Signal Transmission and Impairments Chaiporn Jaikaeo Department of Computer Engineering Kasetsart University 01204325 Data Communications and Computer Networks Based on lecture materials from Data Communications and Networking, 5 th ed., Behrouz A. Forouzan, McGraw Hill, 2012. Revised 2019-07-22
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Signal Transmission and Impairments
Chaiporn Jaikaeo
Department of Computer EngineeringKasetsart University
01204325 Data Communications and Computer Networks
Based on lecture materials from Data Communications and Networking, 5th ed.,Behrouz A. Forouzan, McGraw Hill, 2012.
Revised 2019-07-22
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Outline•Analog and digital data/signals• Time and frequency domain views of signals•Bandwidth and bit rate• Transmitting digital signals as analog• Theoretical data rate• Signal impairments
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Physical Layer
Frame
from Data Link to Data Link
Frame
01001011 01001011
Transmission medium(bits)
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Analog vs. Digital Data•Analog data
◦ Data take on continuous values◦ E.g., human voice, temperature reading
•Digital data◦ Data take on discrete values◦ E.g., text, integers
•A channel allows frequencies from 4000 to 7000 Hz to pass. Can the above signal pass through?
𝑥 𝑡 = 2 + sin 2000𝜋𝑡 +13 sin(6000𝜋𝑡)
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Low-Pass and Band-Pass Channels• Low-pass channel
•Band-pass channel
frequencyf1 f2
gain
frequencyf1
gain
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Baseband vs. Broadband• In baseband transmission, a digital signal is transmitted
over a channel directly•A low-pass channel is required
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Baseband vs. Broadband• In broadband transmission, a digital signal gets modulated
into an analog signal• The signal can pass through a band-pass channel
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0 1 0 0 1 0 1 1
t
amplitude
1 sec
Properties of Digital Signals• Bit rate – number of bits per second
• Symbol rate – number of signal level changes per second
• Symbol interval – time duration of one symbol
01 00 10 11
t
amplitude
1 sec
One bit per symbol#symbols = 2Bit rate = 8 bpsSymbol rate = 8 symbols/s (baud)Symbol interval = 1/8 s
Two bits per symbol#symbols = 4Bit rate = 8 bpsSymbol rate = 4 symbols/s (baud)Symbol interval = 1/4 s
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Digital vs. Analog Bandwidth•Digital bandwidth
◦ Expressed in bits per second (bps)
•Analog bandwidth◦ Expressed in Hertz (Hz)
Bit rate and bandwidth are proportional to each other
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1 0 1 0 1 0
fmax = 3 Hz
Digital vs. Analog Bandwidth•Allowing one harmonic to pass
1 1 1 1 1 1
1 sec
Bit rate = 6
Digital
1 0 1 0 1 0
Bit rate = 6
1 1 1 1 1 1
f = 0 Hz
Analog
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Bit Rate: Noiseless Channels•Nyquist Theorem
◦ Bit rate in bps (i.e., digital bandwidth)◦ Bandwidth in Hz (i.e., analog bandwidth)◦ L – number of signal levels
Harry Nyquist(1889-1976)
𝐵𝑖𝑡 𝑅𝑎𝑡𝑒 = 2×𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ× logH 𝐿
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Example: Nyquist Theorem•We need to send 265 kbps over a noiseless channel with a
bandwidth of 20 kHz. How many signal levels do we need?• Solution: From Nyquist Theorem
• Since this result is not a power of 2, we need to either increase the number of levels or reduce the bit rate.◦ If we have 128 levels, the bit rate is 280 kbps.◦ If we have 64 levels, the bit rate is 240 kbps.