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CS601 - Data Communication - Lecture Handout 28

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Transmission Impairments

TX Media are not perfect
Imperfections cause impairments in the signal through the medium
This means that the signal at the beginning and the end of the medium are not the same
What is sent is not what is received

Transmission Impairments

Attenuation

  • Loss of Energy
  • When a signal travels through a medium, it looses some of its energy in order to overcome the resistance of the medium
  • That is why wire carrying electric signals get hot
  • Some of the electrical energy in the signal is converted to heat
  • To compensate for this loss Amplifiers are used to amplify the signal to heat

Attenuation

Decibel (dB)

  • Measures the relative strength of the two signals or a signal at 2 different points
  • dB is negative if a signal is attenuated
  • dB is positive if a signal is Amplified

Relative strengths of two signals or a signal at two points

Decibel (dB)

Negative dB means attenuation

Positive dB means amplification

Example 7.1

Imagine a signal travels through a transmission medium and its power is reduced to half.

Example 7.1

Distortion

  • Distortion means that the signal changes its form or shape
  • Distortion occurs in a composite signal
  • Signal changes its form or shape
  • Occurs in a composite signal, made of different frequencies
  • Each signal component has its own speed

Distortion

NOISE

  • Thermal Noise
    • Due to random motion of electrons in a wire that creates an extra signal not originally sent by TX
  • Induced Noise
    • Comes from sources like Motors and Appliances
  • Crosstalk:
    • Effect of one wire on another
  • Impulse Noise
    • Spike (A signal with high energy in a very short period of timepower lines, lightening etc,.

Distortion1

Performance of Medium

Three oncepts are used to measure the performance of TX Media:

  • Throughput
  • Propagation Speed
  • Propagation Time

Throughput

  • Measurement of how fast data can pass through a point
  • In other words, if we consider any point in the TX Medium as a wall through which bits pass, then throughput is the number of bits that can pass this wall in second

Throughput

Propagation Speed

  • Propagation speed measures the distance a signal or a bit can travel through a medium in one second
  • The propagation speed of EM signals depend on the medium and the frequency of the signal

Propagation Time

  • Measures the time required for a signal (or a bit) to travel from one point of the TX medium to another
  • The propagation time is calculated as:

Propagation Time

WAVELENGTH

  • Another characteristic of a signal traveling through the TX Medium
  • This concept binds the frequency of the signal to the propagation speed of medium
  • It is customary to talk about wavelength when talking about TX of light in Optical Fiber

WAVELENGTH

  • The wavelength is the distance a simple signal can travel in one period
  • The wavelength depends on both the frequency and the medium
    • Wavelength = Propagation speed * period
    • Wavelength = Propagation speed / frequency

Shannon Capacity

In 1944, Claude Shannon introduced a formula to determine the theoretical highest data rate for a channel:

Shannon Capacity

Examples

  • Extremely noisy channel
    • S/N ≈ 0
    • C = B log2 (1+0) = 0
  • Telephone line
    • Bandwidth is 3000 Hz, S/N ratio is 3162 (35 dB)
    • C = 3000 log2 (1+3162) = 34,860 bps

Media Comparison

When evaluating the suitability of a particular medium to a specific application, 5 factors should be kept in mind:

  • COST: This is the cost of materials plus installation
  • SPEED: Speed is the max no. of bits per second that a medium can transmit reliably
    • Speed varies with frequency, with physical size of medium and Transmission Equipment
  • ATTENUATION: Tendency of EM signal to become weak or distorted over signal
  • EM Interference: EMI is the susceptibility of the medium to external EM energy introduced into the link that interferes with the intelligibility of a signal
    • Familiar effects of EMI are static(audio) and snow (visual)
  • SECURITY: How easy it is for an unauthorized device to listen on the link?
    • Twisted pair is intercept able
    • Optical Fiber is more secure

Summary

  • Transmission Impairments
  • Performance of Transmission Medium
  • Wavelength
  • Shannon Capacity
  • Media Comparison

Reading Sections

  • Section 7.4, 7.5, 7.6. 7.7, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan