Qixiang Pang

A rate adaptation algorithm for IEEE 802.11 WLANs based on MAC-layer loss differentiation

In a WLAN subject to variable wireless channel conditions, rate adaptation plays an important role to more efficiently utilize the physical link. However, the existing rate adaptation algorithms for IEEE 802.11 WLANs do not take into account the loss of frames due to collisions. In a WLAN with coexistence of multiple stations, two types of frame losses due to (a) link errors and (b) collisions over the wireless link can coexist and severely degrade the performance of the existing rate adaptation algorithms. In this paper, we propose a new automatic rate fallback algorithm that can differentiate the two types of losses and sharpen the accuracy of the rate adaptation process. Numerical results show that the new algorithm can substantially improve the performance of IEEE 802.11 WLANs.

Design of an effective loss-distinguishable MAC protocol for 802.11 WLAN

In the IEEE 802.11 WLAN standard, the backoff algorithm assumes that all losses are due to collisions, while the automatic rate fallback algorithm assumes that all losses are due to link errors. The coexistence of these two types of losses in real networks reduces the efficiency of currently used algorithms. We propose a loss-distinguishable MAC layer protocol for 802.11 WLAN. No PHY layer modification is needed. Analysis shows that the new protocol is effective and has negligible overhead.

Performance evaluation of an adaptive backoff scheme for WLAN

Summary In this paper, a simple self-adaptive contention window adjustment algorithm for 802.11 wireless local area networks (WLAN) is proposed and analyzed. Numerical results show that the new algorithm outperforms the standard 802.11 window adjustment algorithm. Compared with the standard and previously proposed enhancement algorithms, a salient feature of our algorithm is that it performs well in both heavy and light contention cases regardless of the packet sizes and physical versions. Moreover, the adaptive window adjustment algorithm is simpler than previously proposed schemes in that no live measurement of the WLAN traffic activity is needed. Copyright # 2004 John Wiley & Sons, Ltd.

Service scheduling for general packet radio service classes

While the quality of service (QoS) profiles for a number of general packet radio service (GPRS) classes has been specified by ETSI, how QoS management is provided by means of traffic scheduling, traffic shaping, and connection admission control, in a GPRS network is an implementation issue that is attracting significant current research interest. This paper presents an evaluation of several traffic scheduling methods, including of FIFO, static priority scheduling (SPS) and earliest deadline first (EDF) by simulations, with the objective of meeting the delay profiles defined for a number of GPRS classes. Traffic sources representative of GPRS applications, including e-mail, fleet management and World Wide Web applications are employed. We focus on the forward link which represents the bottleneck of a typical GPRS data connection. Results show that EDF is able to meet the delay requirements at a much higher channel utilization compared to the other alternatives.

Reliable data transport and congestion control in wireless sensor networks

Reliable data delivery and congestion control are two fundamental transport layer functions. Due to the specific characteristics of Wireless Sensor Networks (WSNs), traditional transport layer protocols (e.g. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)) that are widely used in the Internet may not be suitable for WSNs. In this paper, the characteristics of WSNs are reviewed and the requirements and challenges of reliable data transport over WSNs are presented. The issues with applying traditional transport protocols over WSNs are discussed. We then survey recent research progress in developing suitable transport protocols for WSNs. The proposed reliable data transport and congestion control protocols for WSNs are reviewed and summarised. Finally, we describe some future research directions of transport protocol in WSNs.

Improvement of WLAN Contention Resolution by Loss Differentiation

In a realistic WLAN environment, frame losses may be caused by collisions or channel noise. The existence of noise-induced losses reduces the effectiveness of the standard WLAN backoff algorithm for contention resolution, which assumes that all losses are caused by collisions and always doubles the contention window to reduce contention upon a frame loss. In this paper, we propose new backoff algorithms that take advantage of a new capability to differentiate the losses, and thereby sharpen the accuracy of the contention resolution process. Analytical models are developed to analyze the performance of these algorithms under heterogeneous link conditions in a WLAN. Both analysis and simulation results show that significant improvement of throughput and fairness can be obtained for WLANs in which contention resolution is. enhanced by the loss differentiation ability

Channel Clustering and Probabilistic Channel Visiting Techniques for WLAN Interference Mitigation in Bluetooth Devices

Since Bluetooth and wireless local area network (WLAN) technologies both operate at the 2.4-GHz industrial, scientific, and medical (ISM) band, the two types of devices may suffer from mutual interference and performance degradations. In this paper, we propose two new techniques, channel clustering and probabilistic channel visiting, to effectively improve the existing coexistence and interference mitigation mechanisms. The channel clustering technique employs statistical pattern recognition to classify the status of Bluetooth channels more accurately. The probabilistic channel visiting is used to more equitably allocate the channel resources between Bluetooth and WLAN devices. The effectiveness of these techniques is quantified by simulations. Results show that both techniques are beneficial in improving the performance of the existing mechanisms.

Performance improvement of 802.11 wireless network with TCP ACK agent and auto-zoom backoff algorithm

This paper proposes two schemes to improve the TCP performance over 802.11 WLAN access protocol. In the standard 802.11 and TCP protocols, a TCP data segment is acknowledged twice, once at the MAC layer, and once at the TCP layer. By a simple cross-layer design, a TCP ACK agent is installed at the WLAN AP. When a MAC acknowledgment is received by the AP, the AP generates a TCP ACK on behalf of the WLAN stations. Together with the TCP ACK agent scheme, an auto-zoom backoff algorithm is proposed to further improve the access performance. In the auto-zoom backoff, the contention windows can be progressively reduced to a very low value in case of light or asymmetric traffic, and return to normal quickly when collision occurs. With the TCP ACK agent and auto-zoom backoff, we show that significant improvement in TCP throughput performance can be achieved in typical Internet application scenarios.

Performance evaluation of retransmission mechanisms in GPRS networks

This paper studies the performance of the three retransmission mechanisms (i.e., RLC/MAC, LLC and TCP) in a General Packet Radio Service (GPRS) network accessing the Internet. This assessment is done through the end-to-end performance evaluation of Internet applications such as HTTP and FTP over GPRS. A simulation platform including client mobile stations, base station subsystems, GPRS support nodes, servers and an Internet backbone was implemented in OPNET. The low transmission rate and high radio link error rate over the air interface of a GPRS network affect the overall system performance greatly. Simulation results show that whereas the RLC/MAC block retransmission mechanism is crucial to increase the link utilization efficiency, LLC layer user-data retransmission is not necessary for TCP traffic over GPRS.

Improved Channel Classification and Scheduling for Non-collaborative Bluetooth/ WLAN Coexistence

In this paper, we propose two new techniques to enhance the existing Bluetooth and WLAN coexistence mechanisms. The first one is channel clustering, which is used to classify the channel status more accurately. The second one is called probabilistic channel visiting, which is used to more reasonably allocate the channel resources between WLAN and BT devices. Both techniques are beneficial in improving the performance of the existing coexistence mechanisms. The effectiveness of these techniques is quantified by simulations