1 ITGN 235: Principles of Networking ITGN 225: Networking Fall 2007/2008

2 Chapter 6 Long Distance Communication

3 Topics Covered 6.1 Introduction 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

4 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

5 Business Information Types of information –voice –data –image –video Information sources can produce information in digital or analog form

6 Electromagnetic Signals Function of time –Analog –Analog (varies smoothly over time) –Digital –Digital (constant level over time, followed by a change to another level) Function of frequency –Spectrum (range of frequencies) –Bandwidth (width of the spectrum)

7 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

8 Transmission Basics Figure 4-1: Example of an analog signal Frequency = 1Hz.

9 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

10 Transmission Basics Phase –Refers to progress of a wave over time in relationship to a fixed point Figure 4-2: Phase differences

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

12 Analog Signals

13 Digital Signals

14 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

15 Periodic Signals

16 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

17 Varying Sine Waves s(t) = A sin(2  ft +  )

18 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*10 8 ms -1 (speed of light in free space)

19 Analog Data Choices

20 Digital Data Choices

21 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

22 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

23 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

24 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

25 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 connectiontelephone linesPOTS

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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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

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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31 Figure 1.8 Modes of transmission: (b) modulated transmission

32 Amplitude Modulation

33 Frequency Modulation

34 Phase Modulation

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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37 Signal Transmission Two techniques are used: –Baseband Transmission –Broadband Transmission

38 Baseband Systems uses digital signaling over a single frequency entire communication channel capacity is used to transmit a single data signal devices transmit bidirectionally

39 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

40 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.

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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43 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. Video Voice Channel 2 Voice Video Channel 13

44 Frequency Division Multiplexing

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

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

47 S I B U G W X Y Z G4Z G4ZG4Z U Y UYUY B3X B3XB3X I2 I2I2 S1W S1WS1W Time Division Multiplexing Used in digital transmission STDM

bps 2400 bps Time Division Multiplexing Requires data rate of the medium to exceed data rate of signals to be transmitted

49 S I B U G W X Y Z G4Z G4ZG4Z XUY U XY 2B3 B 2 3 1WI I 1 W S S Statistical Time Division Multiplexing

bps 8400 bps 9600 bps Statistical Time Division Multiplexing data rate capacity required is well below the sum of connected capacity