| The Ad-hoc network is a kind of special wireless mobile communication network, the use of the TCP protocol is involved in the process of communication. The TCP protocol is originally developed for the wired network, it can not adapt to the Ad-hoc network environment, so that it is necessary to improve on the TCP protocol. The idiographic implementation versions of the TCP usually include the Tahoe, Reno and Vegas algorithms in which the whole efficiency of the Vegas algorithm is best, the major content of this paper is to modify the Vegas algorithm to make it to be adapted for the Ad-hoc network environment at the most of its best.To increase the work efficiency, the modified algorithms can be simulated by using NS2 software. NS2 (Network Simulator, version 2) is a kind of network simulation software, it can establish the wireless network environment expediently and use each of communication protocols, and the work status of the new protocol in the Ad-hoc network is analysed and checked up in this paper.In the first place, the congestion avoidance policy of the Vegas algorithm has been modified and the VegasE1 algorithm has been put forward. In the Ad-hoc network, the frequency motion of the mobile terminals cause the data packages to be often lost, this leads to the Vegas algorithm considering that the network is in the state of congestion and entering into the stage of congestion avoidance mistakenly, and this leads to the performance of the network reducing. In allusion to this characteristic of the Ad-hoc network, VegasE1 algorithm inceases the size of the congestion window when the run state of the network is all right; not to hold back the Vegas to enter into the state of congestion avoidance when the run state of the network is not all right, but to keep the size of the congestion window to be invariable and this can avoid the fall of the network throughput.In the next place, the slow start policy of the Vegas algorithm has been modified and the VegasE2 algorithm has been put forward. In the Ad-hoc network, the networktopology structure changes rapidly in the reason of the rapid motion of the mobile terminals, the TCP connection between two mobile terminals can be holded in the very small period of time and the data packages are caused to be lost subsequently because the TCP connection is interrupted and enter into the stage of slow start subsequently in this reason, but its start point is very low. If the slow start is executed when the start point is high, the network performance can be enhanced to a certain extent. On the base of this consideration, VegasE2 algorithm does not reduce the threshold of the congestion window in the process of slow start when the congestion occurs, so that the size of the congestion window will not increase from the minimal value, and the network performance will be increased as well as.In the third place, the timeout control policy of the Vegas algorithm has been modified and the VegasE3 algorithm has been put forward. In the Ad-hoc network, the switched data packages between two communication sides are often lost so that the send side often boubles the value of the timeout timer and modifies the congestion window, and this reults in the fall of network throughout. To adapt to the Ad-hoc wireless network environment, VegasE3 algorithm can freeze the timeout timer and congestion window, this can not prolong the setting of timeout timer in the next time, and it also can not affect the number of sent data package in the next time, so this method can enhance the communication performance of the network.In the last place, to check up the impact of TCP data package on the communication performance of the VegasE3. In the Ad-hoc network, the live time of the TCP between two communication sides is short, more data traffic can be transported in the same period of time by prolonging the size of data packages, this is very strongly practical in the Ad-hoc network and can enhance the network throughout to a great extent. |
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