The Asymmetric PCF Problem Analysis Of Gated Service

With the development of network communication, users has higher requirement: arbitrary, ubiquitous, broadband and wireless. That makes the born of Bluetooth. WLAN, WSN, Ad hoc and other wireless network technologies. How to allocate and manage the network resource efficient; how to provide personalized and fair service based on the high Quality of Service, these are the urgent problems.Wireless Ad hoc network have the following features:without the need of any infrastructure support:self-group; temporary and move easily. Because of these features. Ad hoc keeps the important position in military and becomes more popular in the civilian fields. However, as a new network, it also has many problems.IEEE 802.11 standard defines two ways to access the channel:DCF and PCF. Through the traditional carrier sense multiple accesses and the improved collision carrier sense multiple access mechanism, DCF provides the access for asynchronous data based on competition. With the increase of business. these methods will not be able to provide QoS guarantee for the business which requires high quality of real-time. Although the PCF has the polling mechanism with the bandwidth reservation function which can solve the problem of DCF, the channel is low utilization. Therefore, combining the polling multiple access and random multiple access technology will be a good balance of the problem. In recent years, research on multiple access technology shows the features of combination of a variety of ways. The MAC protocol with multi-service, multi-server and multi-access technology integration has become a research hotspot. The Polling technology has become an important access mechanism used in the Ad Hoc network which is rapidly developed in our days.A model of PCF polling systems usually consists of one central server and N terminals, its processing structure consists of the arrival process of message in each terminal, switching process between the terminals and serving process of the message. Polling systems can be sorted into Limited,Gated (K= 1)and Exhaustive discipline systems according to the kinds of the service disciplines. There are three major performance measures used in analysis of polling systems. including mean queue length, mean cyclic time and mean delay of the message, etc. For a long time, a lot of researchers have struggled to improve the performance of the polling systems by the studying polling order, service disciplines and servicing order in the terminals. However, a large number of studies are based on PCF symmetric polling mechanism. Because of its various linkages between the terminal stations, its complexity of the system of random variables, the mechanism of asymmetric PCF polling analysis is very difficult. Currently, the study on asymmetric system does not get a complete mathematical analysis yet. the mainly effort is on getting the approximate optimum solution.My tutor Prof. Zhao Dongfeng has an in-depth study to the polling multiple access communication system. He has analyzed the polling system with exhaustive service, the polling system with limited service, the polling system with gated (K= 1) service and achieves remarkable results. provides a variety of improved model for the analysis of polling system. This paper was researched on the theoretical modeling based on asymmetric PCF gated service polling mechanism model which created by My tutor.In the asymmetric model, it allows to define the distribution of random variables with different parameters, reduces the information requirements of group identity, it is strong practical. With the embedded Markov chain and the probability generating function method, we analyzed the asymmetric gated service polling mechanism of PCF, and obtained two types of the model of 1-order characteristics (continuous time and discrete time-based), the average cycle period Exact expressions and 2-order characteristics, information packet average waiting time delay derived. Ad hoc wireless network and the existing MAC protocols were analyzed, and then we use the model described in this paper to improve the original MAC protocol.After that, the model described in this paper was simulated, and experimental results and theoretical results were compared, and came to the conclusion:for two types of system features,1-order and 2-order characteristics and the average cycle are the same. But because the start time of the information division to the terminal group is determined by the end time of entry into the time slot in the discrete-time systems, so there is a delay.