| Wireless local area network has the advantages of low cost,high throughput,mature industry and wide application.It has gradually become an important bearing technology to support the development of Internet and Internet of Things.However,the rapid development of Internet of Things applications,such as intelligent medical,smart grid and intelligent transportation,has also brought new technical challenges to WLAN,and the emergence of IEEE 802.11 ah technology.IEEE 802.11 ah is a wireless LAN technology that can support work in license-free frequency band below 1GHz.It provides coverage distance of up to 1 kilometer radius,and has stronger wall-penetrating capability.In addition,it has the characteristics of low power consumption and supporting thousands of terminals.In this context,this paper takes the channel access mechanism of IEEE 802.11 ah based on packet technology as the research object,analyses and evaluates the performance of IE802.11ah average packet technology and random backoff access channel mechanism,finds out the hidden nodes and competition conflicts existing in IE802.11ah,and then puts forward improvement schemes for packet technology and backoff technology based on channel access mechanism.The research contents include the following three parts1.The performance of channel access mechanism based on RAW technology in 802.11ah WLAN is studied.The competition-based channel access mechanism of IEEE 802.11 will cause high packet collision rate and serious hidden terminals in dense networks with large number of nodes.To solve these problems,the IEEE 802.11ah channel access mechanism introduces Traffic Indication Map(TIM)technology,Target Wake-up Time(TWT)mechanism and Restric access window(Restric).Ted Access Window(RAW)technology.Using OPNET simulation tools and simulation experiments,the effects of network data transmission rate,RAW duration and node packet size on the performance of the 802.11 ah channel access mechanism are studied.It is found that the average packet technology of the 802.11ah WLAN can not solve the hidden terminal problem,and its backoff mechanism is prone to cause the defects of collision,and the improvement scheme is proposed.2.Aiming at the hidden node problem in the 802.11ah wireless local area network,an HNCC(Hidden Node Culling Clustering)packet technology is proposed to alleviate the hidden terminal problem.HNCC grouping technology takes advantage of the limited communication distance of network nodes.Through repeated iterations,hidden nodes in the formed grouping are discharged.Hidden nodes refer to nodes whose distance between nodes is greater than the communication distance.Then hidden nodes are grouped again,so that there are no hidden nodes in the formed grouping.The simulation results show that the proposed packet technology has better performance than traditional packet technology in terms of collision caused by hidden nodes and channel access efficiency caused by network nodes competition.3.To solve the problem of collision caused by random backoff counters competing for access channels in the 802.11 ah WLAN,an adaptive backoff mechanism is proposed to alleviate the collision.In the energy-saving mode of IEEE 802.11,there are two modes of active and dormant nodes in the network.Considering the fact that there are more active nodes at the beginning of BI and fewer competing nodes at the later stage,and that the distributed coordination function of IEEE 802.11 randomly chooses the competitive channel with backoff counter value,this paper proposes an adaptive contention window backoff mechanism.The optimal competition window value is used to adjust the competition window value adaptively in the process of backoff.Based on the simulation results of network throughput,channel access delay and average number of retransmissions,this paper compares the proposed scheme with traditional backoff technology,and analyses that the proposed scheme can effectively alleviate node competition conflicts and improve network performance. |
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