C3

• Data

  Entities that convey meaning within a computer. Example: A computer file names, address stored in hard disk
• Signals

  Electric or electromagnetic impulses used to encode and transmit data

  
• Signals

  • Transmission of a telephone conversion over a telephone line
  • Live television news interview transmitted over a satellite
  • Transmission of a term paper over the printer cable between printer and computer
  • Downloading of a webpage as it transfers over the wifi connection between your ISP and your home computer
• Noise

  Unwanted electrical or electromagnetic energy that degrades the quality of signals
• Analog waveform

  • Difficult to separate noise from original signals

  
• Digital waveform

  • Ability to separate noise from the signal is one of the great strengths

  
• Amplitude


  Height of the wave above or below a given reference point as the signal's strength or power of wave

  
• Frequency


  Number of times a signal makes a complete cycle within a given time frame

  
• Period


  Length, time intervals of one cycle
• Phase


  Position of a wave form relative to a given moment of time, or relative to time zero that indicate a change in type of bits being transmitted

  
• Data to signals conversion

  1. Analog data to analog signals: amplitude modulation and frequency modulation techniques

  2. Digital data to analog signals: modulation techniques

  3. Digital data to digital signals: encoding techniques

  4. Analog data to digital signals: digitalization techniques

  
• Modulation


  Process of modifying or changing a carrier signal in order to encode information for transmission

  
• Types of modulation methods


  • Analog-to-analog conversion: Amplitude modulation (AM), Frequency modulation (FM), Phase modulation (PM)
  • Digital-to-analog conversion: Amplitude-shift keying (ASK), Frequency-shift keying (FSK), Phase-shift keying (PSK)
  • Analog-to-digital conversion: Pulse amplitude modulation (PAM), Pulse code modulation (PCM)
  • Digital-to-digital conversion: Unipolar encoding, Polar encoding: Non-return-to zero (NRZ) encoding, Bipolar encoding

  
• Amplitude Modulation (AM)


  Changes the height of the signal

  
• Amplitude Modulation (AM)

  • Highly influenced by noise and interference from external sources like thunderstorm
  • May cause changes to the data transmitted (errors occur in the data)
  • Requires error correction method

  
• Frequency Modulation (FM)


  Changes the number of waves in the signal

  
• Frequency Modulation (FM)


  • Less influenced by external sources (would not be interrupted by a passing thunderstorm)
  • Less sensitive to errors
  • Can reduce the need for error correction

  
• Phase Modulation (PM)


  Changes the direction of the signal wave

  
• Phase Modulation (PM)


  • The amplitude and frequency of the signal are the same
  • Phase shift refers to the change in the type of bits being transmitted

  
• Amplitude Shift Keying (ASK)


  The strength of the carrier signal is different to represent binary 1 or 0
• Amplitude Shift Keying (ASK)


  • Highly susceptible/sensitive to noise interference
  • Noise may be caused by heat/electromagnetic
• Frequency Shift Keying (FSK)


  The frequency of the carrier signal is different to represent binary 1 or 0

  
• Frequency Shift Keying (FSK)


  • The amplitude and phase are the same
  • Used for high-frequency (4 to 30 MHz) radio transmission and on LANs using coaxial cable

  
• Phase Shift Keying (PSK)


  The phase of the carrier signal is different to indicate the binary 1 or 0
• Phase Shift Keying (PSK)


  • The amplitude and frequency are the same
  • The phase of the signal for each bit during a duration is the same

  
• Pulse Amplitude Modulation (PAM)


  Based on the height (amplitude) of the electrical pulse to identify the signal to be transmitted

  
• Pulse Amplitude Modulation (PAM)

  • Takes an analog signal, samples it and generates a series of pulses (after the sampling)
  • Sampling - measures the amplitude at equal intervals
  • Not useful in data communication because the signal after conversion is still in analog, need to use PCM to make it digital

  
• Pulse Code Modulation (PCM)

  To create a completely digital signal

  
• Pulse Code Modulation (PCM)


  1. PAM encoding - to get sample signal

  2. Quantization technique - to get an integer value (0 - 255) to the samples

  3. Binary encoding - convert integer value to the binary value in 7 bits + 1 bit (in front) for sign

  4. Digital encoding - use any

  
• Phase of signal for each bit during a duration

  The value depends on the bit (0 or 1)

  
• Phase Shift Keying (PSK)


  Digital-to-analog conversion based on the height (amplitude) of the electrical pulse to identify the signal to be transmitted

  
• Line coding


  • The process of converting digital data to digital signals
  • Converts a sequence of bits to a digital signal
  • At the sender side digital data are encoded into a digital signal and at the receiver side the digital data are recreated by decoding the digital signal

  
• Unipolar encoding


  • Very simple and very primitive
  • Uses only 1 voltage level - positive
  • Presence of voltage pulse - indicate a Binary bit 1 - positive
  • Absence of voltage pulse - indicate a Binary bit 0 - zero
  • The value 1 is encoded as Positive polarity and 0 is encoded as zero voltage
  • It is very inexpensive to implement
• Polar encoding


  • Uses 2 voltage levels, both positive and negative levels
  • Two types - (non-return zero encoding) - NRZ- L (level) & NRZ - I (Invert)
  • NRZ-L: Binary bit 0 - Positive, Binary bit 1 - Negative
  • NRZ-I: Binary bit 0 - no transition, Binary bit 1 - there is a transition - e.g. It will convert positive to negative and negative to positive
• Bipolar encoding


  • Two types: AMI (Alternate Mark Inversion) & Pseudoternary
  • AMI: Binary 0 indicated by zero voltage, Binary 1 indicated by alternating positive and negative voltage
  • Pseudoternary: Opposite of AMI, Binary 0 indicated by alternating positive and negative voltage, Binary 1 indicated by zero voltage
• Simplex transmission


  • The communication is unidirectional, as on a one-way street
  • Only one of the two devices on a link can transmit; the other can only receive
• Half-duplex transmission


  • Each station can both transmit and receive, but not at the same time
  • When one device is sending, the other can only receive, and vice versa
• Full-duplex transmission


  • Both stations can transmit and receive simultaneously
  • The capacity of the channel must be divided between the two directions
• Serial transmission


  • Transmit data one bit at a time
  • Use one communication line to transmit entire character 1 bit at a time
  • When the entire character is breaking into bits to send one, the receiver need to reassemble the bits into a proper character
• Parallel transmission


  • Transmit data one character (8 bits - 1 bit per line) at a time - faster
  • Uses multiple line to transmit the entire character at a time
  • If a character = 8 bits, need 8 lines to transmit 8 bits (each line = 1 bit), where the 8 lines will accommodate to a single line - expensive