Transcript Class 7 - Sept. 28

CSCI 465
Data Communications and Networks
Lecture 7
Martin van Bommel
CSCI 465
Data Communications and Networks
1
Data Communication Basics
• Analog or Digital
• Three Components
– Data
– Signal
– Transmission
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Data Communications and Networks
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Analog Data Choices
analog line
analog voice
telephone
analog voice
digitized voice
digital line
Codec
01101000110
Codec: coder and decoder
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Digital Data Choices
analog line
moduated data
modem
data
data
digital line
DSU
01101000110
DSU: data service unit
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Data Communications and Networks
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Transmission Choices
• Analog transmission
– only transmits analog signals, without regard for
data content
– attenuation overcome with amplifiers
• Digital transmission
– transmits analog or digital signals
– uses repeaters rather than amplifiers
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Data Communications and Networks
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Data, Signals, and Transmission
A
Data
D
D
A
A
D
Transmission
System
Signal
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Data Communications and Networks
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Digital Transmission Advantages
•
•
•
•
The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over one
line
• Higher bandwidth is possible with data
compression
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Data Communications and Networks
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Analog Encoding of Digital Data
• data encoding and decoding to represent data
using the properties of analog waves
• modulation: the conversion of digital signals
to analog form
• demodulation: the conversion of analog data
signals back to digital form
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Data Communications and Networks
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Modem
• an acronym for modulator-demodulator
• uses a constant-frequency signal known as
a carrier signal
• converts a series of binary voltage pulses
into an analog signal by modulating the
carrier signal
• the receiving modem translates the analog
signal back into digital data
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Data Communications and Networks
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Methods of Modulation
• amplitude modulation (AM) or amplitude shift
keying (ASK)
• frequency modulation (FM) or frequency shift
keying (FSK)
• phase modulation or phase shift keying (PSK)
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Amplitude Shift Keying (ASK)
• In radio transmission, known as amplitude
modulation (AM)
• the amplitude (or height) of the sine wave
varies to transmit the ones and zeros
• Up to 1200 bps over voice-grade lines
• major disadvantage
– telephone lines are very susceptible to variations
in transmission quality that affect amplitude
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ASK Illustration
1
0
0
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12
Frequency Shift Keying (FSK)
• in radio transmission, known as frequency
modulation (FM)
• the frequency of the carrier wave varies in
accordance with the signal to be sent
• signal is transmitted at constant amplitude
• more immune to noise than ASK
• requires more analog bandwidth than ASK
• still up to 1200 bps on voice-grade lines
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Data Communications and Networks
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FSK Illustration
1
1
0
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Data Communications and Networks
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Full-Duplex FSK
CS/IS 465: Data Communications and
Networks
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Phase Shift Keying (PSK)
• also known as phase modulation (PM)
• frequency and amplitude of the carrier signal
are kept constant
• the carrier is shifted in phase according to the
input data stream
• each phase can have a constant value, or
value can be based on whether or not phase
changes (differential keying)
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PSK Illustration
0
0
1
1
1
1
Two-level PSK
0
Differential-level PSK
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Data Communications and Networks
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Complex Modulations
• Combining modulation techniques allows us
to transmit multiple bit values per signal
change (baud)
• Increases information-carrying capacity of a
channel without increasing bandwidth
• Increased combinations also leads to
increased likelihood of errors
• Typically, combine amplitude and phase
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Data Communications and Networks
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Multilevel PSK
• Can extend levels beyond one bit at a time
• 9600 bps modem
– 12 different phase angles
– 4 of them use two different amplitude values (ASK)
– 16 different signal types – 4 bits per signal type
– 9600 bps modem / 4 bits per signal type
– 2400 baud modem (signaling speed)
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Quadrature Amplitude Modulation
(QAM)
• the most common method for quadbit
transfer
• combination of 8 different angles in phase
modulation and two amplitudes of signal
• provides 16 different signals, each of which
can represent 4 bits
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Data Communications and Networks
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Quadrature Amplitude Modulation
(QAM)
• the most common method for quadbit
transfer
• combination of 8 different angles in phase
modulation and two amplitudes of signal
• provides 16 different signals, each of which
can represent 4 bits
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Data Communications and Networks
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QAM Illustration
90
135
45
amplitude 1
180
0
amplitude 2
225
315
270
22
QAM Uses
• CCITT V.22 bis modem
– the "bis" is French for "duo" or "twice"
– supports transmission of full-duplex 2400 bps
synchronous or asynchronous data over a
switched, 2-Wire, voice circuit
– the modulation rate is 600 baud, with each baud
representing four data bits
(CCITT – Consultative Committee International for Telephony and Telegraphy)
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56kbps Modems
• asymmetrical; can download at 56 kbps but
upload at 33.6 kbps only
• requires digital T-1 or ISDN PRI connection
at central site or ISP
• ITU Standard V.90 (Feb. 1998)
• early examples
– U.S. Robotics (56K x2)
– Rockwell (56K flex)
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