Chapter 7: Communications

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After finishing this chapter you are responsible for being able to describe, define, and compare these concepts:

 Communications Systems

Engineering Problem Goal: Communicate a text message from one place to another.

  1. Trade-offs for any system: Time vs. Energy
                                >     Speed : How soon will it get there?
                                >     Accuracy ; Will it arrive error free? How much work must be done to fix errors?
                                > C    ost : Hardware and coding issues translate into both time and energy

  1. General Knowledge Required:
    a. What is a communication system?
                    >     A message: any difference that makes a difference!
                    > Transmitters :     Translator (creator of signals) + Emitter
                    >     Channels : The physical link (air, radio, light pulses in fiber optics)
                    > Switches: temporary connections
                    >     Receivers : Collect & Decode

    b. Translating information into bits, and then into waveforms
                    >     What is a signal modulation?

    c. What are some different kinds of signal modulation choices? What are their respective strengths and weaknesses compared to each other?
                   > sine waves
                   > rectangular (square) pulses & on-off keying
                   > complex (multiplexed) signals

    d. How information travels using sound, light (radio, microwaves, IR, & visible) waves, or electrical impulses in wires. The medium chosen is called the propagation channel.

    e. Recovering bits from waveforms & binary codes at the receivers

    f.     What is bandwidth? What is baseband frequency?

  1. Math Required:
    a. Sinusoids : Amplitude & Frequency
    b. Binary Representation of Information
    c. Serial vs. Parallel Communication

  1. VAB Experiments
    a. Air Modem : Transmitter
    b. Air Modem : Receiver

7.1.4: History of Communications

  1. Wired : Telegraph, Conventional Telephone, Optical Fiber

  2. Wireless : Radio, Broadcast TV, Satellite, Cellular Telephone

7.2.2: Air Modem System Operation

  1. Analog Communications
    a. Each letter gets a different frequency
                            > s(t) = a cos (2p f(t + t1)) with each letter mapped to a frequency
    b.     Transmitter plays waveform for some time period
    c. Receiver listens for tones

7.2.2: Receiver Structure

  1. How is the frequency of the tone determined? What logical steps is the receiving worksheet undertaking to map out tones to an ASCII symbol?

7.2.3: Key Concepts in Communication Systems

  1. There must be M different signals to send messages with an M-symbol alphabet

  2. Each signal must be sufficiently different so that it can be recognized

  3. Designer has a lot of freedom to define what different means

7.3.1: Communication Errors

  1. How can communication systems fail? Know these and how filters and other techniques can overcome them:
    a. Weak Signal: Transmitter & receiver are too far away
    b. Noise: Additive random (usually) background signal
                                > How can you overcome a noisy propagation channel?
    c. Interference: Two or more ongoing conversations
                                >     How can you overcome interference?
    d. Blockage: absorption of the signal's energy in the channel

7.3.2: Coordination In Communication

  1. Communication requires a pre-arranged agreement on how signals are constructed
    a. Tone frequencies
    b. Length of tone bursts
    c. Spacing between bursts
    d. Nearly infinite design possibilities

7.4.1: Improving Communication Systems

  1. Design Choices Affect Many Factors
    a. Implementation cost
    b. Transmission speed
    c. Accuracy & reliability
    d. How can we trade-off these factors?

7.4.2: Binary Coding & Parallel Binary Method

  1. Use more than one tone per character
    a. P tones = 2P–1 combinations possible
    b. Select P so that (# of combinations) > (# of characters required in any message)
                        > Example : 5 tones = 25 -1 = 31 possible combinations which is greater than
                           26 letters + a spacebar

  2. Perform a Fast Fourier Transform to identify the multiplexed (combined) tones in the tone burst.
    a. Depending on combination of present or absent tones, use a lookup table to output the matching character

7.4.3: Serial Binary Method or Frequency Shift Keying (FSK)

  1. Use separate, sequential "tone bursts" to represent chunks of binary code.
    a. The sequence here will be critical since only 2 frequencies are required:
                        > Frequency 1= "1" = Fone (say, 770 Hz)
                        > Frequency 2 = "0" = Fzero (say, 1440 Hz)
    b. Ex: To transmit 26 letters + a spacebar, a 5 bit code is needed
                           >  To transmit the code "10110" would mean a tone burst pattern of 770-1440-770-770-1440 Hz.
                        > Such a code might mean the letter "F"

7.4.4: Design Tradeoffs

  1. Comparison of Parallel and Serial Binary Representations (see book p. 28 of Ch. 7)
    a. Implementation Cost (# of filters)
    b. Relative transmission Speed
    c. Complexity of a signal (ease of detection)
    d. Accuracy (how robust or reliable is it)

  2.  Distinguish between serial and parallel communications

7.4.5: Dual Tone Method: the Touch-Tone Telephone

  1. Engineering advantages of two distinct tones for each of 10 numbers, * & #
    a. seven tones required: less than one-tone-per-character
    b. selective: two distinct tones must be identified for each character = a built-in double-check of the data

7.5.1: Other codes: Morse, ASCII, & Unicodes

  1. Why was it the purpose of these codes? What problem(s) do they each attempt to solve? How are they interpreted by web browers?

7.5.2: Binary Data Streams [includes "Communications Primer" material]

  1. Baseband Binary Encoding
    a. On-off Keying
    b. Visualizing Binary Communications
    c.  Protocols
    d. Message Framing
                             > start bits, message length, message, stop bits
     

  2. Clock Synchronization
    a.  What is Manchester encoding so useful?
     

  3. Challenges of transmitting large amounts of digital data:
    a. Making the coding more efficient (pack more into less space)
                            > Ex: assigning binary data groups (say, unique 5 bit chunks) to a unique frequency
                            > Ex: Using the serial binary method (FSK) as electrical square waves in wired systems
                            > Ex: FSK can also use the presence or absence of light at a particular frequency in fiber optic
                                        systems.

7.6: Other Transmission Channels

  1. EM Spectrum: IR remotes, microwaves, visible light in optical fibers (more in Chapter 8 on these)
 

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