About dB & dBm

Question: What is Decibel (dB) and dBm?

Answer:- The decibel, or dB, is a means of expressing either the gain of an active device (such as an amplifier) or the loss in a passive device (such as an attenuator or length of cable).  The decibel was developed by the telephone company to conveniently express the gain or loss in telephone transmission systems. 
   

    The IEEE definition of dBm is "a unit for expression of power level in decibels with reference to a power of 1 milli watt."  Note that no mention is made of the value of circuit impedance; the dBm is merely an expression of power present in a circuit relative to a known fixed amount (i.e., 1 milliwatt) and the circuit impedance is irrelevant.
  


     Ex. :  A length of coaxial transmission line is being fed  with 150 watts from a transmitter, but the power    measured at the output end of the line is only 112 watts.  What is the line loss in dB?

                             
                    G = 10 log(112/150)

                    G = 10 log 0.747

                    G = 10 (-0.127)

                    G = -1.27 dB


 

About Gain

Question: It is found that the difference between the input and output optical power is not 23dB during WBA/WLA/WPA test. It is always much bigger than 23. Why?

Answer: It is because of the noise. When we use WBA/WLA/WPA to solve the problem of the optical power (attenuation), SNR decrease problem is also involved. There are a lot of noise components in the amplified signal. When it is found that the “gain” is quite large in optical power test, in fact the gain of the signal is still 23dB.If a spectrum analyzer is used to make the test, the result will be more accurate.

About Optical Fiber Used by WDM System

Question: Why is G.655 optical fiber used now?

Answer: G.652 optical fiber is still mostly often used. Its dispersion coefficient is high (17~20ps/nm. Km). When the rate of optical signal is high, the dispersion widens the digital signal pulse which fails to be transmitted for a long distance. It is the so-called “dispersion limitation”. The higher the signal rate, the more obvious the “limitation”. So, when 10G SDH signal is added, it is desired that the dispersion coefficient of the optical fiber is lower. The dispersion coefficient of G.653 is quite small, which is 0.So G.653 optical fiber is very suitable for the transmission of high-rate TDM signal, such as STM-64 and STM-256 signals. But, in the wavelength division system, not a single wavelength but multi-wavelength signals are transmitted in the same optical fiber Because the dispersion coefficient of the optical fiber is “0”, multiple wavelengths can easily generate 4-wavelength frequency mixing effect. This limits the transmission of DWDM signal on G.653.So it is desired that there is such an optical fiber which should have some dispersion, but the dispersion is not too large. Then here is the G.655 optical fiber. This kind of fiber is very popular and in wide application now. Its small dispersion coefficient is applicable to the transmission of high-rate TDM signal and can effectively suppress 4-wavelength frequency mixing effect. So it is the most suitable optical fiber for DWDM system.

About DWDM Applications

Question: Wavelength division system includes 3 types: open, half-open and integrated. What is their difference?

Answer: The light source used by the wavelength division system must be G.692 signal. Simply speaking, at least optical signal of a fixed wavelength is needed. Generally, SDH optical interface does not satisfy the requirement. A conversion unit is needed. Generally speaking, if a conversion unit is used to convert non-G.692 signal into G.692 signal at the transmitting end and the signal is recovered at the receiving end, that is an open application. If the optical interface to be added with wavelength division signals satisfy the requirement of G.692 signal, the interface can be used without the optical conversion unit. At the time, it is an integrated application. Sometimes, if a conversion unit is used at the transmitting end but not used at the receiving end, that is a half-open application. For W32 system, the system using TWC and RWC is an open system. The system using neither TWC nor RWC is an integrated one and the system using TWC but no RWC is a half-open one.

About Dispersion Compensation 1

Question: It is known that when STM-64-based DWDM is transmitted on G.652 optical fiber, dispersion compensation technique is needed. If STM-16, STM-4 or STM-1 signals are added, is there any counteraction? Is it allowed?

Answer: The question is actually about dispersion “overcompensation”. The answer is No.Low-rate TDM signal is not compensated, because the interval between the digital pulses is wide, and no compensation is required. Width extension does not affect the receiving of the receiver. So it is not because compensation can not be made,, but usually compensation is unnecessary. If compensation is made, it would be better and “overcompensation” will not occur.

About Dispersion Compensation 2

Question: When STM-64-based DWDM is transmitted on G.652 optical fiber, dispersion compensation technique is needed. So optical fiber with dispersion compensation is used. But compensation is unnecessary for G.655. Is that right?

Answer: No. The dispersion coefficient of G.655 optical fiber is small, but it does not mean that the dispersion coefficient is 0.When the transmission distance is great, dispersion compensation is still necessary.

About Repeater & Regenerator

Question: What is the difference b/w Repeater and Regenerator?

Answer: I think there is no difference. Lets see the definitions of both

Repeater:- A device that regenerates and propagates electric signals b/w two network segments.  Or Device that restores a degraded digital signal for continued transmission; also called a regenerator.

Regenerator:- A Device that restores a degraded digital signal for continued transmission; also called a Repeater. 
 

About Bit rate & Baud rate

Question: What is the difference b/w bit rate and baud rate?

Answer:- Bit rate is the measurement of the signaling rate at which bits are transmitted by the adapter and is usually expressed in bps. Where as Baud rate is the modulation rate of the MODEM. Generally the telephone network can accommodate 2,400 bit signal changes per second. If  each modulation of the modem represents on bit of information per signal change, the bps will equal the baud. over the time these two terms have come to be used synonymously but they don't necessarily mean the same. But also interchangeability has almost become accepted. example:-  2400 baud = 2400 bps

for calculations:

Baud . X = bps         where, X =  bits per modulation

  Example: 1200 baud with 2-bit modulation = 2400 bps

                    2400 baud with 4-bit modulation = 9600 bps

                    2400 baud with 8-bit modulation = 19.2 k bps

About BER (Bit Error Rate)

Question: What is BER & how we can calculate it?

Answer: Percentage of bits in a transmittal received in error. Or no: of coding violations detected in a unit of time, usually measured in one second.

let     x= Error count/km/hour

BER = x / (No. of Bits)

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