DWDM Span Loss Measurement - Technopediasite

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Monday, July 6, 2020

DWDM Span Loss Measurement

DWDM Span Loss Measurement : Prior to this article, I wrote " Fundamentals of DWDM ", which you guys liked it very much, Technopediasite is thankful to you all. " DWDM Span Loss Measurement " is also a very special topic, you have to read it completely to understand it. We all know that it is very important to measure the optical parameters of the system during the system installation, Optical spectrum analyzer (OSA) is used to efficiently measure the power in DWDM networks, wavelength and OSNR of each transmitted channel to ensure transmission quality.

How do measure DWDM span loss? And what is the parameter of measurement, in this article, I will discuss all issues related to DWDM span loss. It would not be appropriate to write only DWDM span loss because the test parameter of DWDM also comes in Power levels of the individual carriers, Channel wavelength / Channel spacing, Signal-to-noise ratio and Overall power. Now I will technically consider all the points.
DWDM Span Loss Measurement details
DWDM Span loss


Bit Error Ratio (BER)

BER has a very special role in getting Span loss of DWDM. We can say that BER is a key objective of the Optical System Design. The goal of BER is to get from Tx to Rx with a BER < BER threshold of the Rx.Typical minimum acceptable rate is 10-12
BER testing performance in DWDM system
BER Testing
The number of incorrect bits divided by the total bits transmitted, received, or processed in a given period. Note 1: Examples of bit error ratios are (a) the transmission BER, namely, the number of incorrect bits divided by the total number of transmitted bits; And (b) information BER, that is, the number of incorrectly decoded (true) bits divided by the total number of decoded (true) bits. Note 2: BER is usually expressed as a coefficient and a power of 10; For example, 2.5 out of 100,000 bits transmitted would be a flawed bits 105 or 2.5 out of 10 × 10-5. Note that : 10-12 = 1 bit error in 10 Trillion bits received.


OSA

Testing with OSA for DWDM
DWDM Testing with OSA 

OSA is a detail topic so I will not be able to give complete information about OSA here, if you want complete information about OSA, then you have to comment in inbox.I will only explain here that what do measure with OSA in 

It is important to measure during system installation optical parameters of the system. Optical Spectrum Analyzer (OSA) is used to efficiently measure the power, wavelength, and OSNR of each transmitted channel to ensure transmission quality. in DWDM networks. OSAs used for field measurements as well as in laboratories.

OSA can measure Channel power in dBm, Power stability, Channel center wavelength (0.1 nm resolution) and spacing, Wavelength stability, Optical signal-to-noise ratio (OSNR) for each channel and OSNR stability, Total optical power.

Q-factor meter

After bringing network components together in the network, lined up and adjusted for optimization of the system. At this stage Q-factor meter are crucial given the high number of measurements to be performed on multichannel systems.

In fact Q-factor is a "value" that provides quality-of-signal (QoS) information. Based on OSNR, the Q-factor method can predict very low BERs when conventional techniques prove to be impractical.

Q-factor measurement presents several advantages over bit-error rate (BER) measurement  including like – Data format independence, Short measuring time, In-service measurement, High measurement range down to ultra low bit-error rates.
Tests with  Q-factor meter in DWDM systems
DWDM testing with Q-factor meter


DWDM Network Measurement Parameters

Purely optical parameters: All channels, Power, OSNR tested by OSA.
Quality parameters (related to a single channel) : ‘Logical-Layer’ parameters: LOS,  LOF, B1, B2, FEC (G709) tested by BERT.
Physical-Layer’ parameters (S/N): bit error rate, jitter, wander,Q-factor measured by BERT, Q-meter.

The following DWDM System Parameters results are presented as below:
- Wave: Channel wavelength, 0.1 nm resolution
- Power: Channel level in dBm
- Power stability: Power fluctuations in the channel. Difference between minimum and maximum power level occurring during measurement interval.
- S/N ratio: Signal-to-noise ratio for channel. Difference between channel power level and noise (within specified scan bandwidth)
- S/N ratio stability: Fluctuations in the S/N ratio.
DWDM System Parameters
Parameters of DWDM


Reading out results
All power level and S/N results for the DWDM channels can be read out under remote control and further processed. The stability is represented using the Max/Min values. The transmitted results are given as floating point values separated by commas.

Storing and managing results
Measurement results can be stored with their setup parameters and recalled for later use. Six memory locations are provided. The table values can also be checked with a View function.

As a digital transmission system, the transmission performance of a WDM system is assessed by data transmission errors in bit-error ratio (BER).


There are two causes of transmission errors:
 1. Optical noise, known as amplified spontaneous emission (ASE), is created and stored in optical amplifiers used as ILAs.

2.Built-in optical fiber waveform distortion. Typically, the effect by ASE is more potent than waveform distortion.

Gain Profile Management 

There are two type of gain profile, Gain profile-1 and other is called the gain profile-2. Gain profile management-1 controls gain profiles, or gain-to-wavelength characteristics, to meet the BER requirements of individual channels. The gain profile of an optical amplifier covering a range of wavelengths usually exhibits uneven characteristics.

The purpose of gain profile management-2 is to compress or equalize the wavelength-dependent gain variation across acceptable ranges using GEQ.

DWDM Performance Degradation Factors

Factors limiting the transmission distance
➤OSNR (20dB for 2.5Gb/s, and 26dB for 10Gb/s)
Attenuation (optical fiber)
Chromatic dispersion
PMD (polarized mode dispersion)
Non-linearity effects

Solutions

Narrow-band laser sources
Optical amplifier EDFA
Dispersion compensation 
PMD (unsolved) –Controlled in fiber at source.

The BER performance of a DWDM channel is determined by OSNR, which is delivered by the receiver. This acceptable OSNR is delivered through a relatively sophisticated analysis of signal strength per channel, amplifier distances, and the frequency spacing between channels.


OSNR=POUT-L-NF-10 Log N-10 Log[h vv 0
POUT: Per channel output power(dBm)
L: Attenuation between two amplifiers (dB)
NF : Noise figure of amplifier(dB)
N: number of spans
10 Log [h v△v 0]= - 58 dBm(1.55μm, 0.1nm spectrum width)
The total transmit power is limited by the present laser technology and fiber non linearities .The key factors are the span (L) and the number of spans(N).



Chromatic Dispersion


Chromatic Dispersion – Adverse Effects on Performance
Chromatic Dispersion – Adverse Effects on Performance


The spectrum of an optical channel signal propagates around the center wavelength, or an optical channel signal includes several different wavelengths.

Transmission errors due to pulse waveform distortion due to pulse transition point dispersion. The effective pulse width of the received signal decreases due to the transition point dispersion. A decrease in the effective pulse width can cause transmission error.

Light of different wavelengths propagates at different velocities through an optical fiber. The above two facts make the dispersion of the pulse transition points of the received optical signal.

If the chromatic dispersion accumulated over the line system exceeds the objective, dispersion compensation fibers (DCFs) are required for insertion into the transmission fiber line to reduce the accumulated dispersion.

Theoretically, the DCF insertion point can be anywhere before the receiving input port of a transponder, or the point of 3R function processing. Practically, a DCF is inserted into a TX amplifier, in-line amplifier or RX amplifier for convenience of installation.

Chromatic dispersion causes pulses to “spread out” into adjacent bit slots (bit time intervals). This is a serious problem at 10 Gb/s and beyond, for two reasons. First, higher rates mean broader spectra and more pulse spreading. Second, the narrower bit slots are more susceptible to the spreading of neighboring pulses. For these reasons, the acceptable amount of chromatic dispersion is inversely proportional to the square of the bit rate.


Dispersion management compensates the received signal for dispersion deposited on a fiber transmission line to meet the BER requirement.

Objective for the accumulated chromatic dispersion depends on the bit rate of the channel signal, for example; 8,000 ps/nm for 2.5 Gbps channel signal, or ±300 ps/nm for 10 Gbps channel signal.

Typical chromatic dispersion coefficient of a single-mode fiber (SMF) is; +18 ps/(nm•km) in 1550 nm region.

Calculation of Dispersion

In G.652 optical fibres the dispersion is 17 ps/nm/km

For a signal transmission distance = 100km the total dispersion: 100km * 17 ps/nm/km = 1700 ps/nm

For a signal transmission distance = 640km the total dispersion: 640km * 17 ps/nm/km = 10880 ps/nm.

Polarization Mode Dispersion (PMD)

Polarization Mode Dispersion (PMD)
PMD status in DWDM


Polarization mode dispersion (PMD) is another type of dispersion that occurs in an optical fiber. A traffic-signal-carrying light propagates across two orthogonal polarization states in an optical fiber.

The two states of light propagate at slightly different velocities, causing pulse dispersion to be referred to as polarization mode dispersion (PMD).

Imperfections in the crystalline structure of the fiber or asymmetric stress will generate birefringent dispersion,Polarization mode dispersion will generate signal pulse dispersion,worsen the BER, which will result in regulatig transmission capacity affecting transmission quality.

Differential grup delay will distort spread and disperse optical signals, vector sum of of polarization modes,Polarization mode dispersion is defined as an average of differential group delay for the optical signal waves.


Different polarizations travel with different group velocities. Distribution of signal and the energy over different SOP slowly changes with time.
The Polarization hole burning impacts the system performance by causing the noise build-up along the amplifier chain to be greater than that which would be predicted from simpler linear theory.

He signal to Noise ratio is redu8ced by polarization hole burning and as for the cases of Polarization mode dispersion and polarization dependent loss, the measured Q fluctuates in time.

Link Budget Calculation

To check the system performance over a certain period of time or within distinct time intervals monitoring needs to be performed. Monitoring may also imply that the user handles equipment remotely from a central place like a Network Operations Center (NOC) for example.

Monitoring is possible when the system provides a tap signal. Pre-maintenance monitoring also indicates small corrosion of QoS and the network operator can react to the incident immediately. Degradation effects may come through aging and mechanical stress for example.
Link Budget Calculation
DWDM link budget 

How to access and monitor live DWDM networks ?

How to access and monitor live DWDM networks?
Live DWDM Network

Issues to be considered for channel isolation
➤Power Budget for Test Access
➤Channel peak power at monitor points
➤Insertion loss of channel isolator
➤Sensitivity of test instrument ( BERT, Q-meter,...) Influence on test signal
➤Data rate
➤Stability Combination of Spectral measurement and channel isolation.

Q-factors: back to basics
The optical Q-factor is a parameter that directly reflects the quality of an optical communications signal and depends not on system parameters. The Q-factor is determined by monitoring the amplitude and phase of the (analog) signal at the electrical level. Mathematically Q is related to the power distribution functions (PDF) of ‘0’ and ‘1’ states and so it depends on the electrical signal to noise ratio.

The formula indicates the mathematical calculation of Q on the basis of the mean values µ1 and µ2 and standard deviations 
1 and 2 of the Probability density functions of 0 and 1 states.To check the system performance over a certain period of time or within distinct time intervals monitoring needs to be performed. Monitoring may also imply that the user handles equipment remotely from a central place like a Network Operations Center (NOC) for example.

Monitoring is possible when the system provides a tap signal. Pre-maintenance monitoring shows even small degradation of QoS and the network operator can react quickly on this occurrence. Degradation effects may come through aging and mechanical stress for example.



To check the system performance over a certain period of time or within distinct time intervals monitoring needs to be performed. Monitoring may also imply that the user handles equipment remotely from a central place like a Network Operations Center (NOC) for example.

Monitoring is possible when the system provides a tap signal. Pre-maintenance monitoring also indicates small corrosion of QoS and the network operator can react to the incident immediately. Degradation effects may come through aging and mechanical stress for example.

DWDM Automatic Power Control

In DWDM the power is controlled by the OSC channel. First of all we assume that this is 32 channel DWDM System.The Transponder O/P will go to the OMUX then the 32 wave lengths will combined in the OMUX and transmitted as single wave length.This wavelength is in C- band.
DWDM Automatic Power Control
APC in DWDM


OMUX detects incoming optical signals
Informs how many active channels to ILAMPs and Remote Terminal
Via Optical Supervisory Channel @1510nm
Easily upgrade TxPND cards

The OSC channel is Operated in 1550 nm wavelength.The supervisory signal and Data communication channel signals are sent by this OSC channel. In this DWDM the Laser will not shutdown if any Fiber path has been broken.The laser power is decreased .This information is carried out by the OSC.

DWDM Testing

A helpful way of picturing the management functions of the OTU and ODU layers is to show the actual parts of the network they address.
DWDM testing process
DWDM testing process


The ODU layer is for Mux-to-Mux, or PATH management and performance monitoring. And as you can see in this diagram there is one path (Path X), so there is one set of ODU monitoring parameters.

The OTU is for (network device) ~ to ~ (network device) SECTION management and performance monitoring. In this diagram there are 3 sections (A,B, and C) so there are 3 sets of OTU monitoring parameters.

To plan and implement flexible and future-proof DWDM systems and components, basic standards are required.

Here we can say that this is the only way to ensure that highly variable components and modules from different manufacturers comply with defined interface parameters and interact properly.

As per technical view current recommendations are limited to 4- to 8-channel systems, a wide variety of possible combinations for the system wavelengths in use are usable. Accordingly, different manufacturers must agree on specific focus wavelengths to ensure problem-free interaction of different system elements.

The wavelength range can be divided between 1530 and 1565 nm into two bands. The band below 1545 nm is known as the short band and above 1545 nm is the long band.

System reference points
According to ITU-T Rec. G.692, reference test points are to be provided in DWDM systems:
➤ S1 to Sn are reference points directly at the output of the individual optical transmitters 1 to n of the DWDM system.

➤ RM1 to RMn are reference points for the individual fibers directly before the input of the WDM multiplexer. S' is the test point directly at the output of the WDM multiplexer and R' a further test point directly at the input of the demultiplexer.

➤ SD1 to SDn are the corresponding reference points directly at the output of the demultiplexer and R1 to Rn are the reference points at the input of the individual receiver modules of the DWDM system.

For more concept on DWDM, you can READ ALSO : Fundamentals of DWDM

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