An overview on application of machine learning techniques in optical networks

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Communications Surveys & Tutorials
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An Overview on Application of Machine Learning
Techniques in Optical Networks
Francesco Musumeci, Member, IEEE, Cristina Rottondi, Member, IEEE, Avishek Nag, Member, IEEE, Irene
Macaluso, Darko Zibar, Member, IEEE, Marco Ruffini, Senior Member, IEEE, and Massimo
Tornatore, Senior Member, IEEE
Abstract—Today’s telecommunication networks have become
sources of enormous amounts of widely heterogeneous data. This
information can be retrieved from network traffic traces, network
alarms, signal quality indicators, users’ behavioral data, etc.
Advanced mathematical tools are required to extract meaningful
information from these data and take decisions pertaining to the
proper functioning of the networks from the network-generated
data. Among these mathematical tools, Machine Learning (ML)
is regarded as one of the most promising methodological ap-
proaches to perform network-data analysis and enable automated
network self-configuration and fault management.
The adoption of ML techniques in the field of optical com-
munication networks is motivated by the unprecedented growth
of network complexity faced by optical networks in the last
few years. Such complexity increase is due to the introduction
of a huge number of adjustable and interdependent system
parameters (e.g., routing configurations, modulation format,
symbol rate, coding schemes, etc.) that are enabled by the usage
of coherent transmission/reception technologies, advanced digital
signal processing and compensation of nonlinear effects in optical
fiber propagation.
In this paper we provide an overview of the application of
ML to optical communications and networking. We classify and
survey relevant literature dealing with the topic, and we also
provide an introductory tutorial on ML for researchers and
practitioners interested in this field. Although a good number of
research papers have recently appeared, the application of ML
to optical networks is still in its infancy: to stimulate further
work in this area, we conclude the paper proposing new possible
research directions.
Index Terms—Machine learning, Data analytics, Optical com-
munications and networking, Neural networks, Bit Error Rate,
Optical Signal-to-Noise Ratio, Network monitoring.
I. INTRODUCTION
Machine learning (ML) is a branch of Artificial Intelligence
that pushes forward the idea that, by giving access to the
right data, machines can learn by themselves how to solve a
specific problem [1]. By leveraging complex mathematical and
statistical tools, ML renders machines capable of performing
independently intellectual tasks that have been traditionally
Francesco Musumeci and Massimo Tornatore are with Po-
litecnico di Milano, Italy, e-mail: francesco.musumeci@polimi.it,
massimo.tornatore@polimi.it
Cristina Rottondi is with Dalle Molle Institute for Artificial Intelligence,
Switzerland, email: cristina.rottondi@supsi.ch.
Avishek Nag is with University College Dublin, Ireland, email:
avishek.nag@ucd.ie.
Irene Macaluso and Marco Ruffini are with Trinity College Dublin, Ireland,
email: macalusi@tcd.ie, ruffinm@tcd.ie.
Darko Zibar is with Technical University of Denmark, Denmark, email:
dazi@fotonik.dtu.dk.
solved by human beings. This idea of automating complex
tasks has generated high interest in the networking field, on
the expectation that several activities involved in the design
and operation of communication networks can be offloaded to
machines. Some applications of ML in different networking
areas have already matched these expectations in areas such
as intrusion detection [2], traffic classification [3], cognitive
radios [4].
Among various networking areas, in this paper we focus
on ML for optical networking. Optical networks constitute
the basic physical infrastructure of all large-provider networks
worldwide, thanks to their high capacity, low cost and many
other attractive properties [5]. They are now penetrating new
important telecom markets as datacom [6] and the access
segment [7], and there is no sign that a substitute technology
might appear in the foreseeable future. Different approaches
to improve the performance of optical networks have been
investigated, such as routing, wavelength assignment, traffic
grooming and survivability [8], [9].
In this paper we give an overview of the application of
ML to optical networking. Specifically, the contribution of
the paper is twofold, namely, i) we provide an introductory
tutorial on the use of ML methods and on their application in
the optical networks field, and ii) we survey the existing work
dealing with the topic, also performing a classification of the
various use cases addressed in literature so far. We cover both
the areas of optical communication and optical networking
to potentially stimulate new cross-layer research directions.
In fact, ML application can be useful especially in cross-layer
settings, where data analysis at physical layer, e.g., monitoring
Bit Error Rate (BER), can trigger changes at network layer,
e.g., in routing, spectrum and modulation format assignments.
The application of ML to optical communication and network-
ing is still in its infancy and the literature survey included
in this paper aims at providing an introductory reference for
researchers and practitioners willing to get acquainted with
existing ML applications as well as to investigate new research
directions.
A legitimate question that arises in the optical networking
field today is: why machine learning, a methodological area
that has been applied and investigated for at least three
decades, is only gaining momentum now? The answer is
certainly very articulated, and it most likely involves not purely
technical aspects [10]. From a technical perspective though, re-
cent technical progress at both optical communication system
and network level is at the basis of an unprecedented growth

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in the complexity of optical networks.
On a system side, while optical channel modeling has
always been complex, the recent adoption of coherent tech-
nologies [11] has made modeling even more difficult by
introducing a plethora of adjustable design parameters (as
modulation formats, symbol rates, adaptive coding rates and
flexible channel spacing) to optimize transmission systems in
terms of bit-rate transmission distance product. In addition,
what makes this optimization even more challenging is that
the optical channel is highly nonlinear.
From a networking perspective, the increased complexity of
the underlying transmission systems is reflected in a series of
advancements in both data plane and control plane. At data
plane, the Elastic Optical Network (EON) concept [12]–[15]
has emerged as a novel optical network architecture able to
respond to the increased need of elasticity in allocating optical
network resources. In contrast to traditional fixed-grid Wave-
length Division Multiplexing (WDM) networks, EON offers
flexible (almost continuous) bandwidth allocation. Resource
allocation in EON can be performed to adapt to the several
above-mentioned decision variables made available by new
transmission systems, including different transmission tech-
niques, such as Orthogonal Frequency Division Multiplexing
(OFDM), Nyquist WDM (NWDM), transponder types (e.g.,
BVT1
, S-BVT), modulation formats (e.g., QPSK, QAM), and
coding rates. This flexibility makes the resource allocation
problems much more challenging for network engineers. At
control plane, dynamic control, as in Software-defined net-
working (SDN), promises to enable long-awaited on-demand
reconfiguration and virtualization. Moreover, reconfiguring the
optical substrate poses several challenges in terms of, e.g.,
network re-optimization, spectrum fragmentation, amplifier
power settings, unexpected penalties due to non-linearities,
which call for strict integration between the control elements
(SDN controllers, network orchestrators) and optical perfor-
mance monitors working at the equipment level.
All these degrees of freedom and limitations do pose severe
challenges to system and network engineers when it comes
to deciding what the best system and/or network design
is. Machine learning is currently perceived as a paradigm
shift for the design of future optical networks and systems.
These techniques should allow to infer, from data obtained
by various types of monitors (e.g., signal quality, traffic
samples, etc.), useful characteristics that could not be easily or
directly measured. Some envisioned applications in the optical
domain include fault prediction, intrusion detection, physical-
flow security, impairment-aware routing, low-margin design,
traffic-aware capacity reconfigurations, but many others can
be envisioned and will be surveyed in the next sections.
The survey is organized as follows. In Section II, we
overview some preliminary ML concepts, focusing especially
on those targeted in the following sections. In Section III
we discuss the main motivations behind the application of
ML in the optical domain and we classify the main areas of
applications. In Section IV and Section V, we classify and
1For a complete list of acronyms, the reader is referred to the Glossary at
the end of the paper.
summarize a large number of studies describing applications
of ML at the transmission layer and network layer. In Section
VI, we quantitatively overview a selection of existing papers,
identifying, for some of the applications described in Section
III, the ML algorithms which demonstrated higher effective-
ness for each specific use case, and the performance metrics
considered for the algorithms evaluation. Finally, Section VII
discusses some possible open areas of research and future
directions, whereas Section VIII concludes the paper.
II. OVERVIEW OF MACHINE LEARNING METHODS USED IN
OPTICAL NETWORKS
This section provides an overview of some of the most
popular algorithms that are commonly classified as machine
learning. The literature on ML is so extensive that even a
superficial overview of all the main ML approaches goes far
beyond the possibilities of this section, and the readers can
refer to a number of fundamental books on the subjects [16]–
[20]. However, in this section we provide a high level view of
the main ML techniques that are used in the work we reference
in the remainder of this paper. We here provide the reader
with some basic insights that might help better understand the
remaining parts of this survey paper. We divide the algorithms
in three main categories, described in the next sections, which
are also represented in Fig. 1: supervised learning, unsuper-
vised learning and reinforcement learning. Semi-supervised
learning, a hybrid of supervised and unsupervised learning, is
also introduced. ML algorithms have been successfully applied
to a wide variety of problems. Before delving into the different
ML methods, it is worth pointing out that, in the context of
telecommunication networks, there has been over a decade
of research on the application of ML techniques to wireless
networks, ranging from opportunistic spectrum access [21] to
channel estimation and signal detection in OFDM systems
[22], to Multiple-Input-Multiple-Output communications [23],
and dynamic frequency reuse [24].
A. Supervised learning
Supervised learning is used in a variety of applications, such
as speech recognition, spam detection and object recognition.
The goal is to predict the value of one or more output variables
given the value of a vector of input variables x. The output
variable can be a continuous variable (regression problem)
or a discrete variable (classification problem). A training
data set comprises N samples of the input variables and
the corresponding output values. Different learning methods
construct a function y(x) that allows to predict the value
of the output variables in correspondence to a new value of
the inputs. Supervised learning can be broken down into two
main classes, described below: parametric models, where the
number of parameters to use in the model is fixed, and non-
parametric models, where their number is dependent on the
training set.
1) Parametric models: In this case, the function y is a
combination of a fixed number of parametric basis functions.
These models use training data to estimate a fixed set of
parameters w. After the learning stage, the training data can

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(a) Supervised Learning: the algorithm is trained on dataset that
consists of paths, wavelengths, modulation and the corresponding
BER. Then it extrapolates the BER in correspondence to new inputs.
(b) Unsupervised Learning: the algorithm identifies unusual patterns
in the data, consisting of wavelengths, paths, BER, and modulation.
(c) Reinforcement Learning: the algorithm learns by receiving
feedback on the effect of modifying some parameters, e.g. the
power and the modulation
Fig. 1: Overview of machine learning algorithms applied to
optical networks.
be discarded since the prediction in correspondence to new
inputs is computed using only the learned parameters w.
Linear models for regression and classification, which consist
of a linear combination of fixed nonlinear basis functions,
X0=1
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h0=1
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X0=1
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Inputs Weights Ac0va0on
Bias
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Fig. 2: Example of a NN with two layers of adaptive param-
eters. The bias parameters of the input layer and the hidden
layer are represented as weights from additional units with
fixed value 1 (x0 and h0).
are the simplest parametric models in terms of analytical and
computational properties. Many different choices are available
for the basis functions: from polynomial to Gaussian, to
sigmoidal, to Fourier basis, etc. In case of multiple output
values, it is possible to use separate basis functions for each
component of the output or, more commonly, apply the same
set of basis functions for all the components. Note that these
models are linear in the parameters w, and this linearity
results in a number of advantageous properties, e.g., closed-
form solutions to the least-squares problem. However, their
applicability is limited to problems with low-dimensional input
space. In the remainder of this subsection we focus on neural
networks (NNs)2
, since they are the most successful example
of parametric models.
NNs apply a series of functional transformations to the
inputs (see chapter V in [16], chapter VI in [17], and chapter
XVI in [20]). A NN is a network of units or neurons. The
basis function or activation function used by each unit is
a nonlinear function of a linear combination of the unit’s
inputs. Each neuron has a bias parameter that allows for any
fixed offset in the data. The bias is incorporated in the set of
parameters by adding a dummy input of unitary value to each
unit (see Figure 2). The coefficients of the linear combination
are the parameters w estimated during the training. The most
commonly used nonlinear functions are the logistic sigmoid
and the hyperbolic tangent. The activation function of the
output units of the NN is the identity function, the logistic
sigmoid function, and the softmax function, for regression,
binary classification, and multiclass classification problems
respectively.
Different types of connections between the units result in
different NNs with distinct characteristics. All units between
the inputs and output of the NN are called hidden units. In
2Note that NNs are often referred to as Artificial Neural Networks (ANNs).
In this paper we use these two terms interchangeably.

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the case of a NN, the network is a directed acyclic graph.
Typically, NNs are organized in layers, with units in each layer
receiving inputs only from units in the immediately preceding
layer and forwarding their output only to the immediately
following layer. NNs with one layer of hidden units and linear
output units can approximate arbitrary well any continuous
function on a compact domain provided that a sufficient
number of hidden units is used [25].
Given a training set, a NN is trained by minimizing an error
function with respect to the set of parameters w. Depending
on the type of problem and the corresponding choice of
activation function of the output units, different error functions
are used. Typically in case of regression models, the sum
of square error is used, whereas for classification the cross-
entropy error function is adopted. It is important to note that
the error function is a non convex function of the network
parameters, for which multiple optimal local solutions exist.
Iterative numerical methods based on gradient information are
the most common methods used to find the vector w that min-
imizes the error function. For a NN the error backpropagation
algorithm, which provides an efficient method for evaluating
the derivatives of the error function with respect to w, is the
most commonly used.
We should at this point mention that, before training the
network, the training set is typically pre-processed by applying
a linear transformation to rescale each of the input variables
independently in case of continuous data or discrete ordinal
data. The transformed variables have zero mean and unit
standard deviation. The same procedure is applied to the target
values in case of regression problems. In case of discrete
categorical data, a 1-of-K coding scheme is used. This form of
pre-processing is known as feature normalization and it is used
before training most ML algorithms since most models are
designed with the assumption that all features have comparable
scales3
.
2) Nonparametric models: In nonparametric methods the
number of parameters depends on the training set. These
methods keep a subset or the entirety of the training data
and use them during prediction. The most used approaches
are k-nearest neighbor models (see chapter IV in [17]) and
support vector machines (SVMs) (see chapter VII in [16] and
chapter XIV in [20]). Both can be used for regression and
classification problems.
In the case of k-nearest neighbor methods, all training
data samples are stored (training phase). During prediction,
the k-nearest samples to the new input value are retrieved.
For classification problem, a voting mechanism is used; for
regression problems, the mean or median of the k nearest
samples provides the prediction. To select the best value of k,
cross-validation [26] can be used. Depending on the dimension
of the training set, iterating through all samples to compute
the closest k neighbors might not be feasible. In this case, k-d
trees or locality-sensitive hash tables can be used to compute
the k-nearest neighbors.
In SVMs, basis functions are centered on training samples;
the training procedure selects a subset of the basis functions.
3However, decision tree based models are a well-known exception.
The number of selected basis functions, and the number of
training samples that have to be stored, is typically much
smaller than the cardinality of the training dataset. SVMs
build a linear decision boundary with the largest possible
distance from the training samples. Only the closest points to
the separators, the support vectors, are stored. To determine
the parameters of SVMs, a nonlinear optimization problem
with a convex objective function has to be solved, for which
efficient algorithms exist. An important feature of SVMs is
that by applying a kernel function they can embed data into a
higher dimensional space, in which data points can be linearly
separated. The kernel function measures the similarity between
two points in the input space; it is expressed as the inner
product of the input points mapped into a higher dimension
feature space in which data become linearly separable. The
simplest example is the linear kernel, in which the mapping
function is the identity function. However, provided that we
can express everything in terms of kernel evaluations, it is not
necessary to explicitly compute the mapping in the feature
space. Indeed, in the case of one of the most commonly used
kernel functions, the Gaussian kernel, the feature space has
infinite dimensions.
B. Unsupervised learning
Social network analysis, genes clustering and market re-
search are among the most successful applications of unsu-
pervised learning methods.
In the case of unsupervised learning the training dataset
consists only of a set of input vectors x. While unsupervised
learning can address different tasks, clustering or cluster
analysis is the most common.
Clustering is the process of grouping data so that the intra-
cluster similarity is high, while the inter-cluster similarity
is low. The similarity is typically expressed as a distance
function, which depends on the type of data. There exists
a variety of clustering approaches. Here, we focus on two
algorithms, k-means and Gaussian mixture model as exam-
ples of partitioning approaches and model-based approaches,
respectively, given their wide area of applicability. The reader
is referred to [27] for a comprehensive overview of cluster
analysis.
k-means is perhaps the most well-known clustering algo-
rithm (see chapter X in [27]). It is an iterative algorithm
starting with an initial partition of the data into k clusters.
Then the centre of each cluster is computed and data points are
assigned to the cluster with the closest centre. The procedure
- centre computation and data assignment - is repeated until
the assignment does not change or a predefined maximum
number of iterations is exceeded. Doing so, the algorithm may
terminate at a local optimum partition. Moreover, k-means is
well known to be sensitive to outliers. It is worth noting that
there exists ways to compute k automatically [26], and an
online version of the algorithm exists.
While k-means assigns each point uniquely to one cluster,
probabilistic approaches allow a soft assignment and provide
a measure of the uncertainty associated with the assign-
ment. Figure 3 shows the difference between k-means and

5
Fig. 3: Difference between k-means and Gaussian mixture
model clustering a given set of data samples.
a probabilistic Gaussian Mixture Model (GMM). GMM, a
linear superposition of Gaussian distributions, is one of the
most widely used probabilistic approaches to clustering. The
parameters of the model are the mixing coefficient of each
Gaussian component, the mean and the covariance of each
Gaussian distribution. To maximize the log likelihood function
with respect to the parameters given a dataset, the expectation
maximization algorithm is used, since no closed form solution
exists in this case. The initialization of the parameters can be
done using k-means. In particular, the mean and covariance
of each Gaussian component can be initialized to sample
means and covariances of the cluster obtained by k-means,
and the mixing coefficients can be set to the fraction of data
points assigned by k-means to each cluster. After initializing
the parameters and evaluating the initial value of the log
likelihood, the algorithm alternates between two steps. In the
expectation step, the current values of the parameters are
used to determine the “responsibility” of each component for
the observed data (i.e., the conditional probability of latent
variables given the dataset). The maximization step uses these
responsibilities to compute a maximum likelihood estimate of
the model’s parameters. Convergence is checked with respect
to the log likelihood function or the parameters.
C. Semi-supervised learning
Semi-supervised learning methods are a hybrid of the pre-
vious two introduced above, and address problems in which
most of the training samples are unlabeled, while only a few
labeled data points are available. The obvious advantage is that
in many domains a wealth of unlabeled data points is readily
available. Semi-supervised learning is used for the same type
of applications as supervised learning. It is particularly useful
when labeled data points are not so common or too expensive
to obtain and the use of available unlabeled data can improve
performance.
Self-training is the oldest form of semi-supervised learning
[28]. It is an iterative process; during the first stage only la-
beled data points are used by a supervised learning algorithm.
Then, at each step, some of the unlabeled points are labeled
according to the prediction resulting for the trained decision
function and these points are used along with the original
labeled data to retrain using the same supervised learning
algorithm. This procedure is shown in Fig. 4.
Since the introduction of self-training, the idea of using la-
beled and unlabeled data has resulted in many semi-supervised
Fig. 4: Sample step of the self-training mechanism, where an
unlabeled point is matched against labeled data to become part
of the labeled data set.
learning algorithms. According to the classification proposed
in [28], semi-supervised learning techniques can be organized
in four classes: i) methods based on generative models4
; ii)
methods based on the assumption that the decision boundary
should lie in a low-density region; iii) graph-based methods;
iv) two-step methods (first an unsupervised learning step to
change the data representation or construct a new kernel; then
a supervised learning step based on the new representation or
kernel).
D. Reinforcement Learning
Reinforcement Learning (RL) is used, in general, to address
applications such as robotics, finance (investment decisions),
inventory management, where the goal is to learn a policy, i.e.,
a mapping between states of the environment into actions to
be performed, while directly interacting with the environment.
The RL paradigm allows agents to learn by exploring the
available actions and refining their behavior using only an
evaluative feedback, referred to as the reward. The agent’s
goal is to maximize its long-term performance. Hence, the
agent does not just take into account the immediate reward,
but it evaluates the consequences of its actions on the future.
Delayed reward and trial-and-error constitute the two most
significant features of RL.
RL is usually performed in the context of Markov deci-
sion processes (MDP). The agent’s perception at time k is
represented as a state sk ∈ S, where S is the finite set of
environment states. The agent interacts with the environment
by performing actions. At time k the agent selects an action
ak ∈ A, where A is the finite set of actions of the agent,
which could trigger a transition to a new state. The agent will
4Generative methods estimate the joint distribution of the input and
output variables. From the joint distribution one can obtain the conditional
distribution p(y|x), which is then used to predict the output values in
correspondence to new input values. Generative methods can exploit both
labeled and unlabeled data.

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receive a reward as a result of the transition, according to the
reward function ρ : S×A×S → R. The agents goal is to find
the sequence of state-action pairs that maximizes the expected
discounted reward, i.e., the optimal policy. In the context of
MDP, it has been proved that an optimal deterministic and
stationary policy exists. There exist a number of algorithms
that learn the optimal policy both in case the state transition
and reward functions are known (model-based learning) and
in case they are not (model-free learning). The most used RL
algorithm is Q-learning, a model-free algorithm that estimates
the optimal action-value function (see chapter VI in [19]). An
action-value function, named Qfunction, is the expected return
of a state-action pair for a given policy. The optimal action-
value function, Q∗, corresponds to the maximum expected
return for a state-action pair. After learning function Q∗, the
agent selects the action with the corresponding highest Q-value
in correspondence to the current state.
A table-based solution such as the one described above
is only suitable in case of problems with limited state-
action space. In order to generalize the policy learned in
correspondence to states not previously experienced by the
agent, RL methods can be combined with existing function
approximation methods, e.g., neural networks.
E. Overfitting, underfitting and model selection
In this section, we discuss a well-known problem of ML
algorithms along with its solutions. Although we focus on
supervised learning techniques, the discussion is also relevant
for unsupervised learning methods.
Overfitting and underfitting are two sides of the same coin:
model selection. Overfitting happens when the model we use is
too complex for the available dataset (e.g., a high polynomial
order in the case of linear regression with polynomial basis
functions or a too large number of hidden neurons for a
neural network). In this case, the model will fit the training
data too closely5
, including noisy samples and outliers, but
will result in very poor generalization, i.e., it will provide
inaccurate predictions for new data points. At the other end of
the spectrum, underfitting is caused by the selection of models
that are not complex enough to capture important features in
the data (e.g., when we use a linear model to fit quadratic
data). Fig. 5 shows the difference between underfitting and
overfitting, compared to an accurate model.
Since the error measured on the training samples is a poor
indicator for generalization, to evaluate the model performance
the available dataset is split into two, the training set and the
test set. The model is trained on the training set and then
evaluated using the test set. Typically around 70% of the
samples are assigned to the training set and the remaining 30%
are assigned to the test set. Another option that is very useful
in case of a limited dataset is to use cross-validation so that as
much of the available data as possible is exploited for training.
In this case, the dataset is divided into k subsets. The model
5As an extreme example, consider a simple regression problem for pre-
dicting a real-value target variable as a function of a real-value observation
variable. Let us assume a linear regression model with polynomial basis
function of the input variable. If we have N samples and we select N as
the order of the polynomial, we can fit the model perfectly to the data points.
Fig. 5: Difference between underfitting and overfitting.
is trained k times using each of the k subset for validation and
the remaining (k − 1) subsets for training. The performance
is averaged over the k runs. In case of overfitting, the error
measured on the test set is high and the error on the training
set is small. On the other hand, in the case of underfitting,
both the error measured on the training set and the test set are
usually high.
There are different ways to select a model that does not
exhibit overfitting and underfitting. One possibility is to train a
range of models, compare their performance on an independent
dataset (the validation set), and then select the one with
the best performance. However, the most common technique
is regularization. It consists of adding an extra term - the
regularization term - to the error function used in the training
stage. The simplest form of the regularization term is the sum
of the squares of all parameters, which is known as weight
decay and drives parameters towards zero. Another common
choice is the sum of the absolute values of the parameters
(lasso). An additional parameter, the regularization coefficient
λ, weighs the relative importance of the regularization term
and the data-dependent error. A large value of λ heavily
penalizes large absolute values of the parameters. It should
be noted that the data-dependent error computed over the
training set increases with λ. The error computed over the
validation set is high for both small and high λ values. In the
first case, the regularization term has little impact potentially
resulting in overfitting. In the latter case, the data-dependent
error has little impact resulting in a poor model performance.
A simple automatic procedure for selecting the best λ consists
of training the model with a range of values for the regular-
ization parameter and select the value that corresponds to the
minimum validation error. In the case of NNs with a large
number of hidden units, dropout - a technique that consists of
randomly removing units and their connections during training
- has been shown to outperform other regularization methods
[29].

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III. MOTIVATION FOR USING MACHINE LEARNING IN
OPTICAL NETWORKS AND SYSTEMS
In the last few years, the application of mathematical
approaches derived from the ML discipline have attracted the
attention of many researchers and practitioners in the optical
communications and networking fields. In a general sense,
the underlying motivations for this trend can be identified as
follows:
• increased system complexity: the adoption of advanced
transmission techniques, such as those enabled by coher-
ent technology [11], and the introduction of extremely
flexible networking principles, such as, e.g., the EON
paradigm, have made the design and operation of optical
networks extremely complex, due to the high number
of tunable parameters to be considered (e.g., modulation
formats, symbol rates, adaptive coding rates, adaptive
channel bandwidth, etc.); in such a scenario, accurately
modeling the system through closed-form formulas is
often very hard, if not impossible, and in fact “margins”
are typically adopted in the analytical models, leading
to resource underutilization and to consequent increased
system cost; on the contrary, ML methods can capture
complex non-linear system behaviour with relatively sim-
ple training of supervised and/or unsupervised algorithms
which exploit knowledge of historical network data, and
therefore to solve complex cross-layer problems, typical
of the optical networking field;
• increased data availability: modern optical networks are
equipped with a large number of monitors, able to provide
several types of information on the entire system, e.g.,
traffic traces, signal quality indicators (such as BER),
equipment failure alarms, users’ behaviour etc.; here, the
enhancement brought by ML consists of simultaneously
leveraging the plethora of collected data and discover
hidden relations between various types of information.
The application of ML to physical layer use cases is mainly
motivated by the presence of non-linear effects in optical
fibers, which make analytical models inaccurate or even too
complex. This has implications, e.g., on the performance pre-
dictions of optical communication systems, in terms of BER,
quality factor (Q-factor) and also for signal demodulation [30],
[31], [32].
Moving from the physical layer to the networking layer, the
same motivation applies for the application of ML techniques.
In particular, design and management of optical networks is
continuously evolving, driven by the enormous increase of
transported traffic and drastic changes in traffic requirements,
e.g., in terms of capacity, latency, user experience and Quality
of Service (QoS). Therefore, current optical networks are
expected to be run at much higher utilization than in the past,
while providing strict guarantees on the provided quality of
service. While aggressive optimization and traffic-engineering
methodologies are required to achieve these objectives, such
complex methodologies may suffer scalability issues, and in-
volve unacceptable computational complexity. In this context,
ML is regarded as a promising methodological area to address
this issue, as it enables automated network self-configuration
and fast decision-making by leveraging the plethora of data
that can be retrieved via network monitors, and allowing net-
work engineers to build data-driven models for more accurate
and optimized network provisioning and management.
Several use cases can benefit from the application of ML
and data analytics techniques. In this paper we divide these use
cases in i) physical layer and ii) network layer use cases. The
remainder of this section provides a high-level introduction to
the main applications of ML in optical networks, as graphically
shown in Fig. 6, and motivates why ML can be beneficial
in each case. A detailed survey of existing studies is then
provided in Sections IV and V, for physical layer and network
layer use cases, respectively.
A. Physical layer domain
As mentioned in the previous section, several challenges
need to be addressed at the physical layer of an optical net-
work, typically to evaluate the performance of the transmission
system and to check if any signal degradation influences
existing lightpaths. Such monitoring can be used, e.g., to
trigger proactive procedures, such as tuning of launch power,
controlling gain in optical amplifiers, varying modulation
format, etc., before irrecoverable signal degradation occurs.
In the following, a description of the applications of ML at
the physical layer is presented.
• QoT estimation.
Prior to the deployment of a new lightpath, a system
engineer needs to estimate the Quality of Transmission
(QoT) for the new lightpath, as well as for the already
existing ones. The concept of Quality of Transmission
generally refers to a number of physical layer param-
eters, such as received Optical Signal-to-Noise Ratio
(OSNR), BER, Q-factor, etc., which have an impact on
the “readability” of the optical signal at the receiver. Such
parameters give a quantitative measure to check if a pre-
determined level of QoT would be guaranteed, and are
affected by several tunable design parameters, such as,
e.g., modulation format, baud rate, coding rate, physical
path in the network, etc. Therefore, optimizing this choice
is not trivial and often this large variety of possible
parameters challenges the ability of a system engineer
to address manually all the possible combinations of
lightpath deployment.
As of today, existing (pre-deployment) estimation tech-
niques for lightpath QoT belong to two categories: 1)
“exact” analytical models estimating physical-layer im-
pairments, which provide accurate results, but incur heavy
computational requirements and 2) marginated formulas,
which are computationally faster, but typically introduce
high marginations that lead to underutilization of network
resources. Moreover, it is worth noting that, due to the
complex interaction of multiple system parameters (e.g.,
input signal power, number of channels, link type, mod-
ulation format, symbol rate, channel spacing, etc.) and,
most importantly, due to the nonlinear signal propagation
through the optical channel, deriving accurate analytical
models is a challenging task, and assumptions about the

8
NETWORK LAYER
PHYSICAL LAYER
CONTROL PLANE
NON-LINEARITY
MITIGATION
AMPLIFIER CONTROL
MODULATION FORMAT
RECOGNITION
TRAFFIC PREDICTION
TRAFFIC
CLASSIFICATION
QoT ESTIMATOR
OPTICAL NETWORKOPTICAL NETWORK
FIELD MEASUREMENTS AND
HISTORICAL DATA
TRAINING DATASETSTRAINING DATASETS
TRAFFIC REQUESTS
ROUTING AND SPECTRUM
ASSIGNMENT
TRAFFIC PATTERN
TRAFFIC TYPE
QoT LEVEL
RECEIVED
SYMBOL
USED
MOD. FORMAT
NOISE FIGURE
GAIN FLATNESS
FAULT DETECTION
AND IDENTIFICATION
ALERTS
Fig. 6: The general framework of a ML-assisted optical network.
system under consideration must be made in order to
adopt approximate models. Conversely, ML constitutes
a promising means to automatically predict whether un-
established lightpaths will meet the required system QoT
threshold.
Relevant ML techniques: ML-based classifiers can be
trained using supervised learning6
to create direct input-
output relationship between QoT observed at the receiver
and corresponding lightpath configuration in terms of,
e.g., utilized modulation format, baud rate and/or physical
route in the network.
• Optical amplifiers control.
In current optical networks, lightpath provisioning is
becoming more dynamic, in response to the emergence
of new services that require huge amount of bandwidth
over limited periods of time. Unfortunately, dynamic set-
up and tear-down of lightpaths over different wavelengths
forces network operators to reconfigure network devices
“on the fly” to maintain physical-layer stability. In re-
sponse to rapid changes of lightpath deployment, Erbium
Doped Fiber Amplifiers (EDFAs) suffer from wavelength-
dependent power excursions. Namely, when a new light-
path is established (i.e., added) or when an existing
lightpath is torn down (i.e., dropped), the discrepancy
of signal power levels between different channels (i.e.,
between lightpaths operating at different wavelengths)
depends on the specific wavelength being added/dropped
into/from the system. Thus, an automatic control of pre-
amplification signal power levels is required, especially
in case a cascade of multiple EDFAs is traversed, to
avoid that excessive post-amplification power discrepancy
6Note that, specific solutions adopted in literature for QoT estimation, as
well as for other physical- and network-layer use cases, will be detailed in
the literature surveys provided in Sections IV and V.
between different lightpaths may cause signal distortion.
Relevant ML techniques: Thanks to the availability of
historical data retrieved by monitoring network status,
ML regression algorithms can be trained to accurately
predict post-amplifier power excursion in response to the
add/drop of specific wavelengths to/from the system.
• Modulation format recognition (MFR).
Modern optical transmitters and receivers provide high
flexibility in the utilized bandwidth, carrier frequency and
modulation format, mainly to adapt the transmission to
the required bit-rate and optical reach in a flexible/elastic
networking environment. Given that at the transmission
side an arbitrary coherent optical modulation format can
be adopted, knowing this decision in advance also at the
receiver side is not always possible, and this may affect
proper signal demodulation and, consequently, signal
processing and detection.
Relevant ML techniques: Use of supervised ML algo-
rithms can help the modulation format recognition at the
receiver, thanks to the opportunity to learn the mapping
between the adopted modulation format and the features
of the incoming optical signal.
• Nonlinearity mitigation.
Due to optical fiber nonlinearities, such as Kerr effect,
self-phase modulation (SPM) and cross-phase modulation
(XPM), the behaviour of several performance param-
eters, including BER, Q-factor, Chromatic Dispersion
(CD), Polarization Mode Dispersion (PMD), is highly
unpredictable, and this may cause signal distortion at the
receiver (e.g., I/Q imbalance and phase noise). Therefore,
complex analytical models are often adopted to react to
signal degradation and/or compensate undesired nonlinear
effects.
Relevant ML techniques: While approximated analytical

9
models are usually adopted to solve such complex non-
linear problems, supervised ML models can be designed
to directly capture the effects of such nonlinearities, typi-
cally exploiting knowledge of historical data and creating
input-output relations between the monitored parameters
and the desired outputs.
• Optical performance monitoring (OPM).
With increasing capacity requirements for optical com-
munication systems, performance monitoring is vital to
ensure robust and reliable networks. Optical performance
monitoring aims at estimating the transmission parame-
ters of the optical fiber system, such as BER, Q-factor,
CD, PMD, during lightpath lifetime. Knowledge of such
parameters can be then utilized to accomplish various
tasks, e.g., activating polarization compensator modules,
adjusting launch power, varying the adopted modula-
tion format, re-route lightpaths, etc. Typically, optical
performance parameters need to be collected at various
monitoring points along the lightpath, thus large number
of monitors are required, causing increased system cost.
Therefore, efficient deployment of optical performance
monitors in the proper network locations is needed to
extract network information at reasonable cost.
Relevant ML techniques: To reduce the amount of mon-
itors to deploy in the system, especially at intermediate
points of the lightpaths, supervised learning algorithms
can be used to learn the mapping between the optical fiber
channel parameters and the properties of the detected
signal at the receiver, which can be retrieved, e.g., by
observing statistics of power eye diagrams, signal ampli-
tude, OSNR, etc.
B. Network layer domain
At the network layer, several other use cases for ML arise.
Provisioning of new lightpaths or restoration of existing ones
upon network failure require complex and fast decisions that
depend on several quickly-evolving data, since, e.g., oper-
ators must take into consideration the impact onto existing
connections provided by newly-inserted traffic. In general, an
estimation of users’ and service requirements is desirable for
an effective network operation, as it allows to avoid over-
provisioning of network resources and to deploy resources
with adequate margins at a reasonable cost. We identify the
following main use cases.
• Traffic prediction.
Accurate traffic prediction in the time-space domain
allows operators to effectively plan and operate their
networks. In the design phase, traffic prediction allows
to reduce over-provisioning as much as possible. During
network operation, resource utilization can be optimized
by performing traffic engineering based on real-time
data, eventually re-routing existing traffic and reserving
resources for future incoming traffic requests.
Relevant ML techniques: Through knowledge of his-
torical data on users’ behaviour and traffic profiles in the
time-space domain, a supervised learning algorithm can
be trained to predict future traffic requirements and conse-
quent resource needs. This allows network engineers to
activate, e.g., proactive traffic re-routing and periodical
network re-optimization so as to accommodate all users
traffic and simultaneously reduce network resources uti-
lization.
Moreover, unsupervised learning algorithms can be also
used to extract common traffic patterns in different por-
tions of the network. Doing so, similar design and man-
agement procedures (e.g., deployment and/or reservation
of network capacity) can be activated also in different
parts of the network, which instead show similarities in
terms of traffic requirements, i.e., belonging to a same
traffic profile cluster.
Note that, application of traffic prediction, and the rel-
ative ML techniques, vary substantially according to
the considered network segment (e.g., approaches for
intra-datacenter networks may be different than those
for access networks), as traffic characteristics strongly
depend on the considered network segment.
• Virtual topology design (VTD) and reconfiguration.
The abstraction of communication network services by
means of a virtual topology is widely adopted by network
operators and service providers. This abstraction consists
of representing the connectivity between two end-points
(e.g., two data centers) via an adjacency in the virtual
topology, (i.e., a virtual link), although the two end-
points are not necessarily physically connected. After the
set of all virtual links has been defined, i.e., after all
the lightpath requests have been identified, VTD requires
solving a Routing and Wavelength Assignment (RWA)
problem for each lightpath on top of the underlying
physical network. Note that, in general, many virtual
topologies can co-exist in the same physical network,
and they may represent, e.g., service required by different
customers, or even different services, each with a specific
set of requirements (e.g., in terms of QoS, bandwidth,
and/or latency), provisioned to the same customer.
VTD is not only necessary when a new service is pro-
visioned and new resources are allocated in the network.
In some cases, e.g., when network failures occur or
when the utilization of network resources undergoes re-
optimization procedures, existing (i.e., already-designed)
virtual topologies shall be rearranged, and in these cases
we refer to the VT reconfiguration.
To perform design and reconfiguration of virtual topolo-
gies, network operators not only need to provision (or
reallocate) network capacity for the required services, but
may also need to provide additional resources according
to the specific service characteristics, e.g., for guaran-
teeing service protection and/or meeting QoS or latency
requirements. This type of service provisioning is often
referred to as network slicing, due to the fact that each
provisioned service (i.e., each VT) represents a slice of
the overall network.
Relevant ML techniques: To address VTD and VT
reconfiguration, ML classifiers can be trained to optimally
decide how to allocate network resources, by simulta-
neously taking into account a large number of different
and heterogeneous service requirements for a variety of

10
virtual topologies (i.e., network slices), thus enabling fast
decision making and optimized resources provisioning,
especially under dynamically-changing network condi-
tions.
• Failure management.
When managing a network, the ability to perform failure
detection and localization or even to determine the cause
of network failure is crucial as it may enable operators
to promptly perform traffic re-routing, in order to main-
tain service status and meet Service Level Agreements
(SLAs), and rapidly recover from the failure. Handling
network failures can be accomplished at different levels.
E.g., performing failure detection, i.e., identifying the
set of lightpaths that were affected by a failure, is a
relatively simple task, which allows network operators
to only reconfigure the affected lightpaths by, e.g., re-
routing the corresponding traffic. Moreover, the ability of
performing also failure localization enables the activation
of recovery procedures. This way, pre-failure network
status can be restored, which is, in general, an optimized
situation from the point of view of resources utilization.
Furthermore, determining also the cause of network fail-
ure, e.g., temporary traffic congestion, devices disruption,
or even anomalous behaviour of failure monitors, is useful
to adopt the proper restoring and traffic reconfiguration
procedures, as sometimes remote reconfiguration of light-
paths can be enough to handle the failure, while in some
other cases in-field intervention is necessary. Moreover,
prompt identification of the failure cause enables fast
equipment repair and consequent reduction in Mean Time
To Repair (MTTR).
Relevant ML techniques: ML can help handling the
large amount of information derived from the continuous
activity of a huge number of network monitors and
alarms. E.g., ML classifiers algorithms can be trained
to distinguish between regular and anomalous (i.e., de-
graded) transmission. Note that, in such cases, semi-
supervised approaches can be also used, whenever labeled
data are scarce, but a large amount of unlabeled data
is available. Further, ML classifiers can be trained to
distinguish failure causes, exploiting the knowledge of
previously observed failures.
• Traffic flow classification.
When different types of services coexist in the same
network infrastructure, classifying the corresponding traf-
fic flows before their provisioning may enable efficient
resource allocation, mitigating the risk of under- and
over-provisioning. Moreover, accurate flow classification
is also exploited for already provisioned services to apply
flow-specific policies, e.g., to handle packets priority, to
perform flow and congestion control, and to guarantee
proper QoS to each flow according to the SLAs.
Relevant ML techniques: Based on the various traffic
characteristics and exploiting the large amount of in-
formation carried by data packets, supervised learning
algorithms can be trained to extract hidden traffic charac-
teristics and perform fast packets classification and flows
differentiation.
• Path computation.
When performing network resources allocation for an
incoming service request, a proper path should be se-
lected in order to efficiently exploit the available network
resources to accommodate the requested traffic with the
desired QoS and without affecting the existing services,
previously provisioned in the network. Traditionally, path
computation is performed by using cost-based routing
algorithms, such as Dijkstra, Bellman-Ford, Yen algo-
rithms, which rely on the definition of a pre-defined
cost metric (e.g., based on the distance between source
and destination, the end-to-end delay, the energy con-
sumption, or even a combination of several metrics) to
discriminate between alternative paths.
Relevant ML techniques: In this context, use of su-
pervised ML can be helpful as it allows to simultane-
ously consider several parameters featuring the incoming
service request together with current network state in-
formation and map this information into an optimized
routing solution, with no need for complex network-
cost evaluations and thus enabling fast path selection and
service provisioning.
C. A bird-eye view of the surveyed studies
The physical- and network-layer use cases described above
have been tackled in existing studies by exploiting several
ML tools (i.e., supervised and/or unsupervised learning, etc.)
and leveraging different types of network monitored data (e.g.,
BER, OSNR, link load, network alarms, etc.).
In Tables I and II we summarize the various physical- and
network-layer use cases and highlight the features of the ML
approaches which have been used in literature to solve these
problems. In the tables we also indicate specific reference
papers addressing these issues, which will be described in the
following sections in more detail. Note that another recently
published survey [33] proposes a very similar categorization
of existing applications of artificial intelligence in optical
networks.
IV. DETAILED SURVEY OF MACHINE LEARNING IN
PHYSICAL LAYER DOMAIN
A. Quality of Transmission estimation
QoT estimation consists of computing transmission quality
metrics such as OSNR, BER, Q-factor, CD or PMD based
on measurements directly collected from the field by means
of optical performance monitors installed at the receiver side
[105] and/or on lightpath characteristics. QoT estimation is
typically applied in two scenarios:
• predicting the transmission quality of unestablished light-
paths based on historical observations and measurements
collected from already deployed ones;
• monitoring the transmission quality of already-deployed
lightpaths with the aim of identifying faults and malfunc-
tions.
QoT prediction of unestablished lightpaths relies on intelli-
gent tools, capable of predicting whether a candidate lightpath

11
TABLE I: Different use cases at physical layer and their characteristics.
Use Case ML category ML methodology Input data Output data Training data Ref.
QoT
estimation
supervised kriging, L2-norm
minimization
OSNR (historical data) OSNR synthetic [34]
OSNR/Q-factor BER synthetic [35], [36]
OSNR/PMD/CD/SPM blocking prob. synthetic [37]
CBR error vector magnitude, OSNR Q-factor real [38]
lightpath route, length, number
of co-propagating lightpaths
Q-factor synthetic [39], [40]
RF lightpath route, length, MF,
traffic volume
BER synthetic [41]
regression SNR (historical data) SNR synthetic [42]
NN lightpath route and length,
number of traversed EDFAs,
degree of destination, used
channel wavelength
Q-factor synthetic [43], [44]
k-nearest neighbor,
RF, SVM
total link length, span length,
channel launch power, MF and
data rate
BER synthetic [45]
NN channel loadings and launch
power settings
Q-factor real [46]
NN source-destination nodes, link
occupation, MF, path length,
data rate
BER real [47]
OPM supervised NN eye diagram and amplitude
histogram param.
OSNR/PMD/CD real [48]
NN, SVM asynchronous amplitude his-
togram
MF real [49]
NN asyncrhonous constellation di-
agram and amplitude his-
togram param.
OSNR/PMD/CD synthetic [50]–[53]
Kernel-based ridge
regression
eye diagram and phase por-
traits param.
PMD/CD real [54]
NN Horizontal and Vertical polar-
ized I/Q samples from ADC
OSNR, MF, symbol rate real [55]
Gaussian Processes monitoring data (OSNR vs λ) Q-factor real [56]
Optical ampli-
fiers control
supervised CBR power mask param. (NF, GF) OSNR real [57], [58]
NNs EDFA input/output power EDFA operating point real [59], [60]
Ridge regression,
Kernelized Bayesian
regr.
WDM channel usage post-EDFA power
discrepancy
real [61]
unsupervised evolutional alg. EDFA input/output power EDFA operating point real [62]
MF
recognition
unsupervised 6 clustering alg. Stokes space param. MF synthetic [63]
k-means received symbols MF real [64]
supervised NN asynchronous amplitude his-
togram
MF synthetic [65]
NN, SVM asynchronous amplitude his-
togram
MF real [66], [67], [49]
variational Bayesian
techn. for GMM
Stokes space param. MF real [68]
Non-linearity
mitigation
supervised Bayesian filtering,
NNs, EM
received symbols OSNR, Symbol error rate real [31], [32], [69]
ELM received symbols self-phase modulation synthetic [70]
k-nearest neighbors received symbols BER real [71]
Newton-based SVM received symbols Q-factor real [72]
binary SVM received symbols symbol decision bound-
aries
synthetic [73]
NN received subcarrier symbols Q-factor synthetic [74]
GMM post-equalized symbols decoded symbols with im-
pairment estimated and/or
mitigated
real [75]
Clustering received constellation with
nonlinearities
nonlinearity mitigated
constellation points
real [76]
NN sampled received signal se-
quences
equalized signal with re-
duced ISI
real [77]–[82]
unsupervised k-means received constellation density-based spatial
constellation clusters and
their optimal centroids
real [83]

12
TABLE II: Different use cases at network layer and their characteristics.
Use Case ML category ML methodology Input data Output data Training data Ref.
Traffic prediction
and virtual topol-
ogy (re)design
supervised ARIMA historical real-time traffic ma-
trices
predicted traffic matrix synthetic [84], [85]
NN historical end-to-end
maximum bit-rate traffic
predicted end-to-end traf-
fic
synthetic [86], [87]
Reinforcement learning previous solutions of a multi-
objective GA for VTD
updated VT synthetic [88], [89]
Recurrent NN historical aggregated traffic at
different BBU pools
predicted BBU pool traffic real [90]
NN historical traffic in intra-DC
network
predicted intra-DC traffic real [91]
unsupervised NMF, clustering CDR, PoI matrix similarity patterns in base
station traffic
real [92]
Failure manage-
ment
supervised Bayesian Inference BER, received power list of failures for all light-
paths
real [93]
Bayesian Inference, EM FTTH network dataset with
missing data
complete dataset real [94], [95]
Kriging previously established light-
paths with already available
failure localization and moni-
toring data
estimate of failure local-
ization at link level for all
lightpaths
real [96]
(1) LUCIDA: Regres-
sion and classification
(2) BANDO: Anomaly
Detection
(1) LUCIDA: historic BER
and received power, notifica-
tions from BANDO
(2) BANDO: maximum BER,
threshold BER at set-up, mon-
itored BER
(1) LUCIDA: failure clas-
sification
(2) BANDO: anomalies in
BER
real [97]
Regression, decision
tree, SVM
BER, frequency-power pairs localized set of failures real [98]
SVM, RF, NN BER set of failures real [99]
regression and NN optical power levels, ampli-
fier gain, shelf temperature,
current draw, internal optical
power
detected faults real [100]
Flow
classification
supervised HMM, EM packet loss data loss classification:
congestion-loss or
contention-loss
synthetic [101]
NN source/destination IP
addresses, source/destination
ports, transport layer protocol,
packet sizes, and a set of
intra-flow timings within the
first 40 packets of a flow
classified flow for DC synthetic [102]
Path computation supervised Q-Learning traffic requests, set of can-
didate paths between each
source-destination pair
optimum paths for each
source-destination pair to
minimize burst-loss prob-
ability
synthetic [103]
unsupervised FCM traffic requests, path lengths,
set of modulation formats,
OSNR, BER
mapping of an optimum
modulation format to a
lightpath
synthetic [104]
will meet the required quality of service guarantees (mapped
onto OSNR, BER or Q-factor threshold values): the problem is
typically formulated as a binary classification problem, where
the classifier outputs a yes/no answer based on the lightpath
characteristics (e.g., its length, number of links, modulation
format used for transmission, overall spectrum occupation of
the traversed links etc.).
In [39] a cognitive Case Based Reasoning (CBR) approach
is proposed, which relies on the maintenance of a knowl-
edge database where information on the measured Q-factor
of deployed lightpaths is stored, together with their route,
selected wavelength, total length, total number and standard

13
deviation of the number of co-propagating lightpaths per link.
Whenever a new traffic requests arrives, the most “similar”
one (where similarity is computed by means of the Euclidean
distance in the multidimensional space of normalized fea-
tures) is retrieved from the database and a decision is made
by comparing the associated Q-factor measurement with a
predefined system threshold. As a correct dimensioning and
maintenance of the database greatly affect the performance
of the CBR technique, algorithms are proposed to keep it up
to date and to remove old or useless entries. The trade-off
between database size, computational time and effectiveness
of the classification performance is extensively studied: in
[40], the technique is shown to outperform state-of-the-art ML
algorithms such as Naive Bayes, J48 tree and Random Forests
(RFs). Experimental results achieved with data obtained from
a real testbed are discussed in [38].
A database-oriented approach is proposed also in [42]
to reduce uncertainties on network parameters and design
margins, where field data are collected by a software defined
network controller and stored in a central repository. Then,
a QTool is used to produce an estimate of the field-measured
Signal-to-Noise Ratio (SNR) based on educated guesses on the
(unknown) network parameters and such guesses are iteratively
updated by means of a gradient descent algorithm, until
the difference between the estimated and the field-measured
SNR falls below a predefined threshold. The new estimated
parameters are stored in the database and yield to new design
margins, which can be used for future demands. The trade-off
between database size and ranges of the SNR estimation error
are evaluated via numerical simulations.
Similarly, in the context of multicast transmission in optical
network, a NN is trained in [43], [44], [46], [47] using as
features the lightpath total length, the number of traversed
EDFAs, the maximum link length, the degree of destination
node and the channel wavelength used for transmission of
candidate lightpaths, to predict whether the Q-factor will
exceed a given system threshold. The NN is trained online with
data mini-batches, according to the network evolution, to allow
for sequential updates of the prediction model. A dropout
technique is adopted during training to avoid overfitting. The
classification output is exploited by a heuristic algorithm for
dynamic routing and spectrum assignment, which decides
whether the request must be served or blocked. The algorithm
performance is assessed in terms of blocking probability.
A random forest binary classifier is adopted in [41] to
predict the probability that the BER of unestablished lightpaths
will exceed a system threshold. As depicted in Figure 7, the
classifier takes as input a set of features including the total
length and maximum link length of the candidate lightpath,
the number of traversed links, the amount of traffic to be
transmitted and the modulation format to be adopted for
transmission. Several alternative combinations of routes and
modulation formats are considered and the classifier identifies
the ones that will most likely satisfy the BER requirements.
In [45], a random forest classifier along with two other tools
namely k-nearest neighbor and support vector machine are
used. The authors in [45] use three of the above-mentioned
classifiers to associate QoT labels with a large set of lightpaths
Fig. 7: The classification framework adopted in [41].
to develop a knowledge base and find out which is the best
classifier. It turns out from the analysis in [45], that the support
vector machine is better in performance than the other two but
takes more computation time.
Two alternative approaches, namely network kriging7
(first
described in [107]) and norm L2 minimization (typically used
in network tomography [108]), are applied in [36], [37] in
the context of QoT estimation: they rely on the installation
of probe lightpaths that do not carry user data but are
used to gather field measurements. The proposed inference
methodologies exploit the spatial correlation between the QoT
metrics of probes and data-carrying lightpaths sharing some
physical links to provide an estimate of the Q-factor of already
deployed or perspective lightpaths. These methods can be
applied assuming either a centralized decisional tool or in a
distributed fashion, where each node has only local knowledge
of the network measurements. As installing probe lightpaths is
costly and occupies spectral resources, the trade-off between
number of probes and accuracy of the estimation is studied.
Several heuristic algorithms for the placement of the probes are
proposed in [34]. A further refinement of the methodologies
which takes into account the presence of neighbor channels
appears in [35].
Additionally, a data-driven approach using a machine learn-
ing technique, Gaussian processes nonlinear regression (GPR),
is proposed and experimentally demonstrated for performance
prediction of WDM optical communication systems [49].
The core of the proposed approach (and indeed of any ML
technique) is generalization: first the model is learned from the
measured data acquired under one set of system configurations,
and then the inferred model is applied to perform predictions
for a new set of system configurations. The advantage of
the approach is that complex system dynamics can be cap-
tured from measured data more easily than from simulations.
Accurate BER predictions as a function of input power,
transmission length, symbol rate and inter-channel spacing are
reported using numerical simulations and proof-of-principle
experimental validation for a 24 × 28 GBd QPSK WDM
optical transmission system.
7Extensively used in the spatial statistics literature (see [106] for details),
kriging is closely related to Gaussian process regression (see chapter XV in
[20]).

14
Fig. 8: EDFA power mask [60].
Finally, a control and management architecture integrating
an intelligent QoT estimator is proposed in [109] and its
feasibility is demonstrated with implementation in a real
testbed.
B. Optical amplifiers control
The operating point of EDFAs influences their Noise Figure
(NF) and gain flatness (GF), which have a considerable
impact on the overall ligtpath QoT. The adaptive adjustment
of the operating point based on the signal input power can
be accomplished by means of ML algorithms. Most of the
existing studies [57]–[60], [62] rely on a preliminary amplifier
characterization process aimed at experimentally evaluating
the value of the metrics of interest (e.g., NF, GF and gain
control accuracy) within its power mask (i.e., the amplifier
operating region, depicted in Fig. 8).
The characterization results are then represented as a set
of discrete values within the operation region. In EDFA im-
plementations, state-of-the-art microcontrollers cannot easily
obtain GF and NF values for points that were not measured
during the characterization. Unfortunately, producing a large
amount of fine grained measurements is time consuming. To
address this issue, ML algorithms can be used to interpolate
the mapping function over non-measured points.
For the interpolation, authors of [59], [60] adopt a NN im-
plementing both feed-forward and backward error propagation.
Experimental results with single and cascaded amplifiers re-
port interpolation errors below 0.5 dB. Conversely, a cognitive
methodology is proposed in [57], which is applied in dynamic
network scenarios upon arrival of a new lightpath request: a
knowledge database is maintained where measurements of the
amplifier gains of already established lightpaths are stored,
together with the lightpath characteristics (e.g., number of
links, total length, etc.) and the OSNR value measured at the
receiver. The database entries showing the highest similarities
with the incoming lightpath request are retrieved, the vectors
of gains associated to their respective amplifiers are considered
and a new choice of gains is generated by perturbation of such
DP-BPSK DP-QPSK DP-8-QAM
Fig. 9: Stokes space representation of DP-BPSK, DP-QPSK
and DP-8-QAM modulation formats [68].
values. Then, the OSNR value that would be obtained with the
new vector of gains is estimated via simulation and stored in
the database as a new entry. After this, the vector associated
to the highest OSNR is used for tuning the amplifier gains
when the new lightpath is deployed.
An implementation of real-time EDFA setpoint adjustment
using the GMPLS control plane and interpolation rule based
on a weighted Euclidean distance computation is described in
[58] and extended in [62] to cascaded amplifiers.
Differently from the previous references, in [61] the issue of
modelling the channel dependence of EDFA power excursion
is approached by defining a regression problem, where the
input feature set is an array of binary values indicating the
occupation of each spectrum channel in a WDM grid and the
predicted variable is the post-EDFA power discrepancy. Two
learning approaches (i.e., the Ridge regression and Kernelized
Bayesian regression models) are compared for a setup with 2
and 3 amplifier spans, in case of single-channel and superchan-
nel add-drops. Based on the predicted values, suggestion on
the spectrum allocation ensuring the least power discrepancy
among channels can be provided.
C. Modulation format recognition
The issue of autonomous modulation format identification in
digital coherent receivers (i.e., without requiring information
from the transmitter) has been addressed by means of a
variety of ML algorithms, including k-means clustering [64]
and neural networks [66], [67]. Papers [63] and [68] take
advantage of the Stokes space signal representation (see Fig.
9 for the representation of DP-BPSK, DP-QPSK and DP-8-
QAM), which is not affected by frequency and phase offsets.
The first reference compares the performance of 6 unsuper-
vised clustering algorithms to discriminate among 5 different
formats (i.e. BPSK, QPSK, 8-PSK, 8-QAM, 16-QAM) in
terms of True Positive Rate and running time depending on the
OSNR at the receiver. For some of the considered algorithms,
the issue of predetermining the number of clusters is solved by
means of the silhouette coefficient, which evaluates the tight-
ness of different clustering structures by considering the inter-
and intra-cluster distances. The second reference adopts an
unsupervised variational Bayesian expectation maximization
algorithm to count the number of clusters in the Stokes space
representation of the received signal and provides an input
to a cost function used to identify the modulation format.
The experimental validation is conducted over k-PSK (with

15
k = 2, 4, 8) and n-QAM (with n = 8, 12, 16) modulated
signals.
Conversely, features extracted from asynchronous amplitude
histograms sampled from the eye-diagram after equalization
in digital coherent transceivers are used in [65]–[67] to train
NNs. In [66], [67], a NN is used for hierarchical extraction
of the amplitude histograms’ features, in order to obtain a
compressed representation, aimed at reducing the number of
neurons in the hidden layers with respect to the number of
features. In [65], a NN is combined with a genetic algorithm
to improve the efficiency of the weight selection procedure
during the training phase. Both studies provide numerical
results over experimentally generated data: the former obtains
0% error rate in discriminating among three modulation for-
mats (PM-QPSK, 16-QAM and 64-QAM), the latter shows
the tradeoff between error rate and number of histogram bins
considering six different formats (NRZ-OOK, ODB, NRZ-
DPSK, RZ-DQPSK, PM-RZ-QPSK and PM-NRZ-16-QAM).
D. Nonlinearity mitigation
One of the performance metrics commonly used for optical
communication systems is the data-rate×distance product.
Due to the fiber loss, optical amplification needs to be em-
ployed and, for increasing transmission distance, an increasing
number of optical amplifiers must be employed accordingly.
Optical amplifiers add noise and to retain the signal-to-noise
ratio optical signal power is increased. However, increasing
the optical signal power beyond a certain value will enhance
optical fiber nonlinearities which leads to Nonlinear Inter-
ference (NLI) noise. NLI will impact symbol detection and
the focus of many papers, such as [31], [32], [69]–[73] has
been on applying ML approaches to perform optimum symbol
detection.
In general, the task of the receiver is to perform optimum
symbol detection. In the case when the noise has circularly
symmetric Gaussian distribution, the optimum symbol de-
tection is performed by minimizing the Euclidean distance
between the received symbol yk and all the possible symbols
of the constellation alphabet, s = sk|k = 1, ..., M. This type
of symbol detection will then have linear decision boundaries.
For the case of memoryless nonlinearity, such as nonlinear
phase noise, I/Q modulator and driving electronics nonlinear-
ity, the noise associated with the symbol yk may no longer be
circularly symmetric. This means that the clusters in constel-
lation diagram become distorted (elliptically shaped instead of
circularly symmetric in some cases). In those particular cases,
optimum symbol detection is no longer based on Euclidean
distance matrix, and the knowledge and full parametrization of
the likelihood function, p(yk|xk), is necessary. To determine
and parameterize the likelihood function and finally perform
optimum symbol detection, ML techniques, such as SVM,
kernel density estimator, k-nearest neighbors and Gaussian
mixture models can be employed. A gain of approximately
3 dB in the input power to the fiber has been achieved,
by employing Gaussian mixture model in combination with
expectation maximization, for 14 Gbaud DP 16-QAM trans-
mission over a 800 km dispersion compensated link [31].
Furthermore, in [71] a distance-weighted k-nearest neigh-
bors classifier is adopted to compensate system impairments
in zero-dispersion, dispersion managed and dispersion unman-
aged links, with 16-QAM transmission, whereas in [74] NNs
are proposed for nonlinear equalization in 16-QAM OFDM
transmission (one neural network per subcarrier is adopted,
with a number of neurons equal to the number of symbols).
To reduce the computational complexity of the training phase,
an Extreme Learning Machine (ELM) equalizer is proposed
in [70]. ELM is a NN where the weights minimizing the
input-output mapping error can be computed by means of
a generalized matrix inversion, without requiring any weight
optimization step.
SVMs are adopted in [72], [73]: in [73], a battery of
log2(M) binary SVM classifiers is used to identify decision
boundaries separating the points of a M-PSK constellation,
whereas in [72] fast Newton-based SVMs are employed to
mitigate inter-subcarrier intermixing in 16-QAM OFDM trans-
mission.
All the above mentioned approaches lead to a 0.5-3 dB
improvement in terms of BER/Q-factor.
In the context of nonlinearity mitigation or in general,
impairment mitigation, there are a group of references that
implement equalization of the optical signal using a variety of
ML algorithms like Gaussian mixture models [75], clustering
[76], and artificial neural networks [77]–[82]. In [75], the
authors propose a GMM to replace the soft/hard decoder
module in a PAM-4 decoding process whereas in [76], the
authors propose a scheme for pre-distortion using the ML
clustering algorithm to decode the constellation points from
a received constellation affected with nonlinear impairments.
In references [77]–[82] that employ neural networks for
equalization, usually a vector of sampled receive symbols act
as the input to the neural networks with the output being
equalized signal with reduced inter-symbol interference (ISI).
In [77], [78], and [79] for example, a convolutional neural
network (CNN) would be used to classify different classes of
a PAM signal using the received signal as input. The number of
outputs of the CNN will depend on whether it is a PAM-4, 8,
or 16 signal. The CNN-based equalizers reported in [77]–[79]
show very good BER performance with strong equalization
capabilities.
While [77]–[79] report CNN-based equalizers, [81] shows
another interesting application of neural network in impair-
ment mitigation of an optical signal. In [81], a neural network
approximates very efficiently the function of digital back-
propagation (DBP), which is a well-known technique to solve
the non-linear Schroedinger equation using split-step Fourier
method (SSFM) [110]. In [80] too, a neural network is
proposed to emulate the function of a receiver in a nonlinear
frequency division multiplexing (NFDM) system. The pro-
posed NN-based receiver in [80] outperforms a receiver based
on nonlinear Fourier transform (NFT) and a minimum-distance
receiver.
The authors in [82] propose a neural-network-based ap-
proach in nonlinearity mitigation/equalization in a radio-over-
fiber application where the NN receives signal samples from
different users in an Radio-over-Fiber system and returns a

16
impairment-mitigated signal vector.
An example of unsupervised k-means clustering technique
applied on a received signal constellation to obtain a density-
based spatial constellation clusters and their optimal centroids
is reported in [83]. The proposed method proves to be an
efficient, low-complexity equalization technique for a 64-
QAM long-haul coherent optical communication system.
E. Optical performance monitoring
Artificial neural networks are well suited machine learning
tools to perform optical performance monitoring as they can
be used to learn the complex mapping between samples or
extracted features from the symbols and optical fiber chan-
nel parameters, such as OSNR, PMD, Polarization-dependent
loss (PDL), baud rate and CD. The features that are fed
into the neural network can be derived using different ap-
proaches relying on feature extraction from: 1) the power
eye diagrams (e.g., Q-factor, closure, variance, root-mean-
square jitter and crossing amplitude, as in [49]–[53], [69]);
2) the two-dimensional eye-diagram and phase portrait [54];
3) asynchronous constellation diagrams (i.e., vector diagrams
also including transitions between symbols [51]); and 4)
histograms of the asynchronously sampled signal amplitudes
[52], [53]. The advantage of manually providing the features
to the algorithm is that the NN can be relatively simple, e.g.,
consisting of one hidden layer and up to 10 hidden units and
does not require large amount of data to be trained. Another
approach is to simply pass the samples at the symbol level
and then use more layers that act as feature extractors (i.e.,
performing deep learning) [48], [55]. Note that this approach
requires large amount of data due to the high dimensionality
of the input vector to the NN.
Besides the artificial neural network, other tools like Gaus-
sian process models are also used which are shown to perform
better in optical performance monitoring compared to linear-
regression-based prediction models [56]. The authors in [56]
also claims that sometimes simpler ML tools like the Gaussian
Process (compared to ANN) can prove to be robust under
noise uncertainties and can be easy to integrate into a network
controller.
V. DETAILED SURVEY OF MACHINE LEARNING IN
NETWORK LAYER DOMAIN
A. Traffic prediction and virtual topology design
Traffic prediction in optical networks is an important phase,
especially in planning for resources and upgrading them opti-
mally. Since one of the inherent philosophy of ML techniques
is to learn a model from a set of data and ‘predict’ the
future behavior from the learned model, ML can be effectively
applied for traffic prediction.
For example, the authors in [84], [85] propose Autore-
gressive Integrated Moving Average (ARIMA) method which
is a supervised learning method applied on time series data
[111]. In both [84] and [85] the authors use ML algorithms to
predict traffic for carrying out virtual topology reconfiguration.
The authors propose a network planner and decision maker
(NPDM) module for predicting traffic using ARIMA models.
The NPDM then interacts with other modules to do virtual
topology reconfiguration.
Since, the virtual topology should adapt with the variations
in traffic which varies with time, the input dataset in [84] and
[85] are in the form of time-series data. More specifically,
the inputs are the real-time traffic matrices observed over
a window of time just prior to the current period. ARIMA
is a forecasting technique that works very well with time
series data [111] and hence it becomes a preferred choice
in applications like traffic predictions and virtual topology
reconfigurations. Furthermore, the relatively low complexity of
ARIMA is also preferable in applications where maintaining a
lower operational expenditure as mentioned in [84] and [85].
In general, the choice of a ML algorithm is always governed
by the trade-off between accuracy of learning and complexity.
There is no exception to the above philosophy when it comes
to the application of ML in optical networks. For example,
in [86] and [87], the authors present traffic prediction in
an identical context as [84] and [85], i.e., virtual topology
reconfiguration, using NNs. A prediction module based on
NNs is proposed which generates the source-destination traffic
matrix. This predicted traffic matrix for the next period is then
used by a decision maker module to assert whether the current
virtual network topology (VNT) needs to be reconfigured.
According to [87], the main motivation for using NNs is their
better adaptability to changes in input traffic and also the
accuracy of prediction of the output traffic based on the inputs
(which are historical traffic).
In [91], the authors propose a deep-learning-based traffic
prediction and resource allocation algorithm for an intra-data-
center network. The deep-learning-based model outperforms
not only conventional resource allocation algorithms but also
a single-layer NN-based algorithm in terms of blocking per-
formance and resource occupation efficiency. The results in
[91] also bolsters the fact reflected in the previous paragraph
about the choice of a ML algorithm. Obviously deep learning,
which is more complex than a regular NN learning will be
more efficient. Sometimes the application type also determines
which particular variant of a general ML algorithm should be
used. For example, recurrent neural networks (RNN), which
best suits application that involve time series data is applied in
[90], to predict baseband unit (BBU) pool traffic in a 5G cloud
Radio Access Network. Since the traffic aggregated at different
BBU pool comprises of different classes such as residential
traffic, office traffic etc., with different time variations, the
historical dataset for such traffic always have a time dimension.
Therefore, the authors in [90] propose and implement with
good effect (a 7% increase in network throughput and an 18%
processing resource reduction is reported) a RNN-based traffic
prediction system.
Reference [112] reports a cognitive network management
module in relation to the Application-Based Network Op-
erations (ABNO) framework, with specific focus on ML-
based traffic prediction for VNT reconfiguration. However,
[112] does not mention about the details of any specific ML
algorithm used for the purpose of VNT reconfiguration. On
similar lines, [113] proposes bayesian inference to estimate
network traffic and decide whether to reconfigure a given

17
Programmable End-to-End Optical Network
SDN Controller
Flow Info
Network
State Info
outgoing APIData
Collection and
Storage
ML Algorithm 1
ML Algorithm 2
ML Algorithm N
Policy 1
Policy 2
Policy N
incoming API
(reward/
feedback)
Control
Signal
Local
Exchange
Metro/Core CO Metro/Core CO
Local
Exchange…
…
…
…
…
…
…
…
Fig. 10: Schematic diagram illustrating the role of control plane housing the ML algorithms and policies for management of
optical networks.
virtual network.
While most of the literature focuses on traffic prediction
using ML algorithms with a specific view of virtual network
topology reconfigurations, [92] presents a general framework
of traffic pattern estimation from call data records (CDR). [92]
uses real datasets from service providers and operates matrix
factorization and clustering based algorithms to draw useful
insights from those data sets, which can be utilized to better
engineer the network resources. More specifically, [92] uses
CDRs from different base stations from the city of Milan.
The dataset contains information like cell ID, time interval of
calls, country code, received SMS, sent SMS, received calls,
sent calls, etc., in the form of a matrix called CDR matrix.
Apart from the CDR matrix, the input dataset also includes
a point-of-interest (POI) matrix which contains information
about different points of interests or regions most likely visited
corresponding to each base station. All these input matrices
are then applied to a ML clustering algorithm called non-
negative matrix factorization (NMF) and a variant of it called
collective NMF (C-NMF). The output of the algorithms factors
the input matrices into two non-negative matrices one of which
gives the different types basic traffic patterns and the other
gives similarities between base stations in terms of the traffic
patterns.
While many of the references in the literature focus on
one or few specific features when developing ML algorithms
for traffic prediction and virtual topology (re)configurations,
others just mention a general framework with some form of
‘cognition’ incorporated in association with regular optimiza-
tion algorithms. For example, [88] and [89] describes a multi-
objective Genetic Algorithm (GA) for virtual topology design.
No specific machine learning algorithm is mentioned in [88]
and [89], but they adopt adaptive fitness function update for
GA. Here they use the principles of reinforcement learning
where previous solutions of the GA for virtual topology design
are used to update the fitness function for the future solutions.
B. Failure management
ML techniques can be adopted to either identify the exact
location of a failure or malfunction within the network or even
to infer the specific type of failure. In [96], network kriging is
exploited to localize the exact position of failure along network
links, under the assumption that the only information available
at the receiving nodes (which work as monitoring nodes)
of already established lightpaths is the number of failures
encountered along the lightpath route. If unambiguous local-
ization cannot be achieved, lightpath probing may be operated
in order to provide additional information, which increases

18
the rank of the routing matrix. Depending on the network
load, the number of monitoring nodes necessary to ensure
unambiguous localization is evaluated. Similarly, in [93] the
measured time series of BER and received power at lightpath
end nodes are provided as input to a Bayesian network which
individuates whether a failure is occurring along the lightpath
and try to identify the cause (e.g., tight filtering or channel
interference), based on specific attributes of the measurement
patterns (such as maximum, average and minimum values,
presence and amplitude of steps). The effectiveness of the
Bayesian classifier is assessed in an experimental testbed:
results show that only 0.8% of the tested instances were
misclassified.
Other instances of application of Bayesian models to de-
tect and diagnose failures in optical networks, especially
GPON/FTTH, are reported in [94] and [95]. In [94], the
GPON/FTTH network is modeled as a Bayesian Network
using a layered approach identical to one of their previous
works [114]. The layer 1 in this case actually corresponds
to the physical network topology consisting of ONTs, ONUs
and fibers. Failure propagation, between different network
components depicted by layer-1 nodes, is modeled in layer
2 using a set of directed acyclic graphs interconnected via the
layer 1. The uncertainties of failure propagation are then han-
dled by quantifying strengths of dependencies between layer
2 nodes with conditional probability distributions estimated
from network generated data. However, some of these network
generated data can be missing because of improper measure-
ments or non-reporting of data. An Expectation Maximization
(EM) algorithm is therefore used to handle missing data for
root-cause analysis of network failures and helps in self-
diagnosis. Basically, the EM algorithm estimates the missing
data such that the estimate maximizes the expected log-
likelihood function based on a given set of parameters. In [95]
a similar combination of Bayesian probabilistic models and
EM is used for failure diagnosis in GPON/FTTH networks.
In the context of failure detection, in addition to Bayesian
networks, other machine learning algorithms and concepts
have also been used. For example, in [97], two ML based
algorithms are described based on regression, classification,
and anomaly detection. The authors propose a BER anomaly
detection algorithm which takes as input historical information
like maximum BER, threshold BER at set-up, and monitored
BER per lightpath and detects any abrupt changes in BER
which might be a result of some failures of components along
a lightpath. This BER anomaly detection algorithm, which is
termed as BANDO, runs on each node of the network. The
outputs of BANDO are different events denoting whether the
BER is above a certain threshold or below it or within a pre-
defined boundary.
This information is then passed on to the input of another
ML based algorithm which the authors term as LUCIDA.
LUCIDA runs in the network controller and takes historic
BER, historic received power, and the outputs of BANDO
as input. These inputs are converted into three features that
can be quantified by time series and they are as follows:
1) Received power above the reference level (PRXhigh);
2) BER positive trend (BERTrend); and 3) BER periodicity
(BERPeriod). LUCIDA computes these features’ probabilities
and the probabilities of possible failure classes and finally
maps these feature probabilities to failure probabilities. In this
way, LUCIDA detects the most likely failure cause from a set
of failure classes.
Another notable use case for failure detection in optical
networks using ML concepts appear in [98]. Two algorithms
are proposed viz., Testing optIcal Switching at connection
SetUp time (TISSUE) and FailurE causE Localization for
optIcal NetworkinG (FEELING). The TISSUE algorithm takes
the values of estimated BER calculated at each node across a
lightpath and the measured BER and compares them. If the
differences between the slopes of the estimated and theoretical
BER is above a certain threshold a failure is anticipated. While
it is not clear from [98] whether the estimation of BER in the
TISSUE algorithm is based on ML methods, the FEELING
algorithm applies two very well-known ML methods viz.,
decision tree and SVM.
In FEELING, the first step is to process the input dataset
in the form of ordered pairs of frequency and power for
each optical signal and transform them into a set of features.
The features include some primary features like the power
levels across the central frequency of the signal and also
the power around other cut-off points of the signal spectrum
(interested readers are encouraged to look into [98] for fur-
ther details). In context of the FEELING algorithm, some
secondary features are also defined in [98] which are linear
combinations of the primary features. The feature-extraction
process is undertaken by a module named FeX. The next
step is to input these features into a multi-class classifier in
the form of a decision tree which outputs a predicted class
among three options: ‘Normal, ‘LaserDrift and ‘FilterFailure;
and ii) a subset of relevant signal points for the predicted class.
Basically, the decision tree contains a number of decision rules
to map specific combinations of feature values to classes. This
decision-tree-based component runs in another module named
signal spectrum verification (SSV) module. The FeX and SSV
modules are located in the network nodes. There are two more
modules called signal spectrum comparison (SSC) module and
laser drift estimator (LDE) module which runs on the network
controller.
In the SSC module, a similar classification process takes
place as in SSV. But here a signal is diagnosed based on the
different classes of failures just due to filtering. Here the three
classes are: Normal, FilterShift and TightFiltering. The SSC
module uses Support Vector Machines to classify the signals
based on the above three classes. First, the SVM classifies
whether the signal is ‘Normal’ or has suffered a filter-related
failure. Next, the SVM classifies the signal suffering from
filter-related failures into two classes based on whether the
failure is due to tight filtering or due to filter shift. Once
these classifications are done, the magnitude of failures related
to each of these classes are estimated using some linear
regression based estimator modules for each of the failure
classes. Finally, all these information provided by the different
modules described so far, are used in the FEELING algorithm
to return a final list of failures.
A similar multi-ML algorithm based framework like [98]

19
Fig. 11: The failure detection and identification framework
adopted in [99].
for failure detection and classification is also proposed in [99]
and [100]. In [99] several ML algorithms are used and, by
tuning several model parameters, such as BER sampling time
and amount of BER data needed to train the models, one or
more proper optimized algorithm(s) is/are chosen from Binary
and Multiclass SVMs, Random Forests and neural networks.
Moreover, in paper [99], the authors propose the detection
and cause identification algorithms suggesting that a network
operator, able to early-detect a failure (and identify its cause)
before a critical BER threshold is reached, can proactively
re-route the affected traffic onto a new lightpath, so as to
minimize SLA violation and enhance (i.e., speed up) failure
recovery procedures (see Fig. 11).
In [100], optical power levels, amplifier gain, shelf temper-
ature, current draw, internal optical power are used to predict
failures using statistical regression and neural network based
algorithms that sit in the SDN controllers.
C. Flow classification
Another popular area of ML application for optical networks
is flow classification. In [101] for example, a framework is
described that observes different types of packet loss in optical
burst-switched (OBS) networks. It then classifies the packet
loss data as congestion loss or contention loss using a Hidden
Markov Model (HMM) and EM algorithms.
Another example of flow classification is presented in [102].
Here a NN is trained to classify flows in an optical data
center network. The feature vector includes a 5-tuple (source
IP address, destination IP address, source port, destination
port, transport layer protocol). Packet sizes and a set of intra-
flow timings within the first 40 packets of a flow, which
roughly corresponds to the first 30 TCP segments, are also
used as inputs to improve the training speed and to mitigate
the problem of ‘disappearing gradients’ while using gradient
descent for back-propagation.
The main outcome of the NN used in [102] is the classi-
fication of mice and elephant flows in the data center (DC).
The type of neural network used is a multi-layer perceptron
(MLP) with four hidden layers as MLPs are relatively simpler
to implement. The authors of [102] also mention the high
levels of true negative classification associated with MLPs, and
comment on importance of ensuring that mice do not flood
the optical interconnections in the DC network. In general,
mice flows do actually outnumber elephant flows in a practical
DC network, and therefore the authors in [102] suggest to
overcome this class imbalance between mice and elephant
flows by training the NN with a non-proportional amount of
mice and elephant flows.
D. Path computation
Path computation or selection, based on different physical
and network layer parameters, is a commonly studied problem
in optical networks. In Section IV for example, physical
layer parameters like QoT, modulation format, OSNR, etc.
are estimated using ML techniques. The main aim is to
make a decision about the best optical path to be selected
among different alternatives. The overall path computation
process can therefore be viewed as a cross-layer method with
application of machine learning techniques in multiple layers.
In this subsection we identify references [103] and [104] that
addresses the path computation/selection in optical networks
from a network layer perspective.
In [103] the authors propose a path and wavelength selection
strategy for OBS networks to minimize burst-loss probability.
The problem is formulated as a multi-arm bandit problem
(MABP) and solved using Q-learning. An MABP problem
comes from the context of gambling where a player tries to
pull one of the arms of a slot machine with the objective to
maximize sum of rewards over many such pulls of arms. In the
OBS network scenario, the authors in [103] use the concept
of path selection for each source-destination pair as pulling
of one of the arms in a slot machine with the reward being
minimization of burst-loss probability. In general the MABP
problem is a classical problem in reinforcement learning and
the authors propose Q-learning to solve this problem because
other methods does not scale well for complex problems.
Furthermore, other methods of solving MABP, like dynamic
programming, Gittins indices, and learning automata prove to
be difficult when the reward distributions (i.e., the distributions
of the burst-loss probability in case of the OBS scenario) are
unknown. The authors in [103] also argue that the Q-learning
algorithm has a guaranteed convergence compared to other
methods of solving the MABP problem.
In [104] a control plane decision making module for QoS-
aware path computation is proposed using a Fuzzy C-Means
Clustering (FCM) algorithm. The FCM algorithm is added to
the software-defined optical network (SDON) control plane in
order to achieve better network performance, when compared
with a non-cognitive control plane. The FCM algorithm takes
traffic requests, lightpath lengths, set of modulation formats,
OSNR, BER etc., as input and then classifies each lightpath
with the best possible parameters of the physical layer. The
output of the classification is a mapping of each lightpath
with a different physical layer parameter and how closely a
lightpath is associated with a physical layer parameter in terms
of a membership score. This membership score information is

20
then utilized to generate some rules based on which real time
decisions are taken to set up the lightpaths.
As we can see from the overall discussion in this section,
different ML algorithms and policies can be used based on
the use cases and applications of interest. Therefore, one
can envisage a concise control plane for the next generation
optical networks with a repository of different ML algorithms
and policies as shown in Fig. 10. The envisaged control
plane in Fig. 10 can be thought of as the ‘brain’ of the
network that interacts constantly with the ‘network body’ (i.e.,
different components like transponders, amplifies, links etc.)
and react to the ‘stimuli’ (i.e., data generated by the network)
and perform certain ‘actions’ (i.e., path computation, virtual
topology (re)configurations, flow classification etc.). A setup
on similar lines as discussed above is presented in [115]
where a ML-based monitoring module drives a Reconfigurable
Optical Add/Drop Multiplexer (ROADM) controller which is
again controlled by a SDN controller.
VI. EVALUATION OF MACHINE LEARNING ALGORITHMS IN
OPTICAL NETWORKS
In this section, we provide a more quantitative comparison
of some of the ML applications described in Section III. To do
this, we first provide an overview of the typical performance
metrics adopted in ML. Then, we select some of the studies
discussed in Sections IV and V, and we concentrate on how the
ML algorithms used these papers are quantitatively compared
using these performance metrics. For each paper, we also
provide a quick description of the main outcome of this
comparison.
A. Performance metrics
When applying ML to a classification problem, a common
approach to evaluate the ML-algorithm performance is to show
its classification accuracy and a meadure of the algorithm
complexity, usually expressed in the form of training-phase
duration. Classification accuracy represents the fraction of
the test samples which are correctly classified. Although this
metric is intuitive, it turns out to be a poor metric in complex
classification problems, especially when the available dataset
contains an amount of samples largely unbalanced among the
various classes (e.g., a binary dataset where 90% of samples
belongs to one class). In these cases, the following and other
measures can be used:
• Confusion matrix: Given a binary classification problem,
where samples in the test set belong to either a posi-
tive or a negative class, the confusion matrix gives a
complete overview of the classifier performance, showing
1) the true positives (TP) and true negatives (TN),
i.e., the number of samples of the true and false class,
respectively, which have been correctly classified, and
2) the false positives (FP) and false negatives (FN),
i.e., the number of samples of the true and false class,
respectively, which have been misclassified. Note that,
using these definitions, accuracy can be expressed as
(TP + TN)/(TP + TN + FP + FN).
• True Positive Rate, TPR = TP/(TP + FN): This
metric falls in the [0, 1] range and captures the ability of
identifying actually positive samples in the test set (i.e.,
the larger, the better).
• False Positive Rate, FPR = FP/(FP + TN): Also
this metric falls in the [0, 1] range, and it represents
the fraction of negative samples in the test set that
are incorrectly classified as positive (i.e., the lower, the
better).
• Receiver operating characteristic (ROC) curve: In a bi-
nary classifier, an arbitrary threshold γ can be set to
distinguish between true and false instances; by increas-
ing the value of γ, we reduce the number of instances
that we classify as positive and increase the number of
samples that we classify as negative; this has the effect
of decreasing TP while correspondingly increasing FN,
and increasing TN while correspondingly decreasing
FP; hence, both the TPR and the FPR are reduced.
For different values of γ, the ROC curve plots the
TPR (on the vertical axis) against the FPR (on the
horizontal axis). For γ = 1, all samples are classified
as negative, therefore TPR = FPR = 0. Conversely,
for γ = 0, all samples are classified as positive, hence
TPR = FPR = 1. For any classifier, its ROC curve
always connects these two extremes. Classifiers capturing
useful information yield a ROC curve above the diagonal
in the (FPR, TPR) plane, and aim at approaching the
ideal classifier, which interconnects points (0,0), (0,1) and
(1,1).
• Area under the ROC curve (AUC): The AUC takes values
in the [0, 1] range and captures how much a given clas-
sifier approaches the performance of an ideal classifier.
While the ROC curve is an efficient graphical means
to evaluate the performance of a classifier, the AUC
is a synthetic numerical measure to indicate algorithm
performance independently from the specific choice of
the threshold γ.
• Akaike Information Criteria (AIC): This is a metric that
captures the goodness of fit for a particular model. It
measures the deviation of a chosen statistical model
from the ‘true model’ by defining a criteria which is
a mathematical function of the number of estimated
parameters by the model and the maximum likelihood
function. The model with minimum AIC is considered as
the best model to fit a given dataset [116].
• Metrics from the optical networking field: Besides nu-
merical and graphical metrics traditionally used in the
ML context, measures from the networking field can
be also adopted in combination with such metrics, in
order to have a quantitative understanding of how the
ML algorithm impacts on the optical network/system.
E.g., an operator might be interested in the minimum
number of optical performance monitors to deploy along
a lightpath to correctly classify a degraded transmission
with a given accuracy; similarly, the minimum OSNR
and/or signal power level required at an optical receiver
to correctly recognize the adopted MF. Furthermore, an
operator might also wonder how often BER samples

21
TABLE III: Comparison of ML algorithms and performance metrics for a selection of existing papers.
Use Case Ref. Adopted algorithms Metrics Outcome
QoT estimation (BER
classification)
[40] Naive Bayes, Decision tree,
RF, J4.8 tree, CBR
Accuracy, false posi-
tives
CBR has highest accuracy (above 99%) with low false
positive (0.43%), decision tree reaches lowest false positive
(0.02%) at the price of much lower accuracy (86%)
QoT estimation (BER
classification)
[41] KNN, RF Accuracy, AUC, run-
ning time
RF has higher AUC and accuracy than KNN, the training
time of RF is higher than KNN but the testing time is at
least one order of magnitude lower than RNN
QoT estimation (BER
classification)
[45] KNN, RF, SVM Accuracy, Confusion
Matrix, ROC curves
SVM has the best accuracy among all three ML algorithms,
accuracy improves with size of Knowledge Base (KB)
MF recognition in
Stokes space
[63] K-means, EM, DBSCAN,
OPTICS, spectral cluster-
ing, Maximum-likelihood
Running time,
minimum OSNR to
achieve 95% accuracy
Maximum likelihood requires lowest OSNR level and has
very low running time (comparable to OPTICS, which has
lowest running time but requires much higher OSNR level)
Failure Management [94], [95] Bayesian Inference, EM Confusion Matrix The failure detection based on learning of the network
parameters is more accurate compared to the case where
an expert sets the parameters based on certain deterministic
rules
Failure Management [99] NN, RF, SVM Accuracy versus
model parameters
(BER sampling time,
amount of BER data
etc.)
With right model parameters, binary SVM can reach up to
100% accuracy for failure detection
Flow Classification
(Loss classification in
OBS networks)
[101] HMM, EM Misclassification
probability (similar
to FPR)
HMM has better accuracy and has lower misclassification
probability for static traffic type compared to dynamic
traffic, the misclassification probability also goes down
with increasing number of wavelengths per link
should be collected to predict or correctly localize an
optical failure along a lightpath with a certain accuracy.
B. Quantitative algorithms comparison
We now provide a schematic comparison of some ML
algorithms focusing on some of the use cases discussed in
Section III. To perform this comparison, we select, among the
papers surveyed in Sections IV and V, those where different
ML algorithms have been applied and compared with a same
data set. Note that a fair quantitative comparison between
algorithms in different papers is hard due to the fact that, in
each paper, the various algorithms have been designed to fit
with the specific available data set. As a consequence, a given
algorithm may perform incredibly well if applied to a certain
data set, but at the same time it may exhibit poor performance
if the data set is changed, though not substantially.
Table III provides such overview, highlighting, for each
considered use case and corresponding reference, the ML
algorithms and the evaluation metrics used for the comparison.
In the table we also provide a synthetic description of the paper
outcome.
VII. DISCUSSION AND FUTURE DIRECTIONS
In this section we discuss our vision on how this research
area will expand in next years, focusing on some specific areas
that we believe will require more attention during the next
years.
ML methodologies. We notice how the vast majority of
existing studies adopting ML in optical networks use offline
supervised learning methods, i.e., assume that the ML algo-
rithms are trained with historical data before being used to take
decisions on the field. This assumption is often unrealistic for
optical communication networks, where scenarios dynamically
evolve with time due, e.g., to traffic variations or to changes
in the behavior of optical components caused by aging. We
thus envisage that, after learning from a batch of available
past samples, other types of algorithms, in the field of semi-
supervised and/or unsupervised ML, could be implemented to
gradually take in novel input data as they are made available
by the network control plane. Under a different perspective,
re-training of supervised mechanisms must be investigated to
extend their applicability to, e.g., different network infrastruc-
tures (the training on a given topology might not be valid for
a different topology) or to the same network infrastructure at
a different point in time (the training performed in a certain
week/month/year might not be valid anymore after some time).
In a more general sense, novel ML techniques, developed ad-
hoc for optical-networking problems might emerge. Consider,
e.g., active ML algorithms, which can interactively ask the
user to observe training data with specific characteristics. This
way, the number of samples needed to build an accurate
prediction model can be consistently reduced, which may lead
to significant savings in case the dataset generation process is
costly (e.g., when probe lightpaths have to be deployed).
Data availability. As of today, vendors and operators have
not yet disclosed large set of field data to test the practi-
cality of existing solutions. This problem might be partially
addressed by emulating relevant events, as failures or signal
degradations, over optical-network testbeds, even though it
is simply impossible to reproduce the diversity of scenarios
of a real network in a lab environment. Moreover, even in
situations of complete access to real data, for some of the
use cases mentioned before, in practical assets it is difficult to

22
collect extensive datasets during faulty operational conditions,
since networks are typically dimensioned and managed via
conservative design approaches which make the probability of
failures negligible (at the price of under-utilization of network
resources).
Timescales. Scarce attention has so far been devoted to
the fact that different applications might have very different
timescales over which monitored data show observable and
useful pattern changes (e.g., aging would make component
behaviour vary slowly over time, while traffic varies quickly,
and at different timescales, e.g., burst, daily, weekly, yearly
level. Understading the right timescale for the monitoring of
the parameters to be fed into ML algorithms is not only
important to optimize the accuracy of the algorithm (and hence
system performance), but it is fundamental to dimension the
amout of control/monitoring bandwidth needed to actually
implement the ML-based system. If a ML-algorithm works
perfectly, but it requires a huge amount of data to be sampled
extremely frequently, then the additional control bandwidth
required will hinder the practical application of the algorithm.
A complete cognitive control system. Another important
consideration is that all existing ML-based solutions have
addressed specific and isolated issues in optical communi-
cations and networking. Considering that software defined
networking has been demonstrated to be capable of success-
fully converging control through multiple network layers and
technologies, such a unified control could also coordinate
(orchestrate) several different applications of ML, to provide
a holistic design for flexible optical networks. In fact, as seen
in the literature, ML algorithms can be adopted to estimate
different system characteristics at different layers, such as
QoT, failure occurrences, traffic patterns, etc., some of which
are mutually dependent (e.g., the QoT of a lightpath is highly
related to the presence of failures along its links or in the
traversed nodes), whereas others do not exhibit dependency
(e.g., traffic patterns and fluctuations typically do not show
any dependency on the status of the transmission equipment).
More research is needed to explore the applicability and assess
the benefits of ML-based unified control frameworks where all
the estimated variables can be taken into account when making
decisions such as where to route a new lightpath (e.g., in terms
of spectrum assignment and core/mode assignment), when
to re-route an existing one, or when to modify transmission
parameters such as modulation format and baud rate.
Failure recovery. Another promising and innovative area for
ML application paired with SDN control is network failure
recovery. State-of-the-art optical network control tools are
tipically configured as rule-based expert systems, i.e., a set
of expert rules (IF <conditions> THEN <actions>) covering
typical failure scenarios. Such rules are specialized and deter-
ministic and usually in the order of a few tens, and cannot
cover all the possible cases of malfunctions. The application
of ML to this issue, in addition to its ability to take into
account relevant data across all the layers of a network, could
also bring in probabilistic characterization (e.g., making use
of Gaussian processes, output probability distributions rather
than single numerical/categorical values) thus providing much
richer information with respect to currently adopted threshold-
based models.
Visualization. Developing effective visualization tools to
make the information-rich outputs produced by ML algorithms
immediately accessible and comprehensible to the end users
is a key enabler for seamless integration of ML techniques
in optical network management frameworks. Though some
preliminary research steps in such direction have been done
(see, e.g., [117], where bubble charts and spectrum color maps
are employed to visualize network links experiencing high
BER), design guidelines for intuitive visualization approaches
depending on the specific aim of ML usage (e.g., network
monitoring, failure identification and localization, etc.) have
yet to be investigated and devised.
Commercialization and standardization. Though in its
infancy, applications of ML to optical networking have al-
ready attracted the interest of network operators and optical
equipment vendors, and it is expected that this attention will
grow rapidly in the near future. Among the others, we notice
some activities on QoT estimation optimization for margin re-
duction and error-aware rerouting [118], on low-margin optical
network design [119], on traffic prediction [120] and anomaly
detection [121]. Furthermore, also standardization bodies have
started looking at the application of ML for the resolution of
networking problems. Although, to the best of our knowledge,
no specific activity is currently undergoing with dedicated
focus on optical networks, it is worth mentioning, e.g., ITU-T
focus group on ML [122], whose activities are concentrated
on various aspects of future networking, such as architectures,
interfaces, protocols, algorithms and data formats.
Optics for Machine Learning (vs. Machine Learning for
optics). Finally, an interesting, though speculative, area of
future research is the application of ML to all-optical devices
and networks. Due to their inherent non-linear behaviour,
optical components could be interconnected to form structures
capable of implementing learning tasks [123]. This approach
represents an all-optical alternative to traditional software
implementations. In [124], for example, semiconductor laser
diodes were used to create a photonic neural network via
time-multiplexing, taking advantage of their nonlinear reaction
to power injection due to the coupling of amplitude and
phase of the optical field. In [125], a ML method called
“reservoir computing” is implemented via a nanophotonic
reservoir constituted by a network of coupled crystal cavities.
Thanks to their resonating behavior, power is stored in the
cavities and generates nonlinear effects. The network is trained
to reproduce periodic patterns (e.g., sums of sine waves).
To conclude, the application of ML to optical networking
is a fast-growing research topic, which sees an increasingly
strong participation from industry and academic researchers.
While in this section we could only provide a short discussion
on possible future directions, we envisage that many more
research topic will soon emerge in this area.
VIII. CONCLUSION
Over the past decade, optical networks have been growing
‘smart’ with the introduction of software defined networking,
coherent transmission, flexible grid, to name only few arising

23
technical and technological directions. The combined progress
towards high-performance hardware and intelligent software,
integrated through an SDN platform provides a solid base for
promising innovations in optical networking. Advanced ma-
chine learning algorithms can make use of the large quantity
of data available from network monitoring elements to make
them ‘learn’ from experience and make the networks more
agile and adaptive.
Researchers have already started exploring the application
of machine learning algorithms to enable smart optical net-
works and in this paper we have summarized some of the
work carried out in the literature and provided insight into
new potential research directions.
GLOSSARY
ABNO Application-Based Network Operations
ANN Artificial Neural Network
API Application Programming Interface
AUC Area Under the ROC Curve
ARIMA Autoregressive Integrated Moving Average
BBU Baseband Unit
BER Bit Error Rate
BPSK Binary Phase Shift Keying
BVT Bandwidth Variable Transponders
CBR Case Based Reasoning
CD Chromatic Dispersion
CDR Call Data Records
CNN Convolutional Neural Network
C-NMF Collective Non-negative Matrix Factorization
CO Central Office
DBP Digital Back Propagation
DC Data Center
DP Dual Polarization
DQPSK Differential Quadrature Phase Shift Keying
EDFA Erbium Doped Fiber Amplifier
ELM Extreme Learning Machine
EM Expectation Maximization
EON Elastic Optical Network
FCM Fuzzy C-Means Clustering
FN False negatives
FP False positives
FTTH Fiber-to-the-home
GA Genetic Algorithm
GF Gain flatness
GMM Gaussian Mixture Model
GMPLS Generalized Multi-Protocol Label Switching
GPON Gigabit Passive Optical Network
GPR Gaussian processes nonlinear regression
HMM Hidden Markov Model
IP Internet Protocol
ISI Inter-Symbol Interference
LDE Laser drift estimator
MABP Multi-arm bandit problem
MDP Markov decision processes
MF Modulation Format
MFR Modulation Format Recognition
ML Machine Learning
MLP Multi-layer perceptron
MPLS Multi-Protocol Label Switching
MTTR Mean Time To Repair
NF Noise Figure
NFDM Nonlinear Frequency Division Multiplexing
NFT Nonlinear Fourier Transform
NLI Nonlinear Interference
NMF Non-negative Matrix Factorization
NN Neural Network
NPDM Network planner and decision maker
NRZ Non-Return to Zero
NWDM Nyquist Wavelength Division Multiplexing
OBS Optical Burst Switching
ODB Optical Dual Binary
OFDM Orthogonal Frequency Division Multiplexing
ONT Optical Network Terminal
ONU Optical Network Unit
OOK On-Off Keying
OPM Optical Performance Monitoring
OSNR Optical Signal-to-Noise Ratio
PAM Pulse Amplitude Modulation
PDL Polarization-Dependent Loss
PM Polarization-multiplexed
PMD Polarization Mode Dispersion
POI Point of Interest
PON Passive Optical Network
PSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
Q-factor Quality factor
QoS Quality of Service
QoT Quality of Transmission
QPSK Quadrature Phase Shift Keying
RF Random Forest
RL Reinforcement Learning
RNN Recurrent Neural Network
ROADM Reconfigurable Optical Add/Drop Multiplexer
ROC Receiver operating characteristic
RWA Routing and Wavelength Assignment
RZ Return to Zero
S-BVT Sliceable Bandwidth Variable Transponders
SDN Software-defined Networking
SDON Software-defined Optical Network
SLA Service Level Agreement
SNR Signal-to-Noise Ratio
SPM Self-Phase Modulation
SSC Signal spectrum comparison
SSFM Split-Step Fourier Method
SSV Signal spectrum verification

24
SVM Support Vector Machine
TCP Transmission Control Protocol
TN True negatives
TP True positives
VNT Virtual Network Topology
VT Virtual Topology
VTD Virtual Topology Design
WDM Wavelength Division Multiplexing
XPM Cross-Phase Modulation
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