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General computer network
(structure)
Host
Communication
system
Channel
(trasm.
media)
A particular
case
Host
Host
Trasm.
media
Host
Communication channels are fundamental
components of a communication system of a computer
network.
 What signals are used (continuous or discrete)?
 What is the highest signal frequency?
 What is the noise level?
 What is the propagation delay?
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Basic concepts of signals: two points of view
Signal as functions of time
(time-domain view of signals)
a) Continuous (analog) signal.
Amplitude
(volts)
time
b) Discrete signal (3 values).
Amplitude
+a
time
-a
c) Discrete binary signal (2 values).
Amplitude
+a
time
2
Signals in frequency domain
a)
Simple
periodic
signal
A
s(t)=A sin(2ft)
A
time
f
frequency
T=1/f
A1
s1(t)=A1 sin(2ft)
time
T=1/f
s2(t)=A2 sin(23ft)
A2
time
T=1/3f
b)
Complex
periodic
signal
A1
time
A2
frequency
f
3f
T=1/f
s(t)=A1 sin(2ft) + A2 sin(23ft)
3
c) Nonperiodic signal.
Amplitude
Power of signal
frequency
time
f1
f2
bandwidth = f2 – f1
(range of frequencies)
4
Attenuation and distortion (corruption) of a signal
Data bits
(transmitted)
1
0
0
1
0
1
1
+5v
time
Transmitted
signal
-5v
Attenuation
+3v
time
-3v
Bandwidth
limitations
time
time
Delay
distortion
Distortion by
noise
(received signal)
time
Sampling signal
at receiving side
time
Received data
1
0
1
1
0
1
1
error
5
Attenuation of a signal
(decrease of the signal amplitude)
Let P1 be the power of a transmitted signal (watts), and
P2 be the power of the received signal.
Then:
Attenuation = 10 log10 P2 /P1 (decibels, or dB).
Example 1 (attenuation):
Transmitted
signal is 400 mW
Communication
channel
Received signal
is 10 mW
So P1 = 400 mW, P2 = 10 mW
Attenuation = 10 log10 10/400  -10*1.6 = -16 dB
Example 2 (amplification):
Input signal is
P1 =10 mW
Amplifier
Output signal is
P2 =1000 mW
Amplification = 10 log10 P2/P1 = 20 dB (positive)
6
Bandwidth limitation of a signal
Filter
Output
signal
Amplification
Amplitude
Input
signal
Amplitude
f1
f2
f3
frequency
f1
frequency
f3
f1
f3
frequency
An example:
A binary signal of rate 1000 Bps is transmitted over a
communication channel.
What should be the minimum bandwidth of the channel
to receive the 1st and 2nd harmonics of the signal?
Solution:
For the worst case of the signal 10101010…, the 1st
harmonic is 1000/2 = 500 Hz, and the 2nd harmonic is
500*2 = 1000 Hz.
Thus, the minimum bandwidth should be 0-1000 Hz.
7
Effect of the channel bandwidth on the received signal
Source of
signal
channel
Destination
An example:
0
1
0
0
0
1
0
a) Digital
signal at
source
time
b) Channel has
bandwidth of
500 Hz
time
Signal at
destination
c) 1300 Hz
d) 4000 Hz
time
time
8
Distortion of a signal by noise
Ideal Binary Signal (no noise)
time
time
line noise
signal + noise
line noise
A communication channel is
characterized by a signal/noise ratio
The Shannon’s formula defines the theoretical
maximum data rate of a transmission channel:
C = W log2 (1 + S/N) bps
Where
C is the data rate, bps (bits per second)
W is the bandwidth of the channel, Hz
S is the average signal power, watts
N is the noise power, watts
9
Propagation delay of a signal
in a transmission media
Source
Destination
A
Sent
by A
Transmission
media
B
t1
time
Propagation delay =
Propagation
delay
t2
time
Received
by B
Length of transm. media
Velocity of electromagnetic waives
10
Fundamental characteristics of
communication channels
1. Bandwidth W (measured in Hz)
 For a telephone channel: 300 – 3000 Hz
 Voice “channel”: 60 – 7000 Hz
2. Signal to noise ratio, S/N
(usually measured in decibels)
3. Propagation delay
About 200 000 km/s in a conductor
About 300 000 km/s in air
4. Capacity (measured in bits per second)
The fundamental law of communication
(Shannon’s law):
max capacity C = W.log2(1 + S/N) bps
An example:
For a telephone channel:
With W=3000 Hz, S/N = 1000/1 (30 dB),
We have Cmax = 3000.log2 (1001) = 30 000 bps
11
Data transmission speeds and uses
Speed, bit/s
Up to 1 200
Typical use
Telegraph, telemetry
1 200 – 9 600
For PCs (keyboard <> CPU)
9 600 – 57 600
Connecting to Internet via
modems (for file transfer)
32 000 – 64 000
Digital voice, simple video
64 000 – 1 544 000 Backbone links for networks
Over 1 544 000
Multimedia
Network 1
Network 3
Backbone link
Network 2
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