Data Communication Computer Network 简明教程
Wireless Transmission in Computer Network
无线传输是一种非导引介质形式。无线通信涉及两台或多台设备通过无线进行通信而无需建立物理链路。无线信号散布在空气中，并由合适的接收天线接收和解读。
Wireless transmission is a form of unguided media. Wireless communication involves no physical link established between two or more devices, communicating wirelessly. Wireless signals are spread over in the air and are received and interpreted by appropriate antennas.
当天线连接到计算机或无线设备的电气电路时，它会将数字数据转换成无线信号并在其频率范围内全部传播。另一端的接收器接收这些信号并将其转换回数字数据。
When an antenna is attached to electrical circuit of a computer or wireless device, it converts the digital data into wireless signals and spread all over within its frequency range. The receptor on the other end receives these signals and converts them back to digital data.
电磁频谱的一小部分可用于无线传输。
A little part of electromagnetic spectrum can be used for wireless transmission.
Radio Transmission
射频更容易产生，并且由于其波长较长，它可以穿透墙壁和类似的结构。电波的波长可以从 1 毫米到 100,000 公里，频率范围从 3 赫兹（极低频）到 300 吉赫兹（极高频）。射频分为六个频段。
Radio frequency is easier to generate and because of its large wavelength it can penetrate through walls and structures alike.Radio waves can have wavelength from 1 mm – 100,000 km and have frequency ranging from 3 Hz (Extremely Low Frequency) to 300 GHz (Extremely High Frequency). Radio frequencies are sub-divided into six bands.
较低频率的电波可以穿透墙壁，而较高的射频可以直线传播并反弹。低频波的功率在长距离传播时会急剧下降。高频电波的功率更大。
Radio waves at lower frequencies can travel through walls whereas higher RF can travel in straight line and bounce back.The power of low frequency waves decreases sharply as they cover long distance. High frequency radio waves have more power.
极低频、低频、中频等较低频率可以在地面上传播长达 1000 公里，跨越地球表面。
Lower frequencies such as VLF, LF, MF bands can travel on the ground up to 1000 kilometers, over the earth’s surface.
高频电波容易被雨水和其他障碍物吸收。它们利用地球大气层的电离层。高频电波如高频和甚高频波段向上传播。到达电离层时，它们会被折射回地球。
Radio waves of high frequencies are prone to be absorbed by rain and other obstacles. They use Ionosphere of earth atmosphere. High frequency radio waves such as HF and VHF bands are spread upwards. When they reach Ionosphere, they are refracted back to the earth.
Microwave Transmission
100 兆赫以上的电磁波倾向于直线传播，并且可以通过将这些波束射向一个特定的工作站来发送信号。由于微波以直线传播，发送器和接收器都必须严格地对准在视线范围内。
Electromagnetic waves above 100 MHz tend to travel in a straight line and signals over them can be sent by beaming those waves towards one particular station. Because Microwaves travels in straight lines, both sender and receiver must be aligned to be strictly in line-of-sight.
微波的波长范围为 1 毫米 – 1 米，频率范围为 300 MHz 到 300 GHz。
Microwaves can have wavelength ranging from 1 mm – 1 meter and frequency ranging from 300 MHz to 300 GHz.
微波天线通过将波聚焦汇聚成一个波束。如图所示，可以对多个天线进行对齐以实现更远的传输距离。微波具有较高的频率，并且不会像障碍物一样穿透墙壁。
Microwave antennas concentrate the waves making a beam of it. As shown in picture above, multiple antennas can be aligned to reach farther. Microwaves have higher frequencies and do not penetrate wall like obstacles.
微波传输很大程度上取决于天气条件及其使用的频率。
Microwave transmission depends highly upon the weather conditions and the frequency it is using.
Infrared Transmission
红外波位于可见光谱和微波之间。它的波长为 700-nm 至 1-mm，频率范围为 300-GHz 至 430-THz。
Infrared wave lies in between visible light spectrum and microwaves. It has wavelength of 700-nm to 1-mm and frequency ranges from 300-GHz to 430-THz.
红外波用于电视和遥控器等非常短程通信目的。红外波呈直线传播，因此本质上是定向的。由于频率范围高，红外波无法穿透墙等障碍物。
Infrared wave is used for very short range communication purposes such as television and it’s remote. Infrared travels in a straight line hence it is directional by nature. Because of high frequency range, Infrared cannot cross wall-like obstacles.
Light Transmission
用于数据传输的最高电磁频谱是光或光学信号。这是通过激光实现的。
Highest most electromagnetic spectrum which can be used for data transmission is light or optical signaling. This is achieved by means of LASER.
由于频率光线的使用，它倾向于严格地沿直线传播。因此，发送器和接收器必须处于视线范围内。由于激光传输是单向的，因此需要在通信的两端安装激光器和光电探测器。激光束通常宽 1 毫米，因此精确对齐两个遥远的接收器以指向激光源是一项精细工作。
Because of frequency light uses, it tends to travel strictly in straight line.Hence the sender and receiver must be in the line-of-sight. Because laser transmission is unidirectional, at both ends of communication the laser and the photo-detector needs to be installed. Laser beam is generally 1mm wide hence it is a work of precision to align two far receptors each pointing to lasers source.
激光器用作 Tx（发送器），光电探测器用作 Rx（接收器）。
Laser works as Tx (transmitter) and photo-detectors works as Rx (receiver).
激光不能穿透障碍物，如墙壁、雨和浓雾。此外，激光束会因风、大气温度或路径中的温度变化而失真。
Lasers cannot penetrate obstacles such as walls, rain, and thick fog. Additionally, laser beam is distorted by wind, atmosphere temperature, or variation in temperature in the path.
激光对于数据传输是安全的，因为在不中断通信信道的情况下窃听 1 毫米宽的激光非常困难。
Laser is safe for data transmission as it is very difficult to tap 1mm wide laser without interrupting the communication channel.