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Data Transmission

Data Transmission. The basics of media, signals, bits, carries, and modems (Part II). Long-Distance Communication. Encoding used by RS-232 cannot work in all situations over long distances Electric current becomes weaker as it travels on wire

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Data Transmission

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  1. Data Transmission The basics of media, signals, bits, carries, and modems (Part II)

  2. Long-Distance Communication • Encoding used by RS-232 cannot work in all situations over long distances • Electric current becomes weaker as it travels on wire • Resulting signal loss may prevent accurate decoding of data • Signal loss prevents use of RS-232 over long distances • Different encoding strategies needed

  3. Long Distance Communication • Important fact: an oscillating signal travels farther than direct current • For long-distance communication • Send a sine wave (called a carrier wave) • Change (modulate) the carrier to encode data • Note: modulated carrier technique used for radio and television

  4. Terminology • Signal: means to transport data • A signal can be periodic or aperiodic

  5. Terminology (cont’d) • Frequency (f): rate (cycles/sec or Hz) a signal repeats. If periodic, period T = 1/f • Spectrum: range of frequencies. A signal may contain a number of frequencies. • Bandwidth: width of spectrum signal(s) spans over. E.g., traditional telephone line can carry frequencies between 300 Hz and 3300 Hz, then the bandwidth = 3000 Hz.

  6. Terminology (cont’d) • Analog signal: electromagnetic waveform (continuous) representation of (analog or digital) data (e.g., voice sound, human speech: 20 ~ 3300 Hz) • Digital signal: data are represented with sequence of (discrete) voltage pulses (i.e., bit-stream: constant positive voltage level may represent binary 0, constant negative level may represent binary 1).

  7. Illustration Of A Carrier • Carrier • Usually a sine wave • Oscillates continuously • Frequency of carrier fixed

  8. Encoding Data With A Carrier • Modifications to basic carrier encode data for transmission • Technique called modulation • Same idea as in radio, television transmission • Carrier modulation used with all types of media - copper, fiber, radio, infrared, laser

  9. Types Of Modulation • Amplitude modulation - strength, or amplitude of carrier is modulated to encode data • Frequency modulation - frequency of carrier is modulated to encode data • Phase shift modulation (used in data) - changes in timing, or phase shifts encode data

  10. Illustration Of Amplitude Modulation • Strength of signal encodes 0 or 1 • One cycle of wave needed for each bit • Data rate limited by carrier bandwidth

  11. Illustration Of Phase-Shift Modulation • Change in phase encodes K bits • Data rate higher than carrier bandwidth

  12. Phase-Shift Example • Section of wave is omitted at phase shift • Data bits determine size of omitted section

  13. Modem • Hardware device • Useful for long-distant communication • Contains separate circuitry for • Modulation of outgoing signal • Demodulation of incoming signal • Name abbreviates modulator/demodulator

  14. Illustration Of Modems Used Over A Long Distance • One modem at each end • Separate wires carry signals in each direction • Modulator on one modem connects to demodulator on other

  15. Types Of Modems • Conventional • Use four wires • Transmit modulated electrical wave • Optical • Use glass fibers • Transmit modulated light • Wireless • Use air/space • Transmit modulated RF (Radio Frequency) wave

  16. Types Of Modems (cont’d) • Dialup • Use voice telephone system • Transmit modulated audio tone

  17. Dialup Modem • Circuitry for sending data • Circuitry to mimic telephone operation • Lifting handset • Dialing • Hanging up • Detecting dial tone • Full duplex on one voice channel • Different carrier frequencies for each direction • Filters eliminate interference

  18. Operation of Dialup Modems • Receiving modem waits for call in answer mode Other modem, in call mode: • Simulates lifting handset • Listens for dial tone • Sends tones (or pulses) to dial number • Answering modem: • Detects ringing • Simulates lifting handset • Sends carrier • Calling modem: • Sends carrier • Data exchanged

  19. 56K Modems • Traditional Modems: • limitation on the data rate of 33.6 Kbps maximum, both uploading and downloading • determined by narrow bandwidth of the local telephone line with up to 4 KHz • 56K Modems: • Uploading with 33.6 Kbps • downloading with 56 Kbps

  20. Cable Modems • Cable TV provides a coaxial cable with bandwidth up to 750 MHz • With frequency division multiplexing (discussed on Part III), two channels with bandwidth of 6 MHz each can be used for data transmission • Speeds: • downloading -- 3 ~ 10 Mbps currently • uploading -- currently 500 Kbps ~ 1 Mbps

  21. Modern Technology • Full-duplex modem • Provides 2-way communication • Allows simultaneous transmission • Uses four wires • Half-duplex modem • Does provide 2-way communication • Transmits in one direction at any time • Uses two wires

  22. Reading Materials • Chapter 6: Sections 6.1-6.5

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