基于双忙音的多址接入协议的研究

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。双忙音多址接入（DBTMA）协议是采用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，解决了隐藏终端和暴露终端问题。本文在两个方面进一步对DBTMA进行改进：第一，使用一个控制分组RTS来预约信道，通过目的节点所发接收忙音来响应RTS分组；第二，采用时隙ALOHA协议对RTS分组进行传输，通过改进，提高了控制分组成功传输的概率，经仿真研究分析表明，提高了网络的吞吐量，增强了网络性能。     

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.     

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。     

解决隐终端和暴露终端问题的几种方法

介绍了自组网中存在的隐终端和暴露终端问题,并详细讨论了单信道RTS-CTS握手法、双/多信道RTS-CTS握手法以及忙音检测法,特别提出了利用相控阵天线的窄波束定向特性实现空分多址从而解决隐终端和暴露终端问题的方法,可供工程人员参考。     

基于载波监听的Ad Hoc网络双信道接入协议研究

双信道接入协议在解决隐终端和暴露终端问题上具有独特的优势.基于报文监听的Ad Hoc网络双信道接入协议无法消除数据报文的冲突.本文对基于载波监听的Ad Hoc网络双信道接入协议进行了研究,提出了DCMA载波监听信道接入协议.其中DCMA_CSBI通过载波监听和BI控制报文完全解决了隐终端和暴露终端问题.文章通过仿真对DCMA协议的性能进行了分析和比较.分析结果表明DCMA_CSBI是一种非常高效实用的Ad Hoc网络双信道接入协议,载波监听对Ad Hoc网络信道接入协议而言是至关重要的.     

Ad Hoc网络中MAC协议的研究与展望

MAC协议是无线AdHoc网络的一个重要研究领域，特别是暴露终端和隐终端问题的解决尤为重要。文中介绍了当前AdHoc网络中MAC协议研究的最新成果，并对这些协议进行了分析比较，最后给出了进一步的研究方向。     

Ad Hoc网络信道接入协议

文章首先介绍了AdHoc网络特有的隐终端和暴露终端问题 ,并对可能的解决方法进行了分析。在总结了前人工作的基础上 ,将AdHoc网络信道接入协议划分成基于单信道、双信道和多信道三类。文章还介绍了几种具有代表性的单信道接入协议 ,并给出了AdHoc网络信道接入协议的发展动向     

Ad hoc网络技术讲座第1讲 Ad hoc网络的体系结构和信道接入协议

本文首先介绍Adhoc网络的概貌,然后分析Adhoc网络的体系结构。在介绍了信道接入协议的地位和作用后,对Adhoc网络特有的信道共享方式、隐终端和暴露终端问题进行了介绍和分析。文章把Adhoc网络的信道接入协议划分成基于单信道、双信道和多信道三类,最后给出了Adhoc网络信道接入协议的发展动向。     

基于地理位置信息和功率控制的ad hoc网络并行MAC协议

对暴露终端并行传输产生的冲突干扰问题进行了研究,并在此基础上提出了一种基于地理位置信息和功率控制的ad hoc网络并行MAC协议.该协议利用地理位置信息判断节点的暴露终端身份,并在暴露终端发起的并行传输中引入了功率控制机制.在并行传输发送成功概率较低的情况下,该协议通过随机退避机制限制并行传输发起的次数,避免了失败的并行传输对其他接收节点产生干扰并导致节点产生不必要的能耗.仿真证明,该协议在显著提高ad hoc网络平均吞吐量的同时,还能有效节省节点的平均能耗,从而延长节点的工作时间和网络的生存时间.     

Ad hoc网络技术讲座第1讲Ad hoc网络的体系结构和信道接入协议

本文首先介绍Ad hoc网络的概貌，然后分析Ad hoc网络的体系结构。在介绍了信道接入协议的地位和作用后，对Ad hoc网络特有的信道共享方式。隐终端和暴露终端问题进行了介绍和分析。文章把Ad hoc网络的信道接入协议划分成基于单信道、双信道和多信道三类，最后给出了Ad hoc网络信道接入协议的发展动向。     

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     

Can multiple subchannels improve the delay performance of RTS/CTS-based MAC schemes?

We analyze the delay performance of RTS/CTS-based (Request-To-Send/Clear-To-Send) multi-channel MAC (Medium Access Control) schemes for wireless networks. These schemes usually employ multiple data subchannels for data transmission and one control subchannel to send the RTS/CTS dialogue for channel reservation. Through theoretical analysis and simulations, we show that, in fully-connected networks, such multi-channel MAC schemes suffer longer delays than the corresponding single channel MAC scheme, that puts the RTS/CTS dialogue on the same channel as data packet transmissions. This conclusion holds even when data packets have different priorities and higher priority traffic is sent ahead of lower priority traffic.     

Intelligent medium access for mobile ad hoc networks with busytones and power control

In mobile ad hoc networks (MANETs), one essential issue is how to increase channel utilization while avoiding the hidden-terminal and the exposed-terminal problems. Several MAC protocols, such as RTS/CTS-based and busy-tone-based schemes, have been proposed to alleviate these problems. In this paper, we explore the possibility of combining the concept of power control with the RTS/CTS-based and busy-tone-based protocols to further increase channel utilization. A sender will use an appropriate power level to transmit its packets so as to increase the possibility of channel reuse. The possibility of using discrete, instead of continuous, power levels is also discussed. Through analyses and simulations, we demonstrate the advantage of our new MAC protocol. This, together with the extra benefits such as saving battery energy and reducing cochannel interference, does show a promising direction to enhance the performance of MANETs     

Collision-aware design of rate adaptation for multi-rate 802.11 WLANs

One of the key challenges in designing a rate adaptation scheme for IEEE 802.11 wireless LANs (WLANs) is to differentiate bit errors from link-layer collisions. Many recent rate adaptation schemes adopt the RTS/CTS mechanism to prevent collision losses from triggering unnecessary rate decrease. However, the RTS/CTS handshake incurs significant overhead and is rarely activated in today's infrastructure WLANs. In this paper we propose a new rate adaptation scheme that mitigates the collision effect on the operation of rate adaptation. In contrast to previous approaches adopting fixed rate-increasing and decreasing thresholds, our scheme varies threshold values based on the measured network status. Using the "retry" information in 802.11 MAC headers as feedback, we enable the transmitter to gauge current network state. The proposed rate adaptation scheme does not require additional probing overhead incurred by RTS/CTS exchanges and can be easily deployed without changes in firmware. We demonstrate the effectiveness of our solution by comparing with existing approaches through extensive simulations.     

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.     

Power Control for Distributed MAC Protocols in Wireless Ad Hoc Networks

In centralized wireless networks, reducing the transmission power normally leads to higher network transport throughput. In this paper, we investigate power control in a different scenario, where the network adopts distributed MAC layer coordination mechanisms. We first consider widely adopted RTS/CTS based MAC protocols. We show that an optimal power control protocol should use higher transmission power than the "just enough" power in order to improve spatial utilization. The optimal protocol has a minimal transmission floor area of Theta(d_{ij}d_{max}), where d_{max} is the maximal transmission range and d_{ij} is the link length. This surprisingly implies that if a long link is broken into several short links, then the sum of the transmission floors reserved by the short links is still comparable to that reserved by the long link. Thus, using short links does not necessarily lead to higher throughput. Another consequence of this is that, with the optimal RTS/CTS based MAC, rate control can at best provide a factor of 2 improvement in transport throughput. We then extend our results to other distributed MAC protocols which uses physical carrier sensing or busy-tone as the control signal. Our simulation results show that the optimal power controlled scheme outperforms other popular MAC layer power control protocols.     

POWMAC: a single-channel power-control protocol for throughput enhancement in wireless ad hoc networks

Transmission power control (TPC) has great potential to increase the throughput of a mobile ad hoc network (MANET). Existing TPC schemes achieve this goal by using additional hardware (e.g., multiple transceivers), by compromising the collision avoidance property of the channel access scheme, by making impractical assumptions on the operation of the medium access control (MAC) protocol, or by overlooking the protection of link-layer acknowledgment packets. In this paper, we present a novel power controlled MAC protocol called POWMAC, which enjoys the same single-channel, single-transceiver design of the IEEE 802.11 ad hoc MAC protocol but which achieves a significant throughput improvement over the 802.11 protocol. Instead of alternating between the transmission of control (RTS/CTS) and data packets, as done in the 802.11 scheme, POWMAC uses an access window (AW) to allow for a series of request-to-send/clear-to-send (RTS/CTS) exchanges to take place before several concurrent data packet transmissions can commence. The length of the AW is dynamically adjusted based on localized information to allow for multiple interference-limited concurrent transmissions to take place in the same vicinity of a receiving terminal. Collision avoidance information is inserted into the CTS packet and is used to bound/ the transmission power of potentially interfering terminals in the vicinity of the receiver, rather than silencing such terminals. Simulation results are used to demonstrate the significant throughput and energy gains that can be obtained under the POWMAC protocol.