Research On Key Technologies Of Access Selection Of Wireless Local Area Network

Wireless Local Area Network(WLAN)has been widely used due to its advantages of low cost,fast speed and flexibility.To meet people's demand for wireless Internet access,a large number of WLAN Access Points(APs)are deployed in some public places.In the face of dense deployment of WLAN APs,how to select and access an optimal AP is an important issue affecting users' communication experience and rational utilization of network resources.Aiming at this problem,this thesis studies the key technologies of access selection in AP densely deployed WLAN environment,to ensure that Mobile Station(MS)is able to access AP quickly and smoothly,and to reduce the impact of latency,packet loss,and jitter that caused by the access process on network quality of service(QoS)and stability.Firstly,the basic architecture of WLAN is studied and analyzed.Aiming at the issues of random nodes distribution and random signal attenuation in WLAN,a stochastic geometric simulation model of WLAN is established on the basis of stochastic geometry theory.In this model,the WLAN nodes distribution and signal propagation process are analyzed respectively.Using this model,the impacts of AP transmit power,node distribution density and SINR threshold on WLAN's coverage probability and average throughput are then analyzed.Then,in view of the situation when users access WLAN for the first time,an initial access selection method is designed based on analyzing the requirements and preferences of different user business for network attributes.In this method,the users are firstly determined whether or not to access WLAN according to their moving speed in order to avoid unnecessary handoff.Using the Analytic Hierarchy Process(AHP),the weights of network attributes are then obtained based on the requirements and preferences of different user business.At last,a multi-attribute decision mechanism for optimal access selection is designed.According to the OPNET simulation experiment,this method is able to reduce unnecessary handoff,reduce the probability of handoff failure and improve the network performance.Finally,the handoff problem during communication is studied.Since the handover latency is mainly caused by 802.11's Full-scan method,a new scanning method based on the improved neighbor graph,named NOG-scanning,is proposed.The NOG-scanning is a method that uses the topological relationship between APs as a priori knowledge,and it is obtained by improving the structure of neighbor graph in the NG-scanning algorithm with overlapping theory.The performance of NOG-scanning,NG-scanning,and Full-scan are compared and analyzed by OPNET simulation experiments.The experimental results show that the NOG-scanning algorithm can minimize the number of probings that MaxChannelTime expires,optimize the search delay and reduce the handoff latency.