| Airborne Warning And Control System (AWACS,for short) was an electronic information system which has the ability of target detection,command and control. From it's work characters, the AWACS could rely on wireless network only when it communicated with outside. The communication system of AWACS includes a satellite communications system and a communications suite that incorporates High/Very/Ultra High Frequency transceivers. Because the Lines of Communication (LOC) via HF/VHF/UHF spans unkown space and environment, and more, the enemy would cause interference, the communication's condition of a battlefield would be variable and the Bit Error Rate(BER) would be very high, these may cause the loss of message, and increase the degree of fallibility. Along with the development of science and technology and the requirement of global stratagem, the terminal of satellite communications was used widely in AWACS to realize the communication in global range, but traditional TCP will meet some question because of it's data format and congestion mechanism, this will reduce it's performance. The communication terminal of AWACS should join an Ad hoc network to realize some specifically martial purpose, but an Ad hoc network has some remarkable differences which is not achieved in common network, such as system structure, network organization, protocol design, and so on. When an AWACS used TCP to transmit data, the TCP's performance was studied and analyzed in this paper.Three different versions of TCP had be simulationed in this paper. The analyse about the cwnd, RTT, throughput of this simulation indicated: when the segments of data were lost, TCP always perform the step of congestion control, because of this reason, the throughput of TCP was reduced unnecessarily, so the performance of every TCP versions were very bad in wireless network with high bit error rate. According to the Probability theory, the less size of the segment for TCP is, the smaller likelihood for the loss of segments will be. A method to improve the performance in the LOC via HF/VHF/UHF was proposed based on this viewpoint. Used the famous exponential smoothing, the BER of next time was estimated, then the segment size of TCP was changed adaptively based on this estimation. Later, a simulation was completed about this mechanism, and the analysis of the simulation's result proved that the method was effective.A satellite communications link with zero BER and variable BER were simulated separately, the analyse of the simulation's result indicated that TCP could not differentiate the reason of package loss, it always consider that congestion was occurred, and it continually start congestion control, in addition to this, the TCP was not fit for the satellite communications link with a high bandwidth*delay product even if the BER was zero. TCP need a more large window and a better Algorithm to improve it's throughput in a network with a high bandwidth*delay product.To improve the performance of TCP in Ad hoc network, a new Algorithm was proposed in order to estimate the BER of the next time, so TCP can change its segment size adaptively. Last, a simulation, about this optimized mechanism, was completed. The analysis of the simulation's result proved that the optimized mechanism could forecast the variational direction of the network quality in a certain extent, and the optimized mechanism can improve the performance of TCP in Ad hoc network. |
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