Analyzing And Improving The Performance Of TCP Over Wireless Ad Hoc Networks Based On Network Calculus

Wireless ad hoc networks is composed of mobile nodes with route function, and characterized by no fixed communications infrastructure, all nodes in the network are able to move, and each node is both a terminal and router. Wireless ad hoc networks has very flexible network topology, which represents the future development direction of the wireless communications, and has broad application prospects.As the reliability of data transmission, TCP has now become one of the most widely deployed protocol on the Internet, with the developments of wireless communication technology, using TCP in wireless networks will become a natural choice for people. But the TCP performance in wireless ad hoc networks is very poor, such as low throughput, poor stability and fairness, frequent spurious retransmission and so on. Therefore, analyzing and improving the TCP performance in wireless ad hoc networks environment become very urgent. Network calculus as the emerging QoS theory has got rapid development in recent years, its has show out of powerful functions on analyzing and calculating data flow delay, delay jitter, packet loss rate and the queue buffer size for packet-switched network. In this paper, we use network calculus to analyze and improve the TCP performance for wireless ad hoc networks environment, the paper's main results of the work and innovation are as follows:(1) The data flow delay,delay jitter and throughput upper bounds based on network calculus for TCP in wireless ad hoc networks are proposed and then simulated.Accurate TCP performance upper bounds model over wireless environment can instruct TCP performance improvement. The data flow entering the node was regulated by Leaky Bucket, and the node provided Rate-Latency service for the flow. We had derived the TCP data flow upper delay bound,the upper delay jitter bound and the upper throughput bound for wireless ad hoc networks based on the existing link throughput model using network calculus theroy. Simulation experiments show the TCP performance upper bounds model perform well and also show that the wireless TCP need more improvments.(2) The TCP timer performance in wireless environment is analyzed and the improvement method for TCP timer is proposed.The relationship between Retransmission Time-Out(RTO) and Round-Trip Time(RTT) of TCP data flow in wireless ad hoc networks is simulated and analyzed,then we pointed out that the RTO estimated value is not accurate as the the change of RTO value was late to catch up the change of RTT value over wireless Ad Hoc networks. Using Linear Mean Square theory we had estimated the RTT,and then derived the TCP transmission Minimum Delay Up Bound(MDUB) using network calculus theory and then improved the RTO estimated algorithm based on MDUB.Simulation results show that the MDUB RTO algorithm can estimate TCP minimum transmission round-trip time up bound accurately, so the TCP data flow based on MDUB timer can reduce the TCP spurious retransmission and then promote the TCP performance in wireless ad hoc networks.(3) TCP packet loss rate caused by network congestion is researched and TCP sending windows upper bound that can guarantee the data flow congestion packet loss rate to zero is proposed.The relationship between TCP data flow packet losses caused by network congestion and queue length,TCP sending windows over wireless ad hoc networks is simulated and analyzed,and then pointed out that we can guarantee the network data flow congestion packet loss rate to zero in certain conditions.We had derived the TCP sending windows upper bound using network calculus theory and the simulation results validated the network data flow congestion packet loss rate is zero when TCP sending windows is limited by theoretical WThresh. When the value of sending windows is less than WThresh, TCP-WThresh accurately judges packet losses as non-congestion losses and avoids mal-operation of congestion control. Thus, the improved TCP based on WThresh performs well over special wireless ad hoc networks scenarios.(4)The un-fairness of TCP data flow is simulated and analyzed and the TCP fairness joint improving algorithm is proposed.The un-fairness issue is a widely exist problem in wireless ad hoc networks; We first simulated the TCP data flow over cross topology wireless ad hoc networks and then analyzed the TCP fairness issue;A TCP data flow scheduling algorithm that can guarantee node's resource be distributed fairly for wireless ad hoc networks is provided,which based on Service Curve Earliest Deadline(SCED) first scheduling model, incorporate with the already exist link distributed algorithm between wireless nodes then the TCP flow fairness improving scheme is proposed;Simulation results show that the TCP joint improving scheme based on service curve improved the TCP fairness without significantly impairing TCP throughput in wireless ad hoc networks.