Iec 61280 2 11 2006

NORME INTERNATIONALECEI IEC INTERNATIONAL STANDARD 61280-2-11 Première éditionFirst edition2006-01 Procédures d'essai des sous-systèmes de télécommunications à fibres optiques – Partie

Contexte

Monitoring signal quality is crucial for the operation and maintenance of Optical Transport Networks (OTN) From the network operator's perspective, monitoring techniques are essential for establishing agreements related to connections, protection, recovery, and service levels To implement these functions, the monitoring techniques must meet specific general requirements: non-intrusive in-service measurements, detection of signal deterioration (including SNR degradation and waveform distortion), fault isolation (identifying affected sections or nodes), transparency and prioritization (regardless of signal bit rate and formats), and simplicity (small size and low cost).

There are various methods, both analog and digital, that enable the detection of different deficiencies, including the estimation of the bit error rate (BER), block error detection, optical power measurement, and performance evaluation.

The optical SNR is assessed through spectrum measurement, pilot tone detection, and quality factor monitoring Additionally, pseudo BER estimation is performed using two decision circuits, along with evaluation via histograms and synchronous eye diagram measurement A key parameter for monitoring the performance of any digital transmission system is its end-to-end BER.

However, the Bit Error Rate (BER) can only be accurately assessed through external service BER measurements, utilizing a known test bit representation instead of the actual signal.

Additionally, in-service measurements can only provide rough estimates by assessing numerical parameters, such as Bit Error Rate (BER) estimation, block error detection, and error counting in self-corrections, or by evaluating analog parameters.

(par exemple SNR optique et facteur de qualité)

We have extensively researched various signal quality monitoring methods that provide effective quality measurements without the complexity of termination When the system's Bit Error Rate (BER) is too low to be measured within a reasonable timeframe, adopting quality factor measurements becomes beneficial However, all sampling-based methods require synchronization and subsequent analysis, making them comparable to protocol-related termination in terms of cost and complexity In fact, synchronous sampling necessitates temporal extraction using complex equipment tailored to each specific BER and format.

Recently, the situation has begun to improve with the development of a simple and asynchronous histogram method for measuring quality factors This method allows for the monitoring of various types of degradations, such as SNR degradation and wavelength distortion due to chromatic dispersion, providing insights into the origins of these degradations Asynchronous sampling enables quality factor monitoring independent of bit rate, with the same equipment supporting bit rates of up to 160 Gbit/s Additionally, the monitoring applies to both NRZ and RZ optical signals and is independent of the bit rate and signal format used in wavelength division multiplexed (WDM) channels Performance monitoring can be conducted at various points, including optical line repeater sites, regenerators, or optical switching points.

This method requires a pre-measurement and is intended for monitoring points where electrical termination is not feasible In the context of a future all-optical network, an optical switching point can monitor performance without the need for electrical regeneration.

The measurement of the average quality factor, Q avg, through asynchronous sampling offers a cost-effective alternative to Bit Error Rate (BER) measurements This method represents a promising approach for performance monitoring in optical transmission systems.

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

Signal quality monitoring is crucial for the effective operation and maintenance of optical transport networks (OTN) Network operators need reliable monitoring techniques to facilitate connections, ensure protection, enable restoration, and uphold service level agreements To achieve these objectives, the monitoring methods must meet essential criteria, including in-service (non-intrusive) measurement, detection of signal deterioration such as SNR degradation and waveform distortion, and effective fault isolation.

(localize impaired sections or nodes), transparency and scalability (irrespective of the signal bit rate and signal formats), and simplicity (small size and low cost)

Various methods, including both analog and digital techniques, enable the detection of different impairments Key approaches include bit error rate (BER) estimation, error block detection, and optical power measurement Additionally, evaluating optical signal-to-noise ratio (SNR) through spectrum measurement, pilot tone detection, Q-factor monitoring, and pseudo BER estimation using dual decision circuits are also effective strategies.

A key performance metric for digital transmission systems is the end-to-end Bit Error Rate (BER), which can only be accurately assessed through external service measurements using a known test bit pattern In contrast, in-service measurements offer only approximate estimates by evaluating digital parameters such as BER estimation, error block detection, and error count in forward error correction, as well as analog parameters like optical Signal-to-Noise Ratio (SNR) and Q-factor.

Effective signal quality monitoring methods are highly sought after, aiming to deliver accurate assessments of signal quality without the need for complex termination processes In situations where the system's Bit Error Rate (BER) is too low to be measured in a practical timeframe, adopting alternative approaches becomes essential.

Q-factor measurements involve sampling-based methods that necessitate synchronization and subsequent analysis, leading to costs and complexities akin to protocol-aware termination Specifically, synchronous sampling demands intricate timing extraction using specialized equipment tailored to each Bit Error Rate (BER) and format.

Recent advancements have introduced a simple, asynchronous histogram method for measuring Q-factor, allowing for the monitoring of various degradation types, such as SNR degradation and wavelength distortion caused by chromatic dispersion This method provides valuable insights into the origins of these degradations.

Formule du facteur de qualité moyenné

La Figure 1 utilise un diagramme de l'œil asynchrone type et son histogramme d'amplitude obtenu par l'échantillonnage optique asynchrone pour illustrer le principe de la méthode

Among the sampling points that make up the histogram, those with a level exceeding a predetermined threshold of th1 are classified as belonging to the "Mark" level (i.e., 1), while points with a level below the predetermined threshold of th0 are classified as belonging to the "Space" level (i.e., 0).

Le facteur de qualité moyenné, Q avg , est défini par

The average quality factor, denoted as \$Q_{avg}\$, is calculated using the formula \$Q_{avg} = \frac{|à_{1,avg} - à_{0,avg}|}{(\sigma_{1,avg} + \sigma_{0,avg})}\$ (1) Here, \$à_{i,avg}\$ and \$\sigma_{i,avg}\$ represent the mean and standard deviation of the brand (i = 1) and space (i = 0) level distributions, respectively [12-17] The data obtained through asynchronous sampling includes involuntary crossover point data in the eye diagram, which diminishes the measured value of the averaged quality factor Two threshold levels, \$à_{th1}\$ and \$à_{th0}\$, are established to eliminate crossover point data.

Figure 1 – Diagramme de l'œil asynchrone et histogramme d'amplitude

The essence of this method lies in the analysis of eye diagrams without the use of temporal extraction This approach offers a signal format, modulation format, and flexibility in bit rate.

Marque à 1,avg σà σ 1,avg σà σ 0,avg à 0,avg

The averaged Q-factor, derived from amplitude histogram parameters such as standard deviation and average level, effectively monitors the overall impact of optical signal quality degradations caused by factors like ASE and chromatic dispersion This measure remains unaffected by timing jitter due to the asynchronous sampling scheme The subsequent sections will define the averaged Q-factor and outline a method for assessing optical signal quality through this metric Additionally, amplitude histogram parameters enable the identification of the sources of Bit Error Rate (BER) degradation, including SNR degradation and waveform distortion Detailed information regarding the relationship between amplitude histogram parameters, OSNR, and chromatic dispersion is provided in the Annex.

Figure 1 demonstrates the principle of asynchronous optical sampling through a typical asynchronous eye-pattern and its amplitude histogram In this histogram, sampling points above a specified threshold level, denoted as àth1, are classified as level "Mark" (or "1"), while points below another threshold level, àth0, are categorized as level "Space" (or "0").

The averaged Q-factor, Q avg , is defined by

The average Q-factor is calculated using the formula \$ Q_{avg} = \frac{|à_{1,avg} - à_{0,avg}|}{(σ_{1,avg} + σ_{0,avg})} \$, where \$ à_{i,avg} \$ and \$ σ_{i,avg} \$ represent the mean and standard deviation of the Mark (i = 1) and Space level (i = 0) distributions, respectively Asynchronous sampling can introduce unwanted cross-point data in the eye-diagram, leading to a reduced measured value of the averaged Q-factor To mitigate this issue, two threshold levels, \$ à_{th1} \$ and \$ à_{th0} \$, are established to eliminate the cross-point data.

Figure 1 – Asynchronous eye-pattern and amplitude histogram

This method evaluates asynchronous eye diagrams without relying on timing extraction, offering flexibility in signal format, modulation format, and bit rate.

Mark à 1,av g σà σ 1,av g σà σ 0,av g à 0,av g

PROCÉDURES D'ESSAI DES SOUS-SYSTÈMES DE TÉLÉCOMMUNICATIONS À FIBRES OPTIQUES –

Partie 2-11: Systèmes numériques – Détermination du facteur de qualité moyenné par l'évaluation d’histogramme d'amplitude pour la surveillance de la qualité des signaux optiques

This section of IEC 61280 defines the average quality factor and provides a procedure for measuring it using amplitude histogram parameters such as standard deviation and mean level.

The average quality factor and amplitude histogram parameters are essential for monitoring changes in optical signal quality within installed optical networks The average quality factor is correlated with the traditional Q parameter of a given optical channel.

Using the averaged quality factor testing method, it is possible to monitor the degradation of optical signal quality due to the decline in optical signal-to-noise ratio (OSNR) and waveform distortion.

• la dégradation du rapport signal à bruit optique (OSNR) due aux causes suivantes:

– accumulation d’émission spontanée amplifiée (ESA);

– perte de ligne de transmission;

• distorsion de la forme d'onde due aux causes suivantes (généralement présentes simultanément):

– dispersion en mode polarisation (polarization mode dispersion – PMD);

In some cases, the primary cause of signal degradation, whether it be OSNR degradation or waveform distortion, can be identified through appropriate processing of the measured data.

NOTE 2 Cette méthode n'est pas sensible aux variations de la qualité des signaux optiques créées par la l'instabilité de synchronisation

The following reference documents are essential for the application of this document For dated references, only the cited edition is applicable For undated references, the latest edition of the referenced document (including all amendments) is applicable.

CEI 61280-2-2: Procédures d'essai des sous-systèmes de télécommunications à fibres optiques – Partie 2-2: Systèmes numériques – Mesure du diagramme de l'œil optique, de la forme d'onde et du taux d'extinction

Recommandation UIT T G.959.1: Interfaces de la couche physique des réseaux de transport optique

Part 2-11: Digital systems – Averaged Q-factor determination using amplitude histogram evaluation for optical signal quality monitoring

This part of IEC 61280 defines the averaged Q-factor and provides a procedure to measure it by using amplitude histogram parameters such as the standard deviation and average level

The averaged Q-factor and amplitude histogram parameters are essential for tracking variations in optical signal quality within existing optical networks The average Q-factor is directly related to the conventional Q parameter of a specific optical channel.

With the averaged Q-factor test method signal, quality degradations due to optical signal-to- noise ratio (OSNR) degradation and to waveform distortion can be monitored:

• OSNR degradation, due to the following causes:

– accumulation of amplified spontaneous emission (ASE);

• waveform distortion, due to the following causes (usually simultaneously present):

NOTE 1 In some cases the main cause of signal degradation (OSNR degradation or waveform distortion) can be identified by suitable processing of the measured data

NOTE 2 This method is insensitive to the optical signal quality variations created by timing jitter

The following referenced documents are indispensable for the application of this document

For dated references, only the edition cited applies For undated references, the latest edition of the referenced document (including any amendments) applies

IEC 61280-2-2: Fibre optic communication subsystem basic test procedures – Part 2-2: Test procedures for digital systems – Optical eye pattern, waveform, and extinction ratio

ITU-T Recommendation G.959.1: Optical transport network physical layer interfaces

Pour les besoins du présent document, les définitions et les termes suivants s’appliquent

3.1 facteur de qualité moyenné paramètre mesuré de manière asynchrone sur des signaux numériques optiques actifs pour surveiller la qualité de ces signaux

NOTE Ce paramètre est corrélé au traditionnel paramètre de facteur de qualité

ASE émission spontanée amplifiée (amplified spontaneous emission)

BER taux d'erreur binaire (bit error ratio)

EDFA amplificateur à fibre dopée à l'erbium (Erbium-doped fibre amplifier)

IM-DD modulé en intensité – détection directe (intensity modulated direct detection)

NRZ non-retour à zéro (Non-return-to-zero)

OBPF filtre passe-bande optique

OSNR rapport signal sur bruit optique (optical signal-to-noise ratio)

OTN réseaux de transport optiques (optical transport networks)

PMD dispersion en mode de polarisation (polarization mode dispersion)

RZ retour à zéro (return-to-zero)

SNR rapport signal sur bruit (Signal-to-noise ratio)

WDM multiplexage par répartition en longueur d'onde (wavelength division multiplexing)

The fundamental components of the measurement system include an optical bandpass filter, a receiver, a clock pulse generator, an electrical pulse generator, a sampling module, and a signal processing circuit.

In typical cases, the average quality factor measurement is conducted immediately after the optical amplifier in the line or following regenerative repeaters, optical interconnections, and other matrix nodes The signal power used for measuring signal quality can be adjusted to ensure it does not fall below the optical signal-to-noise ratio (OSNR) at the input of the optical amplifier.

Des descriptions plus détaillées de l'équipement sont données aux paragraphes suivants

For the purposes of this document, the following term and definition apply

3.1 averaged Q-factor parameter measured asynchronously on live optical digital signals for the purpose of monitoring the quality of those signals

NOTE This parameter is correlated with the traditional Q-factor parameter

EDFA Er-doped fibre amplifier

IM-DD intensity modulated direct detection

NRZ non-return-to-zero

OSNR optical signal-to-noise ratio

SNR signal-to-noise ratio

The primary components of the measurement system are an optical bandpass filter, a receiver, a clock oscillator, an electrical pulse generator, a sampling module and a signal processing circuit, as shown in Figure 2

In the typical case, the average Q-factor measurement is performed just after the optical amplifier of the line or regenerator repeaters, optical cross-connects, and other fabric nodes

Filtre passe-bande optique

The optical bandpass filter (OBPF) is essential for minimizing unwanted ASE noise from the optical amplifier and for isolating a specific channel from the WDM signal It is important that the optical bandwidth, \$B_{opt}\$, is wider than the bit rate of the optical signal The relationship between the average quality factor, \$Q_{avg}\$, and \$B_{opt}\$ is detailed in Annex A The design of the OBPF is outlined in ITU-T G.959.1, specifically in Figure B.2, which defines two key parameters: the adjacent channel power suppression ratio and the central frequency deviation Characteristics influenced by these parameters are provided in Annex B.

Récepteur

Le récepteur est normalement une photodiode rapide suivie par une amplification électrique

Le récepteur est équipé d'un connecteur optique approprié pour permettre la connexion au point d'interface optique soit directement soit via une jarretière optique

It is challenging to provide precise specifications due to the wide variety of possible installations However, the receiver should adhere to general guidelines based on IEC 61280-2-2, which include: a) an acceptable input wavelength range suitable for the intended application; b) appropriate sensitivity to generate an asynchronous eye diagram.

For instance, consider measuring a non-return-to-zero (NRZ) optical data stream with an average optical power of -15 dBm If the signal processing circuit's sensitivity with the sampling module is 10 mV, this sensitivity plays a crucial role in the performance of the system.

790 V/W est nécessaire pour produire un diagramme de l'œil de 50 mV crête à crête c) puissance optique équivalente du bruit, suffisamment faible pour donner une mesure précise

For instance, consider measuring a non-return-to-zero (NRZ) optical data stream with an average optical power of -15 dBm If the effective noise bandwidth of the measurement system is 470 MHz and the displayed noise squared must be less than 5% of the height of the asynchronous eye diagram, the equivalent noise power must be 145 pW-Hz$^{-1/2}$ or lower.

Circuit de traitement du signal

Mesure du facteur de qualité moyenne Répéteur ou noeud de commutation

Figure 2 – Averaged Q-factor measurement configuration

The optical bandpass filter (OBPF) is essential for minimizing unwanted amplified spontaneous emission (ASE) noise from optical amplifiers and for isolating desired channels from wavelength-division multiplexing (WDM) signals It is crucial that the bandwidth of the optical filter, denoted as \$B_{opt}\$, exceeds the bit rate of the optical signal to ensure optimal performance.

Q avg on B opt is described in Annex A The shape of OBPF is defined in ITU-T G.959.1/Figure

B.2, where two parameters, the power suppression ratio of adjacent channel and the central frequency deviation, are defined The characteristics depending on these parameters are shown in Annex B

The receiver consists of a high-speed photodiode paired with electrical amplification, and it features a suitable optical connector for direct connection to the optical interface point or through an optical jumper cable.

Precise specifications are precluded by the wide variety of possible implementations

The receiver must adhere to the general guidelines outlined in IEC 61280-2-2, which include ensuring an acceptable input wavelength range suitable for the intended application and achieving responsivity sufficient to generate an asynchronous eye-pattern.

To measure a non-return-to-zero (NRZ) optical data stream with an average optical power of –15 dBm, a responsivity of 790 V/W is necessary to achieve an asynchronous eye-pattern of 50 mV peak-to-peak, given that the signal processing circuit's sensitivity is 10 mV Additionally, the optical noise-equivalent power must be sufficiently low to ensure accurate measurements.

To measure a non-return-to-zero (NRZ) optical data stream with an average optical power of –15 dBm, the effective noise bandwidth of the measurement system must be 470 MHz To ensure that the displayed root-mean-square noise remains below 5% of the asynchronous eye-pattern height, the optical noise-equivalent power should be 145 pW-Hz$^{-1/2}$ or lower.

Averaged Q-factor measurement circuit Repeater or switching node

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU. d) fréquence supérieure de coupure (–3 dB), B re Hz;

Pour assurer la répétabilité et la précision, il convient que la fréquence supérieure de coupure (largeur de bande), B re , du récepteur soit indiquée explicitement dans les spécifications particulières

For NRZ format signals, this type of receiver typically has a bandwidth slightly less than the clock frequency A low-pass filter with a -3dB bandwidth of 0.75/T (where T is the binary interval in seconds of the data signal) is commonly employed for this measurement In contrast, RZ format signals can exhibit significantly higher spectral content compared to NRZ signals at the same bit rate.

Ceci peut donner une largeur de bande de récepteur dépassant la fréquence d'horloge e) fréquence inférieure de coupure (–3 dB), 0 Hz;

DC coupling is essential for two main reasons Firstly, it allows for accurate extinction ratio measurements that cannot be achieved through other methods Secondly, using AC coupling can lead to significant distortion in the measured signal due to low-frequency spectral components, which can affect the asynchronous eye diagram through amplitude modulation It is important that transient response, positive overshoot, negative overshoot, and other waveform aberrations remain minimal to avoid interference with measurements.

The upper cutoff frequency (bandwidth), B re, of the receiver primarily influences the system's transient response Additionally, it is essential to have a sufficiently high output impedance matching factor to effectively eliminate reflections from the sampling module that follows the receiver, ranging from 0 Hz to a frequency significantly higher than the receiver's bandwidth.

In the time domain, measurements can be highly inaccurate due to significant multiple reflections It is advisable to maintain a minimum adaptation factor of 15 dB when multiple components are used after the receiver The effective output adaptation factor of the receiver can be enhanced using inline electrical attenuators, although this may result in reduced signal levels Additionally, specifying the adaptation factor is crucial for direct current (DC) applications; otherwise, a DC offset in the waveform may lead to errors in averaged quality factor measurements.

Générateur d'impulsions d'horloge

The clock pulse generator delivers a clock signal that matches the sampling rate This clock signal is not synchronized with the optical signal for asynchronous sampling and is transmitted to both an electrical pulse generator and a signal processing circuit The frequency of the clock signal can range from 1 MHz to 1 GHz.

Générateur d'impulsions électriques

The electric pulse generator must deliver a sequence of electrical pulses to the sampling module The repetition rate of these electrical pulses should align with the sampling rate for optimal performance.

Module d'échantillonnage

The sampling module must sample optical signals at a specified repetition rate and sampling time width (sampling window) using a train of electrical pulses generated by a pulse generator, while detecting the level of the sampled signals The sampled values are then sent to the signal processing circuit.

La précision de Q avg qui dépend de la fenêtre d'échantillonnage ou de la résolution temporelle

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU. d) upper cut-off (–3 dB) frequency, B re Hz;

In order to ensure repeatability and accuracy, the upper cut-off frequency (bandwidth), B re , of the receiver should be explicitly stated in the detail specifications

In NRZ format signals, receivers generally have a bandwidth slightly below the clock frequency, often utilizing a low-pass filter with a –3 dB bandwidth of 0.75/T, where T represents the bit interval in seconds Conversely, RZ format signals can exhibit significantly higher spectral content than NRZ signals at the same bit rate, resulting in a receiver bandwidth that may exceed the clock frequency The lower cut-off frequency is typically set at 0 Hz.

DC coupling is essential for accurate extinction ratio measurements and to prevent significant distortion from low-frequency spectral components when using AC coupling Additionally, it is important that transient response, overshoot, undershoot, and other waveform aberrations remain minimal to avoid interference with the measurement.

The upper cut-off frequency, or bandwidth (B re), of the receiver is crucial in defining the system's transient response Additionally, it is essential to ensure that the output electrical return loss is sufficiently high to effectively suppress reflections from the sampling module that follows the receiver, covering a range from 0 Hz to a frequency well above the receiver's bandwidth.

Accurate time-domain measurements can be compromised by significant multiple reflections, making a minimum return loss of 15 dB advisable when multiple components are used after the receiver In-line electrical attenuators can enhance the effective output return loss of the receiver, although this may lead to lower signal levels Additionally, the return loss specification must include direct current (d.c.) to prevent waveform shifts that could result in erroneous averaged Q-factor measurements.

The clock oscillator generates a clock signal that matches the sampling rate, operating asynchronously with the optical signal This clock signal is transmitted to both an electrical pulse generator and a signal processing circuit, with frequencies varying between 1 MHz and 1 GHz.

The electrical pulse generator must deliver a pulse train to the sampling module, ensuring that the repetition rate aligns with the sampling rate for optimal performance.

The sampling module captures optical signals at a defined repetition rate and sampling time width using an electrical pulse train from a generator, allowing for the detection of sampled signal levels These sampled values are then transmitted to the signal processing circuit for further analysis.

The accuracy of Q avg depending on the sampling window or temporal resolution T res is shown in Annex A

Circuit de traitement de signal

The signal processing circuit is responsible for calculating the amplitude histogram using the sampled values from the sampling module and the clock signal from the clock pulse generator It then computes the averaged quality factor based on the amplitude histogram A detailed description of the relevant signal processing is provided in Article 6.

Paramètres du système de surveillance

To measure the average quality factor, \$Q_{avg}\$, it is essential to appropriately select the measurement system parameters The optical filter bandwidth, \$B_{opt}\$, determines the bandwidth and optical signal-to-noise ratio (SNR) of the optical signal being processed The receiver bandwidth, \$B_{re}\$, is influenced by the O/E converter and the low-pass filter, affecting the waveform and SNR of the electrical signal from the receiver The temporal resolution, \$T_{res}\$, refers to the resolution of the opening process, which is determined by the width of the electrical sampling pulses This resolution is independent of the sampling clock instability, as the sampling in this case is asynchronous.

The sampling number, \$N_{\text{samp}}\$ , refers to the total number of points sampled to create the amplitude histogram The sampling rate, \$R_{\text{samp}}\$ , indicates the clock repetition rate of the sampling process Although it is not directly related to the average quality, \$Q_{\text{avg}}\$ , it affects the total sampling time, \$T_{\text{samp}}\$ , or the overall measurement time The relationship between the parameters \$T_{\text{samp}}\$ , \$N_{\text{samp}}\$ , and \$R_{\text{samp}}\$ is given by the equation: \$$T_{\text{samp}} = \frac{N_{\text{samp}}}{R_{\text{samp}}}\$$ These monitoring system parameters are summarized in Table 1.

Tableau 1 – Paramètres du système de surveillance

Q avg Facteur de qualité moyenné

B opt Largeur de bande du filtre optique

B re Largeur de bande du récepteur

Connexions de l'appareillage

Connect the equipment as shown in Figure 2 An EDFA is only required if the power from the transmission line is insufficient to deliver a sufficiently high signal level to the receiver.

When an EDFA is utilized, the ASE from the EDFA alters the OSNR Therefore, it is essential to verify that the required measurement of the average quality factor can be achieved.

Background

Signal quality monitoring is crucial for the effective operation and maintenance of optical transport networks (OTN) Network operators need reliable monitoring techniques to facilitate connections, ensure protection, enable restoration, and uphold service level agreements To achieve these objectives, the monitoring methods must meet essential criteria, including in-service (non-intrusive) measurement, detection of signal deterioration such as SNR degradation and waveform distortion, and effective fault isolation.

(localize impaired sections or nodes), transparency and scalability (irrespective of the signal bit rate and signal formats), and simplicity (small size and low cost)

Various methods, including both analog and digital techniques, enable the detection of different impairments These methods include bit error rate (BER) estimation, error block detection, optical power measurement, and optical signal-to-noise ratio (SNR) evaluation through spectrum measurement Additionally, pilot tone detection, Q-factor monitoring, and pseudo BER estimation using two decision circuits are also employed for effective impairment detection.

A key performance metric for digital transmission systems is the end-to-end Bit Error Rate (BER), which can only be accurately assessed through external service measurements using a known test bit pattern In contrast, in-service measurements offer only approximate estimates by evaluating digital parameters such as BER estimation, error block detection, and error count in forward error correction, as well as analog parameters like optical Signal-to-Noise Ratio (SNR) and Q-factor.

Effective signal quality monitoring methods are highly sought after, aiming to deliver accurate assessments of signal quality without the complications of termination In situations where the system's Bit Error Rate (BER) is too low to be measured in a practical timeframe, adopting alternative approaches becomes essential.

Q-factor measurements involve sampling-based methods that necessitate synchronization and subsequent analysis, leading to costs and complexities akin to protocol-aware termination Specifically, synchronous sampling demands intricate timing extraction using specialized equipment tailored to each Bit Error Rate (BER) and format.

Recent advancements have led to the development of a simple, asynchronous histogram method for measuring Q-factor This method allows for the monitoring of various types of degradation, such as signal-to-noise ratio (SNR) degradation and wavelength distortion caused by chromatic dispersion, thereby offering insights into the sources of these degradations.

Asynchronous sampling enables bit-rate independent Q-factor monitoring, accommodating bit rates of up to 160 Gbit/s This monitoring technique is applicable to both NRZ and RZ optical signals, remaining unaffected by the bit rate and signal format of the wavelength division multiplexed (WDM) channel Performance monitoring can be conducted at various points, including optical line repeaters, regenerators, or optical switching nodes, which may require pre-measurement This approach is particularly beneficial for monitoring points where electrical termination is not feasible In the context of future all-optical networks, optical switching nodes can perform monitoring without the need for electrical regeneration.

The average Q-factor, denoted as Q avg, can be measured through asynchronous sampling, offering a cost-effective alternative to traditional Bit Error Rate (BER) measurements This method is a promising approach for performance monitoring in intensity modulated direct detection (IM-DD) optical transmission systems, enabling the assessment of both relative and absolute optical signal quality values.

Licensed to MECON Limited for internal use in Ranchi and Bangalore, this document is supplied by Book Supply Bureau The method of intensity modulated direct detection (IM-DD) can be utilized for monitoring both relative and absolute values of optical signal quality.

The average quality factor derived from amplitude histogram parameters (standard deviation and mean level) allows for monitoring the overall impact of optical signal quality degradations caused by various factors, such as ASE and chromatic dispersion Due to the asynchronous sampling scheme, the average quality factor remains unaffected by signal quality variations resulting from synchronization instability The following sections define the average quality factor and outline a procedure for measuring optical signal quality using this factor Additionally, amplitude histogram parameters enable the identification of the sources of Bit Error Rate (BER) degradation, including Signal-to-Noise Ratio (SNR) degradation and waveform distortion Information regarding the dependence of amplitude histogram parameters on OSNR and chromatic dispersion is provided in Appendix F.

0.2 Formule du facteur de qualité moyenné

La Figure 1 utilise un diagramme de l'œil asynchrone type et son histogramme d'amplitude obtenu par l'échantillonnage optique asynchrone pour illustrer le principe de la méthode

Among the sampling points that make up the histogram, those with a level exceeding a predetermined threshold, th1, are classified as belonging to the "Mark" level (i.e., 1), while points with a level below the predetermined threshold, th0, are classified as belonging to the "Space" level (i.e., 0).

Le facteur de qualité moyenné, Q avg , est défini par

The average quality factor, denoted as \$Q_{avg}\$, is calculated using the formula \$Q_{avg} = \frac{|à_{1,avg} - à_{0,avg}|}{(\sigma_{1,avg} + \sigma_{0,avg})}\$ (1) Here, \$à_{i,avg}\$ and \$\sigma_{i,avg}\$ represent the mean and standard deviation of the brand (i = 1) and space (i = 0) level distributions, respectively [12-17] The data obtained through asynchronous sampling includes information from involuntary crossover points in the eye diagram, which diminishes the measured value of the averaged quality factor Two threshold levels, \$à_{th1}\$ and \$à_{th0}\$, are established to eliminate crossover point data.

Figure 1 – Diagramme de l'œil asynchrone et histogramme d'amplitude

The essence of this method lies in the absence of temporal extraction, focusing instead on the analysis of eye diagrams This approach offers a signal format, modulation format, and flexibility in bit rate.

Marque à 1,avg σà σ 1,avg σà σ 0,avg à 0,avg

The averaged Q-factor, derived from amplitude histogram parameters such as standard deviation and average level, effectively monitors the overall impact of optical signal quality degradations caused by factors like ASE and chromatic dispersion This measure remains unaffected by timing jitter due to the asynchronous sampling scheme The subsequent sections will define the averaged Q-factor and outline a method for assessing optical signal quality through this metric Additionally, amplitude histogram parameters enable the identification of the sources of Bit Error Rate (BER) degradation, including SNR degradation and waveform distortion Detailed information regarding the relationship between amplitude histogram parameters, OSNR, and chromatic dispersion is provided in the Annex.

Averaged Q-factor formula

Figure 1 demonstrates the principle of asynchronous optical sampling through a typical asynchronous eye-pattern and its amplitude histogram In this histogram, sampling points above a specified threshold level, denoted as àth1, are classified as level "Mark" (or "1"), while points below a different threshold level, àth0, are categorized as level "Space" (or "0").

The averaged Q-factor, Q avg , is defined by

The average Q-factor is calculated using the formula \$ Q_{avg} = \frac{|à_{1,avg} - à_{0,avg}|}{(σ_{1,avg} + σ_{0,avg})} \$, where \$ à_{i,avg} \$ and \$ σ_{i,avg} \$ represent the mean and standard deviation of the Mark (i = 1) and Space (i = 0) distributions, respectively Asynchronous sampling can introduce unwanted cross-point data in the eye-diagram, leading to a reduced measured value of the averaged Q-factor To mitigate this issue, two threshold levels, \$ à_{th1} \$ and \$ à_{th0} \$, are established to eliminate the cross-point data.

Figure 1 – Asynchronous eye-pattern and amplitude histogram

This method evaluates asynchronous eye diagrams without relying on timing extraction, offering flexibility in signal format, modulation format, and bit rate.

Mark à 1,av g σà σ 1,av g σà σ 0,av g à 0,av g

PROCÉDURES D'ESSAI DES SOUS-SYSTÈMES DE TÉLÉCOMMUNICATIONS À FIBRES OPTIQUES –

Partie 2-11: Systèmes numériques – Détermination du facteur de qualité moyenné par l'évaluation d’histogramme d'amplitude pour la surveillance de la qualité des signaux optiques

This section of IEC 61280 defines the average quality factor and provides a procedure for measuring it using amplitude histogram parameters such as standard deviation and mean level.

The average quality factor and amplitude histogram parameters are essential for monitoring changes in optical signal quality within installed optical networks The average quality factor is correlated with the traditional Q parameter of a given optical channel.

Using the average quality factor test method, it is possible to monitor the degradation of optical signal quality due to the decline in optical signal-to-noise ratio (OSNR) and waveform distortion.