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[问题]在铺设一个室内无线局域网的时候，我们发现有些地方在信号强度极好的情况下数据的吞吐量仍会下降。后来我们通过把频段从1改到6或者11来使用三个互不重迭的频段，我们的数据吞吐量变得稳定了。我们检查了可能出现的干扰源，但是，没有发现任何干扰源。我们正在使用的设备是采用802.11g协议的思科的Aironet 1200系列接入点。当信号强度在-60 dbm或者-55 dbm的时候，数据吞吐量从每秒钟54MB下降到了每秒钟5.5或者2MB。你能解释一下造成这个问题的原因吗?如何准确地找到这个原因?
[解答]首先检查干扰是正确的步骤。信号强度不是干扰的唯一指标，你要查看噪声(背景能量)和信噪比。你还需要检查过度的重传和冲突。你可以使用现场勘测工具（如艾尔麦AirMagnet Surveyor），这类工具能够发现同信道干扰和信道噪声。如果你通过了这个检查过程，验证了比较好的信噪比并且没有发现同信道干扰，就要考虑其它的可能性。
在安装一个802.11g无线局域网时，是可以通过改变频段克服这类问题的，我有个预感，你的问题可能是由于出现了802.11b站点而降低了802.11g的网络通信功能。802.11g使用一种保护机制以便在同一个频段的802.11b站点附近工作时减少冲突。如果没有启用这个保护功能，冲突就会造成数据吞吐量的显著下降。另一方面，当使用这种保护功能时，802.11b站点的存在只能增加无线局域网的开销，造成数据吞吐量的下降。
 Lisa Phifer, Core Competence Inc.
Lisa Phifer owns Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of networking, security, and management products for over 20 years. At Core Competence, she has advised companies large and small regarding security needs, product assessment, and use of emerging technologies and best practices. Before joining Core Competence, Phifer was a Member of Technical Staff at Bell Communications Research, where she won a president's award for her work on ATM Network Management. Phifer teaches about wireless LANs, mobile security, and VPNs at many industry conferences and webinars. She has written extensively about network infrastructure and security technologies for numerous publications, including Wi-Fi Planet, ISP-Planet, Business Communications Review, Information Security, and SearchSecurity. Phifer's monthly WLAN Advisor column is published by searchMobileComputing.
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