A cross-layer congestion and contention window control scheme for TCP performance improvement in wireless LANs

Neither the current TCP protocol nor the standard backoff algorithm of IEEE 802.11 protocol is able to distinguish corruption loss from congestion or collision loss. Hence, high transmission errors and a varying latency inherent in wireless channel would have a seriously adverse effect on the performance of TCP. In this paper, we propose a novel and pragmatic cross-layer approach with joint congestion and contention window control scheme to improve the performance of TCP in IEEE 802.11 wireless environments. In addition to theoretical analysis, simulations are conducted to evaluate the proposed scheme. As it turns out, our design indeed provides a more efficient solution for frequent transmission loss and enables TCP to distinguish between congestion loses and transmission errors, thus to take proper remedial actions.     

Congestion control with dynamic threshold adaptation and cross‐layer response for TCP Vegas over IEEE 802.11 wireless networks

Wireless technologies provide mobile access and enable rapid andcost‐effective network deployment. But a wireless link is generally accompanied by high interference, transmission errors and a varying latency. The erratic packet losses usually lead to a curbing of the flow of segments on the TCP connection, and thus limit TCP performance. This paper presents a threshold control mechanism with cross‐layer response approach for improving TCP Vegas performance in IEEE 802.11 wireless networks. By making slight modifications to the legacy IEEE 802.11 MAC and TCP, the numerical results reveal that the proposed scheme provides a significant improvement in TCP performance under IEEE 802.11 wireless environments. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

TCP Performance in IEEE 802.11-Based Ad Hoc Networks with Multiple Wireless Lossy Links

We propose a packet-level model to investigate the impact of channel error on the transmission control protocol (TCP) performance over IEEE-802.11-based multihop wireless networks. A Markov renewal approach is used to analyze the behavior of TCP Reno and TCP Impatient NewReno. Compared to previous work, our main contributions are listed as follows: 1) modeling multiple lossy links, 2) investigating the interactions among TCP, Internet Protocol (IP), and media access control (MAC) protocol layers, specifically the impact of 802.11 MAC protocol and dynamic source routing (DSR) protocol on TCP throughput performance, 3) considering the spatial reuse property of the wireless channel, the model takes into account the different proportions between the interference range and transmission range, and 4) adopting more accurate and realistic analysis to the fast recovery process and showing the dependency of throughput and the risk of experiencing successive fast retransmits and timeouts on the packet error probability. The analytical results are validated against simulation results by using GloMoSim. The results show that the impact of the channel error is reduced significantly due to the packet retransmissions on a per-hop basis and a small bandwidth delay product of ad hoc networks. The TCP throughput always deteriorates less than ~ 10 percent, with a packet error rate ranging from 0 to 0.1. Our model also provides a theoretical basis for designing an optimum long retry limit for IEEE 802.11 in ad hoc networks.     

PEEC: a channel-adaptive feedback-based error

Reliable transmission is a challenging task over wireless LANs since wireless links are known to be susceptible to errors. Although the current IEEE802.11 standard ARQ error control protocol performs relatively well over channels with very low bit error rates (BERs), this performance deteriorates rapidly as the BER increases. This paper investigates the problem of reliable transmission in a contention free wireless LAN and introduces a packet embedded error control (PEEC) protocol, which employs packet-embedded parity symbols instead of ARQ-based retransmission for error recovery. Specifically, depending on receiver feedback, PEEC adaptively estimates channel conditions and administers the transmission of (data and parity) symbols within a packet. This enables successful recovery of both new data and old unrecovered data from prior transmissions. In addition to theoretically analyzing PEEC, the performance of the proposed scheme is extensively analyzed over real channel traces collected on 802.11b WLANs. We compare PEEC performance with the performance of the IEEE802.il standard ARQ protocol as well as contemporary protocols such as enhanced ARQ and the hybrid ARQ/FEC. Our analysis and experimental simulations show that PEEC outperforms all three competing protocols over a wide range of actual 802.11b WLAN collected traces. Finally, the design and implementation of PEEC using an adaptive low-density-parity-check (A-LDPC) decoder is presented.     

Optimal MAC packet size in networks without cut-through routing

This paper presents an analytical method of optimal breaking of a transmission control protocol (TCP)/Internet protocol (IP) message into medium access control (MAC) packets in networks without cut-through routing (such as networks compliant with the IEEE 802.11 wireless local area network standard). The method accounts for the transmission delay of acknowledgement frames, the sliding window flow control in TCP/IP protocol, error control via retransmissions, and heterogeneity of transport parameters (link-to-link and upstream-downstream) along a multihop network path. Mathematically, the problem consists in minimizing the TCP/IP message transaction time, a nonlinear function of the MAC packet size, in the presence of a set of linear restrictions. Throughput calculations illustrating this method are performed using IEEE 802.11 data.     

Stabilizing TCP performance over bursty wireless links through the combined use of link-layer techniques

Improving TCP performance over wireless fidelity (WiFi) networks is recognized as a crucial issue, specially in those links prone to suffer from high bit error rate (BER) and bursty error losses. This paper shows how TCP throughput over a real IEEE 802.11b wireless local area network (WLAN) in a lossy office environment is stabilized by a combination of an adaptive forward error correction (FEC) scheme in conjunction with a Snoop agent, both added to the idle repeat request (RQ) mechanism inherently used by such technology. The most innovative aspects of this work are that these techniques operate simultaneously and they have been tested over a real scenario.     

Vertical optimization of data transmission for mobile wireless terminals

A major problem for TCP connections over wireless links is that errors introduced by the wireless channel interfere with the TCP protocol, leading to reduced data rates and power wastage. Based on accurate simulations for the TCP and IEEE 802.11 MAC protocols, we discuss recipes to optimize transmission. It is argued that the best approach is to restrict modifications to the mobile device. While this requires separate solutions for the uplink and downlink, the results of optimization are then available when roaming into any WLAN obeying the relevant MAC protocol. Simulation results show that the combination of specific strategies with a vertical interaction between the protocol layers can lead to the required improvements, giving a promising approach to enhance the performance of wireless mobile terminals.     

A packet-reordering solution to wireless losses in transmission control protocol

The wireless medium may cause substantial packet losses, rendering Transmission Control Protocol (TCP) inefficient. We propose a cross-layer solution by combining link-layer retransmission techniques and a solution for TCP packet reordering. It is costly to conduct link-layer retransmission with the constraint of orderly packet delivery. We require the link layer to provide reliable packet delivery, but without orderly delivery guarantee, thus transforming the problem of high packet error rates to the problem of packet reordering. The latter is dealt with by enhancing TCP with a solution for packet reordering. We justify our design by analyzing both general scenarios and the case of IEEE 802.11n. Our simulation results demonstrate that the proposed method is effective in improving TCP connection goodput in wireless networks.     

TCP在无线通信中存在的问题及解决方案

TCP和IP协议非常简单且可靠,它们的组合决定了目前的大多数通信方式(从有线骨干网到混合网)。TCP协议最初是为有线网络而设计的,目前已成为大多数应用事实上的标准。在有线网络中随机比特差错率可以忽略,拥塞主要由包丢失造成。很多研究都表明未修改的TCP协议在无线环境中的性能很差,因为它无法区分数据包的丢失是由于拥塞还是传输差错造成的。文章分析了TCP在无线IP通信中存在的问题,详细给出了相应的解决方案。     

Enhancement of TCP over wired/wireless networks with packet loss classifiers inferred by supervised learning

TCP is suboptimal in heterogeneous wired/wireless networks because it reacts in the same way to losses due to congestion and losses due to link errors. In this paper, we propose to improve TCP performance in wired/wireless networks by endowing it with a classifier that can distinguish packet loss causes. In contrast to other proposals we do not change TCP’s congestion control nor TCP’s error recovery. A packet loss whose cause is classified as link error will simply be ignored by TCP’s congestion control and recovered as usual, while a packet loss classified as congestion loss will trigger both mechanisms as usual. To build our classification algorithm, a database of pre-classified losses is gathered by simulating a large set of random network conditions, and classification models are automatically built from this database by using supervised learning methods. Several learning algorithms are compared for this task. Our simulations of different scenarios show that adding such a classifier to TCP can improve the throughput of TCP substantially in wired/wireless networks without compromizing TCP-friendliness in both wired and wireless environments.     

TCP在无线通信中的问题以及解决方案

由于TCP/IP协议非常简单且可靠,所以它们的组合决定了目前的大多数通信方式（从有线骨干网到混合网）。现在TCP协议已经成为大多数应用事实上的标准。TCP协议最初是为有线网络而设计的。在有线网络中随机比特差错率是可以忽略的。拥塞主要是由包丢失造成的。很多研究都表明未修改的标准TCP协议在无线环境中的性能是很差的,因为它无法区分出数据包的丢失是由于拥塞还是传输差错。分析了TCP在无线IP通信环境中存在的问题,并详细给出相应的解决方案。     

A study on the influence of transmission errors on WLAN IEEE 802.11 MAC performance

Since the advent of the first IEEE 802.11 standard for WLANs, several papers have been presented that evaluate the IEEE 802.11 DCF access method. In realistic WLAN environments frame errors usually occur due to non‐ideal channel conditions; in this way, papers including adverse transmission conditions in the evaluation have been published later in the literature. In this paper, we review existent analytical models that include the influence of transmission errors in IEEE 802.11 DCF performance. We modify current models and provide a more accurate analysis, thus allowing the evaluation in single rate and multi‐rate scenarios with stations subject to different link error conditions. Moreover, this paper exposes the unfairness problem that arises in IEEE 802.11 DCF networks with stations subject to different transmission conditions through analytical and simulation results. Stations are not able to distinguish collisions from failed transmissions due to link errors; both result in a missing ACK and, consequently, the transmitting stations apply the exponential backoff algorithm. This fact leads to a lower performance for stations in worse transmission conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Modeling TCP SACK performance over wireless channels with semi-reliable ARQ/FEC

Providing reliable data communications over wireless channels is a challenging task because time-varying wireless channel characteristics often lead to bit errors. These errors result in loss of IP packets and, consequently, TCP segments encapsulated into these packets. Since TCP cannot distinguish packet losses due to bit corruption from those due to network congestion, any packet loss caused by wireless channel impairments leads to unnecessary execution of the TCP congestion control algorithms and, hence, sub-optimal performance. Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC) try to improve communication reliability and reduce packet losses by detecting and recovering corrupted bits. Most analytical models that studied the effect of ARQ and FEC on TCP performance assumed that the ARQ scheme is perfectly persistent (i.e., completely reliable), thus a frame is always successfully transmitted irrespective of the number of transmission attempts it takes. In this paper, we develop an analytical cross-layer model for a TCP connection running over a wireless channel with a semi-reliable ARQ scheme, where the amount of transmission attempts is limited by some number. The model allows to evaluate the joint effect of stochastic properties of the wireless channel characteristics and various implementation-specific parameters on TCP performance, which makes it suitable for performance optimization studies. The input parameters include the bit error rate, the value of the normalized autocorrelation function of bit error observations at lag 1, the strength of the FEC code, the persistency of ARQ, the size of protocol data units at different layers, the raw data rate of the wireless channel, and the bottleneck link buffer size.     

Increasing throughput in dense 802.11 networks by automatic rate adaptation improvement

Rate control algorithms for commercial 802.11 devices strongly rely on packet losses for their adaptation. As a result, they give poor performance in dense networks because they are not able to distinguish packet losses related to channel error from packet losses due to collision. In this paper, we evaluate automatic rate adaptation algorithms in IEEE 802.11 dense networks. A certain number of works in the literature address this problem, but they demand modifications of the IEEE standard, or depend on some special feature not available in off-the-shelf devices. In this context, we propose a new automatic rate control algorithm which is simple, easy to implement, standards-compliant, and well-suited for crowded 802.11 networks. Our approach consists of measuring the contention level, inferring the collision probability, and choosing transmission rates which maximize throughput. Results from simulation and real experiments show throughput improvement of up to 100% from our mechanism.     

Delayed duplicate acknowledgements: a TCP‐Unaware approach to improve performance of TCP over wireless

Since a TCP sender cannot distinguish between packet losses arising from transmission errors from those due to congestion, TCP tends to perform poorly on wireless links that are prone to transmission errors. Several techniques have previously been proposed to improve TCP performance over wireless links. Existing schemes typically require an intermediate node (typically, a base station) to be TCP‐aware. For instance, the Snoop scheme requires the base station to interpret TCP headers and take appropriate action to help improve TCP performance. This paper proposes an alternative TCP‐unaware technique that attempts to mimic the behavior of the Snoop protocol. Performance evaluation shows that the proposed Delayed Dupacks scheme performs quite well. Copyright © 2001 John Wiley & Sons, Ltd.     

Improving protocol capacity for UDP/TCP traffic with model-based frame scheduling in IEEE 802.11-operated WLANs

In this paper, we develop a model-based frame scheduling scheme, called MFS, to enhance the capacity of IEEE 802.11-operated wireless local area networks (WLANs) for both transmission control protocol (TCP) and user datagram protocol (UDP) traffic. In MFS each node estimates the current network status by keeping track of the number of collisions it encounters between its two consecutive successful frame transmissions, and computes accordingly the current network utilization. The result is then used to determine a scheduling delay to be introduced before a node attempts to transmit its pending frame. MFS does not require any change in IEEE 802.11, but instead lays a thin layer between the LL and medium access control (MAC) layers. In order to accurately calculate the current utilization in WLANs, we develop an analytical model that characterizes data transmission activities in IEEE 802.11-operated WLANs with/without the request to send/clear to send (RTS/CTS) mechanism, and validate the model with ns-2 simulation. All the control overhead incurred in the physical and MAC layers, as well as system parameters specified in IEEE 802.11, are figured in. We conduct a comprehensive simulation study to evaluate MFS in perspective of the number of collisions, achievable throughput, intertransmission delay, and fairness in the cases of TCP and UDP traffic. The simulation results indicate that the performance improvement with respect to the protocol capacity in a WLAN of up to 300 nodes is 1) as high as 20% with the RTS/CTS and 70% without the RTS/CTS in the case of UDP traffic and 2) as high as 10% with the RTS/CTS and 40% without the RTS/CTS in the case of TCP traffic. Moreover, the intertransmission delay in MFS is smaller and exhibits less variation than that in IEEE 802.11; the fairness among wireless nodes in MFS is better than, or equal to, that in IEEE 802.11.     

基于无线连接的TCP/IP性能研究

TCP/IP是Internet的主要技术,也希望能在无线连接中很好地应用,通过一种模拟系统研究了基于IEEE802.11无线介质访问控制协议的TCP/IP的性能.     

Cross-Layer Enhancement to Support TCP-Based Traffics in WLANs

Widespread deployment of wireless local area networks and a gradual increase in streaming applications have brought about a demand for improved quality of service (QoS) in wireless networks. However, increasing user datagram protocol based high priority multimedia traffic and the class differentiation introduced in QoS protocols, has resulted into transmission control protocol (TCP) starvation and increased spurious timeouts. While today’s Internet traffic is still dominated by TCP based applications, the negative effects of IEEE 802.11e enhanced distributed coordination function (EDCF) scheme on TCP performance in the presence of high priority traffic have not been extensively explored. In this paper, the performance of TCP in 802.11e WLAN competing with high priority traffic is examined. The prioritised adaptive enhanced scheme (PAD_EDCF) is proposed. The proposed scheme gives priority to TCP control packets in order to improve the low traffic transmission flow and acquires additional capability of adjusting the MAC parameters based on the traffic load condition. Simulation results demonstrate that the proposed scheme significantly improves TCP performances in terms of traffic efficiency, throughput and reduces delay.     

An Asymmetric Access Point for Solving the Unfairness Problem in WLANs

In a typical deployment of IEEE 802.11 wireless LANs in the infrastructure mode, an access point acts as a bridge between the wireless and the wired part of the network. Under the current IEEE 802.11 DCF access method, which provides equal channel access probability to all devices in a cell, the access point cannot relay all the frames it receives on the downlink. This causes significant unfairness between uplink and downlink flows, long delays, and frame losses. 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 and 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. Moreover, we provide measurement data gathered on an experimental prototype that uses wireless cards implementing the proposed method.