【摘要】：IEEE 802.11 DCF協(xié)議作為一種分布式的隨機(jī)接入?yún)f(xié)議,旨在為網(wǎng)絡(luò)中各鏈路提供均等的接入機(jī)會(huì)。盡管IEEE 802.11 DCF協(xié)議最初是為單跳網(wǎng)絡(luò)而設(shè)計(jì),隨著無線網(wǎng)絡(luò)技術(shù)的飛速發(fā)展,其基本接入思想現(xiàn)已被廣泛應(yīng)用于無線ad hoc網(wǎng)絡(luò)、無線Mesh網(wǎng)絡(luò)、無線傳感網(wǎng)等多跳環(huán)境中。然而,當(dāng)DCF協(xié)議應(yīng)用于多跳網(wǎng)絡(luò)時(shí)將出現(xiàn)嚴(yán)重的鏈路不公平性問題,即網(wǎng)絡(luò)中有些鏈路可以取得較高的吞吐量而有些鏈路吞吐量幾乎為零。自IEEE 802.11標(biāo)準(zhǔn)制定以來,對(duì)DCF協(xié)議的性能分析一直都是無線網(wǎng)絡(luò)領(lǐng)域的熱點(diǎn)問題。本文主要工作即圍繞多跳網(wǎng)絡(luò)中IEEE802.11 DCF協(xié)議的單鏈路吞吐量及鏈路公平性性能研究展開,主要?jiǎng)?chuàng)新性成果包括:(1)對(duì)多跳網(wǎng)絡(luò)中IEEE 802.11 DCF協(xié)議單鏈路吞吐量建模難點(diǎn)進(jìn)行分析,并在此基礎(chǔ)上提出了適用于多跳網(wǎng)絡(luò)中的定長時(shí)隙馬爾科夫鏈模型。該模型可分為退避過程、掛起過程、沖突過程以及成功發(fā)送過程。退避過程模擬了DCF協(xié)議的二進(jìn)制退避機(jī)制;在成功發(fā)送過程以及沖突過程中考慮了多跳網(wǎng)絡(luò)中兩種不同的沖突狀況;利用連續(xù)時(shí)間馬爾科夫鏈模型計(jì)算得出網(wǎng)絡(luò)中各條鏈路發(fā)送節(jié)點(diǎn)的掛起概率以及每次掛起的時(shí)間期望,將其嵌入離散時(shí)間四維馬爾科夫鏈中,進(jìn)而得出馬爾科夫鏈模型中各個(gè)穩(wěn)定狀態(tài)的概率。根據(jù)模型求解,即可得出多跳網(wǎng)絡(luò)中各鏈路的發(fā)送概率、沖突概率以及吞吐量性能。Qual Net網(wǎng)絡(luò)仿真環(huán)境下的仿真結(jié)果與模型推導(dǎo)所得的理論結(jié)果的對(duì)比驗(yàn)證了本文所提模型的有效性。(2)將多跳網(wǎng)絡(luò)中DCF協(xié)議的鏈路不公平性問題的原因概括為發(fā)送概率的不平等和沖突概率的不平等,并且結(jié)合本文所提模型以及仿真實(shí)驗(yàn),分析了競爭窗口的配置對(duì)鏈路的發(fā)送概率和沖突概率的影響。在多跳環(huán)境下,處于網(wǎng)絡(luò)中間的鏈路可能因?yàn)椴煌泥従痈蓴_而無法監(jiān)聽到空閑的信道,從而缺乏發(fā)送數(shù)據(jù)的機(jī)會(huì)。另外,有些鏈路即使有足夠的發(fā)送機(jī)會(huì),仍可能因?yàn)殡[藏終端的存在而使得發(fā)送的數(shù)據(jù)一直遭遇沖突,從而使其吞吐量幾乎為零。在模型推導(dǎo)的基礎(chǔ)上,本文以鏈路公平性為目標(biāo)計(jì)算得出各鏈路競爭窗口的最優(yōu)配置,增大了吞吐量較大鏈路的競爭窗口,使得吞吐量較小鏈路有更多的機(jī)會(huì)接入信道,并減小了沖突概率。Qual Net網(wǎng)絡(luò)仿真環(huán)境下的仿真結(jié)果表明,兩類鏈路不公平性問題均可通過對(duì)各鏈路競爭窗口的合理配置得到解決。
[Abstract]:As a distributed random access protocol, IEEE 802.11 DCF protocol aims to provide equal access opportunities for all links in the network. Although IEEE 802.11 DCF protocol was originally designed for single hop network, with the rapid development of wireless network technology, its basic access idea has been widely used in wireless ad hoc network, wireless Mesh network, wireless sensor network and other multi-hop environment. However, when DCF protocol is applied to multi-hop networks, there will be a serious link unfairness problem, that is, some links in the network can achieve higher throughput and some links have almost zero throughput. Since the establishment of IEEE 802.11 standard, the performance analysis of DCF protocol has been a hot issue in the field of wireless network. The main work of this paper is to study the single link throughput and link fairness performance of IEEE802.11 DCF protocol in multi-hop networks. The main innovative results are as follows: (1) the difficulties in modeling single link throughput of IEEE802.11 DCF protocol in multi-hop networks are analyzed, and a fixed length time slot Markov chain model suitable for multi-hop networks is proposed. The model can be divided into avoidance process, hanging process, conflict process and successful sending process. The avoidance process simulates the binary back-off mechanism of DCF protocol, and two different conflict situations in multi-hop networks are considered in the process of successful transmission and conflict. The hanging probability of each link sending node in the network and the time expectation of each suspension are calculated by using the continuous time Markov chain model, and it is embedded into the discrete time four-dimensional Markov chain, and then the probability of each stable state in the Markov chain model is obtained. According to the solution of the model, the transmission probability, conflict probability and throughput performance of each link in the multi-hop network can be obtained. The comparison between the simulation results in the Qual Net network simulation environment and the theoretical results derived from the model verifies the effectiveness of the model proposed in this paper. (2) the reasons for the link unfairness of the DCF protocol in the multi-hop network are summarized as the inequality of the transmission probability and the inequality of the conflict probability. Combined with the model and simulation experiments proposed in this paper, the influence of the configuration of competitive window on the transmission probability and conflict probability of the link is analyzed. In multi-hop environment, the link in the middle of the network may not be able to monitor the idle channel because of the interference of different neighbors, thus lacking the opportunity to send data. In addition, even if some links have enough transmission opportunities, the transmitted data may still encounter conflicts because of the existence of hidden terminals, so that their throughput is almost zero. Based on the derivation of the model, the optimal configuration of each link competition window is calculated with the goal of link fairness, which increases the competition window of the link with larger throughput, makes the link with smaller throughput have more opportunities to access the channel, and reduces the simulation results of the collision probability. Qual Net network simulation environment. The simulation results show that the two kinds of link unfairness problems can be solved by the reasonable configuration of each link competition window.
【學(xué)位授予單位】：南京航空航天大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2015
【分類號(hào)】：TN929.5

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