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WXET1143

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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WXET1143

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  1. WXET1143 Lecture2: Basic Communication

  2. 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

  3. 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

  4. 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

  5. Modem1 Modem2 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).

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 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.

  12. 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.

  13. 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

  14. 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.

  15. 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 – • 1 3 5 9

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