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  We already know that: Power is measured in watts o 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 o W=Watts; V=Volts; I=Amps Milliwatt (mW) o 1/1000

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

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 o Dbw stands for dB with reference to 1 W o dBm stands for d with reference to 1 Milliwatt (mW) o 10log 10 1=0dB 10log 10 (1/1000)=-30 dB o 10log 10 10=10dB 10log 10 (10/1000)=-20dB o 10log =20dB 10log 10 (100/1000)=-10dB o 10log =30dB 10log 10 (1000/1000)=0dB

Practice W/mWdBW/dBm *2+3 /2-3 *10+10 /10-10 mWdBm 100 mW= 10 * 10 mW10+10=20dBm 100/1000 W=(10*10)/(10*10*10) =-10dB 50 W= 100/2=10*10/ =17dB == 47dBm 1W=?dBm

Gain and Loss  When the output power P 2 is greater than the input power P 1, then the gain G in decibels is  G=10log 10 (P 2 /P 1 )dB  And when P 2 <P 1, there is loss or attenuation, given by:  L=10log 10 (P 1 /P 2 )dB  If however the input and output circuits have the same impedence, then  P 2 /P 1 =(V 2 /V 1 ) 2 =(I 2 /I 1 ) 2  G=20log 10 (V 2 /V 1 )=20log 10 (I 2 /I 1 )

Neper  Gain/Loss is also at times defined in Nepers  A gain of 1 Neper equal 8.69dB  G(N)=log e (I 2 /I 1 ) N

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

Example 2.1  An Amplifier has an input resistance of 600 Ω and a resistive load of 75 Ω. 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?

Terminal Station Line Intermediate Station LineTerminal Station

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?

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

 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

Noise and Jitter  The receiver compares the signal voltage vs, with a threshold value of ½ V  If a noise voltage, v n, is added, an error occurs if  Iv n I>(1/2)V  If bipolar signal is used then error occurs when  Iv n I>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

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

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

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?

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.

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.

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.

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

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