通过跟踪源代码，分析NS-2无线物理层的C++类，对NS-2分组收发仿真实现原理进行研究。针对NS-2仅实现理想信道而未考虑信道衰落效应的问题，给出扩展信道衰落功能的一般计算步骤，并以瑞利衰落为例在NS-2无线物理层中实现。UDP和TCP传输性能在引入瑞利衰落后有较大不同，体现了无线传输的不确定性。单链路的吞吐量随链路长度增加而下降；在无线局域网中，距离AP远的节点受衰落效应影响大，吞吐量下降；距离AP近的节点受衰落效应影响小，共享带宽占用了前者的剩余带宽，吞吐量反而上升。仿真结果显示，NS-2信道衰落功能扩展有助于准确验证无线网络协议性能。%This paper studies the implementation of packets transmission and reception of the NS-2 wireless physical layer by tracing its C++ classes, which offers a guideline to extend the NS-2 wireless functionality. As NS-2 only implements an ideal channel without considering channel fading, the paper gives the general computation method of fading in NS-2 and implements Rayleigh fading as an example. The performance of UDP and TCP flows under Rayleigh fading is different from those without fading, which simulates the uncertainty of wireless transmissions. The throughputs of a single link decrease as the link length increases. In Wireless Local Area Networks(WLAN), the throughputs of the nodes far from the AP are affected significantly by fading causing throughputs degradation, while the throughputs of the nodes nearby the AP are affected slightly by fading and its throughput increases due to bandwidth sharing. Simulation result shows that the fading extension in NS-2 makes wireless network protocols more accurately than before.
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