Transcript Modem - Tsinghua University

CONSYL
Transmission Impairments
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 1
CONSYL
Type of impairments

Attenuation

Delay distortion

Noise
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 2
CONSYL
Attenuation
P1 watts
transmitter
P2 watts
receiver
Attenuation
10 log10 (P1/P2) dB
Amplification
10 log10 (P2/P1) dB
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 3
CONSYL
Delay distortion
1
1
0
0
1
0
1
Department of Computer Engineering, Kasetsart University
0
0
0
1
1
1
0
1
1
1
0
0
1
204325 Introduction to Computer Communications and Networks
4/11/40 page 4
CONSYL
Noise

Effect


distorted a transmitted signal
attenuated a transmitted signal

signal-to-noise ratio to quantify noise
S= average signal power
S/Ndb = 10 log10 S
N= noise power
N

Type of noise



crosstalk
impulse noise
thermal noise
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 5
CONSYL
Crosstalk

NEXT (near-enf crosstalk )


interference in a wire at the transmitting end of a signal
sent on a different wire
FEXT (far-end crosstalk)

interference in a wire at the receiving end of a signal
sent on a different wire
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 6
CONSYL
Effect of noise
Signal
Noise
Signal+Noise
Sampling times
0 1
0 1
1
0
1 1 0
1 1 0
0
0
0
1
0 1
0 1
Data Received
Original data
Bit error
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 7
CONSYL
Nyquist formula
C=
2W log2 M
W = bandwidth
M = number of discrete signal
Theoritical capacity for Noiseless channel
Channel capacity
calculation for voice
bandwidth (3100 Hz)
Department of Computer Engineering, Kasetsart University
M
2
4
8
16
Max data rate (C)
6200 bps
12400 bps
18600 bps
24800 bps
204325 Introduction to Computer Communications and Networks
4/11/40 page 8
CONSYL
Shannel-Hartley law
C=
W log2 (1+ S )
N
Theoritical maximum channel capacity with noise
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 9
CONSYL
Nyquist theorem
?In a perfectly noiseless channel, if f is the
maxmimum frequency the medium can
transmit, the receiver can completely
reconstruct a signal by sampling it 2*f times
per second
Nyquist, 1920
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 10
CONSYL
Bit rate and Baud rate

Bit rate A number of bits that are tranmitted in a second

Baud rate A number of line signal changed variation per second
If a modem transmits 1 bit for every signal change
bit rate = baud rate
If a signal change represents 2 or more or n bits
bit rate = baud rate *n
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 11
CONSYL
Bandwidth efficiency
B=
R/W
Bandwidth of the channel
Data rate

Typical values range from 0.25 to 3.0 bps Hz-1
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 12
CONSYL
Effect of noise

Eb/N0 = signal energy to noise energy ratio
Eb
= S*W = S*W
N0
R* N N* R
S= signal power in watts
R= data rate
W= bandwidth
N= noise power in received signal
= S/N + 10 logW - 10 logR (dB)
Department of Computer Engineering, Kasetsart University
204325 Introduction to Computer Communications and Networks
4/11/40 page 13