基于双忙音的无冲突多址接入协议

在Ad Hoc网络中,隐终端和暴露终端的存在严重影响了网络的通信能力.文中详细讨论了隐终端和暴露终端问题.在此基础上,介绍一种新的可完全避免分组冲突的MAC层协议--双忙音多址接入(DBTMA)协议.该协议采用RTS/CTS对话机制来预约信道,通过引入两个窄带带外忙音信号来避免分组间冲突,彻底解决了隐终端和暴露终端问题,提高了网络的吞吐量.     

双忙音多址接入-Ad Hoc网络中的多址接入控制机制的研究

在Ad Hoc网络中,隐藏终端和暴露终端的存在,严重影响了网络的通信能力。但是通常只采用RTS／CTS对话的MAC层机制不能完全解决隐藏终端和暴露终端的影响,正如单纯的分组侦听的MAC机制在全连通的网络中并不安全一样。为了解决这些问题,介绍一种新的MAC层协议——双忙音多址接入(DBTMA)机制。他的运行借助2个窄带带外忙音。通过使用RTS分组和接收端的接收忙音完全解决了隐藏终端的影响,接收端的CTS分组和接收忙音解决了暴露终端的影响。发送端建立的忙音保护了RTS分组,提高了接收RTS成功的概率,因此也就提高了吞吐量。     

基于多信道预约可冲突避免的多址接入协议

本文为多跳分布多无线网络提出了一套灵活而有效的自适应获取冲突避免（AACA）的多址接入协议。在该协议中，各节点竞争公共信道，利用RTS/CTS对话形式来预约各业务信道，成功预约后的分组传输不会受到其他节点的干扰。它使用任意确定数目信道，以异步方式工作，并且使得各节点利用半双工无线电台就可以灵活、简便地实现资源预约。分析和比较结果说明，所提出的多信道预约协议可以有效地解决隐藏终端、暴露终端以及侵入终端的问题。     

一种Ad Hoc网络隐藏终端问题的解决方案

介绍了存在于Ad Hoc网络中的隐藏和暴露终端问题,指出了解决问题的思路,并在双忙音的基础上提出了DBTMAC协议.仿真结果证明DBTMAC协议可以解决隐藏和暴露终端问题,提高网络的吞吐量,消除链路失效事件的发生.     

一种新的支持定向天线应用的忙音和功率控制多址接入协议

该文针对移动Ad Hoc网络(MANET),提出了支持定向天线应用的忙音和功率控制多址接入协议(BT-DMACP)。协议充分考虑到定向天线应用中存在的特殊隐藏终端问题和旁瓣干扰,利用RTS/CTS短分组对话机制、定向忙音信号和基于信噪比门限的功率控制策略,对数据分组的定向发送功率进行实时调整。同时为准确估计接收节点处的最大干扰功率,设计了自适应干扰功率估计算法。仿真结果证明,BT-DMACP协议能有效支持定向天线在MANET中的应用,在降低了系统功耗同时,具有很高的信道利用率。     

Ad Hoc网中的一种新型MAC协议

在移动Ad Hoc网络中,一个最基本的问题就是如何在避免隐藏终端和暴露终端问题的同时,增加信道利用率．．针对这个问题．在现有的DBTMA协议以及采用定向天线的一些协议的基础上．提出了一种把信道划分与定向天线相结合的新型MAC协议．DC／A协议（Dual Channel／Directional Antennas)一仿真结果表明这种新的MAC协议可以有效改善网络性能.     

多跳结构分组无线网络的性能分析

本文提出了一种应用IEEE 802.11 MAC中RTS/CTS协议的多跳分组无线网络,分析了它的吞吐量,并与时隙ALOHA、CSMA多址接入协议下的网络性能作了比较.最小ID号的分群算法可以通过较少的控制信息快速得到顽健的分群网络结构,选用合适的MAC协议能充分有效地利用多信道.不过,两跳的分群结构使得在群内使用CSMA仍无法避免隐藏终端问题,IEEE 802.11 MAC协议中的RTS/CTS接入方式大大减小了隐藏间的分组碰撞时间,从而提高了网络性能.     

一种并行传输的水声Ad hoc网络多址接入(PTMA)协议

由于存在"隐藏终端"和"暴露终端"的问题,无线Ad hoc网络的应用受到极大的限制.进一步,由于水声信道的特点,例如有用的频率带宽很窄,强时变、多途和长传播时延,使得设计水声Ad hoc网络具有自身特点和难度.本文中,我们为此提出了一种新的多址接入控制(MAC)协议,即并行传输多址接入(PTMA)协议.该协议基于如下思想:为消除冲突而进行多信道预约,这样能在多个信道上并行地传输数据;为缩短等待clear_-to_-send(CTS)的时间,当request_-to_-send(RTS)刚发送完就立即发送分组数据,而不必收到CTS后再发.通过使用OPNET Modeler/Radio仿真器得到的仿真结果显示:PTMA协议能取得较之于适合无线Ad hoc网络的多址接入协议更高的吞吐性能和较低的端到端传播时延.     

Ad-hoc网络中一种低控制开销的多信道MAC协议

针对Ad-hoc网络中多信道MAC机制引起的控制信道瓶颈问题和隐终端问题,该文提出一种低控制开销MAC协议(LCO-MAC)。与基于信道使用表一类的MAC机制不同,LCO-MAC参考Meshhadany提出的RTS/CTS(Request To Send/Clear To Send)信道分配机制,将数据信道映射为帧中时隙,但不同的是LCO-MAC不限制RTS的发送时间,且一旦申请信道后即可发送数据。仿真结果表明,LCO-MAC无需传输太多的控制信息用于预约信道,有效缓解了控制信道瓶颈问题和多信道隐终端问题,网络吞吐量也得到明显提升。     

基于OFDM系统的随机信道接入方案

双忙音多址信道接入(DBTMA)协议是一种信道利用率较高的无线局域网信道随机接入控制方案。但是, 该协议的硬件实现复杂度较高。该文在对DBTMA协议进行简单修改的基础上,结合节能意识多址接入(PAMAS) 协议提出了一种基于OFDM系统的DBTMA信道接入系统,并对此系统做了简要分析。分析结果显示,该文提出的随机信道接入系统的性能良好且实现方式简单。     

Dual busy tone multiple access (DBTMA)-a multiple access controlscheme for ad hoc networks

In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure "packet sensing" MAC schemes are not safe even in fully connected networks. We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20-40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA     

Collision reduction mechanism for masked node problem in ad hoc networks

IEEE 802.11 MAC uses RTS/CTS mechanism to avoid DATA packet collisions. RTS/CTS mechanism has been introduced to solve the problems of carrier sense multiple access (CSMA) in ad hoc networks such as hidden/exposed node problem. However, it creates a new problem called masked node problem. In this paper, a collision reduction mechanism named RTS/CTS/TTM with resume is introduced. This mechanism aims to minimize the probability of DATA packet collisions due to the masked nodes in an ad hoc network. We develop a new control packet called time-to-mask (TTM), which contains the time that the node will be masked. The proposed mechanism has been evaluated with a mathematical analysis and a simulation on a small IEEE 802.11 ad hoc network. The numerical results indicate that the RTS/CTS/TTM with resume reduces the probability of DATA packet collision.     

Effectiveness of RTS/CTS handshake in IEEE 802.11 based ad hoc networks

IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as “virtual carrier sensing”). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the “virtual carrier sensing” implemented by RTS/CTS handshake cannot prevent all interference as we expect in theory. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. In this paper, we investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Two independent solutions are proposed in this paper. One is a simple enhancement to the IEEE 802.11 MAC protocol. The other is to utilize directional antennas. Simulation results verify that the proposed schemes indeed can help IEEE 802.11 resolve most interference caused by large interference range.     

A combined approach for detecting hidden nodes in 802.11 wireless LANs

Hidden node is a fundamental problem that severely degrades the performance of wireless networks. The problem occurs when nodes that do not hear each other transmit at the same time, which leads to data packet collision. IEEE 802.11 Wireless Local Area Networks (WLANs) tries to solve this problem through the Request to Send/Clear to Send (RTS/CTS) mechanism. However, the mechanism is not wholly successful. The RTS/CTS idea is based on the assumption that all nodes in the vicinity of Access Points will hear CTS packets and consequently defer their transmissions. The shortcoming of RTS/CTS stems from the fact that such packets introduce high overhead if extensively used. In this article, we propose a hybrid approach for detecting hidden nodes in 802.11 WLANs. The approach is mainly based on adaptive learning about collisions in the network. We think that the approach will be useful for controlling the tuning of RTS/CTS threshold and therefore reduce the overhead those packets introduce. Detailed simulation experiments have shown the strength of the proposed approach compared with other approaches.     

Neighbor initiated approach for avoiding deaf and hidden node problems in directional MAC protocol for ad-hoc networks

In this paper we propose a MAC called “Neighbor Initiated Approach for avoiding Deaf and Hidden node problems in directional MAC protocol for Ad Hoc networks”, which takes advantage of the multi beam smart antennas. Through the antenna, a node can simultaneously transmit/receive a packet to/from all the directions around it. Thus the antenna switches itself in transmission and reception mode. In our scheme all transmission and reception will be directional. We discussed the hidden and deaf node problems with directional MAC and proposed the scheme to overcome those shortcomings. Our scheme has been inspired by the IEEE 802.11, which includes a new scheme to inform its neighbors who was deaf due to other communication. Moreover, the simultaneous transmission of the RTS/CTS through it’s all beams prevent the hidden node problem. In our scheme the idle nodes stay in reception mode for sensing the channel through its M non overlapping beams, as a substitute of omnidirectional antenna. It prevents the hidden node problem due to asymmetry in gain. We have simulated our scheme by OPNET 16.0, and compared our results with CDR MAC, DMAC and IEEE 802.11 protocols. Our results show that NIADH performs better than that of the existing protocols in majority of the scenarios.     

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.