an issue of the ACM Mobile Computing and Communications Review, as a TPC member for conferences such as IEEE GLOBECOM, IEEE LCN and IEEE PerCom, will serve as Session Chair for the IEEE GLOBECOM 2006 Symposium on Satellite & Space Communications and serves as a reviewer for most of the major journal publications focused on his research field. Moisis Vafiadis was born in Elefsina, Greece, in 1980. He has recently completed his studies towards the Diploma in Electronic Engineering at the Technological Educational Institute of Crete, Greece. His research interests focus on wireless personal communication networks, and especially on the MAC layer and on the development and testing of wireless multimedia applications.     

Adaptive coordination function for IEEE 802.11 wireless LANs

Multiple access control (MAC) protocols play a significant role in wireless LANs. The IEEE 802.11 MAC protocol specifies two coordination functions that are Distributed Coordination Function (DCF) and Point Coordination Function (PCF). While both DCF and PCF are available in a wireless cell, we propose a novel access mechanism called Adaptive Coordination Function (ACF) to support various classes of traffic. The ACF superframe comprises two periods, one TDMA period designed for real-time traffic and followed by an adaptive period which adaptively employs DCF or PCF to support non-real-time traffic. In this paper, we apply the theory of M/G/1 queues to analyze the performance of adaptive period in terms of queuing delay, end-to-end delay, and saturation throughput. With our analytic model, DCF or PCF can be invoked appropriately according to the number of stations, packet arrival rate, packet payload size, and effective channel bit rate. Analytical results are derived for an extensive throughput and delay performance evaluation of both DCF and PCF.     

Throughput Analysis and Measurements in IEEE 802.11 WLANs with TCP and UDP Traffic Flows

There is a vast literature on the throughput analysis of the IEEE 802.11 media access control (MAC) protocol. However, very little has been done on investigating the interplay between the collision avoidance mechanisms of the 802.11 MAC protocol and the dynamics of upper layer transport protocols. In this paper, we tackle this issue from an analytical, simulative, and experimental perspective. Specifically, we develop Markov chain models to compute the distribution of the number of active stations in an 802.11 wireless local area network (WLAN) when long-lived transmission control protocol (TCP) connections compete with finite-load user datagram protocol (UDP) flows. By embedding these distributions in the MAC protocol modeling, we derive approximate but accurate expressions of the TCP and UDP throughput. We validate the model accuracy through performance tests carried out in a real WLAN for a wide range of configurations. Our analytical model and the supporting experimental outcomes show that 1) the total TCP throughput is basically independent of the number of open TCP connections and the aggregate TCP traffic can be equivalently modeled as two saturated flows; and 2) in the saturated regime, n UDP flows obtain about n times the aggregate throughput achieved by the TCP flows, which is independent of the overall number of persistent TCP connections.