Transcript CS412 Computer Networks - Computer Science | Winona State

CS 313 Introduction to
Computer Networking &
Telecommunication
Modulation, Multiplexing, &
Public Switched Telephone
Network
Chi-Cheng Lin, Winona State University
Topics

Modulation

Multiplexing

Telephone System

Switching
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Modulation
Analog signals are devised to send
digital information
 Digital modulation

Process of converting between bits and
signals that represent them

Schemes
Baseband transmission
Passband transmission (our focus)
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Passband Transmission

Fact
Baseband signal from 0 to B Hz and be
shifted up to a passband of S to S+B Hz
for transmission
Amount of information carried not affected
Receiver can shift the signal back down to
0 to B Hz for decoding
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Passband Transmission

Modulation
Superimpose information signals on to the
carrier signal at transmitting end

Basic modulation techniques
Amplitude shift keying (ASK)
Different amplitudes representing different symbols
(e.g., 0 and 1)
Problem: vulnerable to noise
Frequency shift keying (FSK)
Different frequencies representing different symbols
Problem: limited by physical capacity of carrier
Phase shift keying (PSK)
Different phases representing different symbols
Problem: Hard to distinguish small phase shift
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Binary
Signal
ASK
FSK
PSK
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Passband Transmission

Quadratic phase shift keying (QPSK)
4 symbols  2 bits per symbol

Combination of ASK and PSK
QAM-16: 16 symbols  4 bits per symbol
QAM-64: 64 symbols  __ bits per symbol

Constellation patterns
Diagrams showing legal combinations of
amplitude and phase
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More Constellation Diagrams
(a) QPSK (4-QAM).
(b) QAM-16.
(c) QAM-64.
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Passband Transmission

Gary-code
 Adjacent symbols differ in only one bit position
 At most 1 bit error in the expected case
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Multiplexing

Multiplexing
Set of techniques allowing multiple signals
to share a single data link
Dividing total available bandwidth over a
link into multiple channels

Why multiplexing?
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Multiplexing

Frequency Division Multiplexing (FDM)
Dividing bandwidth of a link into separate
channels

Orthogonal FDM (OFDM)
Problem of FDM: guard bands needed to
separate channels well  bandwidth wasted
OFDM uses subcarriers that send data
independently without using guard bands

Time Division Multiplexing (TDM)
Combining signals from low speed channels to
share time on a high-speed link
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Frequency Division Multiplexing (FDM)
(a) The original bandwidths. (b) The bandwidths raised in frequency.
(c) The multiplexed channel.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
OFDM

Used in 802.11, cable networks, power
line networking, (& 4G cellular systems)
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
Time Division Multiplexing

Round-robin
Each user takes turn
Each one periodically gets the entire
bandwidth

Used in telephone and cellular networks
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
Structure of the Telephone System
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
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Structure of the Telephone System

A typical circuit route for a mediumdistance call.
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Major Components of the Telephone
System

Local loops
Analog twisted pairs going to houses and
businesses – “last mile”

Trunks
Digital fiber optics connecting the
switching offices

Switching offices
Where calls are moved from one trunk to
another
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Digital Transmission

Why digital?
Low error rate
Mix signals from different sources
(multimedia)
Cheaper
Maintenance is easier
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Modem
•
The use of both analog and digital transmissions for
a computer to computer call. Conversion is done by
the modems and codecs.
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Modem


How can we transmit digital data over
analog local loop?
Modulator-demodulator
A device that accepts a serial stream of bits as
input and produces a modulated (analog)
carrier signal as output (or vice versa)
Each high-speed modem standard has its own
constellation pattern

Baud rate
Number of signal units transmitted per sec
Number of symbols per second
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Examples

A modem uses QPSK and sends data at the rate of
2400 baud. What is the data rate?

Compute the bit rate for a 2400-baud 16-QAM signal.

Compute the bit rate for a 2400-baud 64-QAM signal.
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Telephone Modems

A telephone line has a bandwidth of
3000 Hz (3300 – 300) for voice
2400 Hz (3000 – 600) for data

Modem standards
V.32: 9,600 bps
V.32bis: 14,400 bps
V.34bis: 28,800 ~ 33,600 bps
V.90: download up to 56kbps (56K modem)
V.92: adjustable speed, call waiting, etc.
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Trellis Coded Modulation


1 parity per symbol to reduce error
Examples
V.32 standards uses 32 constellation points
to transmit data at the rate of 2400 baud and
1 bit per symbol. What is the bit rate?
Ans: 2400 x (log2 32 - 1) = 2400 x (5 - 1) =
9600 bps
V.32 bis standards uses 128 constellation
points to transmit data at the rate of 2400
baud and 1 bit per symbol. What is the bit
rate?
Ans: 2400 x (log2 128 - 1) = 2400 x (7 - 1) =
14400 bps
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Trunk and Muliplexing

TDM – Why is T1 line 1.544 Mbps?

Frame size / duration of frame =
193bits / (12510-6sec) = 1.544 Mbps
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TDM

Multiplexing T1 streams onto higher
carriers
24 voice channels
___ voice channels
___ voice channels
___ voice channels
Capacity of each digital channel = 64 Kbps
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Wavelength Division Multiplexing (WDM)

Used over fiber optics, similar to FDM
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Switching

Switch
Device creating connections between
devices linked to it

Switching
Forwarding data from a switch to another
device
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Switching Techniques

Techniques
Circuit switching
Packet switching

Circuit switching
End-to-end path has to be set up BEFORE
any data can be sent
Data follow the same path
No danger of congestion (except in path
setup phase)
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Switching Techniques

Packet switching
Store-and-forward
Tight upper limit on block size
 allowing packets to be buffered in router
main memory
No single block can tie up a link for too long
Shorter delay and higher throughput
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Circuit Switching Vs. Packet Switching
Circuit switching
Packet switching
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# hops=3
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Circuit switching
Packet switching
Circuit Switching Vs. Packet Switching
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