# Chapter 6 Long-Distance Communication  Property of long-distance transmission - a continuous, oscillating carrier signal (fig 6.1) will propagate farther.

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Chapter 6 Long-Distance Communication  Property of long-distance transmission - a continuous, oscillating carrier signal (fig 6.1) will propagate farther than other signals(fig 6.1)

Modulation – modification of a carrier signal. – modulator (transmitter) is a hardware circuit that modifies a continuous sine wave (carrier) according to a sequence of the data bits being sent. – demodulator(Receiver) is a hardware circuit that reads an incoming modulated carrier wave and reconstructs the original sequence of data bits that was used to modulate the carrier signal. The carrier signal is discarded

Modulation techniques  Amplitude modulation (fig 6.2)(fig 6.2)  Frequency modulation  Phase shift modulation (fig 6.3)(fig 6.3)

Dialup analog modem  used to connect two remote computers via ordinary phone lines or leased line (fig 6.4)(fig 6.4)  Mimics the telephone’s lifting of handset, dialing, and hanging up of phone (fig 6.5)(fig 6.5)  Uses an audible carrier wave.  Achieve full-duplex transmission on 2 wire by using different carrier frequency in each direction or via time-multiplexing.

Carrier Frequency  Two more signals that use different carrier frequencies can be transmitted over the same medium simultaneously without interference. -Broadcast and Cable television transmission. Each TV channel uses a unique carrier frequency for broadcast.  To receive any channel, TV receiver must tune to the carrier frequency of the transmitter of that particular channel.

Multiplexing  Mixing multiple signals over a shared medium (fig 6.6)(fig 6.6)  Frequency Division Multiplexing(FDM) -used by Broadcast and CableTV - Wave Division Multiplexing (WDM)  Time Division Multiplexing (TDM) - senders take turn sending data (ANIM05_1.MOV) (ANIM05_1.MOV) - round robin(synchronous): guarantees fairness by sending small amounts from each source  eg. A leased T1 line (1.544 mbps) is dedicated 4- wire serial data circuit (1 pair for TX, 1 pair for Rx) connected by CSU/DSU at each end. - Statistical(asynchronous)eg. Ethernet

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