A MAC protocol to support hybrid antennas in a wireless LAN

Hybrid Distributed Coordination Function （HDCF）, a modified medium access control protocol of IEEE 802.11 standard, is proposed in this paper to support both smart adaptive array antennas and normal omni-directional antennas simultaneously in one wireless LAN. Omni-directional antennas follow the standard Distributed Coordination Function （DCF） and smart antennas follow the Directional DCF （DDCF）. The proposed DDCF is based on Hybrid Virtual Carrier Sense （HVCS） mechanism, which includes Omni-directional Request-To-Send/Clear-To-Send （ORTS/OCTS） handshake mechanism and directional data transmission. HDCF is compatible with DCF. When a node transmits in a directional beam, the other nodes can multiplex the physical channel. Hence, HDCF supports Space Division Multiple Access （SDMA）. Simulation results show that HDCF can support hybrid antennas effectively and provide much higher network throughput and lower delay and jitter than DCF does.     

IEEE 802.11协议中分布式协调机制的性能模型

IEEE 802.11采用异步传输方式作为媒体层的主要技术,而基于载波检测碰撞避免的分布式接入机制则是其最大的特点.关于分布式接入机制的研究,目前已经有了许多的模型,但是,大部分的模型都是研究终端所产生的数据包是固定长度,很少有模型来研究终端数据包是可变长度的情况.这种情况下的难点就是不易求得碰撞发生时信道所消耗的时间长度.本文则研究在终端数据包长度的分布函数为f(x)下协议的吞吐量和延迟性能模型.首先本文将原标准协议的退避算法看成是有固定大小的竞争窗口,用以求得站点的发送概率;然后,分析信道的工作状态,给出了性能模型,重点在求解碰撞消耗的信道时间,在文章的最后,我们通过仿真试验来验证了模型的正确性.     

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.     

A Simple and Closed-Form Access Delay Model for Reliable IEEE 802.11-Based Wireless Industrial Networks

With the ongoing popularity of the IEEE 802.11 standard, many analytical studies for the distributed coordination function (DCF) have been reported but due to lack of a comprehensive model, the research has been going on. In this paper, three probabilistic analytical access delay models have been proposed for the IEEE 802.11 DCF mechanism in saturated traffic and noisy industrial applications. The first and second one provide an accurate packet delay model by solving non-linear equations at low and high Signal to Noise Ratios (SNRs) respectively. The third one on the other hand offers an approximate and simple closed-form model which does not need solving any non-linear equation and therefore can easily be utilized in the distributed adaptive network quality of service provisioning algorithms for industrial nodes which usually have limited processing capabilities. Delay-reliability, delay-packet payload and delay-data rate tradeoffs has also been studied. Simulation results match the theoretical derivations in most SNRs, confirming the effectiveness of the proposed models.     

Packet Delay Metrics for IEEE 802.11 Distributed Coordination Function

In this paper, we introduce a comprehensive packet delay analysis for wireless networks based on IEEE 802.11 Distributed Coordination Function (DCF). We develop mathematical models that calculate a set of packet delay metrics, namely a) the average packet delay for successfully transmitted packets, b) the average packet delay of successfully transmitted packets experiencing a specific number of collisions, c) the average packet drop time, d) the delay jitter and e) the delay distribution by computing the probability of a packet to be successfully transmitted experiencing delay time lower than a given value. All the developed models are based on calculating station’s delay time at the transmission slot(s) plus the average time that station defers at backoff slots before successful transmission. The mathematical models are simple, computationally fast and can be used to build admission control algorithms. Simulation results show that our proposed mathematical analysis is highly accurate.     

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.     

A dual reservation CDMA-based MAC protocol with power control for ad hoc networks

This paper proposes a new multi-channel Medium Access Control （MAC） protocol named as Dual Reservation Code Division Multiple Access （CDMA） based MAC protocol with Power Control （DRCPC）. The code channel is divided into common channel, broadcast channel and several data channels. And dynamic power control mechanism is implemented to reduce near-far interference. Compared with IEEE 802.11 Distributed Coordination Function （DCF） protocol, the results show that the proposed mechanism improves the average throughput and limits the transmission delay efficiently.     

A smart exponential‐threshold‐linear backoff mechanism for IEEE 802.11 WLANs

Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff mechanism, called Smart Exponential‐Threshold‐Linear (SETL) Backoff Mechanism, to enhance the system performance of contention‐based wireless networks. In the IEEE 802.11 DCF scheme, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the proposed SETL scheme, a threshold is set to determine the behavior of CW after each transmission. When the CW is smaller than the threshold, the CW of a competing station is exponentially adjusted to lower collision probability. Conversely, if the CW is larger than the threshold, the CW size is tuned linearly to prevent large transmission delay. Through extensive simulations, the results show that the proposed SETL scheme provides a better system throughput and lower collision rate in both light and heavy network loads than the related backoff algorithm schemes, including Binary Exponential Backoff (BEB), Exponential Increase Exponential Decrease (EIED) and Linear Increase Linear Decrease (LILD). Copyright © 2011 John Wiley & Sons, Ltd.     

A delay based backoff scheme for WLAN with QoS guarantee and its performance

Based on the Distributed Coordination Function (DCF) and the Enhanced DCF (EDCF) mechanisms in IEEE 802.11 and 802.11e, a novel backoff scheme for Wireless Local Area Network (WLAN) is proposed. The scheme is to solve the problem of the packet loss and the decrease of performance due to the increasing Contention Window (CW) when there are continuous collisions. In the proposed scheme, the CW of the packet will change dynamically with different delay for the different traffics. Mathematic formulas are presented to indicate the relationship between the CW and the delay character. The performance of the new scheme is also discussed with simulation results. The results show that it helps WLAN system handle multimedia simultaneously.     

DCF传输MPEG-4视频流的研究

IEEE 802.11无线局域网的基本接入方式是分布式协调功能(DCF)。当无线局域网中的负载变大时,采用DCF的站点可能在成功发送之前需要延迟几秒以上。这将严重影响MPEG-4视频流等实时应用的性能。该文针对实时应用提出了一种DCF的改进机制L-DCF,并介绍了在L-DCF下如何传输MPEG-4视频流。仿真结果表明,与DCF相比,采用L-DCF的MPEG-4视频流将经历较少的时延和抖动,同时又具有与采用DCF时相同的吞吐量与公平性,因此能够获得更好的播放质量。     

Distributed point coordination function for IEEE 802.11 wireless ad hoc networks

The Distributed Point Coordination Function (DPCF) is presented in this paper as a novel Medium Access Control Protocol (MAC) for wireless ad hoc networks. DPCF extends the operation of the Point Coordination Function (PCF) defined in the IEEE 802.11 Standard to operate over wireless networks without infrastructure. In PCF, a central point coordinator polls the users to get access to the channel and data collisions are completely avoided, thus yielding high performance. In order to extend its high performance to networks without infrastructure, the DPCF is proposed in this paper as a combination of the Distributed Coordination Function (DCF) and the PCF. The general idea is to combine a dynamic, temporary, and spontaneous clustering mechanism based on DCF with the execution of PCF within each cluster. The backwards compatibility of DPCF with legacy 802.11 networks is also assessed in this paper. Comprehensive computer-based simulations demonstrate the high performance of this new protocol in both single-hop and multi-hop networks.     

Enhancing channel utilization by improving media access coordination in wireless local area networks

Distributed coordination function (DCF) is the basis protocol for IEEE 802.11 standard wireless local area networks. It is based on carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. DCF uses backoff process to avoid collisions on the wireless channel. The main drawback with this process is that packets have to spend time in the backoff process which is an additional overhead in their transmission time. The channel is rendered idle when all the stations defer their transmissions due to their backoff process. Therefore, the channel utilization and the total throughput on the channel can be improved by reducing the average time spent by the packets in the backoff process. In this paper, we propose a new media access coordination function called proposed media access protocol (PMAP) that will improve the channel utilization for successful packet transmission and therefore, the total achievable throughput. In addition, we propose an analytical model for PMAP under saturated conditions. We use this model to analyze the performance of PMAP under saturated conditions. To substantiate the effectiveness of our model, we have verified the model by simulating PMAP in NS‐2. Simulation and analytical results show that under saturated conditions, PMAP shows profound improvement in the throughput performance compared to DCF. In addition, the throughput performance of PMAP under unsaturated conditions is presented. We have also presented the delay performance of PMAP and DCF through simulation in both saturated and unsaturated conditions. Simulation results show that the average delay experienced by the packets is less in PMAP compared to DCF. Further, the variance in the packet delay is same for both PMAP and DCF protocols under unsaturated conditions. From the performance results obtained for PMAP under both saturated and unsaturated conditions, it can be concluded that PMAP is superior in performance compared to DCF. Copyright © 2006 John Wiley & Sons, Ltd.     

A Delay-Aware Congestion Control Protocol for Wireless Sensor Networks

In wireless sensor networks, congestion leads to buffer overflowing, and increases delay. The tradi-tional solutions use rate adjustment to mitigate congestion, thus increasing the delay. A Delay-aware congestion con-trol protocol (DACC) was presented to mitigate congestion and decrease delay. In order to improve the accuracy of the existing congestion detection model which is based on the buffer occupancy of a single node, DACC presents a new model considering both the real-time buffer occupancy and the average transmission time of packets. DACC uses the untapped bits in the IEEE 802.11 Distributed coordination function (DCF) frames header to carry congestion infor-mation. During the congestion alleviation period, DACC presents a channel occupancy mechanism which is based on the real-time buffer occupancy for the purpose of decreas-ing delay and preventing packet loss. Simulation results indicate that in terms of delay, packet delivery ratio, col-lision and buffer load, DACC has comparative advantages than those of 802.11 DCF, Priority-based congestion con-trol protocol (PCCP) and Decoupling congestion control and fairness (DCCF).     

Novel Adaptive DCF Protocol Using the Computationally-Efficient Optimization with the Feedback Network Information for Wireless Local-Area Networks

In this paper, we design a novel computationallyefficient linear programming (LP) algorithm to maximize the throughput with respect to the minimum contention window size for the IEEE 802.11 Distributed Coordination Function (DCF) protocol. Based on our LP scheme, a new DCF protocol which can select the best access mode and the optimal size of the minimum contention window is proposed by considering the channel condition and the number of competing stations jointly. The numerical results demonstrate that our proposed DCF protocol significantly outperforms the conventional method.     

Adaptive Priority Sliding Admission Control and Scheduling Scheme for DCF and EDCA WLANs

In this paper an Adaptive Priority Sliding Admission Control and Scheduling (APSAS) scheme is proposed to provide QoS over the existing IEEE802.11 WLANs which operate on Distributed Coordination Function (DCF) and Enhanced Distributed Channel Access (EDCA) mechanisms. The roles of this scheme are generally two folds: (1) To control the number of delay-sensitive real time flows that can be admitted into the WLAN Basic Service Set network and (2) To adjust the priority of selected real time flows in order to accommodate more real time flows without violating the QoS requirement. Extensive simulation studies show that APSAS improves the total throughput, flow throughput ratio, packets end-to-end delay, and jitter of the real time applications when compared with conventional best effort and scheduling-enhanced DCF/EDCA. APSAS also offers near to unity average throughput ratio, lower mean VoIP end-to-end packet delay (<130 ms) and lower mean video packet jitter (<130 ms) over DCF and EDCA.     

BTAC: A Busy Tone Based Cooperative MAC Protocol for Wireless Local Area Networks

Cooperative communications has been actively studied as an effective approach to achieve multi-user/spatial diversity gains and better overall system performance by coordinating multiple users in a dynamic wireless network to share their resources and capabilities. Based on the concept of cooperative communications, this paper proposes and analyzes a Busy Tone based cooperative Medium Access Control (MAC) protocol, namely BTAC, for multi-rate Wireless Local Area Networks (WLANs). A cross-layer Markov chain model is then developed to evaluate the performance of BTAC under dynamic wireless channel conditions. Analytical and simulation results show our BTAC protocol is simple, robust, fully compatible with the IEEE 802.11b standard and can achieve better throughput and delay performance than the standard Distributed Coordination Function (DCF) protocol and the recently-proposed CoopMAC protocol.     

Throughput study for admission control in IEEE 802.11 DCF with ARF

We propose an analytical approach to determining the admission of new stations to a WLAN operating in IEEE 802.11 Distributed Coordination Function (DCF) with Auto Rate Fallback (ARF). The proposed approach is based on a cross layer analytical model of how the throughput of existing stations in the WLAN is affected by admitting the new station in non-saturated as well as saturated traffic conditions. The effectiveness of the proposed approach is verified by simulations.     

Enhancement of QoS differentiation over IEEE 802.11 WLAN

IQD, a distributed coordination function (DCF) with integrated quality of service (QoS) differentiation, is proposed in this letter to enhance QoS over IEEE 802.11 WLAN. DCF does not support any QoS differentiation. Enhanced DCF (EDCF) only supports delay differentiation. IQD can achieve both delay and packet-loss-rate differentiation by differentiating the initial window size and the retry limit. Simulation results show that IQD performs better than DCF and EDCF in enhancing QoS, and the proposed analytical model is valid.     

Impact of bursty error rates on the performance of wireless local area network (WLAN)

With an increasing popularity of DCF based wireless LAN, the modeling of 802.11 distributed coordination function (DCF) has attracted lots of research attention. Existing analysis of 802.11 DCF has been focused on the determination of the throughput and the packet delay under saturated traffic and ideal channel conditions. Although some recent papers address the saturated performance under a simple uniform error model, they can hardly capture the impact of bursty characteristics of wireless fading on the performance of 802.11 DCF. This paper presents exact formulae for the throughput and the delay in DCF for various traffic conditions when either saturated or unsaturated traffic load is present. A two-state Markov channel model is incorporated to present the bursty characteristics of channel errors. With our analysis, the impact of bursty channel error on unsuccessful transmission probability and the DCF performance can be determined. The results of our analytical framework reveal that the four-way handshaking scheme does not improve throughput substantially for light traffic load. However, for heavy traffic load, the four-way handshaking scheme is advantageous as compared to the basic access scheme. Also, extensive simulation is done to substantiate the accuracy of our analytical model.     

Exponential and generalized Dolph-Chebyshev functions for flat-top array beampattern synthesis

For the flat-top array beampattern synthesis, a conceptually simple closed-form method without iterative work and heavy numerical calculations is desired. We are therefore motivated to look for a good template function for flat-top beampattern synthesis, such as Dolph-Chebyshev Function (DCF) for pencil-beam pattern synthesis. In this paper, two new template functions, Exponential Dolph-Chebyshev Function (EDCF) and Generalized Dolph-Chebyshev Function (GDCF) are derived from the original DCF, and the corresponding synthesis procedures in the Quadratic Programming (QP) method are also presented. From the simulation results using the proposed templates, it is shown that although the resultant beampatterns are not equi-ripple, both the mainbeam width and the Side-Lobes Level (SLL) of the synthesized beampatterns are controlled well, simultaneously, and the proposed methods can also be applied to synthesize arrays of different geometries including Uniform Linear Arrays (ULA) and Uniform Circular Arrays (UCA). The comparison between the two proposed template functions is also presented so that one can choose the proper synthesis template based on specified requirements.