• Journal of Communications and Networks
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• v.10 no.3
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• pp.287-296
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• 2008

The IEEE 802.11 wireless standard uses the carrier sense multiple access with collision avoidance (CSMA/CA) as its MAC protocol (during the distributed coordination function period). This protocol is an adaptation of the CSMA/CD of the wired networks. CSMA/CA mechanism cannot guarantee quality of service (QoS) required by the application because orits random access method. In this study, we propose a new MAC protocol that considers different types of traffic (e.g., voice and data) and for each traffic type different priority levels are assigned. To improve the QoS of IEEE 802.11 MAC protocols over a multi-channel CSMA/CA, we have developed a new admission policy for both voice and data traffics. This protocol can be performed in direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS). For voice traffic we reserve a channel, while for data traffic the access is random using a CSMA/CA mechanism, and in this case a selective reject and push-out mechanism is added to meet the quality of service required by data traffic. To study the performance of the proposed protocol and to show the benefits of our design, a mathematical model is built based on Markov chains. The system could be represented by a Markov chain which is difficult to solve as the state-space is too large. This is due to the resource management and user mobility. Thus, we propose to build an aggregated Markov chain with a smaller state-space that allows performance measures to be computed easily. We have used stochastic comparisons of Markov chains to prove that the proposed access protocol (with selective reject and push-out mechanisms) gives less loss rates of high priority connections (data and voices) than the traditional one (without admission policy and selective reject and push-out mechanisms). We give numerical results to confirm mathematical proofs.

• Journal of the Institute of Convergence Signal Processing
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• v.10 no.3
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• pp.181-185
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• 2009

Recently, much attention to a self-organizing mobile ad-hoc network is escalating along with progressive deployment of wireless networks in our everyday life. Being readily deployable, the MANET (mobile ad hoc network) can find its applications to emergency medical service, customized calling service, group-based communications, and military purposes. In this paper we investigate a time slot exchange problem found in the time slot based MAC, that is designed for IEEE 802.11b interfaces composing a MANET. The paper provides a method to maintain the quality of voice call by providing a new time slot when the channel assigned for that time slot gets noisy with interferences induced from other nodes, which belong to the same and/or other subgroups. In order to assess the performance of the proposed algorithm, a set of simulations using the OPNET modeler has been performed assuming that the IEEE 802.11b interfaces are operating under a modified MAC, which is a time slot based reservation MAC implemented in the PCF part of the superframe. In a real-time voice call service over a MANET of a size 500 ${\times}$ 500 meter squares with the number of nodes up to 100, the simulation results are collected and analyzed with respect to the packet loss rate and packet delay. The results show us that the proposed time slot exchange protocol improves the quality of voice call over that of plain DCF.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.13 no.8
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• pp.1561-1570
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• 2009

Being different from traditional mobile communication networks or wireless LAN with access points, ad hoc network can provide direct communications between homogeneous stations. Thus, it can be considered as a much advanced network. Somehow, many researches have been done to enhance system performance of ad hoc networks up to now, one of which is cooperative communications. In this paper, a cooperative MAC protocol, called CO-MAC protocol, is proposed and its performance is evaluated with a mathematical approach. Numerical results show that this scheme provides more enhanced system throughput especially when frame transmission error probability is high than DCF scheme, which is the key protocol in IEEE 802.11 standard, and rDCF scheme.

• Journal of KIISE:Information Networking
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• v.32 no.5
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• pp.633-640
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• 2005

This paper proposes and analyzes a message scheduling scheme and corresponding bandwidth allocation method for the hard real-time communication on dual standard 802.11 Wireless LANs. By making the superframeof one network precede that of the other by half, the dual network architecture can minimize the effect of deferred beacon and reduce the worst case waiting timeby half. The effect of deferred beacon is formalized and directly considered to decide the polling schedule of PCF phase. Simulation results executed via ns-2 show that the proposed scheme can improve the schedulability by 3$36\%$ for real-time messages and give $9\%$ more bandwidth to non-real-time messages for the given stream sets, compared with the network whose bandwidth is just doubled with the same MAC.

• Proceedings of the Korean Institute of Communication Sciences Conference
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• 2004.11a
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• pp.31-31
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• 2004

• Proceedings of the Korea Information Processing Society Conference
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• 2007.05a
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• pp.1294-1296
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• 2007

The IEEE 802.11 DCF forces neighboring nodes of an active transmitter to switch into inactive state. This conservative nature brings frame latency at transmitter neighborhood. This work exploits the IEEE 802.11n Frame Aggregation scheme to allow simultaneous transmissions from nodes that are neighbors to each-other. This is accomplished by the synchronization of control and data transmissions in slots of fixed length. Proposed scheme reduces the frame latency and improves aggregated network throughput.

• Journal of Computing Science and Engineering
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• v.5 no.4
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• pp.324-330
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• 2011

To avoid collisions, IEEE 802.11 medium access control (MAC) uses predetermined inter-frame spaces and the random back-off process. However, the retransmission strategy of IEEE 802.11 MAC results in considerable time wastage. The hidden node problem is well known in wireless networks; it aggravates the consequences of time wastage for retransmission. Many collision prevention and recovery approaches have been proposed to solve the hidden node problem, but all of them have complex control overhead. In this paper, we propose a fast retransmission scheme as a recovery approach. The proposed scheme identifies collisions caused by hidden nodes and then allows retransmission without collision. Analysis and simulations show that the proposed scheme has greater throughput than request-to-send and clear-to-send (RTS/CTS) and a shorter average waiting time.

• Proceedings of the KIEE Conference
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• 2001.11c
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• pp.380-382
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• 2001

This paper presents the implementation of IEEE 802.11 Medium Access Control and Physical Layer Protocol that can be applied to wireless LAN system. We have used PRISM2 chipsets from Intersil to build the baseband, IF, and RF parts. DSSS(Direct Sequence Spread Spectrum) physical layer at 2.4GHz ISM band is adopted in the hardware prototype. To meet the high-speed requirement of physical layer, we have designed the MAC protocol layer with embedded firmware and FPGA. The prototype board is shown to be able to support the physical layer of 5GHz and 600Hz wireless LAN systems.

• Journal of IKEEE
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• v.11 no.4
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• pp.171-177
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• 2007

Although the Distributed Coordination Function is the fundamental access protocol of IEEE 802.11, it cannot meet the Quality of Service (QoS) requirements in general. So, the Point Coordinate Function is provided to support QoS related services. However, it has inherent problems. Access point (AP) has no knowledge of the queue status and instantaneous channel condition of stations in the system. In this paper we propose an efficient and versatile polling scheduler that shows excellent throughput and fairness performance. Comparison with well known polling schemes is provided through computer simulation under various channel situations including error prone environments.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.38 no.6
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• pp.11-17
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• 2001

Design and implementation of the MAC protocol processor prototype for high speed wireless LAN, which has interface with 5GHz OFDM PHY layer, is presented. We analyze the IEEE 802.11 MAC protocol specification and then separate the MAC protocol functions to be implemented by hardware and firmware and define the interface in which frames can be exchanged. That is, it is considered that high speed queue processing and interfaces with RISC processor and OFDM PHY layer. Protocol control and transmission/reception functions of the MAC functions are implemented in hardware in order to guarantee high speed processing in MAC layer. The developed MAC hardware block operates at 10MHz main clock. Therefore, transmission rate in PHY layer is about 80Mbps because data transmission/reception between MAC layer and PHY layer is performed as unit of octet. The designed FPGA MAC function chip has been implemented in wireless LAN test board and it is verified that DCF function is operated correctly.