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Bilingual Course
Network Communications

(For Master Students in the Department of Electronic Engineering)
Chapter 3 Data Transmission
Instructor,Dr,Tianshuang Qiu
School of the Electronic and Information Engineering
Fall of 2004
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§ 3-1 Concepts and Terminologies
Expressions of signals
In time domain
Continuous time signal
Discrete time signal
Periodic signal
Aperiodic signal
In frequency domain
Fourier series
Fourier transform
Discrete Fourier series
Discrete time Fourier transform
Discrete Fourier transform
Fast Fourier transform
)(tx
)(nx
)()( Ttxtx )()( Nnxnx
ttxTa T tjkk de)(1 0
ttxjX tj de)()(
Nn njkk enxNa 0][1?
m tjj enxeX ][)(
10 2][)( Nn NnkjenxkX?
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Relations

)()(
)()(
][)(
zXsX
eXjX
aa
nxtx
j
kk

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Examples of Signals and spectra (Figure 3.3,E.6)
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Figure 3.4
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Figure 3.6
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Figure 3.5
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Other Concepts
Guided media（导向媒体）

（ twisted pair），同轴电缆（ coaxial cable），光纤（ optical fiber）
Unguided media（非导向媒体）

Point-to-Point（点对点）

Multipoint （多点）

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The relations between the
Bandwidth and the Data rate

P.77）；

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T
1
0
1
11
12

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Examples
Case1,Seeing Figure 3.7(a) in P.77,the base frequency of the
signal is f=1MHz,the period is T=1us,and Tb=0.5us,Taking 5
harmonics,B=fmax-f=4MHz,Then the data rate is
R=1/Tb=2Mbps.
Case 2,Seeing Figure 3.7(a) in P.77,the base frequency of the
signal is f=2MHz,the period is T=0.5us,and Tb=0.25us,Taking
5 harmonics,B=10-2=8MHz,Then the data rate is
R=1/Tb=4Mbps.
Case3,Base frequency f=2MHz,T=0.5s,Tb=0.25us,Taking 3
harmonics,B=fmax-f=6-2=4MHz,then R=1/Tb=4Mbps
（ Tb为一个 bit信号波形的时间宽度）
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Figure 3.7
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§ 3-2 Analog and Digital Data
Transmission
Data,传达某种意义的实体
Signaling,信号沿适当媒体的实际传播动作
Transmission,进行数据通信的过程
Data--Signal

·直接传输
·模拟调制模拟数据有编解码器产生数字 bit流数字数据数字数据竟 Modem产生模拟信号
·基带
·编码
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Signal--Transmission

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Video Interlacing Signals
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Spectrum of Speech and Music (Figure 3.9)
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§ 3-3 Transmission Impairments(传输损伤 )
Attenuation

Delay Distortion

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Noises
Classification
Thermal noise,导体内电子热运动引起，是无法消除的白噪声
Intermodulation noise,由不同频率的信号共享一个传输媒体而引起（非线性效应），其结果是产生额外的信号
Crosstalk:是不希望有的信道之间的耦合现象。载有多路信号的相邻双绞线之间发生电耦合。例如打电话时听到了别人的谈话。
Impulse noise,由非连续的、不规则的脉冲或噪声尖峰所组成，

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The Calculation of Thermal Noises
1Hz带宽内的热噪声值

Example

)Hz/(0 WkTN?
0N )Hz/(W
)/(103 8 0 3.1 23?KJk
)(0 WBNkT BN
BT
BTkN
k T BN
B
N
l o g10l o g10d B W6.228
l o g10l o g10l o g10
WHz,
d B W / H z204W / H z104290)1038.1(
2123
0

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Example

d B W6.138
70206.228
10l o g1010l o g10d B W6.228 72

N
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§ 3-4 Channel Capacity

Data rate,是指数据能够通信的速率，用 bps表示。
Bandwidth：是指在发送器和传输媒体的特性限制下的带宽，

Noise：通信通路上的平均噪声电平。
Error rate：这里的差错是指发送的 0而接收的为 1，或者相反。
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Nyquist Bandwidth

Bandwidth为：
Example

Nyquist公式的缺点：没有考虑噪声对信道的影响。但是实际上，

（对照香农公式：）
)bps(lo g2 2 MBC?
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Shannon Channel Capacity
Nyquist公式的缺点

)Hz(
)b p s(,
)dB()S N R1(l o g 2

B
C
BC
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B=4-3=1MHz,(SNR)dB=24dB,SNR=251

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Eb— 每 bit信号的能量
N0— 每 Hz噪声的功率密度

0/NEb
TkS
N
E
TRb i tTS
k T R
S
RN
S
N
ST
N
E
b
bb
bb
101010d B WdB
0
000
l o g10l o g10l o g10)(
/1,,,

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R=2400bps。

10log10290
S=-161.9dBW
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Homework for Chapter 3
3.6,3.11,3.17
Due by Next Monday
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End of Chapter 3