Chapter 3 : Data communication.
Lecture 6
Data communication
- Data communication is the transfer of data from one
device to another via some form of transmission
medium.
- To be transmitted, data must be transformed to
electromagnetic signals.
Analog and Digital Data
- Data can be analog or digital.
- Analog data are continuous and take continuous
values.
- Digital data have discrete states and take discrete
values.
Analog and Digital Data
-Analog data, such as the sounds made by a human
voice, take on continuous values. When someone
speaks, an analog wave is created in the air. This can
be captured by a microphone and converted to an
analog signal or sampled and converted to a digital
signal.
Analog and Digital Data
- Digital data take on discrete values. For example, data
are stored in computer memory in the form of Os and
1s. They can be converted to a digital signal or
modulated into an analog signal for transmission
across a medium.
Analog and Digital Signals
- Signals can be analog or digital.
- Analog signals can have an infinite number of values
in a range.
- Digital signals can have only a limited number of
values.
Comparison of analog and digital signals
Frequency and Time period:
Frequency:
Frequency is the number of complete cycles per second. The
standard unit of frequency is Hertz, abbreviated Hz.
Example:
If a signal completes one cycle per second, then the
frequency is 1 Hz; 60 cycles per second equals 60 Hz.
Time Period:
The inverse of Frequency is called Time period.
T = 1/F.
Frequency and Time period:
Frequency and Time period:
Units of period and frequency:
Example:
Express a period of 100 ms in microseconds, and express the
corresponding frequency in kilohertz.
1100 ms = 100  10-3 s = 100  10-3  106 ms = 105 ms
2Now we use the inverse relationship to find the frequency,
changing hertz to kilohertz.
100 ms = 100  10-3 s = 10-1 s
f = 1/10-1 Hz = 10  10-3 KHz = 10-2 KHz
Modem :
Modem (Modulator/Demodulator):
Device that convert digital and analog signals. Modems
allow computer data (digital) to be transmitted over voicegrade telephone lines (analog).
Modem :
Modulation:
Modulation is the addition of information to an electronic or
optical signal carrier.
Demodulation :
Demodulation is the separation of electronic information
from carrier signal.
Bandwidth:
- Bandwidth refers to how much data you can send through
a network or internet connection.
• In digital systems, bandwidth is expressed as data speed in
bits per second (bps).
• In analog systems, Bandwidth is determined by finding the
difference between the highest and lowest frequency
components.
B = Fh – F l
Bandwidth:
Example1:
If a periodic signal is decomposed into five sine waves with
frequencies of 100, 300, 500, 700, and 900 Hz, what is the
bandwidth?
B = fh - fl = 900 - 100 = 800 Hz
Example2:
A signal has a bandwidth of 20 Hz. The highest frequency is 60
Hz. What is the lowest frequency?
B = fh - fl
20 = 60 - fl
fl = 60 - 20 = 40 Hz