Research On Some Models And Key Schemes In Internet And Wireless Local Area Networks

This thesis studies on the models and schemes of some key issues in the communication between users and server via Wireless Local Area Network (WLAN) and Internet. Its contribution is as follows:1. The model of 802.11 MAC layer in finite load condition. When users access to Internet server via WLAN, the performance of WLAN directly influences the performance of users-server communication. The MAC layer protocol plays a crucial role in WLAN. We propose a novel model of 802.11 MAC in finite load condition. This model implements a 2-D Markov chain to analyze the backoff periods and waiting periods of nodes in 802.11 WLAN, and the stationary distribution of the Markov chain is calculated. By this model, not only the system throughput but also the access delay in MAC layer can be obtained. The instability problem of 802.11 WLAN in the saturated-unsaturated border field is firstly proposed based on this model, and this model is validated by simulations.2. The enhanced MAC protocol for multihop WLAN. There are some shortages in the single-hop WLAN currently used, such as limited coverage. IEEE 802.11s Task Group is developing a standard for multihop Mesh WLAN, so it is an advanced topic to study on the multihop 802.11 WLAN MAC. This thesis analyzes the asymmetry of transmission failures in different directions (which means in different directions a node may encountes different kinds of failures) and points out the reasons why current 802.11 MAC which uses symmetrical backoff mechanism operates inefficiently in the multihop WLAN. We propose an enhanced MAC protocol called DCF-CD to use asymmetrical backoff mechanisms in different directions. The basic idea of DCF-CD is:based on the estimation of transmission failure probability and slot busy probability in backoff period, when a node transmits unsuccessfully it increases its backoff period with a calculated probability. This protocol can improve the channel spatial reuse in multihop 802.11 WLAN and increase the total throughput. The proposed protocol does not require any additional overheads in physical layer, so it is easy to be implemented.3. Research on model and scheme of Active Queue Management (AQM) in Internet router. Congestion control is a key issue when users communicate with server via Internet. AQM is an important Internet congestion control mechanism. In this thesis, based on the fluid model we propose a sufficient condition for stability of TCP/AQM system. For some shortages in fluid model, we propose a novel model of TCP/AQM delayed closed-loop feedback system based on 2-D Markov Chain. This model can estimate the mean values of TCP congestion window, router queue length (queuing delay) and packets dropped probability. The distribution of some key parameters can also be obtained from the equilibrium distribution of the Markov chain. For TCP-UDP mixed flows, we study on the multi-queues mechanism, and extend the TCP/AQM Markov model to a TCP-UDP/AQM model to analyze the TCP/AQM system with UDP flows existing. For the UDP queue, we propose a differentiated services (DiffServ) AQM scheme for multi-priorities UDP flows, which can decrease the UDP packets dropped ratio by estimating the queue states.4. The user behavior and traffic model and a scheme to optimize server's parameters. In addition to the issues in WLAN and Internet, congestion control in server is another key issue affecting the performance of users-server communication. We propose a model analyzing the user behavior to estimate the messages traffic in server. Firstly, we analyze the user behavior in a day, and then propose a user behavior model, and based on the relationship between user behavior and system traffic and the relationship among messages, we propose a model of the messages traffic in server. Moreover, we analyze a special messages queue QCVBA (Queue with Controlled Vacation and Batched Arrival) in server, and based on analysis results we propose a scheme to optimize the server parameters. By the proposed scheme, when handling QCVBA queue, the server can obtain optimal vacation time with satifying the basic QoS requirements, to make server performance improved.