Data Communication Computer Network 简明教程

Transmission Media in Computer Network

传输媒体只不过是在计算机网络中进行通信的物理媒体。

The transmission media is nothing but the physical media over which communication takes place in computer networks.

Magnetic Media

在网络诞生之前，从一台计算机向另一台计算机传输数据的最方便的方法之一就是将其保存在某些存储媒体上，然后在不同站点之间进行物理传输。尽管在当今高速互联网时代，这似乎是一种老套的方法，但是当数据量巨大时，磁性媒体就会发挥作用。

One of the most convenient way to transfer data from one computer to another, even before the birth of networking, was to save it on some storage media and transfer physical from one station to another. Though it may seem old-fashion way in today’s world of high speed internet, but when the size of data is huge, the magnetic media comes into play.

例如，一家银行必须处理和传输其客户的巨量数据，出于安全原因和为了避免不确定的灾难，它在某些遥远的地方存储数据备份。如果银行需要存储其大量的备份数据，那么通过互联网传输是不可行的。广域网链路可能不支持如此高的速度。即使支持，成本也过高。

For example, a bank has to handle and transfer huge data of its customer, which stores a backup of it at some geographically far-away place for security reasons and to keep it from uncertain calamities. If the bank needs to store its huge backup data then its,transfer through internet is not feasible.The WAN links may not support such high speed.Even if they do; the cost too high to afford.

在这些情况下，数据备份存储在磁带上或磁碟上，然后在远程位置进行物理转移。

In these cases, data backup is stored onto magnetic tapes or magnetic discs, and then shifted physically at remote places.

Twisted Pair Cable

双绞线由两根绝缘的塑料铜线绞合在一起形成一个单一介质。在这两根线中，只有一根承载实际信号，另一根用于接地参考。线之间的扭曲有助于减少噪声（电磁干扰）和串扰。

A twisted pair cable is made of two plastic insulated copper wires twisted together to form a single media. Out of these two wires, only one carries actual signal and another is used for ground reference. The twists between wires are helpful in reducing noise (electro-magnetic interference) and crosstalk.

twisted pairs

双绞线电缆有两种类型：

There are two types of twisted pair cables:

  1. Shielded Twisted Pair (STP) Cable

  2. Unshielded Twisted Pair (UTP) Cable

STP 电缆带金属箔覆盖的绞合线对。这使得它对噪音和串扰更不敏感。

STP cables comes with twisted wire pair covered in metal foil. This makes it more indifferent to noise and crosstalk.

UTP 有七类，每类适用于特定的用途。在计算机网络中，最常使用 Cat-5、Cat-5e 和 Cat-6 电缆。UTP 电缆通过 RJ45 连接器连接。

UTP has seven categories, each suitable for specific use. In computer networks, Cat-5, Cat-5e, and Cat-6 cables are mostly used. UTP cables are connected by RJ45 connectors.

Coaxial Cable

同轴电缆有两根铜线。芯线位于中心，由实心导体制成。芯线被包在绝缘护套中。第二根线缠绕在护套上，反过来也被绝缘护套包覆。这一切都覆盖着塑料护套。

Coaxial cable has two wires of copper. The core wire lies in the center and it is made of solid conductor.The core is enclosed in an insulating sheath.The second wire is wrapped around over the sheath and that too in turn encased by insulator sheath.This all is covered by plastic cover.

coaxial cable

同轴电缆由于其结构，能够承载比双绞线电缆更高的频率信号。环绕结构为其提供了针对噪声和串扰的良好屏蔽。同轴电缆提供高达 450 mbps 的高带宽速率。

Because of its structure,the coax cable is capable of carrying high frequency signals than that of twisted pair cable.The wrapped structure provides it a good shield against noise and cross talk. Coaxial cables provide high bandwidth rates of up to 450 mbps.

同轴电缆有三个类别，即 RG-59（有线电视）、RG-58（细以太网）和 RG-11（粗以太网）。RG 代表无线电政府。

There are three categories of coax cables namely, RG-59 (Cable TV), RG-58 (Thin Ethernet), and RG-11 (Thick Ethernet). RG stands for Radio Government.

电缆使用 BNC 连接器和 BNC-T 连接。BNC 终端用于在远端端接电线。

Cables are connected using BNC connector and BNC-T. BNC terminator is used to terminate the wire at the far ends.

Power Lines

电力线通信（PLC）是一种使用电源线传输数据信号的第 1 层（物理层）技术。在 PLC 中，调制数据通过电缆发送。另一端的接收器解调并解释数据。

Power Line communication (PLC) is Layer-1 (Physical Layer) technology which uses power cables to transmit data signals.In PLC, modulated data is sent over the cables. The receiver on the other end de-modulates and interprets the data.

由于电力线的广泛部署，PLC 可以对所有带电设备进行控制和监控。PLC 采用半双工工作模式。

Because power lines are widely deployed, PLC can make all powered devices controlled and monitored. PLC works in half-duplex.

有两种类型的 PLC：

There are two types of PLC:

  1. Narrow band PLC

  2. Broad band PLC

窄带 PLC 提供较低的数据速率，最高可达数百 kbps，因为它们的工作频率较低（3-5000 kHz）。它们可以延伸数公里。

Narrow band PLC provides lower data rates up to 100s of kbps, as they work at lower frequencies (3-5000 kHz).They can be spread over several kilometers.

宽带 PLC 提供较高的数据速率，最高可达数百 Mbps，并以较高的频率工作（1.8 – 250 MHz）。它们的延伸距离不如窄带 PLC。

Broadband PLC provides higher data rates up to 100s of Mbps and works at higher frequencies (1.8 – 250 MHz).They cannot be as much extended as Narrowband PLC.

Fiber Optics

光纤利用光线的特性工作。当光线以临界角射入时，它会倾向于以 90 度折射。这种特性已应用于光纤中。光纤电缆的纤芯由高品质玻璃或塑料制成。从它的一端发射光，光穿过它并在另一端由光探测器探测光流并将其转换为电气数据。

Fiber Optic works on the properties of light. When light ray hits at critical angle it tends to refracts at 90 degree. This property has been used in fiber optic. The core of fiber optic cable is made of high quality glass or plastic. From one end of it light is emitted, it travels through it and at the other end light detector detects light stream and converts it to electric data.

光纤提供最高的速度模式。它有两种模式，一种是单模光纤，另一种是多模光纤。单模光纤可以传输单束光，而多模光纤可以传输多束光。

Fiber Optic provides the highest mode of speed. It comes in two modes, one is single mode fiber and second is multimode fiber. Single mode fiber can carry a single ray of light whereas multimode is capable of carrying multiple beams of light.

fiber optics

光纤还具有单向和双向传输功能。要连接和访问光纤，需要使用特殊类型的连接器。这些可以是用户信道 (SC)、直头 (ST) 或 MT-RJ。

Fiber Optic also comes in unidirectional and bidirectional capabilities. To connect and access fiber optic special type of connectors are used. These can be Subscriber Channel (SC), Straight Tip (ST), or MT-RJ.

Difference between Wired Media and Wireless Media

Wired Media

Wireless Media

The signal energy is contained and guided or propagated inside a solid medium.

The signal energy propagates in the structure of unguided electromagnetic waves in the air.

It can be used for point to point communication.

It can be used for radio advertising in all directions.

It makes discrete network topologies.

It makes continuous network topology.

This capacity can be further increased by inserting more cables for transmission.

In this capacity of transmission cannot be increased beyond the limit.

It is expensive, time-consuming and difficult to install.

Its installation is less time-consuming.

Examples are twisted pair, Co-axial cable, and Optical fiber.

An example is Radio & Infrared Waves.