非导向传输媒体| 计算机网络

Transmission media can be categorized in two ways,

传输媒体可以通过两种方式进行分类：

Guided Transmission Media
引导传输媒体
Unguided Transmission Media
非引导传输媒体

1)非引导传输媒体 (1) Unguided Transmission Media)

It is also called wireless communication or unbounded transmission, they transmit electromagnetic waves without using a physical conductor. In this medium signals are radiated through the air (or, in a few cases, water) and therefore, are reaching to anyone with a device capable of accepting them.

也称为无线通信或无限制传输，它们无需使用物理导体即可传输电磁波。在这种介质中，信号是通过空气(或在少数情况下是水)辐射的，因此会通过能够接收信号的设备到达任何人。

We can categorize wireless transmission into the following groups,

我们可以将无线传输分为以下几类：

Radio waves
无线电波
Micro waves
微波
Infrared waves
红外波

a)无线电波 (a) Radio Waves)

Radio waves are electromagnetic waves and are omnidirectional. When an antenna transports radio waves they are propagated in all directions in free space which means the sending and receiving antennas do not have to be aligned that is any receiving antenna can receive that transmitted wave.

无线电波是电磁波，是全向的。当天线传输无线电波时，它们会在自由空间的所有方向上传播，这意味着发送天线和接收天线不必对齐，因为任何接收天线都可以接收该发射波。

The frequency of radio waves about 30 hertz (Hz) to 300 gigahertz (GHz) and like all other electromagnetic waves radio waves travel at the speed of light in vacuum.

无线电波的频率约为30赫兹(Hz)至300吉赫兹(GHz)，与所有其他电磁波一样，无线电波以真空中的光速传播。

Applications of Radio waves

无线电波的应用

These waves are omnidirectional so they are useful for multicasting in which one sender but many receivers.
这些波是全向的，因此它们对于一个发送者但很多接收者的多播很有用。
Examples of radio waves are television, AM and FM radio, cordless phones, and paging.
无线电波的示例是电视，AM和FM广播，无绳电话和寻呼机。

Advantages and disadvantages

的优点和缺点

Radio waves are easy to generate and penetrate buildings also can travel long distances.
无线电波很容易产生，并且穿透建筑物也可以传播很长的距离。
Radio waves cover a large area and can penetrate the buildings. By this, an AM radio can receive signals inside a building.
无线电波覆盖的面积很大，可以穿透建筑物。这样一来，AM收音机就可以接收建筑物内部的信号。
This can also be disadvantageous because we cannot isolate a communication just inside or outside a building. Cause of this, governments strictly legislate the use of radio transmitters.
这也可能是不利的，因为我们无法隔离建筑物内部或外部的通信。因此，政府严格立法使用无线电发射器。

b)微波 (b) Micro Waves)

Micro Waves includes a line of sight transmission that is the sending and receiving antennas that need to be properly aligned with each other. The distance is directly proportional to the height of the antenna which is covered by the signal. In mobile phone communication and television distribution, these are majorly used.

微波包括一条视线传输线，该传输线是需要彼此正确对准的发送和接收天线。该距离与信号覆盖的天线高度成正比。在移动电话通信和电视发行中，主要使用这些。

Applications of Micro Waves

微波的应用

Due to the unidirectional properties of Micro Waves, they are very useful when unicast (one-to-one) communication is needed between the sender and the receiver. Cellular phones, satellite networks, and wireless LANs are using Micro Waves.

由于微波的单向特性，当发送方和接收方之间需要单播(一对一)通信时，它们非常有用。蜂窝电话，卫星网络和无线局域网正在使用微波。

Microwave Transmission

微波传输

Two types of Microwave Transmission are as follows,

微波传输的两种类型如下：

Terrestrial Microwave
地面微波
Satellite Microwave
卫星微波

(i) Terrestrial Microwave

(i)地面微波

The frequency of Electromagnetic waves between 300 MHz and 300 GHz are called microwaves. These waves are unidirectional. Whenever through an antenna microwave are transmitting, they can be narrowly focused. That is the sending and receiving antennas need to be aligned.

300 MHz至300 GHz之间的电磁波频率称为微波。这些波是单向的。每当通过天线发射微波时，它们都可以狭窄地聚焦。也就是说，发送和接收天线需要对齐。

Characteristics of microwaves

微波炉的特性

It is inexpensive for short distance expensive as it requires a higher tower for a longer distance.
对于短距离的昂贵，它是廉价的，因为它需要更长的塔，距离更长。
Due to environmental conditions and antenna size attenuation (loss of signal) occurs.
由于环境条件和天线尺寸的衰减，会发生信号丢失。
There is a capacity in very high-frequency microwaves that they cannot penetrate walls. This characteristic can be a disadvantage of microwaves if the receiver is inside the buildings.
高频微波具有无法穿透墙壁的能力。如果接收器在建筑物内部，则此特性可能是微波的缺点。

Advantages

优点

Microwave transmission is cheaper than using cables.
微波传输比使用电缆便宜。
It does not require any land for the installation of cables that is free from land acquisition.
它不需要任何土地来安装免征土地的电缆。
Microwave transmission provides easy communication.
微波传输可轻松进行通讯。

Disadvantages

缺点

Bandwidth is limited in microwave transmission.
微波传输带宽有限。
A signal can be moved out of phase and any environmental change such as rain, wind can distort the signal so these signals are susceptible to weather conditions.
信号可能会异相，并且任何环境变化(例如雨，风)都会使信号失真，因此这些信号容易受到天气条件的影响。
Cause of eavesdropping insecure communication occurs in which any user can catch the signal in the air by using its antenna.
发生监听不安全通信的原因，其中任何用户都可以使用其天线捕获空中信号。

(ii) Satellite Microwave

(ii)卫星微波

A satellite is an entity that revolves around the earth at a certain height. Satellite communication offers more flexibility than fiber optic and cable systems. We can transmit signals from any point on the globe by using satellite transmission.

卫星是在一定高度绕地球旋转的实体。卫星通信比光纤和电缆系统具有更大的灵活性。我们可以使用卫星传输来传输来自全球任何一点的信号。

How does a Satellite work?

卫星如何工作？

The satellite receives the signal that is transmitted from the earth station, and it amplifies these signals. It is retransmitted the amplified signal to another earth station.

卫星接收从地球站发送的信号，并放大这些信号。它会将放大的信号重新传输到另一个地球站。

Satellite transmission is much like the line-of-sight transmission in which one of the stations is a satellite orbiting the earth. The principle is the same as the terrestrial microwave. Signals still travel in straight lines in satellite transmission.

卫星传输与视线传输非常相似，在视线传输中，一个站是环绕地球运行的卫星。其原理与地面微波相同。在卫星传输中，信号仍沿直线传播。

Features of Satellite Microwave

卫星微波的特点

It provides transmission capability to and from any location on earth.
它提供往返于地球上任何位置的传输能力。
Deployment of Satellite microwaves for orbiting satellites is difficult.
用于轨道卫星的卫星微波的部署是困难的。

Advantages of Satellite Microwave

卫星微波的优点

High-quality communication available to undeveloped parts of the world without requiring a huge investment in the ground-based infrastructure.
高质量的通信可用于世界上不发达的地区，而无需在地面基础设施上进行大量投资。
It is used in a variety of applications such as radio/TV signal broadcasting, weather forecasting, radio/TV signal broadcasting, mobile communication and mobile, and wireless communication applications.
它用于各种应用程序，例如无线电/电视信号广播，天气预报，无线电/电视信号广播，移动通信和移动以及无线通信应用。
The coverage area of a terrestrial microwave is less than the terrestrial microwave.
地面微波的覆盖面积小于地面微波。

Disadvantages of Satellite Microwave

卫星微波的缺点

The manufacturing cost is very high of satellite and very expensive to launch a satellite.
卫星的制造成本非常高，而发射卫星的成本非常高。
Transmission can go down in bad weather.
在恶劣的天气下，传输会中断。

c)红外波 (c) Infrared Waves)

The frequency of Infrared waves is about 300 GHz to 430 THz, which can be used for short-range communication. Infrared waves of high frequencies cannot penetrate walls. This characteristic of Infrared waves prevents interference between one system and another. This means a short-range communication system in a room cannot be affected by another system in the adjacent room.

红外线的频率约为300 GHz至430 THz，可用于短距离通信。高频红外线无法穿透墙壁。红外线的这一特性可防止一个系统与另一个系统之间的干扰。这意味着一个房间中的短距离通信系统不会受到相邻房间中另一个系统的影响。

If we are using the infrared remote control, we do not interfere with the use of the remote by our neighbors. However, by this characteristic, infrared signals become useless for long-range communication. Also, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with communication.

如果我们使用红外遥控器，则不会干扰我们的邻居使用该遥控器。但是，由于该特性，红外信号对于远程通信变得毫无用处。另外，我们不能在建筑物外使用红外线，因为太阳光线中的红外线会干扰通讯。

Characteristics of infrared waves

红外线的特征

This type of wide bandwidth can be used to transmit digital data with a very high data rate.
这种宽带宽可用于以很高的数据速率传输数字数据。
The Infrared Data Association (IrDA) has established standards for using these signals for communication between devices such as keyboards, mouse, PCs, and printers and it is also responsible for sponsoring the use of infrared waves.
红外数据协会(IrDA)已经建立了使用这些信号在键盘，鼠标，PC和打印机等设备之间进行通信的标准，并且还负责赞助使用红外波。
This type of communication provides better security with minimum interference.
这种类型的通信以最小的干扰提供了更好的安全性。

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