引导性传输介质

有界或引导传输媒体 (Bounded or Guided Transmission Media)

Guided media, which are those that provide a conduit from one device to another, include Twisted-Pair Cable, Coaxial Cable, and Fibre-Optic Cable.

提供从一个设备到另一设备的导管的引导介质包括双绞线电缆 ， 同轴电缆和光纤电缆 。

A signal travelling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fibre is a cable that accepts and transports signals in the form of light.

沿着这些介质中的任何一种传播的信号都受到介质的物理限制的控制。 双绞线和同轴电缆使用金属（铜）导体，以电流形式接收和传输信号。 光纤是一种以光的形式接收和传输信号的电缆。

双绞线 (Twisted Pair Cable)

This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed easily, and they support many different types of network. Some important points :

这是最常用的电缆，比其他电缆便宜。 它重量轻，价格便宜，易于安装，并且支持许多不同类型的网络。 一些要点：

• Its frequency range is 0 to 3.5 kHz.
  它的频率范围是0到3.5 kHz。
• Typical attenuation is 0.2 dB/Km @ 1kHz.
  典型衰减为0.2 dB / Km @ 1kHz。
• Typical delay is 50 µs/km.
  典型延迟为50 µs / km。
• Repeater spacing is 2km.
  中继器间距为2km。

A twisted pair consists of two conductors(normally copper), each with its own plastic insulation, twisted together. One of these wires is used to carry signals to the receiver, and the other is used only as ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference(noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources. This results in a difference at the receiver.

双绞线由两根导体（通常为铜）组成，每根导体都有自己的塑料绝缘层，并绞在一起。 这些导线中的一根用于将信号传输到接收器，另一根仅用作接地参考。 接收器使用两者之间的差异。 除了发送者在一根导线上发送的信号之外，干扰（噪声）和串扰还可能影响两条导线并产生有害信号。 如果两条线是平行的，则这些有害信号的影响在两条线上都不相同，因为它们相对于噪声或串扰源位于不同的位置。 这导致接收器的差异。

Twisted Pair is of two types:

双绞线有两种类型：

• Unshielded Twisted Pair (UTP)非屏蔽双绞线（UTP）
• Shielded Twisted Pair (STP)屏蔽双绞线（STP）

非屏蔽双绞线电缆 (Unshielded Twisted Pair Cable)

It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which consists of two conductors usually copper, each with its own colour plastic insulator. Identification is the reason behind coloured plastic insulation.

与屏蔽双绞线电缆相比，它是最常见的电信类型，屏蔽双绞线电缆由两个通常是铜的导体组成，每个导体都有自己的彩色塑料绝缘子。 识别是彩色塑料绝缘背后的原因。

UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable use RJ-45 connector.

UTP电缆由2或4对双绞线组成。 2对电缆使用RJ-11连接器，4对电缆使用RJ-45连接器。

非屏蔽双绞线电缆的优点 (Advantages of Unshielded Twisted Pair Cable)

• Installation is easy
  安装容易
• Flexible
  灵活
• Cheap
  贱
• It has high speed capacity,
  它具有高速能力，
• 100 meter limit
  限制100米
• Higher grades of UTP are used in LAN technologies like Ethernet.
  更高等级的UTP用于诸如以太网之类的LAN技术中。

It consists of two insulating copper wires (1mm thick). The wires are twisted together in a helical form to reduce electrical interference from similar pair.

它由两根绝缘铜线（1毫米粗）组成。 电线以螺旋形式扭曲在一起，以减少来自类似电线对的电干扰。

非屏蔽双绞线电缆的缺点 (Disadvantages of Unshielded Twisted Pair Cable)

• Bandwidth is low when compared with Coaxial Cable
  与同轴电缆相比带宽低
• Provides less protection from interference.
  提供较少的干扰防护。

屏蔽双绞线电缆 (Shielded Twisted Pair Cable)

This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk (explained in KEY TERMS Chapter).

该电缆具有金属箔或编织网覆盖层，该覆盖层包裹了每对绝缘导体。 金属外壳可防止电磁噪声穿透。 屏蔽还可以消除串扰（在“关键术语”章节中有解释）。

It has same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is more expensive than coaxial and unshielded twisted pair.

它具有与非屏蔽双绞线相同的衰减。 非屏蔽同轴电缆速度更快。 它比同轴和非屏蔽双绞线贵。

屏蔽双绞线电缆的优点 (Advantages of Shielded Twisted Pair Cable)

• Easy to install
  易于安装
• Performance is adequate
  性能足够
• Can be used for Analog or Digital transmission
  可用于模拟或数字传输
• Increases the signalling rate
  增加信令速率
• Higher capacity than unshielded twisted pair
  容量比非屏蔽双绞线高
• Eliminates crosstalk
  消除串扰

屏蔽双绞线的缺点 (Disadvantages of Shielded Twisted Pair Cable)

• Difficult to manufacture
  难以制造
• Heavy
  重

屏蔽双绞线电缆的性能 (Performance of Shielded Twisted Pair Cable)

One way to measure the performance of twisted-pair cable is to compare attenuation versus frequency and distance. As shown in the below figure, a twisted-pair cable can pass a wide range of frequencies. However, with increasing frequency, the attenuation, measured in decibels per kilometre (dB/km), sharply increases with frequencies above 100kHz. Note that gauge is a measure of the thickness of the wire.

测量双绞线电缆性能的一种方法是比较衰减与频率和距离的关系。 如下图所示，双绞线电缆可以通过多种频率。 但是，随着频率的增加，以100公里（dB / km）为分贝的衰减随频率的增加而急剧增加。 注意，量规是电线厚度的度量。

屏蔽双绞线电缆的应用 (Applications of Shielded Twisted Pair Cable)

• In telephone lines to provide voice and data channels. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables.
  在电话线上提供语音和数据通道。 电话公司用于提供高数据速率连接的DSL线路也使用非屏蔽双绞线电缆的高带宽功能。
• Local Area Network, such as 10Base-T and 100Base-T, also use twisted-pair cables.
  诸如10Base-T和100Base-T之类的局域网也使用双绞线电缆。

同轴电缆 (Coaxial Cable)

Coaxial is called by this name because it contains two conductors that are parallel to each other. Copper is used in this as centre conductor which can be a solid wire or a standard one. It is surrounded by PVC installation, a sheath which is encased in an outer conductor of metal foil, barid or both.

之所以用同轴电缆来命名，是因为同轴电缆包含两个相互平行的导体。 铜用作中心导体，它可以是实心线或标准线。 它被PVC装置包围，护套被包裹在金属箔，金属丝或两者的外部导体中。

Outer metallic wrapping is used as a shield against noise and as the second conductor which completes the circuit. The outer conductor is also encased in an insulating sheath. The outermost part is the plastic cover which protects the whole cable.

外层金属包裹层被用作抗噪声的屏蔽层，并被用作完成电路的第二导体。 外部导体也被包裹在绝缘护套中。 最外面的部分是保护整个电缆的塑料盖。

Here the most common coaxial standards.

这里是最常见的同轴标准。

• 50-Ohm RG-7 or RG-11 : used with thick Ethernet.
  50欧姆RG-7或RG-11：与厚以太网一起使用。
• 50-Ohm RG-58 : used with thin Ethernet
  50欧姆RG-58：与瘦以太网一起使用
• 75-Ohm RG-59 : used with cable television
  75欧姆RG-59：与有线电视配合使用
• 93-Ohm RG-62 : used with ARCNET.
  93-Ohm RG-62：与ARCNET一起使用。

同轴电缆标准 (Coaxial Cable Standards)

Coaxial cables are categorized by their Radio Government(RG) ratings. Each RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and the type of the inner insulator, the construction of the shield, and the size and type of the outer casing. Each cable defined by an RG rating is adapted for a specialized function, as shown in the table below:

同轴电缆按其无线电政府（RG）等级分类。 每个RG号代表一组唯一的物理规格，包括内部导体的线规，内部绝缘体的厚度和类型，屏蔽的结构以及外壳的尺寸和类型。 RG额定值定义的每条电缆均适用于特殊功能，如下表所示：

同轴电缆连接器 (Coaxial Cable Connectors)

To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector used today is the Bayonet Neill-Concelman (BNC) connector. The below figure shows 3 popular types of these connectors: the BNC Connector, the BNC T connector and the BNC terminator.

要将同轴电缆连接到设备，我们需要同轴连接器。 今天使用的最常见的连接器类型是刺刀Neill-Concelman（BNC）连接器。 下图显示了这些连接器的3种常见类型：BNC连接器，BNC T连接器和BNC终结器。

The BNC connector is used to connect the end of the cable to the device, such as a TV set. The BNC T connector is used in Ethernet networks to branch out to a connection to a computer or other device. The BNC terminator is used at the end of the cable to prevent the reflection of the signal.

BNC连接器用于将电缆的一端连接到设备，例如电视机。 BNC T连接器在以太网中用于分支到计算机或其他设备的连接。 BNC端接器用于电缆的末端，以防止信号反射。

There are two types of Coaxial cables:

同轴电缆有两种类型：

1.基带 (1. BaseBand)

This is a 50 ohm (Ω) coaxial cable which is used for digital transmission. It is mostly used for LAN's. Baseband transmits a single signal at a time with very high speed. The major drawback is that it needs amplification after every 1000 feet.

这是50欧姆（Ω）同轴电缆，用于数字传输。 它主要用于局域网。 基带以很高的速度一次发送单个信号。 主要缺点是每1000英尺后需要放大一次。

2.宽带 (2. BroadBand)

This uses analog transmission on standard cable television cabling. It transmits several simultaneous signal using different frequencies. It covers large area when compared with Baseband Coaxial Cable.

这在标准有线电视电缆上使用模拟传输。 它使用不同的频率传输多个同时信号。 与基带同轴电缆相比，它覆盖的面积更大。

同轴电缆的优点 (Advantages of Coaxial Cable)

• Bandwidth is high
  带宽高
• Used in long distance telephone lines.
  用于长途电话线。
• Transmits digital signals at a very high rate of 10Mbps.
  以10Mbps的高速率传输数字信号。
• Much higher noise immunity
  更高的抗噪能力
• Data transmission without distortion.
  数据传输无失真。
• The can span to longer distance at higher speeds as they have better shielding when compared to twisted pair cable
  与双绞线电缆相比，它们具有更好的屏蔽性能，因此可以以更高的速度跨度更长的距离

同轴电缆的缺点 (Disadvantages of Coaxial Cable)

• Single cable failure can fail the entire network.
  单根电缆故障可能会使整个网络瘫痪。
• Difficult to install and expensive when compared with twisted pair.
  与双绞线相比，安装困难且价格昂贵。
• If the shield is imperfect, it can lead to grounded loop.
  如果屏蔽不完善，则可能导致接地环路。

同轴电缆的性能 (Performance of Coaxial Cable)

We can measure the performance of a coaxial cable in same way as that of Twisted Pair Cables. From the below figure, it can be seen that the attenuation is much higher in coaxial cable than in twisted-pair cable. In other words, although coaxial cable has a much higher bandwidth, the signal weakens rapidly and requires the frequent use of repeaters.

我们可以用与双绞线相同的方法来测量同轴电缆的性能。 从下图可以看出，同轴电缆的衰减比双绞线电缆的衰减高得多。 换句话说，尽管同轴电缆具有更高的带宽，但是信号会Swift减弱并且需要频繁使用中继器。

同轴电缆的应用 (Applications of Coaxial Cable)

• Coaxial cable was widely used in analog telephone networks, where a single coaxial network could carry 10,000 voice signals.
  同轴电缆已广泛用于模拟电话网络，其中一个同轴网络可以承载10,000个语音信号。
• Cable TV networks also use coaxial cables. In the traditional cable TV network, the entire network used coaxial cable. Cable TV uses RG-59 coaxial cable.
  有线电视网络也使用同轴电缆。 在传统的有线电视网络中，整个网络都使用同轴电缆。 有线电视使用RG-59同轴电缆。
• In traditional Ethernet LANs. Because of it high bandwidth, and consequence high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs. The 10Base-2, or Thin Ethernet, uses RG-58 coaxial cable with BNC connectors to transmit data at 10Mbps with a range of 185 m.
  在传统的以太网LAN中。 由于它的高带宽以及由此带来的高数据速率，因此在早期的以太网LAN中选择同轴电缆进行数字传输。 10Base-2（即瘦以太网）使用带有BNC连接器的RG-58同轴电缆，以10Mbps的速度传输185 m的数据。

光纤电缆 (Fiber Optic Cable)

A fibre-optic cable is made of glass or plastic and transmits signals in the form of light.

光纤电缆由玻璃或塑料制成，并以光的形式传输信号。

For better understanding we first need to explore several aspects of the nature of light.

为了更好地理解，我们首先需要探索光的本质的几个方面。

Light travels in a straight line as long as it is mobbing through a single uniform substance. If ray of light travelling through one substance suddenly enters another substance (of a different density), the ray changes direction.

只要光线穿过单一的均匀物质，光线就会沿直线传播。 如果穿过一种物质的光线突然进入另一种物质（密度不同），则该光线会改变方向。

The below figure shows how a ray of light changes direction when going from a more dense to a less dense substance.

下图显示了当光线从较稠密的物质变为稀疏的物质时光线如何改变方向。

Bending of a light ray

光线弯曲

As the figure shows:

如图所示：

• If the angle of incidence I(the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface.
• 如果入射角I （射线与垂直于两种物质之间的界面的直线所成的角度 ） 小于 临界角 ，则射线会折射并更靠近表面。
• greater than the critical angle, the ray 大于临界角，则射线会reflects(makes a turn) and travels again in the denser substance.反射 （转弯）并在较密的物质中再次传播。
• If the angle of incidence is equal to the critical angle, the ray refracts and moves parallel to the surface as shown.
• 如果入射角等于临界角，则光线会折射并平行于曲面移动 ，如图所示。

Note: 注意： The critical angle is a property of the substance, and its value differs from one substance to another. 临界角是物质的属性，其值因一种物质而异。

Optical fibres use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

光纤使用反射来引导光通过通道。 玻璃或塑料芯被密度较小的玻璃或塑料包层包围。 两种材料在密度上的差异必须使得穿过纤芯的光束从包层反射而不是被包层折射。

Internal view of an Optical fibre

光纤的内部视图

光纤的传输方式 (Propagation Modes of Fiber Optic Cable)

Current technology supports two modes(Multimode and Single mode) for propagating light along optical channels, each requiring fibre with different physical characteristics. Multimode can be implemented in two forms: Step-index and Graded-index.

当前技术支持两种模式（ 多模和单模 ），用于沿光通道传播光，每种模式都需要具有不同物理特性的光纤。 多模式可以两种形式实现： Step-index和Graded-index 。

多模式传播模式 (Multimode Propagation Mode)

Multimode is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the structure of the core as shown in the below figure.

之所以称为多模，是因为来自光源的多束光束以不同的路径穿过纤芯。 这些光束在电缆中的移动方式取决于芯线的结构，如下图所示。

• multimode step-index fibre, the density of the core remains constant from the centre to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding.多模阶跃折射率光纤中 ，纤芯的密度从中心到边缘保持恒定。 光束以恒定的密度沿直线移动，直到到达纤芯和包层的界面。
  The term step-index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fibre.
  术语阶跃折射率是指这种变化的突然性，当信号通过光纤时会导致信号失真。
• multimode graded-index fibre, this distortion gets decreases through the cable. The word index here refers to the index of refraction. This index of refraction is related to the density. A graded-index fibre, therefore, is one with varying densities. Density is highest at the centre of the core and decreases gradually to its lowest at the edge. 多模渐变折射率光纤中 ，这种畸变通过电缆得到减小。 这里的索引一词是指折射率。 该折射率与密度有关。 因此，渐变折射率纤维是一种具有不同密度的纤维。 密度在核心的中心最高，然后在边缘逐渐降低到最低。

单模 (Single Mode)

Single mode uses step-index fibre and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The single-mode fibre itself is manufactured with a much smaller diameter than that of multimode fibre, and with substantially lower density.
The decrease in density results in a critical angle that is close enough to 90 degree to make the propagation of beams almost horizontal.

单模使用阶跃折射率光纤和高度聚焦的光源，可将光束限制在很小的角度范围内，所有角度都接近于水平方向。 单模光纤本身的直径要比多模光纤的直径小得多，密度也要低得多。
密度的降低会导致临界角接近90度，从而使光束的传播几乎是水平的。

光纤电缆的纤维尺寸 (Fibre Sizes for Fiber Optic Cable)

Optical fibres are defined by the ratio of the diameter or their core to the diameter of their cladding, both expressed in micrometers. The common sizes are shown in the figure below:

光纤由直径或纤芯与包层直径的比值（均以微米表示）定义。 常见尺寸如下图所示：

光纤电缆连接器 (Fibre Optic Cable Connectors)

There are three types of connectors for fibre-optic cables, as shown in the figure below.

光纤电缆有三种类型的连接器，如下图所示。

The Subscriber Channel(SC) connector is used for cable TV. It uses push/pull locking system. The Straight-Tip(ST) connector is used for connecting cable to the networking devices. MT-RJ is a connector that is the same size as RJ45.

用户频道（SC）连接器用于有线电视。 它使用推/拉锁定系统。 Straight-Tip（ST）连接器用于将电缆连接到网络设备。 MT-RJ是与RJ45尺寸相同的连接器。

光缆的优点 (Advantages of Fibre Optic Cable)

Fibre optic has several advantages over metallic cable:

与金属电缆相比，光纤具有几个优点：

• Higher bandwidth
  更高的带宽
• Less signal attenuation
  信号衰减小
• Immunity to electromagnetic interference
  抗电磁干扰
• Resistance to corrosive materials
  耐腐蚀材料
• Light weight
  重量轻
• Greater immunity to tapping
  更大的抗攻丝能力

光纤电缆的缺点 (Disadvantages of Fibre Optic Cable)

There are some disadvantages in the use of optical fibre:

使用光纤有一些缺点：

• Installation and maintenance
  安装及保养
• Unidirectional light propagation
  单向光传播
• High Cost
  成本高

光缆的性能 (Performance of Fibre Optic Cable)

Attenuation is flatter than in the case of twisted-pair cable and coaxial cable. The performance is such that we need fewer(actually one tenth as many) repeaters when we use the fibre-optic cable.

衰减比双绞线电缆和同轴电缆更平坦。 这种性能使我们在使用光缆时需要更少（实际上是十分之一）的中继器。

光纤电缆的应用 (Applications of Fibre Optic Cable)

• Often found in backbone networks because its wide bandwidth is cost-effective.
  通常在骨干网中发现是因为其宽带宽具有成本效益。
• Some cable TV companies use a combination of optical fibre and coaxial cable thus creating a hybrid network.
  一些有线电视公司使用光纤和同轴电缆的组合，从而创建了一个混合网络。
• Local-area Networks such as 100Base-FX network and 1000Base-X also use fibre-optic cable.
  诸如100Base-FX网络和1000Base-X之类的局域网也使用光纤电缆。

翻译自: https://www.studytonight.com/computer-networks/bounded-transmission-media

引导性传输介质