Analog and Digital Signal

1

• Introduction
• Analog and Digital Signal
• Advantages Disadvantages Digital Signal
• Transmission Impairment
• Data Rate Limit
• Bandwidth of the human voice
• Telephone System Bandwidth
• Digital Channel Capacity
• Digital Signal Regeneration

2

• Introduction
• Data communications are the exchange of data
  between two devices via some form of transmission
  such as a wire cable.
• For data communications to occur, the
  communicating devices must be part of
  communication system made up of a combination of
  hardware (physical equipment) and software
  (programs). See Figure 1.0
• A data communications system has five components
  such as
• 1. Message
• The message is the information (data) to be
  communicated. Popular forms of information
  include text, numbers, pictures, audio, and
  video.
• 2. Sender
• The sender is the device that sends the data
  message. It can be a computer, workstation,
  telephone handset, video camera, and so on.

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• Introduction (continue )
• 3. Receiver
• The receiver is the device that receives the
  message. It can be a computer, workstation,
  telephone handset, television, and so on.
• 4. Transmission medium
• The transmission medium is the physical path by
  which a message travels from sender to receiver.
  Some examples of transmission media include
  twisted-pair wire, coaxial cable, fiber-optic
  cable and radio waves.
• 5. Protocol
• A protocol is a set of rules that govern data
  communications. It represents an agreement
  between the communicating devices. Without a
  protocol, two devices may be connected but not
  communicating, just as a person speaking French
  cannot be understood by a person speak only
  Japanese.

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• Introduction (continue )

Figure 1.0 Five components of data
communications
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• Analog Digital Signals.
• Data can be analog or digital. Analog data are
  continuous and take continuous values. Digital
  data have discrete states and take discrete
  value.
• Like the data they represent, signals can be
  either analog or digital.
• 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 include an infinite number of values along
  its path.

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Figure 1.1 (a) Analog signals. Example of Analog
signal A typical Commercial Radio system
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• Analog Digital Signals (continue ...)
• A digital signal can have only a limited number
  of defined values. Although each value can be any
  number, it is often as simple as 1 and 0. See
  Figure 1.1 (b).
• The constraint for both analog and digital
  communication is the physical capabilities of the
  communications media.

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• Figure 1.1 (b) Digital electrical signals.
• Example of Digital signal Signal on a typical
  printer cable.

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• Advantages of Digital Signal
• Digital circuits are subject to less distortion
  and interference
• Error correction is possible.
• Encryption and privacy is possible
• Digital circuit is simple and cheap
• The receiver can request a retransmission of bad
  information

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• Disadvantages of Digital Signal
• Interface to analogue is needed.
• A digital system requires a greater bandwidth
  than analogue to carry the same information.
• Generally digital communication system require
  synchronization but analogue do not require.

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• Transmission Impairment.
• Signals travel through transmission media, which
  are not perfect. The imperfection causes signal
  impairment.
• Three causes of impairment are attenuation,
  distortion and noise.

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• Attenuation
• Attenuation means a loss of energy.
• When a signal, simple or composite, travels
  through a medium, it loses some of its energy in
  overcoming the resistance of the medium.
• In simple term means a decrease in the electrical
  signal.
• To composite for this loss, amplifiers are used
  to amplify the signal. See Figure 1.2 (a).

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• Attenuation

Figure 1.2 (a) Attenuation
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• Distortion
• Means that the signal changes its form or shape.
  
• Distortion can occur in a composite signal made
  of different frequencies.
• Each signal component has its own propagation
  speed through a medium, its own delay in arriving
  at the final destination.
• Differences in delay may create a difference in
  phase if the delay is not exactly the same as the
  period duration. See Figure 1.2(b)

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• Distortion

Figure 1.2 (b) Distortion
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• Noise
• Noise is another cause of impairment. Several
  types of noise, such as thermal noise, induced
  noise, crosstalk and impulse noise, may corrupt
  the signal.
• Thermal Noise is the random motion of electrons
  in a wire which creates an extra signal not
  originally sent by the transmitter.
• Induced noise comes from sources such as motors
  and appliances.
• Crosstalk is the effect of one wire on the other.
  One wire acts as a sending antenna and the other
  as the receiving antenna.
• Impulse noise is a spike that comes from power
  lines, lightning and so on.
• Figure 1.2(c) shows the effect of a noise on a
  signal.

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• Noise

Figure 1.2 (c) Noise
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• Data Rate Limit
• A very important consideration in data
  communications is how fast we can send data, in
  bits per second, over a channel. Data rate
  depends on three factors
• The bandwidth available
• The level of the signals we use
• The quality of the channel (the level of noise)
• Two theorectical formulas were developed to
  calculate the data rate
• Nyquist bit rate for a noiseless channel
• BitRate 2 bandwidth log2 L
• Shannon Capacity for a noisy channel
• Capacity bandwidth log2 (1 SNR)

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• Data Rate Limit
• Channel Capacity
• The maximum possible rate information rate that
  data can be transmitted over a given
  communication path or channel under given
  condition.
• Bandwidth
• One characteristics that measures network
  performance is bandwidth. However, the term can
  be used in two different measuring value
  bandwidth in hertz and bandwidth in bits per
  second.
• Bandwidth in Hertz
• Bandwidth in Hertz is the range of frequencies
  contained in a composite signal or the range of
  frequencies a can pass. For example We can say
  the bandwidth of a subscriber telephone line is
  4kHz.
• Bandwidth in Bits per Seconds
• Bandwidth in Bits per seconds refer to the speed
  of bit transmission in a channel, a link or even
  a network can transmit. For example One can say
  the bandwidth of a Fast Ethernet network is a
  maximum of 100 Mbps. This means that the network
  can send 100 Mbps.

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• Data Rate Limit
• Relationship between bandwidth in Hertz and
  bandwidth in bps
• There is an explicit relationship between the
  bandwidth in hertz and bandwidth in bits per
  seconds. Basically, an increase in bandwidth in
  hertz means an increase in bandwidth in bits per
  second. The relationship depends whether we have
  baseband transmission or broadband transmission.
• Baseband Transmission
• Transmission of digital or analog signal without
  modulation using a low-pass channel. Low pass
  channel is a channel with bandwith starts from
  zero. See Figure 1.3.1

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• Data Rate Limit
• Baseband Transmission

Figure 1.3.1 Baseband Transmission
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• Data Rate Limit
• Baseband Transmission

Figure 1.3.2 (a) and (b) Bandwidth of two
low-pass channel
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• Data Rate Limit
• Broadband Transmission
• Transmission of signals using modulation of a
  higher frequency signal. It means changing the
  digital signal to analog for transmission and
  modulation allows us to use a bandpass channel
  a channel with a bandwith that does not start
  with zero. See Figure 1.3.3 (a)

Figure 1.3.3 (a) Bandwidth of a bandpass channel
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• Data Rate Limit
• Broadband Transmission

Figure 1.3.3 (b) Modulation of a digital signal
for a transmission on
a bandpass channel
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Bandwidth of the human voice

• Figure 1.4.1(a) shows the sound power a human
  vocal system can produce at various frequencies.
• The power of human sounds at lower frequencies,
  or the base pitches, is much higher than the
  power at higher frequencies.
• The human frequency range is from near zero to
  over 12,000 Hz i.e. bandwidth of over 12,000Hz.
• Modern stereo equipment can reproduce most of
  this range

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Figure 1.4.1(a) Bandwidth of the human voice.
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• Telephone System Bandwidth
• Due to technology limitations and cost tradeoffs
  the public telephone system can handle only a
  small part of the total bandwidth of the human
  voice.
• The system provides coverage for the portion of
  the voice bandwidth that can produce the greatest
  power.
• The range is only from 300 to 3,300Hz which is
  sufficient to convey messages to distant
  listener.

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• Figure 1.4.1(b) shows frequency range vendors
  use to convey data communications through the
  telephone system

Figure 1.4.1 (b) Telephone signal amplitude
versus frequency.
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• Digital Channel Capacity
• The number of digital values the channel can
  convey in one second.
• A digital communications channel has limitations
  that determine how often the signal can change
  states over a period.
• These limitations establish the maximum rate at
  which data can flow through the channel.

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• A variety of baseband signaling techniques are
  used to convey information or data.
• Digital systems may have more than two discrete
  changes as shown in Figure 1.5.1.
• A binary system has only two discrete energy
  levels
• A digital system can have many discrete energy
  levels.

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Figure 1.5.1 Digital system electrical signals.
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Figure 1.5.2 Digital signal distortion and
attenuation with distance
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• Bandwidth versus length characteristics as shown
  in Figure 1.5.2 can be used to determine the
  length of channel they want to use for specific
  applications.
• high-volume application requires a high bandwidth
  such as a direct connection between two mainframe
  computers, a vendor can limit the length of the
  communications channel to a short distance.
• A low-volume application such as the connection
  between a personal computer and a low speed
  printer, the vendor can specify a longer channel.

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Figure 1.5.3 Medium bandwidth versus length
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• Digital Signal Regeneration
• Provide devices that regenerate a digital signal.
  
• Repeaters receive the signal and rebuild it to
  its original strength and shape.
• The repeater catches the signal before it
  degrades to the point that it is unusable.
• Digital signal cannot be amplified to increase
  their distance range in a channel.

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• If you amplify a digital signal, you also amplify
  the noise that contaminated the signal.
• The amplified noise can become a substantial part
  of the signal.
• A repeater removes the noise from a signal while
  it is regenerating the signal.

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Figure 1.5.4 Digital signal regeneration.