计算机网络-物理层

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4 Periodic and Aperiodic Signals

Signals are broadly classified as
periodic aperiodic (sometimes called nonperiodic) classification depends on whether they repeat

A frequency domain representation can also be used with nonperiodic signals

but aperiodic representation is not essential to an understanding of the subject



A network usually permits multiple pairs of communicating entities to communicate over a given physical medium There are possible ways underlying facilities can be shared We should understand advantages and disadvantages of each and the resulting modes of communication

The function Asin(2pt) is represented by a single line of height A that is positioned at x = t The frequency domain graph in Figure 6.6 represents a composite from Figure 6.5c

most systems use composite signals to carry information a composite signal is created at the sending end and the receiver decomposes the signal into the simple components


For example:

上图是非周期的 下图是周期的

模拟信号波型
V(t)
0
t

数字信号波型
V(t)
0
t
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5.模拟信号——正弦波

周期信号不含有任何信息，但一般传播 距离远，作为信号的传输载体 周期与频率 振幅 相位 时域和频域
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6.5 Sine Waves and Signal Characteristics
第3章 物理层
本章内容



信号 数字信号 模拟信号 传输介质 传输技术
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引论
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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1 引论
1.1 物理层的基本概念 通信子网分为点-点通信线路通信子网与广播信道通 信子网; 广域网主要采用点到点通信线路，局域网与城域网一 般采用广播信道; 由于技术上存在较大的差异，因此在物理层和数据链 路层协议上出现了两个分支，一类是基于点-点通信 线路，另一类是基于广播信道。

A frequency domain graph shows a set of simple sine waves that constitute a composite function The y-axis gives the amplitude, and the x-axis gives the frequency
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Figure 5.3 A conceptual framework for a data communications system. Multiple sources send to multiple destinations through an underlying physical channel

the signals that result from modulation are usually composite signals it is possible to decompose a composite signal into its constituent parts


A mathematician named Fourier discovered that
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数据通信是三个领域的交叉科学
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1.4 Motivation and Scope of the Subject

Three main ideas provide much of the motivation for data communications and help define the scope
1. 2. 3.
The sources of information can be of arbitrary types – 多媒体 传播所使用的物理系统 Multiple sources of information can 共享物理介质

第一点：relevant considering the popularity of multimedia applications:
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2.信号的概念

信号是数据在传输过程中电信号的表示形式； 模拟信号（analog signal）的信号电平是连续变化的； 数字信号（digital signal）是一组不同的固定电平； 按照在传输介质上传输的信号类型，通信系统分为模 拟通信系统与数字通信系统两种。
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3 Analog and Digital Signals

a set of sine functions, each with a frequency, amplitude, and phase

The analysis by Fourier shows that if the composite signal is periodic, the constituent parts will also 介质的特点 物理现象如何能够传输信息 传输时会出现哪些问题 技术上如何发现、解决这些问题
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1.4 Motivation and Scope of the Subject

第三点：共享基础设施 Sharing plays a fundamental role in most computer networks

The figure shows a set of simple periodic signals
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Composite Signals
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Time and Frequency Domain Representations
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Time and Frequency Domain Representations
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信息编码标准



ASCII 码被国际标准化组织ISO接受，成为国际标准 ISO 646，又称为国际5号码; 它用于计算机内码，也用做数据通信中的编码标准; ASCII码采用7位二进制比特编码，可以表示128个字 符; 字符分为图形字符与控制字符两类。图形字符包括数 字、字母、运算符号、商用符号等。

We define the bandwidth of an analog signal to be the difference between the highest and lowest frequencies of the constituent parts (the highest and the lowest frequencies obtained by Fourier analysis)
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5.3 Bandwidth of an Analog Signal

What is network bandwidth? In networking and communication, the definition of bandwidth varies; here we describe analog bandwidth
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6.1 The Importance of Composite Signals and Sine Functions

Why does data communications seem obsessed with sine functions and composite signals? When we discuss modulation and demodulation, we will understand one of the primary reasons:
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Figure 5.3
Processing at the transmitter/sender end
Figure 5.3
Processing at the receiver end
信号与编码
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2 数据通信的基本概念
1.信息的基本概念


通信的目的是交换信息， 信息是对物质存在和运动形式的一 般描述。 信息的载体可以是数字、文字、语音、图形或图像，称作消 息。计算机产生的信息一般是字母、数字、语音、图形或图 像的组合。消息不是物理的； 在计算机中为了传送这些信息，首先要将字母、数字、语音、 图形或图像用二进制代码的数据来表示，称作编码； 信号是消息的表现形式，消息则是信号的具体内容。信号是 消息的载体，是物理的。 为了传输二进制代码的数据，必须将它们用模拟或数字信号 编码的方式表示； 数据通信是指在不同计算机之间，通过某种传输介质传送表 示字母、数字、符号的二进制代码0、1比特序列的模拟或数 字信号的过程。要求：1)传递性；2)准确性；3)及时性。