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Modulation & More

Modulation & More. Signal Propagation. Electric current cannot be transmitted on copper over long distances because the signal gets weaker as it travels over a distance.

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Modulation & More

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  1. Modulation & More

  2. Signal Propagation • Electric current cannot be transmitted on copper over long distances because the signal gets weaker as it travels over a distance. • This degradation in the signal is referred to as signal loss. This loss is due to the internal resistance of the copper wire.

  3. Signal Propagation (cont.) • However, an electrical signal which oscillates at a fixed frequency will be able to propagate farther than a non-oscillating signal. • Signals can be modulated in varying ways: amplitude modulation (AM), frequency modulation (FM), and phase-shift modulation.

  4. Creation of an AM Signal • Here is a continually oscillating signal. This is referred to as the carrier signal. • Here is the bit pattern we wish to send on the carrier signal. • The resultant signal after amplitude modulation. The carrier is reduced to 2/3 to encode a 1, and to 1/3 to encode a 0.

  5. Creation of a Phase-Modulated Signal • Here is the carrier signal. • The bit pattern we wish to send on the carrier signal. • The resultant phase-modulated signal.

  6. Pros: Simple to modulate Simple to demodulate. Easy to “see” what’s happening with the waveform. OK for audio transmission. Cons: Bit encoding requires at least one full cycle of the carrier signal. Pros/Cons of AM

  7. Pros: Simple to modulate. Simple to demodulate. Less audible distortion than AM – finer tuning capabilities. Pretty good for audio transmission. Cons: But encoding requires at least one full cycle of the carrier signal. Pros/Cons of FM

  8. Pros: Very fast! Multiple bit encoding possible in one cycle of the carrier signal. Great for data transmission! Cons: Difficult to “see” what’s happening. Not useful for audio transmission. Pros/Cons of Phase-Shift Mod.

  9. Modems • Modem is really an acronym. It means modulator/demodulator. • Why do we use modems? We can encode more data in a modulated signal than in an unmodulated signal. Plus, a modulated signal can propagate longer distances.

  10. Modems (cont.) • Here is a logical connection between two modems. The middle “stuff” can be a variety of connection types.

  11. The “Middle Stuff” • In the previous example, the modems connected via a leased line. • A leased line is a dedicated connection which does not transit the telephone company’s circuit-switched network. It can be considered a point-to-point circuit between two locations.

  12. The “Middle Stuff” (cont.) • We’re not going to discuss it now, but the “middle stuff” can consist of a Plain-Old-Telephone-Service (POTS), ISDN, DSL, Frame Relay, etc. • Lots of stuff, right? It’ll all come in time.

  13. A Dialup Connection • Here, the “middle stuff” is a POTS network.

  14. An Important Note • Computers don’t necessarily care what medium the modems communicate on. That is left up to the modem hardware. • Computers usually communicate via RS-232 to the modems (both are serial devices, remember). • All the computer has to know is how to send and receive bits on the serial port. The modem does the rest.

  15. Modulation & Multiplexing • Multiple signals can be transmitted on a single medium if the carrier signals use noticeably different frequencies. • One usage of multiplexing is in cable TV reception. All channel frequencies are sent on the cable, but the descrambler (demux) only interprets one at a time. • Three types of multiplexing exist: • Frequency Division Multiplexing (FDM) • Wave Division Multiplexing (WDM) • Time Division Multiplexing (TDM)

  16. FDM • FDM is used when multiple, independent signals traverse a single medium. • FDM can be used with a copper or fiber medium.

  17. FDM (cont.) • FDM allows for real simultaneous communication over a shared medium. • FDM is quite effective as long as each carrier frequency is not: • A multiple of another carrier frequency or • Too close to another

  18. WDM • Wave Division Multiplexing (WDM) is really FDM, since a wave is a fixed frequency. • WDM is used when discussing optical circuits. • Optical FDM uses multiple frequencies of light (colors, if you will) for communication.

  19. TDM • Time Division Multiplexing (TDM) is a strict alternation form of multiplexing. • Each source/receiver pair accesses the shared medium for some small time, and then releases it. This continues in a round-robin fashion. • ASIDE: TDM was heavily used in early operating systems (UNIX), but it was called “timesharing.”

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