TELECOMMUNICATION TRANSMISSION AND SWITCHING SYSTEMSLecture 03 Spring 2013Instructor: 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=VxIo W=Watts; V=Volts; I=Amps

• Milliwatt (mW)o 1/1000

Decibel (dB)

• The Bel is a logarithmic measure of the ratio between two valueso 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 systemso dB=1/10th of a Belo dB=10*log10 (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 dBo Dbw stands for dB with reference to 1 Wo dBm stands for d with reference to 1 Milliwatt (mW)

o 10log101=0dB 10log10(1/1000)=-30 dBo 10log1010=10dB 10log10(10/1000)=-20dBo 10log10100=20dB 10log10(100/1000)=-10dBo 10log101000=30dB 10log10(1000/1000)=0dB

Practice

W/mW dBW/dBm

*2 +3

/2 -3

*10 +10

/10 -10

mW dBm

100 mW= 10 * 10 mW 10+10=20dBm

100/1000 W=(10*10)/(10*10*10)

10+10-10-10-10=-10dB

50 W= 100/2=10*10/2 10+10-3=17dB == 47dBm

1W= ?dBm

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)

Neper

Gain/Loss is also at times defined in Nepers

A gain of 1 Neper equal 8.69dB G(N)=loge(I2/I1) 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 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?

Terminal Station

Line

Intermediate Station

Line

Terminal 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

http://www.southalabama.edu/coe/bset/johnson/lectures/lec15_files/image014.jpg

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 , vn, is added, an error occurs if IvnI>(1/2)V

If bipolar signal is used then error 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

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. 27-28

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