Reliable communication protocols require that all the intended recipients of a message receive the message intact. Automatic Repeat reQuest (ARQ) techniques are used in unicast protocols, but they do not scale well to multicast protocols with large groups of receivers, since segment losses tend to become uncorrelated thus greatly reducing the effectiveness of retransmissions. In such cases, Forward Error Correction (FEC) techniques can be used, consisting in the transmission of redundant packets (based on error correcting codes) to allow the receivers to recover from independent packet losses. Despite the widespread use of error correcting codes in many fields of information processing, and a general consensus on the usefulness of FEC techniques within some of the Internet protocols, very few actual implementations exist of the latter. This probably derives from the different types of applications, and from concerns related to the complexity of implementing such codes in software. To f...

this paper, we investigate the issue of error control in wireless communication networks. We review the alternative error control schemes available for providing reliable end-to-end communication in wireless environments. Through case studies, the performance and tradeoffs of these schemes are shown. Based on the application environments and QoS requirements, the design issues of error control are discussed to achieve the best solution. 1. Introduction In recent years there has been an increasing trend towards personal computers and workstations becoming portable. Desire to maintain connectivity of these portable computers to the existing installation of Local Area Networks (LANs), Metropolitan Area Networks (MANs), and Wide Area Networks (WANs), in a manner analogous to present day computers, is fueling an already growing interest in wireless networks. Wireless networks will be needed to provide voice, video and data communication capability between mobile termina

system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission of the author.

This paper proposes a hybrid ARQ error control scheme based on the concatenation of a Reed--Solomon (RS) code and a rate-compatible punctured convolutional (RCPC) code for low-bit-rate video transmission over wireless channels. The concatenated hybrid ARQ scheme we propose combines the advantages of both type-I and type-II hybrid ARQ schemes. Certain error correction capability is provided in each (re)transmitted packet, and the information can be recovered from each transmission or retransmission alone if the errors are within the error correction capability (similar to type-I hybrid ARQ). The retransmitted packet contains redundancy bits which, when combined with the previous transmission, result in a more powerful RS/convolutional concatenated code to recover information if error correction fails for the individual transmissions (similar to type-II hybrid ARQ).

Since high error rates are inevitable in the wireless environment, energy efficient error control is an important issue for mobile computing systems. When error-correction mechanisms are implemented on general-purpose processors the power consumption that is required to perform the errorcorrection mechanism can be significant. To illustrate this we have studied two different error correction mechanisms with different characteristics and capabilities, i.e. EVENODD and Reed-Solomon.

We consider the energy efficiency of error control mechanisms for wireless communication. Since high error rates are inevitable to the wireless environment, energy efficient error control is an important issue for mobile computing systems. Although good designed retransmission schemes can be optimal with respect to energy efficiency, they can introduce problems to fulfill the required QoS of an application. Since the channel conditions and the requirements vary dynamically, we concentrate on adaptive implementations of error correction mechanisms. Error correction mechanisms traditionally optimize the performance and provide an optimal solution to withstand all possible errors. In the context of energy consumption we have studied the energy efficiency of three different error correction mechanisms and have implemented these in software. We show that the energy efficiency trade-off between error correction and communication can have a great impact on the energy efficiency of a system. It will be shown that from energy efficiency perspective it is not always profitable to minimize the number of bits transmitted over the air. A general strategy will be provided that can be used to determine an energy efficient error correction scheme of a minimal system consisting of a general-purpose processor and a wireless interface. As an example we have investigated a system with a WaveLAN card as wireless interface.

Quality of Service (QoS) is a key problem of today's IP networks. Many frameworks (IntServ, DiServ, MPLS, Trac engineering, etc.) have been proposed to provide service dierentiation 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. IEEE 802.11 wireless LAN (WLAN) is the most widely used WLAN standard today, but 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 report 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, dierent QoS enhancement techniques proposed for 802.11 WLAN are described and classied along with their advantages/drawbacks. Finally, the upcoming IEEE 802.11e QoS enhancement standard is introduced and studied in detail.

In this paper, we investigate the transmission of H.263 video sequences over wireless networks with error recovery provided by a twostep adaptive hybrid ARQ scheme using RS codes. Each video frame is divided into data packets for transmission. For each packet transmission, by using a simple table-driven approach, the best RS code is selected from a given set of codes to minimize the transmission overhead. Further, an additional adaptation step is used to guarantee certain QoS requirement. Simulation results show that the proposed error recovery scheme outperforms the traditional single-code schemes and the single-step error recovery schemes thanks to its adaptability to both the wireless channel condition and the actual frame loss events. Keywords--- Wireless networks, adaptive hybrid ARQ, transmission gain, partial gain, transmission status, pseudo delivery deadline, frame loss rate. I.

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 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.