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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.
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.)
However, an electrical signal which
oscillates at a fixed frequency will be
able to propagate farther than a nonoscillating signal.
Signals can be modulated in varying
ways: amplitude modulation (AM),
frequency modulation (FM), and phaseshift modulation.
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
Here is the carrier
signal.
The bit pattern we
wish to send on the
carrier signal.
The resultant phasemodulated 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 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 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.
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.)
Here is a logical connection between
two modems. The middle “stuff” can be
a variety of connection types.
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.)
We’re not going to discuss it now, but
the “middle stuff” can consist of a PlainOld-Telephone-Service (POTS), ISDN,
DSL, Frame Relay, etc.
Lots of stuff, right? It’ll all come in time.
A Dialup Connection
Here, the “middle stuff” is a POTS
network.
Fax
PC
Modem
Circuit-Sw itched
Telephone
Service
Modem
Mac Classic
Telephone
Telephone
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
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)
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 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
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.
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.”