204325 Data Communications and Computer Networks - HBCC204325数据通信与计算机网络的HBCC

Bit rate and bandwidth are proportional to each other
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Transmission Impairment
Attenuation Distortion Noise
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Signal Attenuation
Attenuation Loss of energy
Throughput
Actual data rate
Latency (delay)
Time it takes for an entire message to completely arrive at the destination
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Digital-to-Analog Conversion
Analog Signal Analog Signal Digital Signal Digital Signal
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Bandwidth
A property of a medium
Indicates the difference between the highest and the lowest frequencies allowed to pass
Types of noise
Thermal
Due to random motion of electrons in a wire
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Noise
Types of noise (cont’d)
Crosstalk
Signal from one line picked up by another
levels in the digital signal. Simplest form On-Off Keying (OOK)
fc – Carrier frequency 0 < d < 1
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Binary Frequency Shift Keying
Or Binary FSK In FSK, the frequency of the carrier signal
<highest freq allowed> – <lowest freq allowed>
Also a property of a single spectrum
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Digital Signals
Properties:
Bit rate – number of bits per second Bit interval – duration of 1 bit
Physical Layer: Data and Signals
Semester: 131 Course: CSET 221 Computer Networking
Instructor: Farhan Khan Computer Science & Engineering Technology Unit
Wire 1
Wire 2
Impulse
Irregular pulses or spikes E.g., lightning Short duration High amplitude
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Network Performance
Bandwidth
Hertz Bits per second (bps)
Also known as "Carrier Frequency"
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Conversion Techniques
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Binary Amplitude Shift Keying
Or Binary ASK In ASK, amplitude of a carrier signal is changed to reflect amplitude
Signal strength falls off with distance
Transmission medium
Attenuation depends on medium Attenuation is an increasing function of
frequency
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Signal Distortion
Required to send digital data over an analog channel
Also known as digital modulation
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Carrier Signals
Carrier signal is a high-frequency signal acting as a base for information signal
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Analog vs. Digital Signals
To be transmitted, data must be transformed to electromagnetic signals
Analog signals
value
have an infinite number of
values in a range
Distortion Change in signal shape
Only happens in guided media
Propagation velocity varies with frequency
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Noise
Noise Undesirable signals added between the transmitter and the receiver
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Quadrature Amplitude Modulation
Or QAM A combination of ASK and PSK
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Summary
Data need to take form of signal to be transmitted
Bit rate is proportional to bandwidth Signals get impaired by attenuation,
is varied to represent digital data.
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Binary Phase Shift Keying
Or Binary PSK In PSK, the phase shift of the carrier
signal is varied to represent digital data.
time
Digital signals
value
Have a limited number of
values
time
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Data and Signals
Analog Data Digital Data Analog Data Digital Data
Telephone
Modem Codec
Digital transmitter
Analog data
Data take on continuous values E.g., human voice, temperature reading
Digital data
Data take on discrete values E.g., text, integers
amplitude 10010110
bit interval
... time
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Digital vs. Analog Bandwidth
Digital bandwidth
Expressed in bits per second (bps)
Analog bandwidth
Expressed in Hertz (Hz)
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Outline
Analog and digital data/signals Bandwidth and bit rate Signal impairment Transmitting digital signals as analog
Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM)
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Analog vs. Digital Data
distortion, and noise Digital data can be sent over analog
channel using digital modulation
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