Mehmet Fatih Tüysüz

A cross layer QoS algorithm to improve wireless link throughput and voice quality over multi-rate WLANs

Voice over IP (VoIP) applications over wireless local area networks (WLANs) has become increasingly popular in recent years. VoIP is sensitive to delay and variations in packet inter-arrival times and require a steady and a dependable stream of packets to provide reasonable voice quality. Therefore, it is important to analyze the performance of the IEEE 802.11 protocol accurately by considering the effects of channel conditions, frame size, number of nodes and collision probability. In this paper, we present an analysis of link layer behavior for ideal and error prone channels by classifying the packet loss caused either by congestion or wireless medium errors and propose an efficient, adaptive, codec and frame size modification algorithm for the optimization of maximum throughput and voice quality on multirate WLANs.

A Beacon-Based Collision-Free Channel Access Scheme for IEEE 802.11 WLANs

In IEEE 802.11 based WLAN standard, distributed coordination function is the fundamental medium access control (MAC) technique. It employs a CSMA/CA with random binary exponential backoff algorithm and provides contention-based distributed channel access for stations to share the wireless medium. However, performance of this mechanism drops dramatically due to random structure of the backoff process, high collision probability and frame errors. That is why development of an efficient MAC protocol, providing both high throughput for data traffic and quality of service (QoS) support for real-time applications, has become a major focus in WLAN research. In this paper, we propose an adaptive beacon-based collision-free MAC adaptation. The proposed scheme makes use of beacon frames sent periodically by access point, lets stations enter the collision-free state and reduces the number of idle slots regardless of the number of stations and their traffic load (saturated or unsaturated) on the medium. Simulation results indicate that the proposed scheme dramatically enhances the overall throughput and supports QoS by reducing the delay, delay variation and dropping probability of frames.

A novel energy-efficient QoS-aware handover scheme over IEEE 802.11 WLANs

Energy efficiency in wireless networks is one of the major issues for mobile devices since they rely on their batteries. In this context, the wireless network interface card (WNIC) has to be taken into account carefully as it consumes a significant portion of the overall system energy. In this paper, we aim to reduce the energy consumption of mobile devices in wireless networks by performing specific solutions, such as reducing the overhead of the periodic background channel scanning, keeping mobile stations in the doze state as much as possible, proposing a smart selective channel scanning during the handover preparation phase and associating with an access point (AP) that is expected to consume the least energy among all APs. Stations performing the proposed scheme can fairly coexist with the other stations in the network. In the proposed scheme, each station makes use of its local information and the information provided by the IEEE 802.21 information server (IS). The proposed scheme can be easily implemented without any changes on the current IEEE 802.11 standard. Performance of the proposed scheme was investigated by numerical analyses and extensive simulations. The results illustrate that the proposed scheme is very efficient and reduces the energy consumptions of mobile stations under a wide range of contention and signal strength levels.

Exploiting the channel using uninterrupted collision-free MAC adaptation over IEEE 802.11 WLANs

IEEE 802.11 wireless local area networks WLANs have reached an important stage and become a common technology for wireless access due to its low cost, ease of deployment, and mobility support. In parallel with the extensive growth of WLANs, the development of an efficient medium access control protocol that provides both high throughput performance for data traffic and quality of service support for real-time applications has become a major focus in WLAN research. The IEEE 802.11 Distributed Coordination Functions DCF/EDCA provide contention-based distributed channel access mechanisms for stations to share the wireless medium. However, performance of these mechanisms may drop dramatically because of high collision probabilities as the number of active stations increases. In this paper, we propose an adaptive collision-free MAC adaptation. The proposed scheme prevents collisions and allows stations to enter the collision-free state regardless of the traffic load saturated or unsaturated and the number of stations on the medium. Simulation results show that the proposed scheme dramatically enhances the overall throughput and supports quality of service for real-time services over 802.11-based WLANs. Copyright

An uninterrupted collision-free channel access scheme over IEEE 802.11 WLANs

IEEE 802.11 distributed coordination function, (DCF), provides contention-based distributed channel access for stations to share the wireless medium. However, Performance of IEEE 802.11 DCF protocol, in terms of delay and throughput, degrades dramatically as the number of active stations increases, especially when each station transmits in its saturated state. Therefore, development of an efficient Medium Access Control (MAC) protocol providing both high throughput for data traffic and quality of service (QoS) for real-time applications has become a major focus in WLAN research. In this paper, we propose an adaptive collision-free MAC adaptation. Proposed scheme prevents collisions and allows stations to enter an uninterrupted collision-free state, regardless of the traffic types (saturated or unsaturated) and the number of stations on the medium. Simulation results indicate that proposed scheme dramatically enhances the overall throughput and supports QoS maintaining real-time services at high level over 802.11-based WLANs.

Network-assisted QoS-based fast handover with smart scanning over IEEE 802.11 WLANs

Nowadays, IEEE 802.11 WLANs have been widely deployed in parallel with the growth of mobile devices. However, due to design choices and requirements, the communication range of the IEEE 802.11 standard is limited. Therefore, WLAN coverage over a large geographic area is provided using multiple access points (APs). Due to the mobility of mobile stations, a station might move and lose the signal of its associated AP. In that case, the station tries to maintain its wireless connectivity by associating with a new AP in a process known as handover. However, providing handover with guaranteed QoS is a challenging task for real-time applications, such as VoIP, video/audio streaming, and video conferencing. In this paper, we propose an efficient handover mechanism that can provide fast handover with guaranteed QoS for real-time applications in WLANs. With the help of the IEEE 802.21 information server (IS), a smart selective channel scanning method is deployed on mobile stations. Selective channel scanning enables stations to filter out the unavailable channels, and organize reliable group of channels to scan during the upcoming handover process. Simulation results show that the proposed scheme reduces the channel scanning delay in handover process, increases the overall throughput and provide QoS for real-time applications compared to the conventional handover scheme.

Smart channel scanning with minimized communication interruptions over IEEE 802.11 WLANs

In IEEE 802.11 wireless networks, periodic channel scanning is an essential procedure to discover available access points (APs) in vicinity and to achieve fast handover. However, this procedure leads to unnecessary overhead in wireless networks and also interrupts ongoing communications of stations. In this paper, we propose a smart channel scanning scheme that minimizes the communication interruptions. In the proposed scheme, periodic channel scanning is scheduled based on the predefined channel scanning intervals and activated after a successful frame transmission only if the mobile station is on move and the received signal strength indicator value (RSSI) drops below a pre-specified threshold value. Analytical and simulation results show that the proposed channel scanning scheme reduces the channel scanning overhead dramatically, minimizes communication interruptions and improves the overall throughputs of stations.

Adaptive channel access tuning and QoS support for multimedia applications over IEEE 802.11e WLANs

IEEE 802.11e Enhanced Distributed Channel Access (EDCA) standard defines multiple Access Categories (AC) to support MAC-level Quality of Service (QoS). However, the values of the main parameters of each AC are static and do not take the wireless channel condition and the channel density into account. Therefore, the default EDCA parameters only yield good performance for few scenarios. In this paper, we propose a new cross-layer adaptation scheme that improves the IEEE 802.11e Quality of Service (QoS) by tuning the Medium Access Control (MAC) layer channel access parameters. Proposed scheme is based on the number of mobile nodes on the channel, their traffic types, application layer QoS requirements and physical layer (PHY) channel conditions. Simulation results indicate that our scheme outperforms the standard implementation and provides desired QoS for multimedia applications.

Access point selection and reducing the handoff latency to support VoIP traffic

In IEEE 802.11 WLANs, a station can associate with more than one access point (AP). Previous algorithms which consider only the received signal strength indicator (RSSI) can cause stations to be associated to a single or only a few number of AP. However, it results in an overall network performance degradation. Besides, Hondoff in wireless networks is one of the critical issues for delay sensitive applications due to the high handoff latency. Therefore, a new algorithm is required both to select the most proper access point (AP) and to reduce the handoff latency to support multimedia traffic. In this paper, we propose a new AP selection algorithm which is based on reducing the handoff latency, improving the perceived voice quality (R value), capacity utilization, transmission rate, fairness, hidden nodes and the number of associated stations to an AP. Proposed algorithm works efficiently in the existence of both real-time and non-real-time traffic. Simulation results show improvements on the overall throughput and the perceived voice quality for users.

A local estimation of node quantity and channel density for VoIP applications over IEEE 802.11e WLANs

IEEE 802.11 WLANs have reached an important stage and have become a common technology for wireless access due to its low cost, ease of deployment and mobility support. However, IEEE 802.11 standard cannot support desired Quality of Service (QoS) for delay-sensitive multimedia applications. Therefore, IEEE 802.11e standard which aims to support QoS by providing different priorities to different types of frames was specified in 2005. However, there are still unsolved performance issues such as optimization of throughput, limiting the delay and packet loss ratio. In this paper, we first present a local estimation method of node quantity and channel density to let the mobile nodes change their parameters according to the channel conditions. Then, we propose a QoS adaptation for delay-sensitive VoIP applications. Proposed algorithm only uses its local information and can be easily implemented without any changes on the current IEEE 802.11e standard. Simulation results show better voice quality and higher overall throughput than the standard IEEE 802.11e.