analog & digital signals © prof. aiman hanna department of computer science concordia...

Analog & Digital SignalsAnalog & Digital Signals

© Prof. Aiman Hanna© Prof. Aiman HannaDepartment of Computer Science Department of Computer Science

Concordia University Concordia University Montreal, CanadaMontreal, Canada

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A A nalog nalog & Digital Signals& Digital Signals

Figure 3.1 – Digital & Analog Signals

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D D igital Encoding Schemesigital Encoding SchemesThere are 10 types of people; those who know binary and those who do not.There are 10 types of people; those who know binary and those who do not.

Digital data are represented by a sequence of 1s & 0sDigital data are represented by a sequence of 1s & 0s

1 refer to a high electrical voltage, and 0 refers to a low electrical voltage1 refer to a high electrical voltage, and 0 refers to a low electrical voltage

Two major digital encoding schemes exist:Two major digital encoding schemes exist:

• NonReturn to ZeroNonReturn to Zero (NRZ)(NRZ) Encoding Encoding

• Manchester EncodingManchester Encoding

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D D igital Encoding Schemesigital Encoding Schemes (continue...)(continue...)

NRZ EncodingNRZ Encoding A 0 voltage is transmitted by raising the voltage level high, while 1 is A 0 voltage is transmitted by raising the voltage level high, while 1 is

transmitted by using a low voltagetransmitted by using a low voltage

Figure 3.2(a) – NRZ Encoding

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NRZINRZI Encoding Encoding An alternative to NRZ is NRZI (Inverted)An alternative to NRZ is NRZI (Inverted)

The voltage changes only when a 1 is to be sentThe voltage changes only when a 1 is to be sent

Figure 3.2(b) – NRZI Encoding

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Both NRZ and NRZI have problems; for example what is the Both NRZ and NRZI have problems; for example what is the exact sequence being transmitted in the sequence below?exact sequence being transmitted in the sequence below?

Is time synchronization possible? Is time synchronization possible?

Figure 3.3 – NRZ Encoding of a Sequence of 0s

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Manchester EncodingManchester Encoding Also called Also called Self-Synchronizing CodeSelf-Synchronizing Code Uses signal changes to keep the sending and receiving devices synchronizedUses signal changes to keep the sending and receiving devices synchronized 0 is represented by a change from high to low in the middle of transmission 0 is represented by a change from high to low in the middle of transmission

and 1 is represented by a low to high change in the middle of transmission and 1 is represented by a low to high change in the middle of transmission

Are there any disadvantages? Are there any disadvantages?

Figure 3.4 – Manchester Encoding

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Differential Manchester EncodingDifferential Manchester Encoding Similar to Manchester encoding, the signal will change in the Similar to Manchester encoding, the signal will change in the

middle, however middle, however

1 causes the signal to remain the same, while 0 causes the signal 1 causes the signal to remain the same, while 0 causes the signal to changeto change

Figure 3.5 – Differential Manchester Encoding

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A A nalog Signalsnalog Signals Adds complexity to data communicationAdds complexity to data communication

Phone lines carry analog signalsPhone lines carry analog signals

Digital computers need to convert their signal to analog before placing it on the wire; this is called Digital computers need to convert their signal to analog before placing it on the wire; this is called ModulationModulation

Similarly, all received signals must be converted to digital; this is called Similarly, all received signals must be converted to digital; this is called DemodulationDemodulation

A A modemmodem is hence needed is hence needed An analog signal is not that simple; in general, an analog signal is characterized by its frequency, amplitude An analog signal is not that simple; in general, an analog signal is characterized by its frequency, amplitude

and phase shiftand phase shift

Different Amplitude & phase shift

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B B it Rateit Rate Describes a medium capacity; that is how many bits can be transferred per unit of timeDescribes a medium capacity; that is how many bits can be transferred per unit of time

Measured as Bits Per Second (bps)Measured as Bits Per Second (bps)

A higher bandwidth medium is capable of a higher bit rateA higher bandwidth medium is capable of a higher bit rate

Transmitter sends a signal representing a bit sting (a component) , while the receiver listens to Transmitter sends a signal representing a bit sting (a component) , while the receiver listens to the medium and creates a bit string based on what it receives the medium and creates a bit string based on what it receives

Once the component is sent, the transmitter gets another bit string (another component) and the Once the component is sent, the transmitter gets another bit string (another component) and the process repeatsprocess repeats

Figure 3.9 – Sending Data via Signals

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B B it Rate it Rate (continue...)(continue...)

The Nyquist Theorem & Noiseless ChannelsThe Nyquist Theorem & Noiseless Channels

Baud RateBaud Rate: the frequency with which components change: the frequency with which components change

Each bit string is composed of Each bit string is composed of nn bits, and hence the signal component may bits, and hence the signal component may have up to have up to 22nn different amplitudes (one for each unique combination for different amplitudes (one for each unique combination for bb11, , bb22, …b, …bnn) )

Are bit rate and baud rate the Same?Are bit rate and baud rate the Same?

No, bit rate depends on the number of bits (No, bit rate depends on the number of bits (nn) as well as the baud rate; more ) as well as the baud rate; more precisely:precisely:

Bit Rate = n * Baud RateBit Rate = n * Baud Rate

Bit rate can then be increased by either increasing the baud rate or Bit rate can then be increased by either increasing the baud rate or nn; ; however only up to a pointhowever only up to a point

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Bit Rate = n * Baud RateBit Rate = n * Baud Rate This result is surprisingly old, back to 1920s, when Harry Nyquist developed his classic theoryThis result is surprisingly old, back to 1920s, when Harry Nyquist developed his classic theory

Nyquist theory showed that if Nyquist theory showed that if f f is the maximum frequency a medium can transmit, then the receiver can is the maximum frequency a medium can transmit, then the receiver can reconstruct the signal by sampling it reconstruct the signal by sampling it 2f 2f times per secondtimes per second

In other words, the receiver can construct the signal by sampling it at intervals of In other words, the receiver can construct the signal by sampling it at intervals of 1 / 2f1 / 2f seconds, or twice seconds, or twice each period (one period is each period (one period is 1 / f1 / f ). ).

For example, if the maximum frequency is 4000 Hz, then the receiver can completely construct it by sampling For example, if the maximum frequency is 4000 Hz, then the receiver can completely construct it by sampling it every 1/8000it every 1/8000thth of a second of a second

Assuming that the transmitter baud rate is Assuming that the transmitter baud rate is 2 2 f f , in other words changes signal each , in other words changes signal each 1 / 2f 1 / 2f intervals, we can intervals, we can statestate

Bit Rate = n * Baud Rate = n * 2 * Bit Rate = n * Baud Rate = n * 2 * ff

This can also be stated based on component; if This can also be stated based on component; if BB is the number of different components, then is the number of different components, then

B = B = 22nn

OrOrnn = log = log22((BB))

Hence,Hence,

Bit Rate = 2 * Bit Rate = 2 * f f * * loglog22((BB))

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Noisy ChannelsNoisy Channels We interject that Nyquist theory assumed that there is absolutely no noise in the We interject that Nyquist theory assumed that there is absolutely no noise in the

channel to alter the signal channel to alter the signal The result of Nyquist Theorem for a maximum frequency of 3300Hz, the The result of Nyquist Theorem for a maximum frequency of 3300Hz, the

approximate upper limit of the telephone system, can then be summarized as approximate upper limit of the telephone system, can then be summarized as follows:follows:

This result seem to imply that there is no upper bound for a bit rate given the This result seem to imply that there is no upper bound for a bit rate given the maximum frequency; unfortunately this is not the case due to two main reasons:maximum frequency; unfortunately this is not the case due to two main reasons:

1.1. More components mean subtler change among themMore components mean subtler change among them

2.2. Channels are subject to noiseChannels are subject to noise


Result of Nyquist Theorem for a maximum of 3300 HzResult of Nyquist Theorem for a maximum of 3300 Hz

NN, number of bits, number of bits BB, number of signal components, number of signal components Maximum bit rate (bps)Maximum bit rate (bps)

11 22 6,6006,600

22 44 13,20013,200

33 88 19,80019,800

44 1616 26,40026,400

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Noisy ChannelsNoisy Channels1.1. More components mean subtler change among themMore components mean subtler change among them

2.2. Channels are subject to noiseChannels are subject to noise The transmitted signal can be distorted due to the channel noiseThe transmitted signal can be distorted due to the channel noise If distortion is too large, the receiver may not be able to reconstruct If distortion is too large, the receiver may not be able to reconstruct

the signal at allthe signal at all


Figure 3.10 – Effect on Noise on Digital Signals (The same applies to analog signals)

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Shannon’s ResultShannon’s Result

How much noise is bad? This depends on its ratio to the signalHow much noise is bad? This depends on its ratio to the signal

We define S/N (We define S/N (Signal-to-Noise-RatioSignal-to-Noise-Ratio) ) A higher S/N (less significant noise) indicates higher qualityA higher S/N (less significant noise) indicates higher quality Because S >> N, the ratio is often scaled down as Because S >> N, the ratio is often scaled down as

R = logR = log1010(S/N)(S/N) bels bels // // belsbels is the measurement unit is the measurement unit

For example, For example, If S is 10 times larger than N, then If S is 10 times larger than N, then

R = logR = log1010(10N/N)(10N/N) = 1 bel = 1 belIf S is 100 times larger than N, then If S is 100 times larger than N, then

R = logR = log1010(100N/N)(100N/N) = 2 bels = 2 bels

Perhaps, a more familiar measurement is the Perhaps, a more familiar measurement is the decibel (dB)decibel (dB)1 dB = 0.1 bel1 dB = 0.1 bel

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Shannon’s ResultShannon’s Result

In 1940, Claude Shannon went beyond Nyquist’s results and considered noisy channelsIn 1940, Claude Shannon went beyond Nyquist’s results and considered noisy channels

Shannon related the maximum bit rate not only to the frequency but also to the S/N ratio; Shannon related the maximum bit rate not only to the frequency but also to the S/N ratio; specifically he showed that:specifically he showed that:

Bit Rate = Bandwidth * logBit Rate = Bandwidth * log22(1 + S/N)(1 + S/N) bps bps

The formula states that a higher BW and S/N ratio allow higher bit rate The formula states that a higher BW and S/N ratio allow higher bit rate

Hence, for the telephone system, which has a frequency of about 3000 Hz and S/N ≈ 35 dB, or Hence, for the telephone system, which has a frequency of about 3000 Hz and S/N ≈ 35 dB, or 3.5 bels, Shannon’s result yields the following3.5 bels, Shannon’s result yields the following

3.5 = 3.5 = log log1010(S/N) (S/N) S = 10 S = 103.53.5N N S S ≈ 3162 ≈ 3162 N N S/N S/N ≈≈ 3162 3162

Bit Rate = Bandwidth * logBit Rate = Bandwidth * log22(1 + S/N)(1 + S/N) = 3000 * = 3000 * loglog22(1 + 3162)(1 + 3162) ≈ ≈ 3000 * 3000 * 11.63 bps11.63 bps ≈ ≈ 34,88034,880 bps bps ≈ 35 kbps≈ 35 kbps

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Shannon’s ResultShannon’s Result Shannon’s result is not just a theoretical result; rather it has its Shannon’s result is not just a theoretical result; rather it has its

very real implicationvery real implication

During the 1980s, 2400 & 9600 bps modems became commonDuring the 1980s, 2400 & 9600 bps modems became common Early 1990s, modems with a rate of 28.8 and 33.6 kbps became Early 1990s, modems with a rate of 28.8 and 33.6 kbps became

common (this matched the maximum bit rate that Shannon’s common (this matched the maximum bit rate that Shannon’s result indicated)result indicated)

Not much longer, modems that supported 56.6 kbps rates were a Not much longer, modems that supported 56.6 kbps rates were a reality. Did this violate Shannon’s results? reality. Did this violate Shannon’s results?

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D D igital-to-Analog-igital-to-Analog-ConversionConversion Computers are digital, however phone lines connecting Computers are digital, however phone lines connecting

them to remote machines/servers are analog them to remote machines/servers are analog

Figure 3.11 – Computer Data Transmitted Over Telephone Lines

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D D igital-to-Analog-igital-to-Analog-ConversionConversion New phone lines use optical fiber, which carry digital signal New phone lines use optical fiber, which carry digital signal

CodecCodec (Coder/Decoder) is needed in such cases (Coder/Decoder) is needed in such cases

Figure 3.12 – Voice Information Transmitted Digitally

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D D igital-to-Analog-igital-to-Analog-ConversionConversionFrequency Modulation (FM)Frequency Modulation (FM)

Simple method of digital to analog conversion, Simple method of digital to analog conversion, Also known as Also known as Frequency Shift Keying (FSK)Frequency Shift Keying (FSK) Assigns digital 0 to one signal and 1 to another based on some ruleAssigns digital 0 to one signal and 1 to another based on some rule In general, if n bits are sent per baud, then we can have In general, if n bits are sent per baud, then we can have 22nn signal combination signal combination The signal is transmitted for a fixed period of timeThe signal is transmitted for a fixed period of time

Figure 3.13 – FSK (Two Frequencies), One Bit per Baud – Analog signal for 01001

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D D igital-to-Analog-igital-to-Analog-ConversionConversionAmplitude Modulation (AM)Amplitude Modulation (AM)

Also known as Also known as Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK) Each bit group is assigned to an analog signal of given Each bit group is assigned to an analog signal of given

magnitudemagnitude The signal is transmitted for a fixed period of time as with FMThe signal is transmitted for a fixed period of time as with FM

Figure 3.14 – ASK (Four Amplitudes), Two Bits per Baud – Analog signal for 00110110

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D D igital-to-Analog-igital-to-Analog-ConversionConversionPhase Modulation (PM)Phase Modulation (PM)

Also known as Also known as Phase Shift Keying (PSK)Phase Shift Keying (PSK) 1 results in a phase change of 1801 results in a phase change of 180°°, while 0 results in 0, while 0 results in 0°° (no (no

change) change) In such case, one bit of information can be transmitted per baud In such case, one bit of information can be transmitted per baud

PSK (One Shift), One Bit per Baud – Analog signal for 0010110

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D D igital-to-Analog-igital-to-Analog-ConversionConversionDifferential Phase Modulation (DPM)Differential Phase Modulation (DPM)

Also known as Also known as DifferentialDifferential Phase Shift Keying (DPSK)Phase Shift Keying (DPSK) Allows transmission of more than 1 bit per baudAllows transmission of more than 1 bit per baud For example, if the phase is shifted by multiples of 90For example, if the phase is shifted by multiples of 90°°, two bits , two bits

at a time can be transmitted at a time can be transmitted

DPSK, Two Bits per Baud – Analog signal for 10001110

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D D igital-to-Analog-igital-to-Analog-ConversionConversionQuadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)

A greater signal variety means a greater bit rate per A greater signal variety means a greater bit rate per baud ratebaud rate

The problem is that the higher the number of signal, The problem is that the higher the number of signal, the smaller the difference among themthe smaller the difference among them

One common approach to avoid that is to use a One common approach to avoid that is to use a combination of frequencies, amplitudes or phase shifts combination of frequencies, amplitudes or phase shifts to allow a large group of bits while marinating a larger to allow a large group of bits while marinating a larger differences among them differences among them

QAM is a common technique in which a group of bits QAM is a common technique in which a group of bits is assigned a signal defined by its amplitude and phase is assigned a signal defined by its amplitude and phase shiftshift

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D D igital-to-Analog-igital-to-Analog-ConversionConversionQuadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)

Bit Value: Amplitude, Phase ShiftBit Value: Amplitude, Phase Shift Bit Value: Amplitude, Phase ShiftBit Value: Amplitude, Phase Shift

000: A1, 0000: A1, 000 phase shift phase shift 001: A2, 0001: A2, 000 phase shift phase shift

010: A1, 90010: A1, 9000 shift shift 011: A2, 90011: A2, 9000 phase shift phase shift



Figure 3.15 – QAM (Two Amplitudes & Four Phases), Three Bits per BaudAnalog Signal for 001 010 100 011 101 000 011 110

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A A nalog-to-Digital-nalog-to-Digital-ConversionConversion Some analog-to-digital conversion is nothing but the Some analog-to-digital conversion is nothing but the

opposite to what has been discussed, however,opposite to what has been discussed, however,

Other conversions are more complex; for example Other conversions are more complex; for example those that are required to convert analog data for voice those that are required to convert analog data for voice or musicor music

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A A nalog-to-Digital-nalog-to-Digital-ConversionConversionPulse Amplitude Modulation (PAM)Pulse Amplitude Modulation (PAM)

An analog signal is sampled at a regular interval, then a An analog signal is sampled at a regular interval, then a pulse with amplitude equal to the sampled signal is pulse with amplitude equal to the sampled signal is generatedgenerated

Figure 3.17 – Pulse Amplitude ModulationAmplitude Modulation

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A A nalog-to-Digital-nalog-to-Digital-ConversionConversionPulse Code Modulation (PCM)Pulse Code Modulation (PCM)

PAM-generated signals look digital, but the signal has analog PAM-generated signals look digital, but the signal has analog characteristics characteristics

PCM allows the signal to truly be digital by assigning amplitude PCM allows the signal to truly be digital by assigning amplitude from a predefined set of digital codesfrom a predefined set of digital codes

By sampling By sampling ss times per second, we obtain a bit rate of times per second, we obtain a bit rate of n * s n * s bpsbps

Figure 3.18 – Pulse Code Modulation, Code Modulation, nn=3, =3, 22nn codes codes

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At the receiving end, the accuracy of the construction depends on: At the receiving end, the accuracy of the construction depends on: • Sampling frequency: a slower sampling frequency may result in some oscillations Sampling frequency: a slower sampling frequency may result in some oscillations

to be missed entirelyto be missed entirely higher sampling rate will produce a higher accuracy; but up to a point higher sampling rate will produce a higher accuracy; but up to a point Based on Nyquist theorem, if the original signal has Based on Nyquist theorem, if the original signal has ff maximum frequency, then maximum frequency, then

sampling at 2sampling at 2ff is enough is enough That is, sampling at a higher rate than 2That is, sampling at a higher rate than 2ff will not produce any better accuracy than 2 will not produce any better accuracy than 2 f f

• Number of amplitude (binary codes) from which to chooseNumber of amplitude (binary codes) from which to choose The higher the difference between the sampled signal and the pulse/code, the higher The higher the difference between the sampled signal and the pulse/code, the higher

the chances that the reconstructed signal become distortedthe chances that the reconstructed signal become distorted This is called, This is called, quantization noisequantization noise

It should be noted however that higher sampling and higher number of pulse amplitude comes with a priceIt should be noted however that higher sampling and higher number of pulse amplitude comes with a price

Figure 3.19 – Sampling at Too Low Frequency

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PAM has several common applications; which includePAM has several common applications; which include• Digitizing of voice signal over long-distance telephone linesDigitizing of voice signal over long-distance telephone lines

8000 samples, with 8-bit sample 8000 samples, with 8-bit sample bit rate of 64 kbps bit rate of 64 kbps

• Compact discs (CDs) Compact discs (CDs) Varies from one device to anotherVaries from one device to another As an example: 44.1 kHz sampling frequency, with a D-A As an example: 44.1 kHz sampling frequency, with a D-A

conversion of 16-bit linearconversion of 16-bit linear 16-bit would allow 64,000 amplitude (16-bit would allow 64,000 amplitude (linearlinear indicates that the indicates that the

differences between these amplitudes are the same)differences between these amplitudes are the same)

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M M odemsodemsTwo important issues are there with modems Two important issues are there with modems

• Software (deliver signals to and from modem)Software (deliver signals to and from modem)

• Compatibility (demodulate what other modems modulated)Compatibility (demodulate what other modems modulated) Several standards were defined for modemsSeveral standards were defined for modems CCITT (ITU) defined V.xx series of modem standardsCCITT (ITU) defined V.xx series of modem standards V.21 uses FSK with 1 frequency for 1 bit, and the resulting bit V.21 uses FSK with 1 frequency for 1 bit, and the resulting bit

rate is equal to the baud rate. (supports up to 300 bps.)rate is equal to the baud rate. (supports up to 300 bps.) V.22 uses PSK with 2 bits for each phase shift; resulting bit rate V.22 uses PSK with 2 bits for each phase shift; resulting bit rate

is twice the baud rate. 600 bauds/sec = 1200bpsis twice the baud rate. 600 bauds/sec = 1200bps Sending and receiving modems use different frequency; that Sending and receiving modems use different frequency; that

enable full-duplex communication enable full-duplex communication

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M M odems odems (continue...)(continue...)

Signal Constellation Signal Constellation Many modems work by changing more than one analog signal’s Many modems work by changing more than one analog signal’s

components, typically phase shift & amplitude (QAM)components, typically phase shift & amplitude (QAM) QAM can visually be described a signal constellation graphQAM can visually be described a signal constellation graph Distance represents amplitude, while angel represents phase shiftDistance represents amplitude, while angel represents phase shift

Figure 3.20 – Quantifying a Point on a signal Constellation

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Signal Constellation Signal Constellation A shift with A shift with xx° ° corresponds to corresponds to x/360°x/360° of a period of a period V.22 uses fixed amplitude with 4 (2V.22 uses fixed amplitude with 4 (222) points and 600 bauds) points and 600 bauds

600 * 600 * 22 = 1200 bps = 1200 bps V.22 bis uses 16 (2V.22 bis uses 16 (244) points and 600 bauds) points and 600 bauds

600 * 600 * 44 = 2400 bps = 2400 bps V.32 uses 5 bits per baud but one of them is for V.32 uses 5 bits per baud but one of them is for Trellis CodingTrellis Coding error detection error detection

16 (216 (244) points are counting and 2400 bauds) points are counting and 2400 bauds 2400 * 2400 * 44 = 9600 bps = 9600 bps

Figure 3.21 – Quantifying a Point on a signal Constellation

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Signal Constellation Signal Constellation Distortion is possible, which may change the amplitude or the Distortion is possible, which may change the amplitude or the

phase shiftphase shift

Figure 3.22 – Distortion of Signal Constellation Points

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Signal Constellation Signal Constellation Depending on the distortion level, the modem may or may not Depending on the distortion level, the modem may or may not

be able to interpret the distorted signal correctlybe able to interpret the distorted signal correctly

Figure 3.23 – Interpreting Interpreting Constellation Points for a distorted signal for a distorted signal

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Modem Standards Modem Standards Few modem standards exist!Few modem standards exist! They differ in baud rate, bit per baud, modulation techniques, error detection, They differ in baud rate, bit per baud, modulation techniques, error detection,

compressions, ..etc.compressions, ..etc. Autobaud modemsAutobaud modems exist and convenient since they are capable of exist and convenient since they are capable of

automatically choose the appropriate standard; they also allow users to automatically choose the appropriate standard; they also allow users to communicate using any of several standardscommunicate using any of several standards

56 kbps modems were achievable when connecting directly with ISP56 kbps modems were achievable when connecting directly with ISP

Figure 3.24 – Connections using a ModemConnections using a Modem

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Cable ModemsCable Modems Driven by many facts Driven by many facts

including the inability including the inability of modem over phone of modem over phone lines to go beyond lines to go beyond 56kbps56kbps

They connect with the They connect with the analoganalog component of a component of a cable TV (CATV) cable TV (CATV) instead of the analog instead of the analog component of the component of the telephone systemtelephone system

Figure 3.25 – Cable Modem Placement Cable Modem Placement

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Cable ModemsCable Modems Utilizes the much higher frequency of CATV, so few Mbps Utilizes the much higher frequency of CATV, so few Mbps

speed is achievedspeed is achieved

However bit rate may differ depending on the number of users However bit rate may differ depending on the number of users (neighbors usually!) that are using the cable line(neighbors usually!) that are using the cable line

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Cable ModemsCable Modems Usually has a frequency of about 750 MHz, divided into many Usually has a frequency of about 750 MHz, divided into many

6-MHz for different channels6-MHz for different channels Information from the Internet can be downloaded onto a 6-Information from the Internet can be downloaded onto a 6-

MHz band somewhere between 42 and 750 MHzMHz band somewhere between 42 and 750 MHz

Figure 3.26 – Cable Modem & Carrier Signals Cable Modem & Carrier Signals

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Cable ModemsCable Modems A number of techniques can be used for modulating & A number of techniques can be used for modulating &

demodulating, but two are more popular:demodulating, but two are more popular:• Quaternary Phase Shift Keying (QPSK)Quaternary Phase Shift Keying (QPSK)

• QAM64, a variation of QAMQAM64, a variation of QAM

QAM64 is typical for high bandwidth requirement – data QAM64 is typical for high bandwidth requirement – data download rate can be 36 Mbps, which many PCs today are download rate can be 36 Mbps, which many PCs today are not capable of handlingnot capable of handling

A more realistic figure is between 1 and 11 Mbps A more realistic figure is between 1 and 11 Mbps

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Cable ModemsCable Modems Can also be used to transmit information on the Can also be used to transmit information on the

opposite direction (Uploading) opposite direction (Uploading)

Frequency used for uploading is usually between 5 & Frequency used for uploading is usually between 5 & 40 MHz40 MHz

• Uploading has lower bit rate, which might be okay. Uploading has lower bit rate, which might be okay. Why?Why?

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D D SLSL DSLDSL - - Digital Subscriber Line Digital Subscriber Line

Fast and does not require cable wiring or dialing up to ISPFast and does not require cable wiring or dialing up to ISP Uses telephone lines!!!! Uses telephone lines!!!! Local loop (last mile) is capable of transmitting signals of up Local loop (last mile) is capable of transmitting signals of up

to 1 MHz frequency, that Local exchange can receiveto 1 MHz frequency, that Local exchange can receive

Figure 3.27 – Local Loop - POTS (Plain Old Telephone Service) ConfigurationLocal Loop - POTS (Plain Old Telephone Service) Configuration

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D D SL SL (continue...)(continue...)Various forms of DSL exist Various forms of DSL exist

Asymmetric DSLAsymmetric DSL

Figure 3.28 – ADSL ConnectionADSL Connection

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D D SL SL (continue...)(continue...)ADSL LiteADSL Lite Designed for residential customers Designed for residential customers

Figure 3.30 – ADSL LiteADSL Lite

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D D SL SL (continue...)(continue...)VDSL – Very High Data Rate DSLVDSL – Very High Data Rate DSL The local loop causes many DSL problemsThe local loop causes many DSL problems Signal degrades over long distance, do DSL is not available if the house is Signal degrades over long distance, do DSL is not available if the house is

more than 3.5 miles from the local officemore than 3.5 miles from the local office New improved local loop using fiber/copper hybrid is used, which New improved local loop using fiber/copper hybrid is used, which

improves quality, reduces the copper length and makes VDSL possible improves quality, reduces the copper length and makes VDSL possible

Figure 3.31 – Local Loop: Fiber/Copper HybridLocal Loop: Fiber/Copper Hybrid

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