Transcript Chapter 1 - Introduction

Computer Networks and Internets with
Internet Applications, 4e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 6
Long Distance Communication
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Topics Covered
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6.1 Introduction
6.2 Sending Signals Across Long Distances
6.3 Modem Hardware Used For Modulation/Demodulation
6.4 Leased Analog Data Circuits
6.5 Optical, Radio Frequency, And Dialup Modems
6.6 Carrier Frequencies And Multiplexing
6.7 Baseband And Broadband Technologies
6.8 Wavelength Division Multiplexing
6.9 Spread Spectrum
6.10 Time Division Multiplexing
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6.1 Introduction
This chapter
• explains why the same scheme does not work for long distances
• describes the hardware needed for long-distance communication
• describes the motivation for using a continuous carrier
• discusses how a carrier can be used to send data
• identifies the purpose of modem hardware
• shows how modems are used for long-distance communication.
• discusses point-to-point digital circuits and how they are used
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6.2 Sending Signals Across Long
Distances (1)
• Electric current cannot be propagated an arbitrary
distance over copper wire
– because the current becomes weaker as it travels
– resistance in the wire causes small amounts of the electrical
energy to be converted to heat
• An interesting property of long-distance transmission
– a continuous, oscillating signal will propagate farther
• long-distance communication systems send a
continuously oscillating signal
– usually a sine wave, called a carrier
• Figure 6.1 illustrates a carrier waveform
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6.2 Sending Signals Across Long
Distances (2)
• To send data, a transmitter modifies the carrier slightly
– Collectively, such modifications are called modulation
• Whether they transmit over wires, optical fibers, MW, or
RF, most long-distance NW
– The transmitter generates a continuously oscillating carrier
signal
• which it modulates according to the data being sent
– The receiver on a long-distance link must be configured to
recognize the carrier that the sender uses
• The receiver
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monitors the incoming carrier
detects modulation
reconstructs the original data
and discards the carrier
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6.2 Sending Signals Across Long
Distances (3)
• Network technologies use a variety of schemes:
– Amplitude modulation (AM, ASK)
• varies the strength/amplitude of the outgoing signal in proportion to
the information being sent
– Frequency modulation (FM, FSK)
• varies the frequency of the outgoing signal
– Phase modulation (PM, PSK)
• varies the phase of the outgoing signal
• Figure 6.2 illustrates how a bit might be encoded
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6.2 Sending Signals Across Long
Distances (4)
• The Nyquist theorem suggests that the rate can be
increased
– if the encoding scheme permits multiple bits to be encoded in a
single cycle
• Figure 6.3 shows a PSK waveform
• HW can measure the amount of shift
– each phase shift can encode more than one bit of data
– sender takes the value of a group of bits
• to determine how much to shift
• The chief advantage of mechanisms like PSK arises
– from their ability to encode more than one bit value at a given
change
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6.3 Modem Hardware Used For Modulation
And Demodulation (1)
• HW that accepts a sequence of data bits and applies
modulation to a carrier wave according to the bits
– called a modulator
• HW that accepts a modulated carrier wave and recreates
the sequence of data bits that was used to modulate the
carrier
– called a demodulator
• To support such full duplex communication,
– each location needs both a modulator and a demodulator
– manufacturers combine both circuits into a single device
• called a modem ( modulator and demodulator).
• Figure 6.4 illustrates how a pair of modems
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6.4 Leased Analog Data Circuits
• Private companies cannot install circuits across long distances
• Government regulations only allow utility companies to run wires
across public property
• Telephone companies allow companies to lease a circuit between
any two locations
– A leased circuit usually consists of four wires
– Bits travel across such circuits one at a time,
– we use the terms serial data circuit, serial line or leased serial line
• The chief advantage of such an arrangement arises from its
constant availability
• The chief disadvantages arise from the limited connectivity and cost
– the leased line only connects two points
– pay the monthly fee even if the line is not being used to send data
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6.5 Optical, Radio Frequency, And Dialup
Modems (1)
• In addition to dedicated/leased wires, also others, such
as:
– RF transmission (RF modem)
• especially attractive because of the increased interest in wireless
– glass fibers (optical modem)
– conventional telephone dail-up connections (dial-up modem)
• As Figure 6.5 illustrates, a dialup modem
• 2-wire dialup modems differ from 4-wire leased-modems:
– A dialup modem contains circuitry that mimics a telephone
– A dialup modem uses a carrier that is an audible tone
– A pair of dialup modems can offer full duplex communication:
• Modems must use different carrier tones or coordinate to avoid
having both modems transmit at the same time.
• Modems that coordinate sending data are called half-duplex
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6.5 Optical, Radio Frequency, And Dialup
Modems (2)
• As an alternative to standard dialup modems,
– A V.90 or V.92 modem uses an asymmetric scheme:
• An ISP has a digital (ISDN) connection
• A subscriber has a standard analog connection
• the asymmetry makes it possible for the downstream path to have
higher throughput
• An application that uses a modem
– might be able to deduce the underlying media by measuring the
delay and bandwidth of the channel
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6.6 Carrier Frequencies And Multiplexing
• Two or more signals that use different carrier frequencies over a
single medium simultaneously without interference
– A receiver configured to accept a carrier at a given frequency will not be
affected by signals sent at other frequencies
– Multiple carriers can pass over the same wire at the same time without
interference
• Frequency division multiplexing (FDM) technology can be used
– when sending signals over copper wire, RF, or fiber optics
• Figure 6.6 illustrates the concept
• Large gaps between the carrier frequencies needed
– underlying HW must tolerate a wide range of frequencies
– consequently, FDM is only used on high-BW systems
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6.7 Baseband And Broadband Technologies
• The primary motivation for using FDM
– desire for high throughput
• When HW uses a larger part of the EM spectrum:
– it is called broadband
• Alternative technology uses a small part of the EM
spectrum and sends only one signal at a time
– baseband
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6.8 Wavelength Division Multiplexing
• The concept of FDM can be applied to optical medium
• Optical FDM
– is known as wavelength division multiplexing wave (WDM)
• When many wavelengths are used,
– the term is Dense Wavelength Division Multiplexing (DWDM)
– carriers can be mixed onto a single medium
– at the receiving end, an optical prism is used to separate them
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6.9 Spread Spectrum (SS)
• SS is used for a variety of reasons:
– One chief reason for using SS is to improve reliability
– If the transmitter and receiver are close to sources of EM
interference or if large objects move around in the area between
them
• the optimum carrier frequency may vary over time
• at a given time, one carrier frequency may work while others do not
• It works as follows
– A transmitter to send the same signal on a set of carrier freq.
– A receiver is configured to check all carrier freq. and to use
whichever is working at present
– If interference damages one or more of the carriers, the modem
can extract the data from the others
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6.10 Time Division Multiplexing
• The general alternative to FDM is
– time division multiplexing (TDM)
• In TDM sources share a medium by ``taking turns''
• There are two types of TDM:
– Synchronous Time Division Multiplexing (STDM)
• arranges for sources to proceed in a round-robin manner
• also known as Slotted Time Division Multiplexing
– Statistical Multiplexing
• Works similar to STDM, but if a given source does not have data to
send, the multiplexor skips that source
• Most NW use a form of statistical multiplexing because computers
do not all generate data at exactly the same rate
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