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ITGN 235: Principles of NetworkingITGN 225: Networking

Fall 2007/2008Fall 2007/2008

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Chapter 6

Long Distance Communication

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Topics Covered

• 6.1 Introduction <Transmission Basics>• 6.2 Sending Signals Across Long Distances• 6.3 Modem Hardware Used For Modulation/Demodulation• 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 • 6.10 Time Division Multiplexing

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6.1 Introduction

This unit• 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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Business Information

• Types of information– voice– data– image– video

• Information sources can produce information in digital or analog form

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Electromagnetic Signals

• Function of time– AnalogAnalog (varies smoothly over time)– DigitalDigital (constant level over time, followed by a change to another

level)

• Function of frequency– Spectrum (range of frequencies)– Bandwidth (width of the spectrum)

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Transmission Basics

• Both analog and digital signals are generated by electrical current, pressure of which is measured in volts

• In analog signals, voltage varies continuously• In digital signals, voltage turns off and on repeatedly

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Transmission Basics

Figure 4-1: Example of an analog signal

Frequency = 1Hz.

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Transmission Basics

• Amplitude– Measure of a signal’s strength

• Frequency– Number of times a signal’s amplitude changes over a period of

time– Expressed in hertz (Hz)

• Wavelength– Distances between corresponding points on a wave’s cycle

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Transmission Basics

• Phase– Refers to progress of a wave over time in relationship to a fixed point

Figure 4-2: Phase differences

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Transmission Basics

Figure 4-3: A complex analog signal representing human speech

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Analog Signals

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Digital Signals

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Transmission Impairments

• signal received may differ from signal transmitted causing:– analog - degradation of signal quality– digital - bit errors

• most significant impairments are– attenuation and attenuation distortion– delay distortion– noise

During transmission, the information contained in analogue signals will be degraded by noise. Conversely, unless the noise exceeds a certain threshold, the information contained in digital signals will remain intact. This represents a key advantage of digital signals over analogue signals

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PeriodicSignals

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Sine Wave

• peak amplitude (A)– maximum strength of signal– volts

• frequency (f)– rate of change of signal– Hertz (Hz) or cycles per second– period = time for one repetition (T)– T = 1/f

• phase ()– relative position in time

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Varying Sine Wavess(t) = A sin(2ft +)

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Wavelength ()

• is distance occupied by one cycle• between two points of corresponding phase in two

consecutive cycles• assuming signal velocity v have = vT• or equivalently f = v especially when v=c

c = 3*108 ms-1 (speed of light in free space)

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Codec

Codec: coder and decoder

telephone

analog voice

analog line

digital line

analog voice

0 1 1 0 1 0 0 0 1 1 0

digitized voice

Analog Data Choices

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DSU

DSU: data service unit

analog line

digital line

moduated data

0 1 1 0 1 0 0 0 1 1 0

data

data

modem

Digital Data Choices

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Transmission Choices

• Analog transmission– only transmits analog signals, without regard for data content– attenuation overcome with amplifiers

• Digital transmission– transmits analog or digital signals– uses repeaters rather than amplifiers

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Advantages of Digital Transmission

• Cheaper (VLSI circuitry)• The signal is exact• Better security and privacy (encryption)• Signals can be checked for errors • Noise/interference are easily filtered out• A variety of services can be offered over one line• Higher bandwidth is possible with data compression• Digital transmission is now preferred! Ex: DSL

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Analog Encoding of Digital Data

• data encoding and decoding technique to represent data using the properties of analog waves

• modulation: the conversion of digital signals to analog form

• demodulation: the conversion of analog data signals back to digital form

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Modem

• an acronym for modulator-demodulator• uses a constant-frequency signal known as a carrier

signal• converts a series of binary voltage pulses into an analog

signal by modulating the carrier signal• the receiving modem translates the analog signal back

into digital data

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Modem

• The most familiar example is a voiceband modem that turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Serial, or Network connection

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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 farthera 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– 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 modulation 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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Figure 1.8 Modes of transmission: (b) modulated transmission

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Amplitude Modulation

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Frequency Modulation

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Phase Modulation

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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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Signal Transmission

Two techniques are used:

– Baseband Transmission

– Broadband Transmission

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Baseband Systems

• uses digital signaling over a single frequency• entire communication channel capacity is used to transmit a single

data signal• devices transmit bidirectionally

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Broadband Systems

• uses analog signaling and a range of frequencies (signals are continuous and nondiscrete)

• if the bandwidth is available, multiple transmission systems can be supported simultaneously

• signal flow is unidirectional

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Multiplexing Techniques

• Frequency Division Multiplexing– divides the bandwidth into multiple low-speed channels.

– The only analog multiplexing technique.

• Time Division Multiplexing– allocates a particular time slot on the high-speed line whether it has

something to transmit or not.

• Statistical Time Division Multiplexing (STDM)– advanced TDM/intelligent TDM

– allocates time slots only as the terminals require them.

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6.6 Frequency division multiplexing (FDM)

• 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• The primary motivation for using FDM

– desire for high throughput

• 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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Frequency Division Multiplexing Television uses FDM and transmits several TV channels from the same antenna. The TV tuner locks onto a particular frequency (channel) and filters out the video signal.

VideoVoice Channel 2

VoiceVideo Channel 13

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Frequency Division Multiplexing

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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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6.10 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• requires digital signaling & transmission• Most NW use a form of statistical multiplexing because computers

do not all generate data at exactly the same rate

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S I B U G1 2 3 4

W X Y ZG4Z

G4Z

U Y

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YB3X

B3X

I2

I2S1W

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Time Division Multiplexing

•Used in digital transmission

STDMSTDM

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9600 bps 9600 bps2400 bps

2400 bps

2400 bps

2400 bps

Time Division Multiplexing

•Requires data rate of the medium to exceed data rate of signals to be transmitted

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S I B U G1 2 3 4

W X Y ZG4Z

G4Z

XUY

U XY

2B3

B2 31WI

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S

S

Statistical Time Division Multiplexing

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9600 bps 9600 bps

9600 bps

8400 bps

9600 bps

9600 bps

Statistical Time Division Multiplexing

•data rate capacity required is well below the sum of connected capacity