telecommunication system engineering telephony signal transmission

7/29/2019 Telecommunication system engineering Telephony Signal Transmission

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3/13/13

Telephony Signal

Transmission

Two-Wire and Four-

Wire TransmissionSystems

BasicImpairments


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The basic building block fortransmission is the telephone

channel or voice channel.The nominal voice channel occupies the band from0 to 4 kHz.

CCITT defines the voice channel as the band of

frequencies between 300 and 3400

Hz.Bell Laboratories states that the optimum trade-offbetween economics and quality transmission occurswhen the telephone speech signal is band-limited tothe range from about

200 to 3200 Hz.


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There are three basic impairments wemust deal with regarding the voicechannel.

Attenuationdistortion Phase

distortionNois

eTwo additionalimpairments are:Echo and Singing.


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Intentionally Left Blank


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THE THREE BASIC IMPAIRMENTS TOVOICE CHANNEL TRANSMISSION

AttenuationDistortion

It is the result of imperfect amplitude-frequencyresponse.

Attenuation distortion can be avoided if all

frequencies within the passband are subjected toexactly the same loss (or gain)

On loaded wire-pair systems, higher frequenciesare attenuated more than lower ones.

Attenuation distortion across the voice channel ismeasured against a reference frequency.

800 Hz as a reference, which is universally used in Europe,Africa, and parts of Hispanic America.

1000 Hz is the common reference frequency in North America


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3/13/13 Typical attenuationfrequency response (attenuationdistortion) for a voice channel. Hatched areas showspecified limits.


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Attenuation increases as theband edges are approached.

European requirement may state thatbetween 600 Hz and 2800 Hz the levelwill vary no more than 1 to +2 dB.

where the plus sign means more lossand the minus sign means less loss.


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DecibelUnits

Attenuation(reduction) of the signal strength arises fromvarious loss mechanisms in a transmission medium.For example:

Electric power is lost through heat generationas an electric signal flows along a wire.

To compensate for these energy losses, amplifiers are usedperiodically along a channel to boost the signal level, as shownin next slide.

A standard and convenient method for measuring attenuation

through a link or a device is to reference the output signallevel to the input level.


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DecibelUnits

Periodically placed amplifiers compensate for energy losses alonga link


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DecibelUnits

For guided media :

The signal strength normally decays

exponentially.

For convenience it is designated in

logarithmic power ratio measured in decibels

(dB).The dB unit is defined by

Power ratio in dB = 10 log(P2/P1)


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DecibelUnits

Representative values of decibel power loss and the remainingpercentages


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Q1: After traveling a certain distancein some transmission medium, thepower of a signal is reduced to half atpoint 2,

that is P2= 0.5P1.

what is the attenuation or loss of powerat point 2?

Q2: Consider the transmission path from point 1 to point 4 shown infig below. Here the signal attenuated by 9 dB between point 1 and 2.After getting a 14 dB boost from an amplifier at point 3, it is againattenuated by 3 dB between points 3 and 4.what is the dB level atpoint 4?


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PhaseDistortion

A voice channel may be regarded as a band-pass filter.

The velocity of propagation also tends to varywith frequency because of the electrical

characteristics associated with the network.

Considering the voice channel, therefore, thevelocity of propagation tends to increasetoward band center and decrease toward bandedge.

Propagation time is different fordifferent frequencies.


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Absolute delay is minimum around 1700 and 1800 Hz inthe voice channel.Around 1700 or 1800 Hz, envelope delay distortion isflattest. It is for this reason that so many data modems use

1700 or 1800 Hz for the characteristic tone frequency whichis modulated by the data.

h


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A modulated signal will not be distorted onpassing through the channel if the phase shift

changes uniformly with frequency.whereas if the phase shift is nonlinear withrespect to frequency, the output signal isdistorted compared to the input.

This phase distortion is often measured by aparameter called envelope delay distortion

(EDD).The maximum difference in the derivative over anyfrequency interval is called envelope delay distortion.

PhaseDistortion

i


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Noise

Any undesired signal in acommunication circuitIt is the major limiting factor in systemperformance.

Noise is broken down into fourcategories:1. Thermal noise2. Intermodulation noise

3. Crosstalk4. Impulse noise

N i


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NoiseThermalNoise

Thermal noise occurs in all transmissionmedia and all communication equipment,including passive devices.It arises from random electron motion and ischaracterized by a uniform distribution of

energy over the frequency spectrum with aGaussian distribution of levels.

Thermal noise is directly proportional tobandwidth and temperature.

Thermal noise is the factor that sets the lowerlimit of sensitivity of a receiving system

N i


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NoiseIntermodulationNoise.

Intermodulation (IM) noise is the result of thepresence of intermodulation products

If two signals with frequencies F1 and F2 arepassed through a nonlinear device or medium, the

result will contain IMproducts that are spurious frequency energycomponents.Intermodulation noise may result from anumber of causes:

Improper level setting. If the level of input to adevice is too high, the device is driven into itsnonlinear operating region. Improper alignment causing a device to functionnonlinearly.

Nonlinear envelope delay.

N i


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NoiseCrosstalk

Unwanted coupling betweensignal paths.

There are essentially three causesof crosstalk:1-Electrical coupling between transmission media,such as between wire pairs on a voice-frequency (VF)cable system.2-Poor control of frequency response (i.e., defective

filters or poor filter design).3-Nonlinear performance in analog (FDM) multiplexsystems.

There are two types of

crosstalk:1. Intelligible: Where at least four words areintelligible to the listener from extraneousconversation in a 7-s period.2. Unintelligible: Crosstalk resulting from any other

form of disturbing effects of one channel on another.

N i


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NoiseCrosstalk

Intelligible crosstalk presents thegreatest impairment because of itsdistraction to the listener.

Distraction is considered to becaused either by fear of loss ofprivacy

N i


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ImpulseNoise

Noise

Impulse noise is noncontinuous,consisting of irregular pulses ornoise spikes of short duration,

broad spectral density, andrelatively high amplitude.

Impulse noise degrades telephony

ordinarily only marginally.

It may seriously degrade data errorperformance on data or other digital

waveforms.


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L


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Level

qIn telephony it is measured in dBm

(decibelsreferenced to 1 milliwatt) or inmilliamperes.

q

In radio (wireless) systems, it is more likelymeasured in dBW

(decibels referenced to 12 watts ) ?

qvideo systems (e.g., television), the unit ofmeasure is voltage. The commonly derived unit isthe dBmV

Meaning decibels referenced to 1 millivolt.

Level means signalmagnitude.Level could be

comparative

L


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If levels are too high, amplifiers becomeoverloaded, resulting in intermodulation and other

types of distortion such as crosstalk

If levels are too low, customer satisfaction maysuffer with a degraded loudness rating.

Level

The power in dBm is an absolute valuedefined by:

Power level (in dBm)= 10 log ( P in

mW / 1 mW)0 dBm = 1mW.

positive values of dBm are greater than 1mW.Negative values are less than 1mW.


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Signal to Noise


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Signal-to-NoiseRatio

S/N ratio expresses in decibels the amount bywhich a signal level exceeds the noise within aspecified bandwidth.

Minimum S/N ratio is required to satisfy thecustomer or to make the receiving instrumentfunction within certain specified criteria.

Following S/N ratios required with thecorresponding end instruments

Signal to Noise


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A 1000-Hz signal has an S/Nratio of 10 dBThe level of the noise is +5 dBm, and the signal level is+15 dBm


Signal to Noise


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Individual talker signal power can

fluctuate widely so that S/N ratio is far fromconstant from telephone call to telephonecall.


In lieu of actual voice, a test tone is usedto measure level and S/N ratio, which has aconstant amplitude and no silentintervals


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TWO-WIRE AND FOUR-WIRE


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TWO-WIRE AND FOUR-WIRETRANSMISSIONTwo-WireTransmission

When both directions are carried on the samepair of wires, it is called two-wiretransmission.

The telephones in our homes and officesare connected to a local switching centerby means of two-wire circuits.

when oppositely directed portions of a singletelephone conversation occur over the sameelectrical transmission channel or path, we callthis two-wire operation.

OR



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Four-WireTransmission

TWO-WIRE AND FOUR-WIRETRANSMISSION

Carrier and radio systems require thatoppositely directed portions of a singleconversation occur over separate transmissionchannels or paths.

Two wires for the transmit path and two wiresfor the receive path.

Output of the local serving exchange towardsthe toll network is four-wire.

The four-wire interconnection could be amultichannel digital carrier system on cable,fiber-optic light guide, or multiplex over radio.


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3/13/13 A typical long-distance (toll)

telephone connection



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Operation of aHybrid

TWO WIRE AND FOUR WIRETRANSMISSION

Hybrid in terms of telephony is a transformer.

Hybrid may be viewed as a power splitter withfour sets of wire-pair connections.

Operation of a


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qSignal energy entering from the two-wire subsetconnection divides equally, half of it dissipating in the

impedance of the four-wire side receive path and theother half going to the four-wire side transmit path.

qThe balancing network is supposed to display the

characteristic impedance of the two-wire line to thehybridqSignal energy entering from the four-wire sidereceive path is also split in half in the ideal situationwhere there is perfect balance. Half of the energy is

dissipated by the balancing network (N) and half atthe two-wire port (L).

In every case, ideally half of the signal energyentering the hybrid is used to advantage and

half is dissipated or wasted.

Operation of a


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There are two losses associatedwith Hybrid

Hybrid insertion loss 0.5 dB

Hybrid dissipation loss 3.0 dB (half ofthe power) 3.5 dB

(total)


Any signal passing through a hybrid suffers a3.5-dB loss

Impedance matching on the two-wire side isimportant . That is the match between thecompromise network (N) and the two-wire side(L).The capability of impedance match ismeasured by return loss. In this particularCase it is called balance return loss:


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Block diagram of two-wire-to-four-wire conversionusing a hybrid

telecommunication system engineering telephony signal transmission

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