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WXET1143. Lecture2: Basic Communication. Communication using electricity. Since electricity was discovered, scientist have researched on ways to use the electrical signal for communication. Digital communication historical stages: Properties of signals on wires

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Lecture2: Basic Communication

communication using electricity
Communication using electricity
  • Since electricity was discovered, scientist have researched on ways to use the electrical signal for communication.
  • Digital communication historical stages:
    • Properties of signals on wires
    • Sending bits and organizing them
    • Error detection and correction
signal on wires
Signal on wires
  • Electrical signal reflects from the end of the metal wire the same way light reflects from mirror – requires terminator device
  • It loses energy as it passes along a wire – length of interconnecting limited or use amplifier
  • Electrical signal in a wire emits electromagnetic radiation that can interfere with signals in nearby wires – high speed network uses cable that encloses the wire in a metal shield
information coding
Information coding
  • When properties of signals understood, encoding of the information are studied.
  • Modulation
    • Transmits voice
    • Uses a modulator
      • Basic electrical signal oscillating back and forth (carrier)
      • Second signal (generated by human voice) to change the carrier signal
    • Demodulator
      • Reverses the function
modem modulator demodulator



Transmission line

Computer A

Computer B

MODEM: Modulator-Demodulator
  • Principal of modulation still in use in modern communication systems
  • Modem contains both modulator (to send info) and a demodulator (for arriving info).
modem modulator demodulator1
MODEM: Modulator-Demodulator
  • Illustration of modem that use modulation to send data across a transmission line.
  • When a computer interacts with a modem, it send and receives digital data, the modem encodes the data for transmission.
two way traffic
Two-way traffic
  • Modem permits data to be sent between them in both directions.
  • Modems either use two carrier signals or agree to take turns sending data.
  • In either case, data appears to flow in both directions simultaneously.
two way traffic1
Two-way traffic
  • In short:
    • A modem is a device needed for communication across a dial-up telephone connection on for long distance communication across a wire.
    • A modem supports two-way communication because it contains a modulator for the signal being sent and a demodulator for the signal being received.
character code
Character code
  • Researchers also studied transmission of digital information.
  • Found ways to encode basic values of bit in an electrical signal.
    • +ive voltage to encode 1
    • -ive voltage to encode 0
  • Devised a sequence of bits to represent each letter and digit.
character code1
Character code
  • This type of encoding is similar to Morse Code.
  • But each character is assigned a code with the same number of bits.
    • A sequence of seven 0’s and 1’s to each letter
  • ASCII – American Standard Code for Information Interchange
character code2
Character code
  • To summarize
    • Many networks use the ASCII code when sending textual information in digital form.
    • ACII assigns a 7-bit code to each letter and digit.
    • Most users never see ASCII because it is an internal detail that remains hidden.
error detection
Error detection
  • Natural phenomena like lighting can cause random electrical signals that distorts the carrier signal.
  • Also when wire carrying the signal passes through a string magnetic field.
  • Even a bit of change is crucial when passing digital information.
error detection1
Error detection
  • Remember:
    • When electrical signals to communicate digital information, electrical or magnetic interference can cause the value of one or more bits to be changed.
  • To detect and correct errors they added an extra bit to the character’s code.
    • Add 1 if code has odd number of 1’s
    • Add 0 if code has odd number of 0’s
    • This is called a parity bit
error detection2
Error detection
  • Example:
    • Letter E – 1000101 (parity bit=1)
    • Letter S – 1010011 (parity bit=0)
  • To make it work, receiver must test parity of each incoming character.
  • Examines all and declares error if wrong number of bits turned on.
  • But does not guarantee that all problems will be detected.
error detection3
Error detection
  • The point is:
    • Adding a parity bit to each character code before transmission can help the hardware detect errors that occur when transmitting the character across a network.
    • However, parity alone is not sufficient to detect all possible errors.
  • Checksum –
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