[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: IEEE 802.11 Wireless LAN (WLAN) has become the pre-vailing solution for wireless Internet access while Transport Con-trol 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., AP-to-station) transmissions in infrastructure-based WLANs. In this paper, we develop a compre-hensive 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 con-tention 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 cor-rectness 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.
Journal of Communications and Networks 10/2009; · 0.75 Impact Factor
Quality of Service in Heterogeneous Networks, 6th International ICST Conference on Heterogeneous Networking for Quality, Reliability, Security and Robustness, QShine 2009 and 3rd International Workshop on Advanced Architectures and Algorithms for Internet Delivery and Applications, AAA-IDEA 2009, Las Palmas, Gran Canaria, Spain, November 23-25, 2009, Proceedings; 01/2009
[Show abstract][Hide abstract] ABSTRACT: IEEE 802.11 Wireless LAN (WLAN) has become a prevailing solution for broadband wireless Internet access while the Transport
Control Protocol (TCP) is the dominant transport-layer protocol in the Internet. Therefore, it is critical to have a good
understanding of the TCP dynamics over WLANs. In this paper, we conduct rigorous and comprehensive modeling and analysis of
the TCP performance over the emerging 802.11e WLANs, or more specifically, the 802.11e Enhanced Distributed Channel Access
(EDCA) WLANs. We investigate the effects of minimum contention window sizes and transmission opportunity (TXOP) limits (of
both the AP and stations) on the aggregate TCP throughput via analytical and simulation studies. We show that the best aggregate
TCP throughput performance can be achieved via AP’s contention-free access for downlink packet transmissions and the TXOP
mechanism. We also study the effects of some simplifying assumptions used in our analytical model, and simulation results
show that our model is reasonably accurate, particularly, when the wireline delay is small and/or the packet loss rate is
Mobile Networks and Applications 01/2009; 14(4):470-485. · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Broadband Communications, Networks and Systems, 2007. BROADNETS 2007. Fourth International Conference on; 10/2007
[Show abstract][Hide abstract] ABSTRACT: 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
Wireless on Demand Network Systems and Services, 2007. WONS '07. Fourth Annual Conference on; 02/2007
[Show abstract][Hide abstract] ABSTRACT: 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.
Communication Systems Software and Middleware, 2007. COMSWARE 2007. 2nd International Conference on; 02/2007
[Show abstract][Hide abstract] ABSTRACT: 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.
Proceedings of the Global Communications Conference, 2007. GLOBECOM '07, Washington, DC, USA, 26-30 November 2007; 01/2007
[Show abstract][Hide abstract] ABSTRACT: 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
[Show abstract][Hide abstract] ABSTRACT: The popular IEEE 802.11 WLAN today does not provide any quality-of-service (QoS) because of its contention-based channel access nature of the medium access control (MAC). Therefore, we proposed a simple software upgradebased solution, called a dual queue scheme, to provide QoS to real-time services such as Voiceover- IP (VoIP) in our previous work. The dual queue scheme operates on top of the legacy MAC. We in this paper present a modified dual queue (MDQ) scheme by considering a practical implementation limitation. On the other hand, currently 802.11e MAC is being standardized in order to support QoS, and it includes a contention-based channel access mechanism, called enhanced distributed channel access (EDCA). Prioritized channel accesses are provided to different traffic classes under EDCA. In this paper, we comparatively analyze the performances of MDQ and EDCA via simulations in terms of delay, jitter, and throughput for various scenarios. We also consider an APÂ¿s fast channel access scheme based on the channel access parameter adaptation of EDCA. The simulation results show that EDCA apparently provides a better performance in most situations. However, MDQ provides an acceptable performance close to that of EDCA so that it can be used when the EDCA is not available and/or the hardware upgrade to 802.11e is not desirable for the cost reason.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Today's 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.
Communications, 2004 IEEE International Conference on; 07/2004
[Show abstract][Hide abstract] ABSTRACT: In this paper, we propose a simple fragmenta-tion/aggregation scheme which combines the MSDU fragmenta-tion (used to enhance the reliability at the cost of more protocol overhead) of the legacy 802.11 and A-MPDU aggregation (used to reduce the overhead) of the emerging high-speed IEEE 802.11n. The proposed scheme increases the transmission range (i.e., dramatically increased throughput in bad channel condition) of high data rates via fragmentation without severely increasing the protocol overhead thanks to aggregation.
[Show abstract][Hide abstract] ABSTRACT: SummaryIEEE 802.11 Wireless LAN (WLAN) is fast rising as a prevailing solution for the broadband wireless networking. However, today's 802.11 WLANs cannot provide any quality-of-service (QoS) guarantee because of its contention-based channel access nature of the mandatory distributed coordination function (DCF). Recently, the needs for real-time services such as Voice over IP (VoIP) and audio/video (AV) streaming over WLANs have been increasing drastically. Accordingly, remarkable research efforts have been carried out to enhance the legacy 802.11 Medium Access Control (MAC) for QoS support. Moreover, the emerging IEEE 802.11e, which is an amendment of the existing 802.11 MAC is expected to provide QoS. In this chapter, we focus only on the schemes based on contention-based channel access. After a short overview of research trends on QoS provisioning in the 802.11 WLAN, we introduce a simple software upgrade-based solution, called a dual-queue scheme, and the emerging IEEE 802.11e standard to support QoS. The former can be considered a short- and mid-term solution to provide a limited QoS to real-time services, and the latter can be considered a long-term solution, which supports QoS at the MAC level. Finally, we compare three schemes, namely, legacy DCF, dual queue, and 802.11e EDCA schemes via simulations.
[Show abstract][Hide abstract] ABSTRACT: In order to improve the Voice over IP (VoIP) capacity of a Wireless Local Area Network (WLAN), we develop a relay-aided solution, which includes a relay-aided admission control scheme, a packet aggregation scheme, and a protocol for packet relay without sacrificing the voice quality. Basically, our admission control scheme utilizes relay stations to admit as many VoIP sessions as possible. When a new VoIP session requests admission, the admission controller checks whether it can be admitted by either using relay or communicating with the AP directly. Moreover, to overcome the overhead increased due to relaying small-size VoIP packets, we employ a packet aggregation scheme for VoIP packet transmissions between the AP and relay stations. Simulation results demonstrate that the proposed relay-aided VoIP provisioning scheme can significantly improve the VoIP capacity without compromising the service quality of the admitted VoIP sessions.