Introduction to Data Communications And Networking

Computer networks are designed to transfer data from one point to another. During transit data is in the form of electromagnetic signals. Hence it is important to study data and signals before we move to further concepts in data communication.


To be transmitted, data must be transformed to electromagnetic signals.

2.2.1. Data can be Analog or Digital.
1. Analog data refers to information that is continuous; ex.
sounds made by a human voice
2. Digital data refers to information that has discrete states.
Digital data take on discrete values.
3. For example, data are stored in computer memory in the
form of Os and 1s

2.2.2. Signals can be of two types:
1. Analog Signal: They have infinite values in a range.
2. Digital Signal: They have limited number of defined

2.2.3. Periodic & Non Periodic Signals

Signals which repeat itself after a fixed time period are called
Periodic Signals.
Signals which do not repeat itself after a fixed time period
are called Non-Periodic Signals.
In data communications, we commonly use periodic
analog signals and non-periodic digital signals.


  • An analog signal has infinitely many levels of intensity over a
    period of time.
  • As the wave moves from value A to value B, it passes
    through and includes an infinite number of values along its
    path as it can be seen in the figure below.
  • A simple analog signal is a sine wave that cannot be further
    decomposed into simpler signals.


A sine wave is characterized by three parameters:

1. Peak Amplitude

2. Frequency

3. Phase

2.3.1 Characteristics of an Analog Signal Peak Amplitude

  • The amplitude of a signal is the absolute value of its
    intensity at time t
  • The peak amplitude of a signal is the absolute value of
    the highest intensity
  • The amplitude of a signal is proportional to the energy
    carried by the signal

fig-amplitude-of-a-sine-wave Frequency

  • Frequency refers to the number of cycles completed by the
    wave in one second.
  • Period refers to the time taken by the wave to complete one

fig-frequency-period-of-a-sine-wave Phase
Phase describes the position of the waveform with respect to time
(specifically relative to time O).


  •  Phase indicates the forward or backward shift of the
    waveform from the axis
  • It is measured in degrees or radian
  • The figure above shows the sine waves with same amplitude
    and frequency but different phases

Digital Signal

Information can also be explained in the form of a digital signal.

A digital signal can be explained with the help of following points

Definition:- A digital is a signal that has discrete values. The signal will have value that is not continuous.

BIT LENGTH or Bit Interval (Tb)

  • It is the time required to send one bit.
  • It is measured in seconds.


  • It is the number of bits transmitted in one second.
  • It is expressed as bits per second (bps).
  • Relation between bit rate and bit interval can be as follows

Bit rate = 1 / Bit interval

Baud Rate

It is the rate of Signal Speed, i.e the rate at which the signal changes.

A digital signal with two levels  0 & 1  will have the same baud rate and bit rate & bit rate.


  • Each composite signal has a lowest possible(minimum) frequency and a highest possible (maximum) frequency.
  • From the point of view of transmission, there are two types of channels:


1 Low pass Channel

  • This channel has the lowest frequency as 0 and highest
    frequency as some non-zero frequency f1.
  • This channel can pass all the frequencies in the range 0 to f1.

2 Band pass channel

  • This channel has the lowest frequency as some non-zero
    frequency  f1 and highest frequency as some non-zero
    frequency  f2.
  • This channel can pass all the frequencies in the range f1 to f2.

Transmission of Digital signal

Digital signal can be transmitted in the following two ways:

1 Baseband Transmission

  • The signal is transmitted without making any change to it (ie. Without modulation)
    In baseband transmission, the bandwidth of the signal to
    be transmitted has to be less than the bandwidth of the
  • Ex. Consider a Baseband channel with lower frequency 0Hz and higher frequency 100Hz, hence its bandwidth is
    100 (Bandwidth is calculated by getting the difference between the highest and lowest frequency).
  • We can easily transmit a signal with frequency below 100Hz, such a channel whose bandwidth is more than
    the bandwidth of the signal is called Wideband channel Logically a signal with frequency say 120Hz will be
    blocked resulting in loss of information, such a channel whose bandwidth is less than the bandwidth of the signal
    is called Narrowband channel

2.Broad band Transmission

  • Given a bandpass channel, a digital signal cannot be transmitted directly through it  In broadband transmission we use modulation, i.e we change the signal to analog signal before transmitting it.
  • The digital signal is first converted to an analog signal, since we have a bandpass channel we cannot directly
    send this signal through the available channel. Ex. Consider the bandpass channel with lower frequency
    50Hz and higher frequency 80Hz, and the signal to be transmitted has frequency 10Hz.
  • To pass the analog signal through the bandpass channel,the signal is modulated using a carrier frequency. Ex.
    The analog signal (10Hz) is modulated by a carrier frequency of 50Hz resulting in an signal of frequency
    60Hz which can pass through our bandpass channel.
  • The signal is demodulated and again converted into an
    digital signal at the other end

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