I Introduction
Monitoring the activity in communication networks has become a popular area of research and particular attention has been paid to detection tasks such as spotting events or anomalies. An effective way to represent the communication activity is via a dynamic graph where the entities are considered to be nodes, and each communication event (or more simply event) to be represented by a set of connecting edges that appear at a specific time interval. Multiple occurring events over time may be seen as a link stream [1] with fast creation and deletion of edges. The use of this representation is mainly motivated by the fact that, in reality, contentspecific features of the communicated messages are usually kept undisclosed so as to preserve privacy. Consequently, most studies on activity monitoring merely deal with linkage information, i.e. who communicated with whom and at which time; the body of work on anomaly detection is not an exception.
The anomaly detection task on graphrelated activity can refer to the node, the subgraph, or the whole graphlevel [2]. To the best of our knowledge, the existing methods consider timeaggregated representations of the dynamic graph. It has been proposed to work with timeseries of static graphs, each of them summarizing the link stream during a time interval. In other words, each edge weight of a static graph is a function of the number of events occurring between two nodes during that time interval. Modeling the weights’ evolution with counting processes [3, 4] is among the standard approaches. The main drawback of any aggregated representation is that it neglects events that involve more than two nodes (e.g. multiple receivers). Besides, a common limitation of the existing literature is the assumption that the communication volume is generated by a stationary underlying distribution.
In this work, we focus on the detection of abnormal communication volume at nodelevel, which is particularly interesting as a change in the behavior of a node may reveal various types of abnormality (e.g. account hack, antenna breakdown, etc.). We put forward a contentagnostic approach supposing access solely to the linkage information observed at each event, that is the set of the involved nodes. The conceptual novelty of our approach is that, contrarily to our predecessors that use timeaggregated representations, we model the activity as a clique stream. We track each event independently, we consider it to be instantaneous and thus to activate an undirected subgraph spanning over the set of equally participating nodes, i.e. there are no special roles such as sender and receivers. Hence, we can represent each event with a binary fingerprint indicating the involved nodes. Subsequently, we propose to statistically model and infer the probability that a node takes part in an event, knowing the observed event fingerprint indicating the other participating nodes. The assumption is that there is a pattern in the fingerprints of the events in which a node participates. This pattern results from the underlying network structure since it is natural for subsets of neighboring nodes to participate frequently together in events.
This modeling allows us to derive confidence levels for the communication volume to which a node participates in a time interval. Our detection approach has two strong aspects. First, it allows the timeseries of the node’s communication volume to be nonstationary, since it only assumes regularity in the corresponding event fingerprints. Specifically, knowing the fingerprint of an event for all nodes but for a reference node, then the conditional probability that this node takes part in that event is constant over time, whereas the marginal probability that the node could participate in the event is not necessarily constant. Second, the anomaly score that our approach outputs is easily interpretable as it is simply based on the prediction error of a regression function.
Ii Related Work
In the literature, the existing detection methods for abnormal node communication volume mostly analyze a timeaggregated representation of the actual dynamic graph of communication activity. This implies a timeseries of static graphs , where is the weighted adjacency matrix representing all the shared communication events between pairs of nodes at in the time interval , and is the total number of nodes in the network. Most methods do not consider selfedges and therefore require each to have zero diagonal. Since these methods consider nodetonode communication events, note that the weighted degree of a node according to gives also the total number of events observed at a node in the time interval . The multivariate timeseries of the total number of events occurring in the network over time can be written as . This is the variable of our interest which we would like to know when it gets abnormal values.
A featurebased approach for detecting anomalies in such timeseries of graphs, is to compute several graph features for each , such as the node degree or centrality, and then apply standard anomaly detection techniques on the derived multivariate timeseries of these features [5, 6, 7]. More generally, the literature of anomaly detection in timeseries of graphs varies in three aspects:
The reader should refer to [2] for a more detailed survey on anomaly detection in dynamic graphs.
As in [3, 21, 22], our work assumes a semisupervised setting and proposes a modelbased approach for nodelevel anomaly detection. Moreover, as in [4], graph edges are considered to be undirected, and each event to be shared by two or more nodes without distinguishable roles (e.g. sender and receivers).
In [1], the link stream framework is presented for the representation of a dynamic graph as a stream where edges are being created and removed. Therein, the nodes are assumed to be fixed and the dynamics affect only the edges between them. An edge is characterized by a triplet noting two communicating nodes , , and a time interval which is not necessarily continuous (may even be a union of noncontiguous time intervals). In this work, we adopt this stream framework. In particular, as we will see, we represent the activity as a clique stream, and is always a finite union of singletons as edges appear instantaneously.
Iii Model Description and Methodology
Iiia The model
Let a communication network have
interconnected entities, referred to as nodes. In terms of notation style, we differentiate a random from an observed variable (respectively vector) with uppercase and lowercase (respectively bold) letters. Moreover, let
denote the size of the input set.Definition 1.
(Communication event): A communication event is denoted by a tuple of elements, which contains the timestamp at which the event occurred and its fingerprint .
Definition 2.
(Event fingerprint): The fingerprint of an event is an dimensional binary vector , where if node is involved in the event, and otherwise.
Note that the involvement of a node in an event implies its participation regardless its communication role (e.g. sender or receiver). From a probabilistic point of view, a fingerprint follows a multivariate Bernoulli distribution. From a graph point of view, we can see that each event creates a
clique with all the involved nodes (see an example in Fig. 1). Formally, a clique is defined as a subset of nodes of the graph that are all pairwise adjacent. Therefore, we regard the communication activity as a clique stream, and each clique appears instantaneously as events have no duration.Definition 3.
(Event stream): An event stream is a sequence of communication events each creating a clique among the involved nodes. We write as the substream with the events that occurred in a certain time interval , and .
Assumption 1.
The communication events are considered to be independent. The total number of events recorded during an event stream is considered to be deterministic.
Let us consider a time interval and the associated event stream consisting of its recorded events. Let the event realizations be denoted by , where . Also, let be the number of events recorded at node over the time interval .
For a given node and time interval , the goal of our method is to be able to decide if the volume of events in which that node participates is abnormal. To solve this problem, the main idea is to provide confidence levels for based on the fingerprints collected from events of the neighboring nodes. This way, an anomaly can be simply spotted whenever the observed value of lies out of the confidence level.
Definition 4.
(Conditional probability function): Let be the fingerprint of the event that indicates the participation of all nodes except from node . Then, we define as the probability that node participates in the event , provided the fingerprint :
(1)  
(2) 
Knowing the fingerprint over all the other nodes allows us to express the behavior of node
as a Bernoulli random variable:
(3) 
Concerning a substream and the number of events recorded at node therein, we can note that is a sum of Bernoulli distributions and, thus, we can use concentration inequalities [23], such as Chernoff’s or Hoeffding’s [24], to derive confidence levels. Below, we apply the bilateral Hoeffding’s inequality to our case:
(4) 
with . Using this inequality and, as mentioned earlier, knowing the event fingerprints of all other nodes, we have with probability at least :
(5) 
This equation provides, with high probability (as is close to
), a good confidence interval for
.IiiB Methodology
Suppose we observe the substream and the associated fingerprints. Let be the observed version of indicating if node participated in the event or not, and be the observed version of . If we suppose the access to (i.e. the true conditional probability for node ), then an intuitive anomaly score for the is:
(6) 
This score is obtained by replacing with in the righthand side of Eq. 4
. Relating to the statistical hypothesis testing theory, this score can be seen as an upper bound on the
value. Then, a threshold can be set, conventionally , to detect anomalies. More specifically, an anomaly is detected when . Note that this method is equivalent to replacing with the chosen threshold value, and then checking if falls out of the confidence interval in Eq. 5. Bear also in mind that, since this method provides an upper bound on the value, it is in fact more conservative than in the standard statistical testing. Indeed, the confidence intervals built with Eq. 5 are larger than the confidence intervals that correspond to probability exactly equal to .In practice, we cannot have access to the true conditional probability functions which need to be estimated. To this end, we suppose that we have access to a training data stream which is an event stream recorded at times of normal communication behavior for all nodes. With our definition of (Definition 4), the estimation problem refers to the task of estimation of conditional probabilities. However, since we deal with a Bernoulli random variable, the problem actually becomes a regression of the unknown function , which can be performed using the previous normal dataset .
In this work we do not discuss the regression procedure, but we still need to note that nonparametric methods do seem suitable. Indeed, the estimation of the conditional distributions for every possible combination of fingerprints would lead to the estimation of
parameters. Note also that the Binary Tree or Random Forest regression algorithms seem welladapted to this setting since the explanatory variables are binary. Let
be our regressor. The first anomaly detection method one can think of is the simple ‘plugin’ method:Remark. In practice, fixing is not trivial and simply taking a value below could lead to bad results. One way to fix is via crossvalidation on the training stream. To do so, one should fix an acceptable false positive rate (e.g. a standard value is ), then via crossvalidation find the value of that generates a false positive rate lower than that fixed value.
However, Eq. 4 is not true for the estimated version of and we must provide a concentration inequality around the estimated expectation . In the following subsection, we give an asymptotic concentration inequality around our predicted number of shared events.
IiiC Modelfree prediction intervals
Theorem 1.
Let be the training (normal) event stream for which we assume that , . Let be another stream for which the distribution may be different but having the same support. Assume that both distributions have the same conditional probability function (Definition 4).
Assume our estimator is weakly consistent [25], and ,
(7) 
where, tends to when tends to infinity such that . Then, we have :
(8) 
Proof.
A sketch follows; the complete proof is provided in the Appendix. The successive use of the triangle, the CauchySchwarz and the Jensen inequalities allows us to upperbound by:
Since tends to as tends to infinity, due to the consistency assumption, we can bound the lefthand side of inequality (8) by:
Applying Hoeffding’s inequality on the first element of the sum, and McDiarmid’s inequality on the second one, leads to the final result.
∎
Remark. Replacing by in the final inequality given by Theorem 1, we obtain:
(9) 
Remarks on Theorem 1.
First of all, as mentioned in the first part of the theorem, the training and test event streams may follow different probability distributions. This is very interesting since, in practice,
is a nonstationary timeseries: i.e. proportion of events in which a node is involved in is not stationary over time. However, we assume that, while in normal state, the probability that a node participates in an event, knowing the participation of the other nodes, does not change over time. From the network viewpoint, this means that the underlying graph structure, on which the events are dynamically created, does not change in that time as well.Therefore, provided that all hypotheses are verified, we test whether the function has changed between the training and the test event streams; we test the stationarity of the conditional distributions. Falling out of the confidence intervals (built with Eq. 8 or (9
)) would indicate a significant change in the conditional probability. Consequently, a property of this method is that it enables the detection of changes in the activity level of a node, having as reference the activity of the other nodes in its close communication environment.
Besides real anomalies, one reason for our statistical test to see the observed communication volume to fall out of the confidence intervals is when the assumptions are not verified. The consistency may has not been reached yet, which means that the number of training samples is not large enough. The other reason may be that the support of the distribution has changed (e.g. nodes sharing events for the first time), which is however important to be able to detect as well.
The consistency assumption is pretty typical for a regression framework. The reader may refer to the large literature that deals with this question [26, 25] in which it has been shown that many regressors are consistent. As clarified earlier, this work does not aim to provide a new regression method, however, we must note that our method largely depends on the convergence rate of the estimator.
The last assumption we need to analyze is the bounded difference of Eq. 7. In simple words, it says that when the size of the training set increases, a change of one sample does not affect much the estimated regression function. The second hypothesis, , is less intuitive. Nonetheless, for many estimators, and thus the hypothesis holds. As an example, take the NadarayaWatson regressor [25]. Let here be the kernel function and the bandwidth. We then have , since converges to .
Iv Experiments
Iva Stateoftheart competitors
For our comparative evaluation, we rely on the anomaly detection literature for dynamic graph (see Sec. II). We choose stateoftheart methods from the literature which, to the best of our knowledge, are the only existing works on the probabilistic anomaly detection at nodelevel, and hence are natural competitors to our work. They use the aggregated representation of the dynamic graph (see Sec. II and Fig. 1). We set the aggregation’s timescale to one day, hence the edge weight between two nodes at a time interval corresponds to the number of events shared by those two nodes in that interval.
Heard’s method [3] consists in fitting, either sequentially (based on all past values) or retrospectively (based on all values but the one to predict), an homogeneous counting process on each edge of the graph independently. However, rather than focusing on edges, here we decided to model the total number of messages received per day by each node. We chose a retrospective fitting, as the number of studied timestamps is not large enough for efficient sequential fitting.
The Scan Statisticsbased method in [14], at each timestamp, builds a statistic on the neighborhood around the node of interest, and normalizes it using past values in a timewindow. The normalized statistic is used directly as an anomaly score. In our experiments, we used a statistic of order , specifically, the weighted degree of the node of interest. With our aggregated graph construction, this corresponds to the sum of weights of the adjacent edges.
Anomaly scores. We can build two anomaly scores. The first one, referred to as bilateral, increases when the observed value is ‘far’ from the expected one, in terms of absolute value. For our method, that simply corresponds to the score described in IIIB, i.e. the Eq. 6 taken negatively so that it increases with the deviation from what is expected.
The second anomaly score, referred to as unilateral, is motivated by the fact that in telecommunication networks an interesting type of anomalous behavior is when a node has an abnormal low level of received messages. That may reflect an antenna breakdown. For this reason, the anomaly score should increase only when the observed value is lower than expected. For our method, we simply take .
IvB IoT dataset
The first results are obtained from a real industrial setting that concerns Sigfox, a telecommunication operator specialized on sensor and InternetofThings (IoT) networks ^{1}^{1}1The datasets and our implementation of all compared anomaly detection methods are publicly available at http://kalogeratos.com/psite/nad2019.. Networks like these are dedicated to cover objects or devices that need to exchange only little information with users avoiding standard transmission protocols (such as WiFi, 4G or Bluetooth) that may not be welladapted to the operational constraints (e.g. for low energy consumption). When a sensor needs to transmit a message, it simply sends a signal which can be received by several nearby Base Stations (BSs) that are in reachable distance. Our objective is to detect abnormal volume of received signals observed at any BS during a day. Hence, we consider that each sent message corresponds to a single event whose fingerprint spans only over the set of receiving BSs of the network. The value of each dimension of the event fingerprint indicates whether the message has been received or not by the corresponding BS ( or , respectively).
In this evaluation study we use the event stream recorded at a subset of BSs over a period of months. Fig. 1 shows the relative geographical locations of the BSs. Each BS is a node in the considered graph representation and each event creates instantaneously a clique in the graph (e.g. see the left column of Fig. 1) among the involved nodes that all receive the same message, sent from the same device).
The results of Fig. 2 concerns two BSs: one with a known anomaly (lying between the two vertical red lines of Fig. 2a), the other with no known issues (Fig. 2b). Note that we have the opinion of Sigfox’s experts only about these two BSs, yet we lack labels for the rest of the BSs. According to the experts, network’s operation has been normal during January 2017, thus, for both reference BSs the learning phase was performed during that period. We used a Random Forest regressor [27] as implemented in [28]. The testing phase was performed independently on a daily basis for the subsequent months. In other words, and this concerns all our results, we report the raw outcome of the independent daily detection for anomalies without applying any postprocessing that could certainly improve the performance of most methods. Fig. 2 refers to the testing phase and shows the evolution of the observed number of received messages (blue) and the evolution of the confidence levels (orange) with .
The results, especially the ROC curves, show that our method (bilateral and unilateral variations) outperforms the compared approaches. As expected, the tests with unilateral score were always better than those with bilateral, for all the detection methods. Fig. 2b suggests that our model is wellsuited for the analysis of the BSs in normal network operation. Indeed, the false positive rates are pretty low in that case.
To prove this latter idea, we applied our method on other BSs which are located close to each other. The predicted confidence region around the predicted value are plotted for each BS in Fig. 3. Once again, we can see that the observed number of received messages falls out of the confidence level very few times. The fact that our method reports long anomalies for many BSs during May 11  25, may be a sign that retraining is needed. However, for the third BS, the observed value is persistently very low compared to the predicted confidence intervals, which is a stronger indication for anomalous behavior during that period of time.
IvC Simulated dataset
Aiming to extend the scale of our experimental study, we developed a data generator that simulates network communication activity. To be consistent with the previous experiment of Sec. IVB, we keep the nodes’ spatial arrangement of Sigfox network. We propose the following simulation process:

Sample the spatial network structure: Draw node (i.e. analogous to BSs) locations, according to a mixture model of
(bivariate) Gaussian distributions.

Sample an event/fingerprint: First generate a transmission location (analogous to a device) , as in Step 1. Then for each node, let its location , draw a Bernoulli with a parameter inversely proportional to the distance . In our experiments, we set the Bernoulli parameters to be equal to , where is a locationdependent visibility parameter that controls the density of the graph.

Generate an event stream (clique stream): At each timestamp , draw event fingerprints by applying Steps 23, where may be constant, or random, over time.

Simulate anomalies through nonstationarity: The simplest way to simulate nonstationarity is to draw the total number of events at each timestamp according to a nonstationary process. To increase the complexity of the phenomena, one may also let the component (or cluster) proportion of the mixture in to vary at each timestamp. That would correspond to the case where devices appear following a nonuniform spatial distribution. To simulate anomalies for a node, it is sufficient to let vary the visibility parameter associated to the node’s location.
In order to demonstrate the robustness of our method, we apply the above generative process in three simulations with different ‘complexity’, whereas sharing the following properties:

S1: communication nodes are drawn, for which, timestamps are then simulated. The number of Gaussian distributions are fixed to .

S2: The same set of constant visibility parameters is used.

S3: The first timestamps are treated as the training stream, while the rest correspond to the test stream.

S4: A single arbitrary node is chosen to be anomalous. For which, anomaly time intervals are simulated: , , and . Each of these intervals imitates an anomalous behavior at a different scale; this is achieved by decreasing only the visibility parameter associated with the anomalous node.
the ROC curves of the nodelevel outlier detection task for the three synthetic streams.
Our three experiments (Exp. 13) differ in their complexity regarding the stationarity of the respective timeseries, i.e. number of events in which the anomalous node (the node that at some point develops an anomalous behavior) participates in each experiment. The top row of Fig. 4 presents the timeseries of the test streams. The timestamps of the beginning and the end of each simulated anomalous behavior are also indicated in the plots with orange and red vertical lines, respectively.
In Exp. 1 (Fig. 4a, d), the process is perfectly stationary: at each timestamp, exactly events are generated with the same process. In Exp. 2 (Fig. 4b, e), the total number of events participated at each timestamp remains , with the difference that a Dirichlet random variable of order is drawn, with parameters all equal to . This corresponds to the mixing variable (i.e. proportion) for the components of . The last one, Exp. 3 (Fig. 4c, f), is meant to be more difficult: it uses the Dirichlet mixing as well, however, at each timestamp of anomaly, the total number of generated events is increased. This is a ‘tricky’ setting for the bare human eye as the timeseries of interest ‘looks’ stationary (Fig. 4c) although there are actual changepoints in the node behavior.
For all three experiments, the threshold value that needs to be fixed for building the confidence levels was estimated using crossvalidation (see details in Sec. IIIB). We fix the acceptable false positive rate at . The light gray vertical lines in the background of the top row plots in Fig. 4 indicate the timestamps at which the observed values fall out of the confidence levels, and as such they can be spotted as outliers.
The bottom row of Fig. 4 shows the ROC curves of the nodelevel outlier detection task for the three synthetic streams. The competitors are the same as those of the experiment on real data (Fig. 2), but here only bilateral scores are plotted. In addition, here we employ a second version of both Heard’s and Scan Statistics methods. The Heard Edges fits an homogeneous Poisson process on each edge independently; each edge denoting the number of common messages received by two nodes. In this case, the node anomaly score is the sum of values of its edges. Moreover, the Scan Batch method simply outputs anomaly scores equal to the normalized deviation of the statistic of interest (see Sec. IVA) from a (unordered) batch of the training stream, hence without sequential analysis.
All the reported results indicate that the proposed method outperforms clearly its competitors. As expected by its design, our approach is shown to be robust to the nonstationarity introduced at arbitrary timestamps during our simulations. The performance of all other methods seems to decrease fast with the increase of nonstationarity (i.e. behavior complexity). An important closing remark is to remind that, in our evaluation, we have been applying anomaly detection independently on each day. As this work is related to the detection of changepoints in nodes’ behavior rather than instantaneous anomalies, postprocessing (such as filtering) of the raw detection outcome could increase the accuracy of most methods.
V Conclusions
In this paper we presented a probabilistic framework for nodelevel anomaly detection in communication networks. We went beyond the aggregated representations that the existing literature has used to model the communication activity. Instead, we modeled such activity as a clique stream where each event creates an instantaneous clique among the communicating nodes of the graph. The detection approach we proposed is to infer the conditional probabilities of cliques to be generated. This allowed the derivation of node anomaly scores which are efficient in detecting when the communication volume deviates from the ‘normal’ behavior (estimated using a training stream of normal communication behavior), while also being statistically interpretable. We applied our method on both realworld and synthetic sensor network data, and demonstrated that it outperforms other probabilistic approaches found in the related literature.
As future work, there is room to further improve the accuracy of the statistical modeling, consider that events can create more complex structures of connected nodes than cliques, include dynamics coming from (dis)appearance of nodes, and finally bring our method closer to the link prediction or structure inference tasks, using for instance the learned conditional probabilities.
Acknowledgments
Part of this work was funded by the IdAML Chair hosted at ENSParisSaclay.
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Appendix
Proof.
In the following, we note . We also assume the distribution described in Theorem 1: , and , . With an abuse of notation, and also refer to the marginal distributions. Using the triangle inequality, we get:
In the above, means that the expectation is taken with distribution for and for . Using Jensen’s inequality and the fact that all are i.i.d., we get:
Using CauchySchwarz inequality:  
With the same support hypothesis:  
Using Jensen inequality:  
Due to the assumption of weak consistency, converges to zero, so as . In the following, we assume that which is always true after a certain rank. We note . Back to the first inequality of the proof, we get :
This is due to the fact that . We now need to find an upper bound on the two elements of the righthand side of the previous inequality. The first element of the sum is easily bounded using Jensen’s inequality:
For the second element, we must note that is a function, with bounded differences, of independent random variables. Thus, we can apply McDiarmid’s inequality to bound our probability:
This implies that:
This is true . Furthermore, since and , passing to the limit on both side of the previous equation, we get our final result:
∎
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