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Data Communication Networks

Physical Layer 

Nguyễn Quốc Tuấn

Network and Communication System Department

Faculty of Electronics and Telecommunications

UNIVERSITY OF ENGINEERING AND TECHNOLOGY

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Responsible for transmission of signals over a link

Focus on point-to-point channels (i.e., channels that connect just two

nodes),

 – There are two broad classes of point-to-point channels:

digital channels

analog channels.

Physical Layer

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Channel

Physical Layer

 Analog channel was modeled as a filtering

- One of the most important distorting effects on most analog channels is linear

time invariant (LTI) filtering

- The defining properties of linear time-invariant filters are as follows:

If input s(t) yields output r(t), then for any  , input s(t -   ) yields output r(t -   ).

If s(t) yields r(t), then for any real number   then, .s(t) yields   r(t).

If s1(t) yields r 1(t) and s2 (t) yields r 2 (t), then s1(t) + s2 (t) yields r 1(t) + r 2 (t).

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Channel

Physical Layer

The problem is that: the impulse response of channel is not longer than a pulse

time, it is caused inter-symbol interference.

- Called h(t) the impulse response of the channel.

- The pulse from s(   ) to s(  +  ) can be viewed as a small impulse of area  .s(   )

at time  

- It called the convolution integral 

Digital channel was converted by DSP

- h = (h1, h

2.. h

L) is channel vector  – L is order of channel

- s = (s1, s2 .., sn) is a symbol to transmite

           d t h st r 

 L

k 

k nhk  snr 1

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Bandwidth of Channel

Physical Layer

Fourier transform to frequency domain

|H( f )| is large only for a frequency band around f = 0 to be the

- Frequency band of symbol have to fit channel

- It is seen that |H( f )| is approximately 1 for small  f and decreases as 1/ f for

large f .

  f  S   f   H   f   Rd t h st r   

      

             d eh  f   H    f    j2

  f    j  f   H et h   t 

   

  

   

2

 

           d e s  f  S    f    j2

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The Sampling Theorem

Physical Layer

It is important to recognize that the sampling theorem specifies the number of 

samples per second that can be utilized on a low-pass channel, but it does not

specify how many bits can be mapped into one sample

- For a given sample rate f s the band limit for perfect reconstruction is B  ≤ f s/2

Looks

Multiplexed - High performance Communications

 M  BC 

SNR BC 

 BC 

2

2

log

1log

2

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Time-Division Multiplexing

Physical Layer

- Time-division multiplexing (TDM) is an alternative technique for splitting a big

channel into many little channels.

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Modulation

Physical Layer

- The modulation techniques of the preceding subsection are equally applicable

whether a bandwidth constraint is imposed by FDM or by channel filtering.

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Modulation

Physical Layer

 Analog Modulation

-  AM 

-  PM 

-  FM 

Digital Modulation- M-ASK 

- M-PSK 

- M-FSK 

- M-QAM 

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Physical Layer