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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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signal propagation
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.
signal propagation cont
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.
creation of an am signal
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.
creation of a phase modulated signal
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.
pros cons of am
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
pros cons of fm
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
pros cons of phase shift mod
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.
modems
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.
modems cont
Modems (cont.)
  • Here is a logical connection between two modems. The middle “stuff” can be a variety of connection types.
the middle stuff
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.
the middle stuff cont
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.
a dialup connection
A Dialup Connection
  • Here, the “middle stuff” is a POTS network.
an important note
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.
modulation multiplexing
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)
slide16
FDM
  • FDM is used when multiple, independent signals traverse a single medium.
  • FDM can be used with a copper or fiber medium.
fdm cont
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
slide18
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.
slide19
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.”