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Telecommunication Transmission and Switching Systems

Telecommunication Transmission and Switching Systems. Lecture 03 Spring 2013 Instructor: Engr. Arifa Saher. Power levels. Wide range of power levels are encountered in telecommunication transmission systems. For convenience logarithmic units, decibels, are used to for power

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Telecommunication Transmission and Switching Systems

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  1. Telecommunication Transmission and Switching Systems Lecture 03 Spring 2013 Instructor: Engr. ArifaSaher

  2. Power levels • Wide range of power levels are encountered in telecommunication transmission systems. • For convenience logarithmic units, decibels, are used to for power • We already know that: • Power is measured in watts • Basic unit of power is a watt (W) • A watt is a power dissipated when 1 Amp flows through a resistor of 1 Ohm, to give a potential difference of 1 Volt. • Electrical power is found as W=VxI • W=Watts; V=Volts; I=Amps • Milliwatt(mW) • 1/1000

  3. Decibel (dB) • The Bel is a logarithmic measure of the ratio between two values • The Bel is the log10 of ratio of two powers • The deciBel (dB) is most commonly use to describe gain or loss in wireless communications systems • dB=1/10th of a Bel • dB=10*log10 (signal/reference)

  4. Standard Power references • The unit dB can also be used to represent an absolute power value, by defining a reference and adding the corresponding suffix to dB • Dbw stands for dB with reference to 1 W • dBm stands for d with reference to 1 Milliwatt (mW) • 10log101=0dB 10log10(1/1000)=-30 dB • 10log1010=10dB 10log10(10/1000)=-20dB • 10log10100=20dB 10log10(100/1000)=-10dB • 10log101000=30dB 10log10(1000/1000)=0dB

  5. Practice

  6. Gain and Loss • When the output power P2 is greater than the input power P1, then the gain G in decibels is • G=10log10(P2/P1)dB • And when P2<P1, there is loss or attenuation, given by: • L=10log10(P1/P2)dB • If however the input and output circuits have the same impedence, then • P2/P1=(V2/V1)2=(I2/I1)2 • G=20log10(V2/V1)=20log10(I2/I1)

  7. Neper • Gain/Loss is also at times defined in Nepers • A gain of 1 Neperequal 8.69dB • G(N)=loge(I2/I1) N

  8. Insertion Loss/Gain • Insertion Loss • If a passive network, such as an attenuator pad or a filter, is inserted in a circuit between its generator and load, the increase in the total loss of the circuit is called the insertion loss • Insertion Gain • If an active network, such as an amplifier, is inserted, the power received by the load may increase, which is known as the insertion gain

  9. Example 2.1 • An Amplifier has an input resistance of 600Ω and a resistive load of 75Ω. When it has an r.m.s. input voltage of 100mV, the r.m.s. output current is 20mA. Find the gain in dB • Input power? • Output power? • Gain is P2/P1=? • In dB?

  10. Intermediate Station Terminal Station Line Line Terminal Station

  11. Digital Transmission: Bandwidth and Equalization • Minimum Bandwidth needed to transmit a digital signal at B bauds is • Wmin=1/2B • If a signal is sent through an ideal low-pass network with this cut-off frequency • Every pulse can be detected without error • No inter-symbol interference • Practically?

  12. BW and Equalization cont.. • Zero inter-symbol interference can be obtained • If gain of the channel changes from unity to zero over a band of frequencies with a gain/frequency response that is skew/symmetrical about f=1/2B • The transfer function of the channel should therefore be equalized so that the out put signal has such a spectrum http://www.southalabama.edu/coe/bset/johnson/lectures/lec15_files/image014.jpg

  13. Gain and Phase Equalization • a special attenuator that has a frequency response that is intentionally not flat. • A device that equalizes the phases of different frequency components in the spectrum. • Time Domain Equalizers • Transversal equalizer • Adaptive equalizer

  14. Noise and Jitter • The receiver compares the signal voltage vs, with a threshold value of ½ V • If a noise voltage , vn, is added, an error occurs if • IvnI>(1/2)V • If bipolar signal is used thenerror occurs when • IvnI>v • Thus same error rate can be obtained with a 3dB lower signal/noise ratio • For telephone transmission error rate of 1 in 10^3 is intolerable but 1 in 10^5 is tolerable

  15. Jitter • Variations in the extracted frequency by regenerative repeaters can cause periodic variations of the times of regenerated pulses, which is known as jitter. • Jitter tolerance in devices has to be cared for in subsequent equipment. • If variation is large then it is known as wander

  16. Frequency-division Multiplexing • In this form of transmission a number of baseband channels are sent over a common wideband transmission path by using each channel to modulate a different carrier frequency • Systems using this process are called multichannel carrier systems • Pg. 27-28

  17. Time Division Multiplexing • In this system each baseband channel is connected to the transmission path by a sampling gate which is opened for short intervals by means of a train of pulses. • For telephony • Binary digits are sent at a rate of 8 x 8 = 64 kilobauds • As samling is carried out at 8 kHz • And 8-bit encoding is used. • Minimum bandwidth required is ? • Quantizing noise?

  18. PDH: PLESIOCHRONOUS DIGITAL HIERARCHY • A TECHNOLOGY USED IN TELECOMMUNICATIONS NETWORK TO TRANSPORT LARGE QUANTITY OF DATA OVER DIGITAL TRANSPORT EQUIPMENT SUCH AS FIBRE OPTIC AND MICROWAVE RADIO WAVE SYSTEMS. • THE TERM “PLESIO(near)CHRONOUS(time)” IS DERIVED FROM Greek • IT MEANS THAT PDH NETWORKS RUN IN A STATE WHERE DIFFERENT PARTS OF THE NETWORK ARE ALMOST, BUT NOT QUITE PERFECTLY SYNCHRONISED.

  19. PDH • SENDING A LARGE QUANTITY OF DATA ON FIBRE OPTIC TRANSMISSION SYSTEM. • TRANSMISSION AND RECEPTION ARE SYNCHRONIZED BUT TIMING IS NOT. • THE CHANNEL CLOCKS ARE DERIVED FROM DIFFERENT MASTER CLOCKS WHOSE RANGE IS SPECIFIED TO LIE WITHIN CERTAIN LIMITS. THE MULTIPLEXED SIGNAL IS CALLED A “PLESIOCHRONOUS” SIGNAL. • PDH SIGNALS ARE NEITHER SYNCHRONOUS NOR ASYNCHRONOUS.

  20. PDH • PDH ALLOWS TRANSMISSION OF DATA STREAMS THAT ARE NOMINALLY RUNNING AT THE SAME RATE, BUT ALLOWING SOME VARIATION ON THE SPEED AROUND A NOMINAL RATE. • BY ANALOGY, ANY TWO WATCHES ARE NOMINALLY RUNNING AT THE SAME RATE, CLOCKING UP 60 SECONDS EVERY MINUTE. • HOWEVER, THERE IS NO LINK BETWEEN WATCHES TO GUARANTEE THEY RUN AT EXACTLY THE SAME RATE. • IT IS HIGHLY LIKELY THAT ONE IS RUNNING SLIGHTLY FASTER THAN THE OTHER.

  21. VERSIONS OF PDH • THERE ARE TWO VERSIONS OF PDH NAMELY • 1) THE EUROPEAN AND • 2 ) THE AMERICAN. • THEY DIFER SLIGHTLY IN THE DETAIL OF THEIR WORKING BUT THE PRINCIPLES ARE THE SAME. • EUROPEAN PCM = 30 CHANNELS • NORTH AMERICAN PCM = 24 CHANNELS • JAPANESE PCM = 24 CHANNELS

  22. EUROPEAN DIGITAL HIERARCHY • 30 Channel PCM = 2 Mbps • 2 Mbps x 4 = 8 Mbps • 8 Mbps x 4 = 34 Mbps • 34 Mbps x 4 = 140 Mbps • 140 Mbps x 4 = 565 Mbps

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