Computer Networks and Internets with Internet Applications 4

Computer Networks and Internets with Internet Applications, 4 e By Douglas E. Comer Lecture Power. Points By Lami Kaya, LKaya@ieee. org © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Chapter 6 Long Distance Communication © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Topics Covered • • • 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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

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 © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.