A Simple Analytical Model for Expected Frame Waiting Time Evaluation in IEEE 802.11e HCCA Mode

IEEE 802.11e standard defines two access modes to provide Quality of Service support for 802.11-based Wireless LANs: Enhanced Distributed Channel Access and HCF Controlled Channel Access (HCCA). Only HCCA can guarantee bandwidth per flow which is suitable for various multimedia application requirements. However, the reference scheme HCCA is designed without awareness of variable bit rate (VBR) transmission. Several enhancement techniques have been proposed to address the VBR problem, but there are few research results for a model to capture the characteristics of HCCA. In this paper, an analytical model for the expected packet waiting time is proposed. The model can capture the system behavior per session leading to suitable admission control process for VBR support. The proposed model has been validated using the Network Simulator. The results confirm that the reference admission control scheme with mean data rate acceptance cannot guarantee the required bound on waiting time. Using the model, suitable parameters can be investigated for admission control with VBR support. The model can also be applied with polling based technologies, such as WiMAX.     

Utilization Model for HCCA EDCA Mixed Mode in IEEE 802.11e

This letter proposes an analytical model to characterize medium utilization in IEEE 802.11e operating in HCCA‐EDCA mixed mode (HEMM). In contrast to existing works which model the backoff process in individual stations, we consider the channel occupancy pattern. Additionally, our work considers the operation of HEMM, which is not widely documented. We show that the proposed model accurately characterizes medium utilization with no more than 5% error.     

Delay guaranteed MDP scheduling scheme for HCCA based on 802.11p protocol in V2R environments

In recent years, the vehicular ad hoc network has attracted worldwide attention from academe and industry. Many researches have been executed to improve the quality of services (QoS) of the intelligent transportation system. However, current existing channel access schemes at the medium access control layer specified in 802.11 protocol, including hybrid coordination function control channel access (HCCA) and enhanced distributed channel access, could not efficiently achieve the QoS requirements in some special situations. This paper proposes a delay guaranteed HCCA (DG‐HCCA) scheduling scheme, which works based on a Markov decision process model and the measurement of historic performance, to guarantee the QoS enhanced data transmission for vehicles to roadside unit. Besides, this paper also presents a performance analysis model to systematically evaluate the system performance of the channel utility and the average delay. The performance of the proposed delay guaranteed HCCA scheduling scheme is compared with that of original HCCA scheme specified in 802.11p standard and other 2 HCCA improved schemes by the simulation experiments. The simulation results demonstrate that the proposed solution could highly fulfill the transmission delay requirements with a better channel utility and less loss rates than those by the standard HCCA scheme and other 2 schemes.     

Performance improvements of HCCA scheduling in V2R environments

This paper addresses the issue of real‐time data transmission in vehicles to roadside environment by the hybrid coordination function controlled channel access (HCCA) scheme with Institute of Electrical and Electronics Engineers 802.11p protocol. The HCCA is one of the medium access protocols specified in 802.11p standard, which uses a polling scheme similar to the point coordination function to provide a reliable QoS. In this paper, we improve the HCCA polling scheme and design a new data transmission scheduling method. We also prepared a mathematical model to systematically evaluate the performance of the HCCA in terms of average delay. Moreover, we analyze the factors that could impact the average delay, throughput, and packet loss rate as well. The simulation results are compared with the original scheme specified in the standard. The simulation results demonstrate that the proposed delay model could estimate the average delay of the HCCA with a high precision, whereas the proposal of the enhancement of the HCCA has lower delay, lower loss rate, and higher throughput than the one specified in the standard. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhancement of QoS support of HCCA schedulers using EDCA function in IEEE 802.11e networks

The IEEE 802.11e standard introduces Quality of Service support for wireless local area networks through two MAC functions: Enhanced Distributed Channel Access (EDCA) and HCF Controlled Channel Access (HCCA). While the former provides prioritized contention-based access to the medium, the latter uses a parameterized contention-free polling scheme. Several studies have proposed enhancements to EDCA or improved scheduling algorithms for HCCA to properly support VBR traffic. However, the cooperation between these functions has only marginally been considered and the solutions vary depending on specific traffic requirements.In this paper we propose a novel approach to address the problem of scheduling VBR traffic streams. Our scheduler, named Overboost, uses HCCA to negotiate a minimum bandwidth and deals with traffic streams that require more bandwidth than the negotiated one by redirecting the excess bandwidth to the EDCA function. An analytical evaluation has been conducted and the results has been corroborated by an extensive set of simulations. They show that the overall scheduler improves the performance with respect to other HCCA schedulers in terms of null rate, throughput, access delay, and queue length.     

Improvement of polling and scheduling scheme for real-time transmission with HCCA of IEEE 802.1 lp protocol

This paper addresses the issue of real-time data transmission in vehicles to roadside （V2R） environment by the hybrid coordination function （HCF） of controlled channel access （HCCA） specified by the IEEE 802.1 lp standard. HCCA is one of the medium access mechanisms in the IEEE 802.11 standard, and uses the polling scheme similar to the point coordination function （PCF） to provide reliable quality of service （QoS）, which may cause resource overshooting and high time consuming. This paper tries to improve the performance of the HCCA polling scheme and designs new data transmission scheduling scheme. The simulation results of the proposal are compared with those of the standard strategy. Simulation results demonstrate that the improved HCCA has lower delay, loss rate, and higher throughput than those of the standard one.     

DPS: An Architecture for VBR Scheduling in IEEE 802.11e HCCA Networks with Multiple Access Points

Recent advances in 802.11 Wireless Local Area Networks (WLANs) have been focused on introducing Quality of Service (QoS) in their performance through the adoption of 802.11e. The Hybrid Controlled Channel Access (HCCA) has been proposed as the mechanism to provide the means for guaranteed QoS in networks controlled by a single Access Point (AP). Moreover, scheduling algorithms have been developed allowing efficient scheduling of Variable Bit Rate (VBR) traffic flows. However, little research may be found regarding the problem of applying HCCA on networks with multiple APs. In this paper, various VBR scheduling mechanisms and methods for multi—AP HCCA are presented and evaluated. In addition, this paper introduces the Dynamic Parallel Scheduler (DPS), a novel HCCA scheduling algorithm for VBR traffic among multiple APs that takes advantage of the rate variability and spatial reuse. DPS’s performance evaluation shows that the proposed scheme achieves improved performance in terms of delay, throughput and packet loss.     

IEEE802．11eWLAN中基于HCCA机制的带宽分配算法的研究

随着使用WLAN（无线局域网）接入因特网的日渐普及,IEEE802．11eWLAN中的HCCA（混合控制信道接入）机制由于在处理实时多媒体业务的高效率受到越来越广泛的关注。在WLAN中为达到高带宽利用率和良好的端到端QoS,介绍了一种基于HCCA机制的带宽分配算法,同时对该算法与802．11PCF（点协调功能）两种接入场景下的网络性能进行仿真比较,其结果验证了该算法可以为WLAN提供良好的QoS保证,对在WLAN中传输语音、视频等实时业务具有一定的指导意义。     

High performing multimedia transmission approach based on QoS support and admission control over IEEE 802.11e networks

Hybrid coordination function controlled channel access (HCCA) is a medium to enhance quality of service (QoS) via the IEEE 802.11e standard. The main limitation of HCAA is that it is only efficient for constant bit rate (CBR) applications. This is due to the nature of its scheduler that allocates transmission opportunities (TXOPs) based on traffic stream (TS) specifications (TSPECs) that are determined during the traffic setup time. Variable bit rate (VBR) traffics used in HCCA have nondeterministic profile, making it not optimally and efficiently supported by HCCA. The result of this inefficiency is a deterioration of the transmission performance of multimedia data as well as a drop in the number of served QoS video traffics. We propose a novel approach to deal with this issue, which is the feedback‐based admission control unit (FACU). FACU works by optimizing the usage of extra bandwidth to ensure optimal transmission performance of multimedia data. FACU achieves this by exploiting piggybacked information concerning sequential video frames in order to accurately assign the TXOP. The proposed approach is evaluated by utilizing various video sequences. It is demonstrated that FACU maximizes the overall number of video streams and optimizes the overall usage of the network without having any adverse effects on the QoS constraints determined.     

On delay constrained CAC scheme and scheduling policy for CBR traffic in IEEE 802.11e wireless LANs

CBR (constant bit rate) traffic is expected to be an important traffic source in wireless networks. Such sources usually have stringent jitter or delay requirements and in many cases they should be delivered exactly as they were generated. In this paper, we propose a strictly guaranteed QoS (quality‐of‐service) provisioning CAC (call admission control) scheme with a polling‐based scheduling policy for CBR traffic in IEEE 802.11e wireless LANs. Under such a scheme, the proposed transmit‐permission policy for HCCA (HCF controlled channel access) method can forecast the maximum suffered delay for each packet and derive sufficient conditions so that all the CBR sources satisfy their time constraints to provide deterministic QoS guarantees. A simple analytical model is carried out to estimate the average queueing delay of the proposed scheme. In addition to theoretical analysis, simulations are conducted to validate its promising performance. Our simulation results show that the proposed scheme maintains a high throughput with respect to the whole range of system load. Copyright © 2009 John Wiley & Sons, Ltd.     

Admission control in IEEE 802.11e wireless LANs

Although IEEE 802.11 based wireless local area networks have become more and more popular due to low cost and easy deployment, they can only provide best effort services and do not have quality of service supports for multimedia applications. Recently, a new standard, IEEE 802.11e, has been proposed, which introduces a so-called hybrid coordination function containing two medium access mechanisms: contention-based channel access and controlled channel access. In this article we first give a brief tutorial on the various MAC-layer QoS mechanisms provided by 802.11e. We show that the 802.11e standard provides a very powerful platform for QoS supports in WLANs. Then we provide an extensive survey of recent advances in admission control algorithms/protocols in IEEE 802.11e WLANs. Our survey covers the research work in admission control for both EDCA and HCCA. We show that the new MAC-layer QoS schemes and parameters provided in EDCA and HCCA can be well utilized to fulfill the requirements of admission control so that QoS for multimedia applications can be provided in WLANs. Last, we give a summary of the design of admission control in EDCA and HCCA, and point out the remaining challenges.     

Removing redundant TCP functionalities in wired‐cum‐wireless networks with IEEE 802.11e HCCA support

The TCP was originally designed for wired networks, assuming transmission errors were negligible. Actually, any acknowledgment time‐out unconditionally triggers the congestion control mechanism, even in wireless networks in which this assumption is not valid. Consequently, in wireless networks, TCP performance significantly degrades. To avoid this degradation, this paper proposes the so‐called split TCP and UDP. In this approach, the access point splits the TCP connection and uses a customized and lighter transport protocol for the wireless segment. It takes advantage of the IEEE 802.11e Hybrid Coordination Function Controlled Channel Access (HCCA) mechanisms to remove redundant TCP functionalities. Specifically, the HCCA scheduler allows disabling of the congestion control in the wireless link. Similarly, the IEEE 802.11e error control service makes possible to eliminate TCP acknowledgments, therefore reducing the TCP protocol overhead. Finally, the usage of an HCCA scheduler permits providing fairness among the different data flows. The proposed split scheme is evaluated via extensive simulations. Results show that split TCP and User Datagram Protocol outperforms the analyzed TCP flavors—specifically designed for wireless environments—and the split TCP solution, achieving up to 95% of end‐user throughput gain. Furthermore, the proposed solution is TCP friendly because TCP flows are not degraded by the presence of flows by using this approach. Copyright © 2013 John Wiley & Sons, Ltd.     

基于IEEE802.11e HCCA的接入控制方案

较传统的IEEE802.11标准的MAC协议,802.11e增强了对QoS的支持力度,主要基于IEEE802.11e的HCCA机制,提出了一套控制接入方案,本方案在全面考虑QoS参数的基础上提出了一个有效的调度算法,从而确保了实时性业务的QoS需要.     

IEEE802.11e EDCA和CFB下饱和吞吐量分析

IEEE802.11eMAC草案规范对IEEE802.11无线局域网标准在QOS方面加以了补充。IEEE802.11e采用2种协调机制基于控制的综合协调可控信道接入方式HCCA和基于竞争的增强型分布式信道接入方式EDCA。主要评估EDCA和竞争空闲脉冲(contention-freeburst)CFB相结合时,在系统负载过载的情况下,改变性能参数后系统饱和吞吐量的变化,并分析系统性能参数对饱和吞吐量的影响,从而达到系统参数优化的作用。     

On centralized schedulers for 802.11e WLANs distribution versus grouping of resources allocation

Wireless LAN is becoming a pervasive wireless access technology that can be found in almost any mobile device such as laptops, PDAs, portable game consoles and mobile phones. Each of these groups of devices have a different set of requirements according to their intended use and applications but most of them share two main requirements: QoS support to satisfy applications' demands and power saving functionality to achieve an operating time according to users' expectations. IEEE 802.11e defines two centralized solutions in order to address these problems: Hybrid Coordination Channel Access (HCCA) for QoS and Scheduled Automatic Power Save Delivery (S‐APSD) for power saving. The focus of our work in this paper is the analysis and evaluation of a proposed centralized scheduler that makes use of both aforementioned IEEE 802.11e QoS and power saving solutions. Our contributions are as follows: (i) Design and analytical modeling of a proposed centralized scheduler (DRA) that maximizes the minimum distance between the resource allocations with pseudo‐polynomial complexity, (ii) Extensive performance evaluation of the QoS and power saving benefits of the Distribution proposal (DRA) as compared to a generic Grouping one (GRA), and (iii) Evaluation of the complexity and scalability of the proposal to assess its feasibility in practice. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance modeling and analysis of the ADHOC MAC protocol for vehicular networks

The ADHOC MAC protocol is a classical MAC protocol for supporting reliable broadcast services in vehicular ad hoc networks (VANETs). This paper develops an analytical model for analyzing the access performance of the ADHOC MAC protocol. A Markov chain is first constructed to describe the number of vehicles which have acquired a timeslot successfully at the end of a frame when using the ADHOC MAC protocol. Based on the Markov model, an analytical model is then derived to describe the relationship between the frame length and the channel utilization. Based on the derived analytical model, an optimal frame length that maximizes the channel utilization is further obtained. Simulation experiments are conducted to verify the effectiveness of the analytical model.     

Study on bandwidth control for multiple transmission rates in IEEE 802.11e EDCAF

IEEE 802.11e supports the guaranteed quality of service (QoS) by providing different transmission priorities. IEEE 802.11e improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). Signal quality may affect the available bandwidth and transmission rate, because the characteristic of communication channel in wireless environment is in random time‐variation manner. Generally a station using a lower transmission rate will occupy communication channel for a longer time and degrade system performance, which causes unfairness and cannot provide the guaranteed QoS for the stations with higher transmission rates. We propose a bandwidth control scheme (BCS) by combining the IEEE 802.11e enhanced distributed channel access function (EDCAF) protocol to overcome the guaranteed bandwidth issue in multirate environments. A multirate discrete Markov chain model is analyzed for the multirate transmission system in this paper. According to the obtained results, BCS improves performance especially in throughput and makes the different QoS requirements be processed efficiently and flexibly. Copyright © 2009 John Wiley & Sons, Ltd.     

Non-line-of-sight ultraviolet single-scatter path loss model

A novel non-line-of-sight ultraviolet single-scatter path loss model for coplanar geometries is proposed on the basis of spherical coordinate system. In comparison with the classical single-scatter analytical model based on the prolate-spheroidal coordinate system, it is of a simple integral form which only depends on the variables of the zenith and receiver elevation angles. Additionally, analytical approximation for the proposed single-scatter path loss model is presented. Numerical examples on path loss are presented for various system geometries. Correspondingly, the results are verified with the classical single-scatter analytical model, which demonstrates the validity of our path loss model and the reasonability of the analytical approximation.     

A fully analytical AC large-signal model of the GaAs MESFET fornonlinear network analysis and design

A fully analytical version of an AC large-signal model for the GaAs MESFET is presented. The source model is based on basic principles and the actual physics and geometry of the device. The analytical version was developed by curve fitting the analytical expressions to the source model. The accuracy of the model for microwave circuits is demonstrated using simulation examples of a power amplifier and a mixer     

NBTI效应导致SET脉冲在产生与传播过程中的展宽

本文研究了负偏置温度不稳定性(NBTI)对单粒子瞬态(SET)脉冲产生与传播过程的影响.研究结果表明:NBTI能够导致SET脉冲在产生与传播的过程中随时间而不断展宽.本文还基于工艺计算机辅助设计模拟软件(TCAD)进行器件模拟,提出了一种在130nm体硅工艺下,计算SET脉冲宽度的解析模型,并结合NBTI阈值电压退化的...