Jeonggyun Yu

Enhancement of VolP over IEEE 802.11 WLAN via dual queue strategy

Todays IEEE 802.11 Wireless LAN (WLAN) is an excellent solution for the broadband wireless networking. However, it lacks of the capability to support real-time services such as voice-over-IP (VoIP) properly. In this paper, we present a simple and viable approach to enhance the VoIP performance over the 802.11 WLAN by implementing two queues along with a strict priority queuing on top of the 802.11 medium access control (MAC) controller, e.g., in the device driver of the 802.11 cards. We find via extensive simulations that the proposed scheme is remarkably effective for the VoIP service in the infrastructure-based WLAN in the coexistence with the nonreal-time traffic thanks to the flow control mechanism of the TCP protocol, which is typically used for the nonreal-time traffic today. Due to its simplicity, the proposed scheme should be readily deployable in the existing WLANs via simple software upgrades for the enhanced VoIP services.

Modeling and analysis of TCP dynamics over IEEE 802.11 WLAN

Today, IEEE 802.11 wireless LAN (WLAN) is a prevailing solution for the wireless Internet access while transport control protocol (TCP) is the dominant transport protocol in the Internet. It is known that, in the infrastructure WLAN with multiple long-lived TCP flow stations, the actual number of contending TCP stations is limited to a small number, and the aggregated TCP throughput is basically independent of the total number of TCP stations. It is due to the behavior of the TCP flow control in the infrastructure WLAN, where the downlink (i.e., access point-to-stations) is the bottleneck link. In this paper, we develop an accurate and realistic analytical model of long-lived TCP over WLAN based on a well-known p-persistent model of the 802.11. We calculate the average number of the active TCP stations as well as the aggregated TCP throughput using our model for a given number of TCP stations and the maximum TCP receive window size. We verify our model via ns-2 simulations. Moreover, we find from our work that the minimum contention window sizes of the standards (i.e., 31 and 15 for IEEE 802.11b and 802.11a, respectively) are not optimal in the sense of the TCP throughput maximization

A novel hidden station detection mechanism in IEEE 802.11 WLAN

The popular IEEE 802.11 wireless local area network (WLAN) is based on a carrier sense multiple access with collision avoidance (CSMA/CA), where a station listens to the medium before transmission in order to avoid collision. If there exist stations which can not hear each other, i.e., hidden stations, the potential collision probability increases, thus dramatically degrading the network throughput. The RTS/CTS (request-to-send/clear-to-send) frame exchange is a solution for the hidden station problem, but the RTS/CTS exchange itself consumes the network resources by transmitting the control frames. In order to maximize the network throughput, we need to use the RTS/CTS exchange adaptively only when hidden stations exist in the network. In this letter, a simple but very effective hidden station detection mechanism is proposed. Once a station detects the hidden stations via the proposed detection mechanism, it can trigger the usage of the RTS/CTS exchange. The simulation results demonstrate that the proposed mechanism can provide the maximum system throughput performance

Probabilistic-Based Rate Adaptation for IEEE 802.11 WLANs

Collision awareness has been recognized as a critical component for effective rate adaptation schemes. Recently, several collision-aware rate adaptation schemes have been proposed for IEEE 802.11 Wireless LANs (WLANs), such as CARA (Collision Aware Rate Adaptation) and RRAA (Robust Rate Adaptation Algorithm). These schemes are able to distinguish between channel- error-induced and collision-induced frame losses via adaptive and appropriate usage of RTS/CTS; hence the multiple transmission rates provided by 802.11 physical layers (PHYs) may be fully exploited. In this paper, we propose a unique collision-aware rate adaptation scheme, called PBRA (Probabilistic-Based Rate Adaptation). The key ideas of PBRA include (i) probabilistic-based adaptive usage of RTS/CTS, which is in direct contrast to trial- based RTS Probing in CARA and window-based adaptive usage of RTS/CTS in RRAA; and (ii) threshold-based rate adjustment, which allows a station to make more appropriate rate adjustment decisions, thanks to its accurate estimation of the channel-error-induced frame loss ratio. Simulation results show that PBRA clearly outperforms all other testing schemes (including CARA and RRAA), particularly in random topology networks with fading wireless channels.

SP-TPC: a self-protective energy efficient communication strategy for IEEE 802.11 WLANs

We propose a novel energy efficient communication strategy, called SP-TPC, for IEEE 802.11 wireless LANs (WLANs). SP-TPC utilizes both transmit power control (TPC) and PHY rate adaptation in order to minimize the communication energy consumption while maximizing the throughput performance. When TPC is employed in the 802.11 WLAN, the overall performance may be degraded due to the potential increase of hidden stations, and hence the request-to-send/clear-to-send (RTS/CTS) exchange is typically used in conjunction with TPC in order to eliminate hidden stations. However, the RTS/CTS exchange is an expensive solution since it itself consumes energy as well as precious bandwidth. By taking advantage of the existing clear channel assessment (CCA) mechanism intelligently, SP-TPC does not introduce any extra hidden stations, and hence RTS/CTS exchange is not needed if hidden stations did not exist without TPC. Through simulations, we demonstrate that SP-TPC outperforms other TPC strategies consistently in terms of both throughput and energy efficiency in small and mid-ranged WLAN environments.

A performance analysis of block ACK scheme for IEEE 802.11e networks

The demand for the IEEE 802.11 wireless local-area networks (WLANs) has been drastically increasing along with many emerging applications and services over WLAN. However, the IEEE 802.11 medium access control (MAC) is known to be limited in terms of its throughput performance due to the high MAC overhead, such as interframe spaces (IFS) or per-frame based acknowledgement (ACK) frame transmissions. The IEEE 802.11e MAC introduces the block ACK scheme for improving the system efficiency of the WLAN. Using the block ACK scheme can reduce the ACK transmission overhead by integrating multiple ACKs for a number of data frames into a bitmap that is contained in a block ACK frame, thus increasing the MAC efficiency. In this paper, we mathematically analyze the throughput and delay performance of the IEEE 802.11e block ACK scheme in an erroneous channel environment. Our extensive ns-2 simulation results validate the accuracy of our analytical model and they further demonstrate that the block ACK scheme enhances the MAC throughput performance at the cost of the resequencing delay at the receiving buffer.

Supporting VoIP Services in IEEE 802.11e WLANs

Voice over Internet Protocol (VoIP) over Wireless Local Area Network (WLAN) is becoming popular thanks to its cost efficiency. However, it has been a challenge to provide good quality of VoIP services in WLANs, which is due mainly to (i) the nature of contention-based channel access of WLAN Medium Access Control (MAC); (ii) the presence of coexisting non-real-time data traffic; and (iii) the time-varying WLAN capacity caused by transmission rate diversity and variation of stations over time. In this paper, we propose a simple, effective and viable solution to improve the quality of VoIP services in 802.11e contention-based WLANs, which basically utilizes the advanced features of 802.11e MAC for QoS support. The key ingredients of our solution include (i) a priority queue to serve the VoIP traffic with higher priority than the non-real-time data traffic; and (ii) a conservative history-based admission control scheme for VoIP services, which accommodates the transmission rate diversity and variation of ongoing VoIP sessions over time. Simulation results demonstrate that our solution admits as many VoIP calls as possible without compromising the quality of their services.

TCP dynamics over IEEE 802.11E WLANs: Modeling and throughput enhancement

Today, IEEE 802.11 Wireless LAN (WLAN) has become a prevailing solution for broadband wireless Internet access while Transport Control Protocol (TCP) is the dominant transport protocol in the Internet. It is known that, in an infrastructure-based WLAN with multiple stations carrying long-lived TCP flows, the number of stations that are actively contending to access the channel is very small. Therefore, the aggregate TCP throughput is basically independent of the total number of stations. This phenomenon is due to the closed-loop nature of TCP flow control and the bottleneck downlink (i.e., AP-to-station) transmissions in infrastructure-based WLANs. In the emerging Enhanced Distributed Channel Access (EDCA)-based IEEE 802.11e WLANs, with a proper configuration, packet congestion at the bottleneck downlink could be alleviated since the AP and stations are allowed to use different channel access parameters. In this paper, we first conduct a rigorous, comprehensive analysis of the TCP dynamics over the 802.11e EDCA. Then, the effects of minimum contention window sizes (of both AP and stations) on the aggregate TCP throughput are evaluated via mathematical analysis and simulation. We also show that the best TCP aggregate throughput performance can be achieved via AP’s contention-free access for downlink packet transmissions. Finally, some of the simplifying assumptions used in our mathematical model are evaluated via simulation, and results show that our model is reasonably accurate when the wireline delay is small and the packet loss rate is low.

Throughput and Delay Analysis of IEEE 802.1le Block ACK with Channel Errors

Recently, along with many emerging applications and services over Wireless LANs (WLANs), the demands for higher-speed WLANs have been growing drastically. However, it is well known that IEEE 802.11 Medium Access Control (MAC) has a high overhead. As a solution to improve the system efficiency, the new IEEE 802.11e MAC introduces Block ACK scheme. In this paper, we mathematically analyze both throughput and delay performances of the 802.11e Block ACK scheme over a noisy channel considering the Block ACK protection scheme. Then, the numerical results are verified with ns-2 simulations.

Analytical modeling of TCP dynamics in infrastructure-based IEEE 802.11 WLANs

IEEE 802.11 wireless local area network (WLAN) has become the prevailing solution for wireless Internet access while transport control protocol (TCP) is the dominant transport-layer protocol in the Internet. It is known that, in an infrastructure-based WLAN with multiple stations carrying long-lived TCP flows, the number of TCP stations that are actively contending to access the wireless channel remains very small. Hence, the aggregate TCP throughput is basically independent of the total number of TCP stations. This phenomenon is due to the closed-loop nature of TCP flow control and the bottleneck downlink (i.e., access point-to-station) transmissions in infrastructure-based WLANs. In this paper, we develop a comprehensive analytical model to study TCP dynamics in infrastructure-based 802.11 WLANs. We calculate the average number of active TCP stations and the aggregate TCP throughput using our model for given total number of TCP stations and the maximum TCP receive window size. We find out that the default minimum contention window sizes specified in the standards (i.e., 31 and 15 for 802.11b and 802.11a, respectively) are not optimal in terms of TCP throughput maximization. Via ns-2 simulation, we verify the correctness of our analytical model and study the effects of some of the simplifying assumptions employed in the model. Simulation results show that our model is reasonably accurate, particularly when the wireline delay is small and/or the packet loss rate is low.