H-SP-MAC: Improvement of media access control mechanism SP-MAC for hidden terminal problem
Abstract and Figures
IEEE802.11規格に基づく無線LANで用いられる代表的なメディアアクセス制御(MAC)としてCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA)が挙げられる．しかし，CSMA/CAにはアクセスポイントに接続する無線端末数の増加に伴ってデータフレームの衝突が頻発し，全無線端末の合計スループットが低下する問題が知られている．この問題を回避するために，我々はこれまで結合振動子の同期現象に基づくMAC方式(以降，SP-MACと示す)を提案してきた．SP-MACを利用する端末は，データフレームの衝突を回避することでスループットを大幅に向上できることを確認している．しかし，これまでのSP-MACに対する評価は，全無線端末がお互いのチャネル利用状況を認知できる，すなわち隠れ端末が存在しない環境でのみ行われてきた．現実の無線LAN環境では隠れ端末の存在が無線端末の合計スループットに影響を及ぼすことが予想される．そこで，本論文では隠れ端末が存在する環境におけるSP-MACの問題点を明らかにし，その解決策であるSP-MACの改良方式を提案する．シミュレーション評価の結果，通常のSP-MACはUDP通信を行う場合に，隠れ端末が存在することで合計スループットが大きく低下するが，改良方式は通常のSP-MAC，及びRTS/CTSを用いないCSMA/CAと比べて高い合計スループットが得られることがわかった．
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Wireless local area networks (LANs) based on the IEEE802.11 standard usually use carrier sense multiple access with collision avoidance (CSMA/CA) for media access control. However, in CSMA/CA, if the number of wireless terminals increases, the back-off time derived by the initial contention window (CW) tends to conflict among wireless terminals. Consequently, a data frame collision often occurs, which sometimes causes the degradation of the total throughput in the transport layer protocols. In this study, to improve the total throughput, we propose a new media access control method, SP-MAC, which is based on the synchronization phenomena of coupled oscillators. Moreover, this study shows that SP-MAC drastically decreases the data frame collision probability and improves the total throughput when compared with the original CSMA/CA method. © Copyright 2015 The Institute of Electronics, Information and Communication Engineers.
Wireless local area networks, based on the distributed coordination function of the IEEE802.11 standard, usually use carrier sense multiple access with collision avoidance (CSMA/CA) for media access control. In the CSMA/CA method, if the number of wireless terminals increases, data frame collision often occurs among wireless terminals, which results in a decrease in the total throughput. Therefore, to improve the total throughput, a media access control method based on the synchronization phenomena of coupled oscillators, which we refer to as SP-MAC, is proposed. However, the performance of terminals based on SP-MAC, which coexist with CSMA/CA terminals, has not been sufficiently evaluated. Therefore, in this study, we evaluate the throughput performance in detail when terminals based on SP-MAC coexist with those based on CSMA/CA.
This survey provides a comprehensive review of existing physical carrier sensing enhancements for IEEE 802.11 wireless networks. The original physical carrier sensing mechanism, used by wireless stations to gain access to the medium, is limited. Consequently, IEEE 802.11 networks are vulnerable to the presence of hidden and exposed nodes. Such nodes can significantly decrease system performance by increasing the collision rate and decreasing the channel spatial reuse. The value of the physical carrier sensing threshold is a key factor influencing the presence of hidden and exposed nodes in a wireless network. Several enhancements have been proposed in the literature, which attempt to mitigate the loss in performance caused by the limited carrier sensing. Firstly, the notion of an optimum carrier sensing threshold has been studied, and results indicate that it can be tuned to an optimum value. Building on the positive early results, further work was performed to develop mechanisms that dynamically adjust the threshold according to varying network conditions. This article presents an in-depth survey of the existing literature in the area, detailing the various approaches and their efficacy in addressing the problem of hidden and exposed nodes (and consequently increasing performance). It offers a comparison of the techniques, by evaluating the models, limitations, assumptions, and performance gains.
In wireless sensor networks (WSNs), contention occurs when two or more nodes in a proximity simultaneously try to access the channel. The contention causes collisions, which are very likely to occur when traffic is correlated. The excessive collision not only affects the reliability and the QoS of the application, but also the lifetime of the network. It is well-known that random access mechanisms do not efficiently handle correlated-contention, and therefore, suffer from high collision rate. Most of the existing TDMA scheduling techniques try to find an optimal or a sub-optimal schedule. Usually, the situation of correlated-contention persists only for a short duration, and therefore, it is not worthwhile to take a long time to generate an optimal or a sub-optimal schedule. We propose a randomized distributed TDMA scheduling (RD-TDMA) algorithm to quickly generate a feasible schedule (not necessarily optimal) to handle correlated-contention in WSNs. In RD-TDMA, a node in the network negotiates a slot with its neighbors using the message exchange mechanism. The proposed protocol has been simulated using the Castalia simulator to evaluate its runtime performance. Simulation results show that the RD-TDMA algorithm consider-ably reduces the time required to schedule.
This paper proposes a new TDMA-based cooperative MAC scheme for a two-hop relay network in which there are several helpers besides a default relay node. Here, a helper has lower obligation than the default relay to help packet forwarding. The proposed method is developed from the existing CR-TDMA scheme, and named as opportunistic CR-TDMA or OCR-TDMA. We derive the throughput performance of CR-TDMA and OCR-TDMA. Both the theoretical analysis and simulation results demonstrate that OCR-TDMA provides superior throughput performance compared with CR-TDMA, even though OCR-TDMA is a limited modification of CR-TDMA. Amazingly, the OCR-TDMA gains remarkable cooperation benefit even when the helpers have low resource and capability to cooperate.
In this letter, we propose and analyze a time division multiple access (TDMA)-based cooperative medium access control (MAC) protocol for wireless multi-hop relaying networks. The fundamental drawback of a reservation-based MAC protocol is channel waste, i.e., the existence of idle slots. The proposed MAC, called cooperative relaying TDMA (CR-TDMA), allows a packet that is pre-assigned to a busy relay node to be reassigned to a neighbor node with an empty queue, i.e., a helper node. With the assistance of helper nodes, the relay node can process multiple packets during only one frame. Furthermore, CR-TDMA does not require additional overhead, such as mini-slots or additional packet header information for negotiations of cooperative communication. The numerical and simulation results show that CR-TDMA can significantly improve performance if helper nodes are available and can also guarantee equivalent or better performance than a conventional TDMA even in the worst cases.
Avoidance of collision in wireless ad-hoc network is important and CSMA/CA MAC protocol is playing a major role to avoid collision. CSMA/CA MAC protocol handle it using sort RTS/CTS control message. Though reliability of data transmission increase, it blocks some nodes unnecessarily due to RTS and CTS control message and also makes existence of hidden-terminal (HT) problem which reduces the network performance. In wireless Ad-hoc network the nodes are mobile, infrastructure less and communicate through their radio range. If we can reduce the number of blockage nodes the network performance will increase by utilizing those nodes as other communication's source, destination or relay. In this paper, we propose two methods by which wireless ad-hoc network performance will increase. First we propose a method for enhancement of CSMA/CA MAC protocol by unblocking the nodes using directional antenna and setting up NAV values of RTS/CTS control message considering directional antenna and modified the CSMA/CA timing diagram accordingly. The second method shows how the hidden-terminal problem is reduced by the proposed method. The analytical model explains the performance of CSMA/CA protocol with reduction of HT problem.