Digital and Analog Transmission Slides

• Learning Objectives


  •Explain the difference between an analog and a digital signal • Compare parallel and serial transmission modes• Understand how coding schemes are used to represent data (e.g., BCD, EBCDIC, ASCII, Unicode) • Contrast unipolar, bipolar and Manchester digital signaling• Explain how an analog waveform can be modulated to form a data signal• Recognize different modulation methods (frequency, amplitude, phase)• Understand the use of symbols and Differentiate between bit and baud rate• Explain how analog data may be digitized, and the cause of and ways to reduce quantizing error
• Digital vs. Analog Transmission

  •Digital transmission involves discrete binary values (i.e., 0 or 1)•Analog transmission involves waves with a continuous range of values

  
• Digital Transmission of Digital Data

  •Binary coding scheme needed for text messages◦ American Standard Code for Information Interchange (ASCII)◦ Unicode (UTF-16 used by windows)• Character represented by a group of bits◦ 7-bit ASCII uses groups of 7 bits◦ Can represent 2^7=128 characters◦ 8-bit ASCII uses groups of 8 bits (bytes)◦ Can represent 2^8=256 characters◦UTF-16 uses 2-byte words◦ Can represent 2^16=65,536 characters◦ Extensive character set includes Cyrillic, Chinese, etc.

  
• Digital Transmission of Digital Data – Transmission Modes

  
• Digital Transmission of Digital Data – Symbols

  • Sender and receiver must agree upon:◦ Set of symbols◦ How bits are encoded as voltages or light pulses◦+5V might represent “1." Symbol rate (Baud)◦ How often symbols are sent◦ Example: Symbol rate of 64 kilohertz (kHz) means one symbol is sent every 1/64,000 of a second◦ A symbol may contain multiple bits

  
• Types of Signaling Techniques (1)

  •Unipolar: voltage is 0 or positive representing binary bits (in some circuits, 0 and negative voltage could be used)

  
• Types of Signaling Techniques (2) 


  •Bipolar NRZ: voltage is positive or negative, but not zeroFewer errors than unipolar because signals are more distinct

  
• Types of Signaling Techniques (3)

  •Bipolar RZ: voltage is positive or negative, returning to zero between each bit◦Fewer synchronization errors than bipolar NRZ 

  
• Types of Signaling Techniques (4)

  •Bipolar AMI: voltage is 0, positive, or negative, returns to zero between each bit, and alternates between positive and negative voltage 

  
• Types of Signaling Techniques (5) 


  •Manchester: voltage is positive or negative and bits are indicated by a mid-bit transition•This technique is most resistant to undetected errors (Why?) Ethernet (most common LAN Technology) uses Manchester Encoding

  
• Analog Transmission of Digital Data


  Telephone system built for analog (vocal) data

  Electrical signals mimic sound waves

  Analog transmissions take on range of values (vs. discrete values of digital transmissions)

  Need a modem (modulator/demodulator) to transmit digital data with analog signal and to recover the digital data at receiver
• Analog Transmission of Digital Data (cont’d.)

  •Three characteristics of waves◦ Amplitude: height of wave (decibels)◦ Frequency: waves per second (hertz)◦ Wavelength is the inverse of frequency◦ Phase: wave direction (degrees) or the point at which the wave begins

  
• Analog Transmission of Digital Data (cont’d.)

  •Carrier wave: the basic wave transmitted through a circuit• Modulation: the modification of a carrier wave’s fundamental characteristics in order to encode information• Three ways to modulate a carrier wave:◦Amplitude Modulation (AM) or Amplitude Shift Keying (ASK)◦ Frequency Modulation (FM) or Frequency Shift Keying (FSK)◦ Phase Modulation (PM) or Phase Shift Keying (PSK)
• Symbols 


  The modulated carrier wave has encoded symbols

  Send 1 bit/interval by defining two different symbols

  Send more bits/interval by defining additional symbols

  More complex data coding schemes

  1 bit of information => 2 symbols 

  2 bits of information => 4 symbols

  3 bits of information => 8 symbols

  n bits of information => 2^n  symbols
• Modulation Illustrated

  
• Two-Bit Amplitude Modulation


  • With four levels of amplitude defined as symbols, two bits can be transmitted per symbol.

  
• Data Rate and Symbol Rate

  Data rate (or bit rate) is the number of bits transmitted per second

  Symbol rate: number of symbols transmitted per second (baud)

    Data rate = symbol rate × (# bits/symbol)  Example:

  Symbol rate = 16,000 symbols/sec

  #bits/symbol = 4 bits/symbol Data rate = 16,000 symbols/sec × 4 bits/symbol 

                     = 64,000 bits/sec = 64Kbps
• Bit Rate and Baud Rate (Symbol Rate) 


  •Bit and baud are often confused.• Bit◦A unit of information◦ Bit rate – the number of bits transmitted per second• Baud (symbol rate)◦ A unit of signaling speed◦ Baud rate – the number of times per second the signal on the communication circuit changes. Replaced by the term symbol rate to avoid confusion
• Capacity of Circuit


  •The data rate sent on a circuit is calculated in bits per second◦ number of bits × maximum symbol rate• The number of bits depends on the modulation technique.• Maximum symbol rate is typically the same as the bandwidth in hertz◦ The difference between the highest and lowest frequencies in a band, or set of frequencies
• How Modems Transmit Data 


  • The term “modem” is a portmanteau for “modulator/demodulator”• Takes digital data (electrical pulses) from computer and convert to analog signal•Several types and standards exist; for data to be transmitted, sender and receiver must use the same type of modem • Throughput date depends on:◦ Modem’s data transmission rate◦Use of data compression (e.g., Lempel-Ziv encoding)
• Digital Transmission of Analog Data

  Codecs (COde, DECode) are devices or software that convert analog signals (voice) into digital and vice-versa 

  Pulse Amplitude Modulation (PAM) converts analog to digital by:

  1. Sampling the analog signal at regular intervals 2. Measuring the amplitude of each sample

  Pulse Code Modulation (PCM)

  1. Encoding (quantizing) the amplitude as 8-bit binary data 2. Used in North American telephone networks
• Digital Transmission of Analog Data

  
• Digital Transmission of Analog Data

  • Quantizing Error is the difference between the original analog signal and the approximated, digital signal. Reducing quantizing error can be accomplished by:

  § Sampling more frequently§ Using more levels of amplitude in encoding

  
• How Voice Data is Transmitted Today

  •Telephone◦ Digital transmission except for in the last loop• Instant messaging◦ Adaptive differential pulse code modulation (ADPCM)◦ Like PCM, but transmits only how the amplitude (8-bit value) has changed since the last transmission◦ Voice over Internet Protocol (VoIP)◦ Uses the same wiring and network as computers◦ Must operate even when power fails for emergency calls (with uninterruptable power supplies (UPS) on all circuits) Common standard: G.722 wideband audio
• Implications for Cyber Security

  •Unauthorized access to physical hardware cannot be protected by firewalls, encryption, or other software measures.•Portable equipment, particularly USB drives, are particularly vulnerable.◦Hackers often place malware on USB drives.•Routers, servers, and mobile devices must also be protected. 
• Additional Optional Material (Beyond Basic Modulation) 


  •Quadrature Amplitude Modulation (QAM) is a more sophisticated modulation method used in digital TV and cable modem Internet services◦ QAM represents symbols using eight different phases (3 bits) and two different amplitudes (1 bit), for a total of 16 (24) different possible values • Watch this video for an overview of QAM when the 802.ac wireless standard came out (Meru Networks, 2012).