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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