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CHAPTER No. 6

LONG DISTANCE

COMMUNICATION

(Carriers, Modulation and Modems)

Long Distance Communication

 Electric current becomes weaker as it travels over copper

 Signal loss occurs when RS232 is attempted

to use to communicate to a remote site

 A continuous, oscillating signal, called a

Carrier, propagates farther than other signals

Types of Modulation

 Three of the modulation schemes are:

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

Modem

 A Modulator is a hardware circuit that

• accepts a sequence of data bits
• applies modulation to a carrier wave according to the bits

 A Demodulator is a hardware circuit that

• accepts a modulated carrier wave and
• recreates the sequence of data bits

 A Modem is a hardware that combines both

modulator and demodulator in a single device

Multiplexing

 Q: How can two or more signals can be

transmitted simultaneously over a single wire?

 A: By using different carrier frequencies

Career Frequencies And

Multiplexing

 Frequency division multiplexing (FDM)

• Using multiple carrier frequencies to allow independent signals, to travel through a medium

Time Division Multiplexing (TDM)

 Sources sharing a medium “take turns” to send data

 Synchronous TDM gives each source an opportunity to send data

 Statistical TDM does not give chance to a source that does not have data to send