Rated Window and Packet Size Differentiation Methods for Per-Rate TCP Fairness Over IEEE 802.11

The paper introduces an analytical model to investigate the energy efficiency of the IEEE 802.11 distributed coordinated function (DCF). Many factors, such as the number of contending nodes, packet size, contention window, packet transmission collision probability and channel condition, that affect the energy efficiency of 802.11 DCF have been considered. We determine the packet transmission probability by a method simpler than the existing Markov chain model. The paper identifies the tradeoff in choosing packet size to optimize the energy efficiency of DCF in error-prone environments. The effects of contention window and packet size on the energy efficiency are obtained for both DCF basic mode and DCF with four-way handshaking. It is shown that under error-prone environments, optimal packet size can improve more on the energy efficiency than optimal contention window. Combining both optimal contention window and optimal packet size can achieve the maximum optimization. To validate our analytical results, we have done extensive simulations, and they seem to match very well with the presented analytical results.

The 802.11 standard supports multiple data transmission rates. To achieve a high performance under varying conditions, wireless stations need to adapt their transmission rate dynamically. Typically, stations with 802.11b products would reduce their bit rate from 11 Mbps to 5.5, 2, or 1 Mbps if they are far away from an access point (AP) and subject to significant signal fading and interference. This leads to considerable rate diversity, particularly when the network is congested. Under such conditions, aggregate throughput of WLANs will be degraded drastically (M. Heusse et al., 2003) because the basic CSMA/CA channel access method would guarantee an equal long term CAP (channel access probability) to all hosts. The main contribution of this paper is the proposal of an algorithm that can be implemented at both the AP and the wireless stations (WSs) with the purpose of guaranteeing fairness and improving the aggregate throughput of 802.11b multi-rate WLANs, We use COT (channel occupancy time) as fairness index instead of the CAP to analyze the fairness of WLANs. With our algorithm, the AP and wireless stations in the WLAN can achieve COT-based fairness by adjusting their packet length, sending the multiple back-to-back packets at one time, or giving up an opportunity to access the channel. Through exclusive simulations, we show that our algorithm leads to significant improvements in aggregate throughput and provides COT-fairness under multi-rate condition

The mobile ad hoc network (MANET) has attracted lots of attention recently. Most of the researches assume that every mobile host in the MANET uses a fixed data rate and follows a distributed coordination function (DCF) to transmit messages. As we know that none of the research has combined multiple data rates and transmission scheduling to minimize waiting time and conserve energy for a MANET with power-saving (PS) mode hosts. IEEE 802.11 has already supported multiple data transmission rate. However, how to decide the transmission rate is still an open question. Here, we propose a data rate selection protocol to select the best available data rate to transmit messages. After the data transmission rate has been selected, we can schedule each transmission according to the data transmission rate and the packet size. Our goal is to minimize the average waiting time of each transmission and thus the PS hosts can switch back to power-saving mode as soon as possible. Therefore, we follow the shortest job first policy to let the transmission with shortest transmission time to access the channel first. Simulation results show that our scheduling protocol can achieve high packet delivery rate, reduce waiting time and conserve lots of energy.

Over the past few years, wireless local area networks (WLANs) have gained an increased attention and a large number of WLANs are being deployed in universities, companies, airports etc. Majority of the IEEE 802.11 based WLANs employ distributed coordination function (DCF) in wireless access points (AP) to arbitrate the wireless channel among Wireless Stations (STAs). However, DCF poses serious unfairness problem between uplink and downlink flows. To overcome this unfairness problem, we propose a simple enhancement to the IEEE 802.11 DCF which provides priority to the AP and thus enables it to acquire a larger share of the channel when required. We have demonstrated the unfairness problem through systematic measurements in an experimental test bed of WLAN using the legacy 802.11 DCF. We also developed analytical models to calculate the throughput of AP and the STAs and verify these results through thorough simulations in ns-2. We observe that our simulation results find in good agreement with our analytical models. Results show that our proposed enhancement achieves a fair distribution of bandwidth and improves the throughput (by nearly 300%) for the downlink flows as compared to the DCF, without severely affecting the performance of uplink flows

The Distributed Coordination Function (DCF) is the fundamental access method defined in the IEEE 802.11 Standard for Wireless Local Area Networks (WLANs). With this standard, the Access Point (AP) and the mobile stations consume a significant amount of energy to contend for access to the shared wireless channel. In order to improve the energy efficiency of WLANs, we investigate in this paper a simple and backwards compatible mechanism, called Bi-Directional DCF (BD-DCF), that enables bidirectional communications between wireless stations with a single channel access invocation. The key idea is to let the AP or any mobile station transmit a data packet together with the acknowledgement upon the successful reception of a data packet. This approach reduces the communication overhead, the channel contention, and better balances uplink and downlink transmission opportunities. We evaluate the performance of the proposed BD-DCF by means of computer-based simulation considering different traffic loads, degrees of traffic symmetry, data packet sizes, and data rates. The results presented in this paper show that the BD-DCF protocol can improve the energy efficiency of DCF up to 50%.

This paper introduces an analytical model to investigate the energy efficiency of the IEEE 802.11 distributed coordinated function (DCF). This model not only accounts for the number of contending nodes, the contention window, but also the packet size, and the channel condition. Based on this model, we identify the tradeoff in choosing optimum parameters to optimize the energy efficiency of DCF in the error-prone environment. The effects of contention window and packet size on the energy efficiency are examined and compared for both DCF basic scheme and DCF with four-way handshaking. The maximum energy efficiency can be obtained by combining both the optimal packet size and optimal contention window. To validate our analysis, we have done extensive simulations in ns-2, and simulation results seem to match well with the presented analytical results.

According to the amendment 5 of the IEEE 802.11 standard, 802.11n still uses the distributed coordination function (DCF) access method as mandatory function in access points and wireless stations (essentially to assure compatibility with previous 802.11 versions). This article provides an accurate two dimensional Markov chain model to investigate the throughput performance of IEEE 802.11n networks when frame aggregation and block acknowledgements (Block-ACK) schemes are adopted. Our proposed model considered packet loss either from collisions or channel errors. Further, it took anomalous slots and the freezing of backoff counter into account. The contribution of this work was the analysis of the DCF performance under error-prone channels considering both 802.11n MAC schemes and the anomalous slot in the backoff process. To validate the accuracy of our proposed model, we compared its mathematical simulation results with those obtained using the 802.11n DCF in the network simulator (NS-2) and with other analytical models investigating the performance of 802.11n DCF. Simulation results proved the accuracy of our model.

In atypical deployment of IEEE 802.11 wireless LANs in the infrastructure mode, an access point acts as abridge between the wireless and the wired parts of the network. Under the current IEEE 802.11 Distributed Coordination Function (DCF) access method, which provides equal channel access probability to all devices in a cell, the access point cannot relay all the frames that it receives on the downlink. This causes significant unfairness between upload and download connections, long delays, and frame losses. This unfairness problem comes from the not-so-complex interaction of transport-layer protocols with the MAC-layer access method. The main problem is that the access point requires more transmission attempt probability than wireless stations for correct operation at the transport layer. In this paper, we propose to solve the unfairness problem in a simple elegant way at the MAC layer. We define the operation of an Asymmetric Access Point that benefits from a sufficient transmission capacity with respect to wireless stations so that the overall performance improves. The proposed method of operation is intrinsically adaptive so that when the access point does not need the increased capacity, it is used by wireless stations. We validate the proposed access method by simulation to compare it with other solutions based on IEEE 802.11e. Unlike many papers in this domain, which only validate MAC-layer modifications through simulation or analytical modeling, we provide measurement data gathered on an experimental prototype that uses wireless cards implementing the proposed method.

The mesh topology based on the standard IEEE 802.11 for wireless LANs appears to be a very promising architecture for achieving a ubiquitous wireless Internet access in the future. However, the current IEEE 802.11 protocol is aimed at single access point (AP) environments and many problems related to the wireless meshed interconnection of APs and mobile terminals (MTs) remain to be solved. Some proposed solutions to build such mesh architectures are based on single-channel ad-hoc oriented schemes in which IEEE 802.11 protocol has been modified. The main problem with this type of schemes, however, lies in the very low performance of the single-channel architecture itself. The task group S of IEEE 802.11 is currently working out standards for IEEE 802.11-compliant mesh architectures in a number of usage scenarios including residential, office and campus/community/public access network but a lot of work remains to be done since the group was established just on last year. In this paper we propose a new multi-channel mesh architecture for hot zones which works using a distributed coordination function (DCF)-based technique for interconnecting APs. A major advantage of our proposed scheme is that, putting routing issues aside, it introduces no change into the MAC protocol of IEEE 802.11. Our simulations results obtained in OPNET show a good performance of our proposed scheme in terms of throughput and delay. We also show interesting results related with the size of packets and the mesh architecture itself that could lead to further research in the future

The IEEE 802.11 standard has been evolved to support multiple transmission rates in wireless local area networks (WLANs) to cope with diverse channel conditions and to increase throughput. However, when stations with different transmission rates coexist, the basic channel access mechanism of WLAN, distributed coordination function (DCF), not only fails to assure airtime fairness among competing stations but also decreases overall network throughput, because DCF was designed to provide fair opportunity of channel access, regardless of transmission rate. As an effective solution to this problem, we propose a hybrid control mechanism that integrates contention window control and frame aggregation. The former adjusts the size of contention window and differentiates the channel access opportunity depending on the transmission rates of stations. The latter controls the number of packets in the aggregated frame to tightly assure per-station airtime fairness with the reduced channel access overheads. Moreover, we derive an analytical model to evaluate the performance of the proposed mechanism in terms of throughput and fairness. Along with the analysis results, the extensive simulation results confirm that the proposed mechanism significantly increases the overall throughput by about three times compared to the conventional DCF, while assuring airtime fairness strictly.

In conventional IEEE 802.11 medium access control protocol, the distributed coordination function is designed for the wireless stations (WSs) to perform channel contention within the wireless local area networks (WLANs). Packet collision is considered one of the major issues within this type of contention-based scheme, which can severely degrade the network performance for the WLANs. Research work has been conducted to modify the random backoff mechanism in order to alleviate the packet collision problem while the WSs are contending for channel access. However, most of the existing work can only provide limited throughput enhancement under specific number of WSs within the network. In this paper, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed to improve the packet collision from the random access schemes. With its adaptable reservation period, the contention-based channel access can be adaptively transformed into a reservation-based system while there are pending packets required to be transmitted from the WSs. Furthermore, according to the designed reservation table within the access point, the fairness for channel access between the WSs is addressed within the ARCR protocol. Numerical results indicate that the proposed ARCR scheme can outperform the existing schemes with enhanced channel utilization and network throughput.

In the IEEE 802.11 distributed coordination function (DCF), the basic service set (BSS) suffers from two unfairness issues: 1) performance anomaly and 2) uplink/downlink unfairness. In this paper, to solve these issues, we propose a window control scheme in the Transmission Control Protocol (TCP) for the IEEE 802.11 DCF mode. The proposed scheme is called channel occupancy time based rate control for TCP (COTRC-TCP). COTRC-TCP controls the maximum window size based on the throughput estimated at the TCP layer so that each station can use the wireless channel for equal duration. The throughput estimation is based on the number of active stations and the channel occupancy period used by each station in the BSS, which are monitored at the Medium Access Control (MAC) layer. The proposed scheme forms a cross-layer approach that involves both MAC and transport layers. The performance of COTRC-TCP over multirate IEEE 802.11 DCF is evaluated and compared with that of previous works through extensive simulations. Simulation results show that COTRC-TCP exhibits fairness in terms of channel-occupancy time among the competing stations, which accordingly remedies the unfairness problems. The proposed scheme also improves the transmission efficiency.

This paper presents a modified proportional fairness (PF) criterion suitable for mitigating the \textit{rate anomaly} problem of multirate IEEE 802.11 Wireless LANs employing the mandatory Distributed Coordination Function (DCF) option. Compared to the widely adopted assumption of saturated network, the proposed criterion can be applied to general networks whereby the contending stations are characterized by specific packet arrival rates, $\lambda_s$, and transmission rates $R_d^{s}$. The throughput allocation resulting from the proposed algorithm is able to greatly increase the aggregate throughput of the DCF while ensuring fairness levels among the stations of the same order of the ones available with the classical PF criterion. Put simply, each station is allocated a throughput that depends on a suitable normalization of its packet rate, which, to some extent, measures the frequency by which the station tries to gain access to the channel. Simulation results are presented for some sample scenarios, confirming the effectiveness of the proposed criterion.

Due to the rapid development of Wi-Fi networks based on the IEEE 802.11 standards, almost every household has installed at least one Wi-Fi access point(AP). Although dense deployment of APs provides high coverage of wireless networks, high power level of signal from neighbor APs causes many problems including interference and hidden terminals resulted from the contention-based distributed coordinated function (DCF) used by 802.11. Therefore, the collaborative mechanisms between APs will be one of important methods to improve the performance of wireless network and power control will especially be key technique to solve these problems. Although there are many researches discussing about power control model, they do not consider the behavior of DCF. In this thesis, we first analyze the characteristics of DCF and model successful transmission for DCF including three steps: accessing channel, data reception, and ACK reception. We define different sets which can accurately represent the interaction and influence between APs for three steps during transmission. Based on the model of successful transmission for DCF, we find that power setting indeed influences the performance of 802.11 network and we propose power control design. The simulation results show that proposed power control design outperform other models for not only probability of successful transmission but also fairness of network. From microscopic view of simulation results, we find that traditional models suffer from problems of hidden terminal and high interference. For our proposed design, these problems can be solved effectively because we consider and analyze the characteristics of DCF precisely.

Recently there have been considerable interests focusing on the enhancement of Mobile Ad-hoc NETworks (MANETs) which are a collection of wireless mobile nodes forming a temporary network without using any centralized access point, infrastructure, or centralized administration. The Distributed Coordination Function (DCF) of IEEE 802.11 Medium Access Control (MAC) protocol has been widely employed in MANETs to manage the shared wireless medium. In DCF, the size of contention window is doubled upon a collision regardless of the network loads. This paper presents a dynamic MAC scheme to improve the performance of MANETs, which applies a threshold of the collision rate to switch between two different functions for increasing the size of contention window and two different mechanisms of resting the size of the contention window based on the status of network loads. The performance of this scheme is investigated and compared to the original DCF using the network simulator NS-2. Moreover, the Random WayPoint (RWP) mobility model is adopted to investigate the effects of mobility on network performance. The performance results reveal that the dynamic scheme is able to achieve the higher throughput and energy efficiency as well as lower end-to-end delay than the original DCF with and without mobility.