1、Introduction to WirelessNetworking Concepts1- 无线网络概念简介Wireless Local-Area Networks(无线局域网 )Sir William Herschel (1738 to 1822) made a discovery that infrared light(红外线 ) existed The discovery of infrared light led the way to the electromagnetic wave（电磁波） theory electromagnetic waves travel at the spe
2、ed of light and that electricity can be carried on these waveshow they relate to wireless local-area networks (WLANs)？ In standard LANs, data is propagated over wires such as an Ethernet cable（以太网线路） , in the form of electrical signals（电信号） . The discovery opens the airways to transfer the same data
3、, as electrical signals, without wires. This is accomplished by way of Radio Frequencies (RF)（射频） . With RF, the goal is to send as much data as far as possible and as fast as possible.The problem is the numerous influences on radio frequencies that need to be either overcome or dealt with. One of t
4、hese problems is interference（干扰）How Bandwidth Is Achieved from RF SignalsTo achieve bandwidth（带宽） from RF signals, you need to send data as electrical signals using some type of emission method（发送方式） . One such emission method is known as Spread Spectrum（扩展频谱） .In 1986, the FCC agreed to allow the
5、use of spread spectrum in the commercial market using what is known as the industry, scientific, and medical (ISM) frequency bands. To place data on the RF signals, you use a modulation technique（调制技术） . Modulation is the addition of data to a carrier signal. You are probably familiar with this alre
6、ady. To send music, news, or speech over the airwaves, you use frequency modulation (FM)（调频） or amplitude modulation (AM)（调幅） .Bandwidth（带宽） In wireless networking, the word bandwidth can mean two different things. In one sense of the word, it can refer to data rates（数据率） . In another sense of the w
7、ord, it can refer to the width of an RF channel（ RF频道宽度） .This book uses the term bandwidth to refer to the width of the RF channel and not to data rates.the standard unit of measure is the Hertz (Hz)（赫兹）例如： FM radio is generally a higher quality, using about 175 kHz of bandwidth. Compare that to a
8、television signal, which sends both voice and video over the air. The TV signal you receive uses almost 4500 kHz of bandwidth.As Figure 1-1 illustrates, the electromagnetic spectrum spans from Extremely Low Frequency (ELF)（极低频） to Extremely High Frequency (EHF)（极高频） . In fact, the data you send using WLANs is either in the 900-MHz, 2.4-GHz, or 5-GHz frequency ranges.
