A Reinforcement Learning Based Framework for Prediction of Near Likely Nodes in Data-Centric Mobile Wireless Networks
© Yingying Chen et al. 2010
Received: 5 September 2009
Accepted: 8 May 2010
Published: 10 June 2010
Data-centric storage provides energy-efficient data dissemination and organization for the increasing amount of wireless data. One of the approaches in data-centric storage is that the nodes that collected data will transfer their data to other neighboring nodes that store the similar type of data. However, when the nodes are mobile, type-based data distribution alone cannot provide robust data storage and retrieval, since the nodes that store similar types may move far away and cannot be easily reachable in the future. In order to minimize the communication overhead and achieve efficient data retrieval in mobile environments, we propose a reinforcement learning-based framework called PARIS, which utilizes past node trajectory information to predict the near likely nodes in the future as the best content distributee. Our framework can adaptively improve the prediction accuracy by using the reinforcement learning technique. Our experiments demonstrate that our approach can effectively and efficiently predict the future neighborhood.
The development of data-centric storage has enabled efficient data dissemination of wireless networks. In data-centric storage, data is stored by attributes or types (e.g., geographic location and event type) at nodes within the network [1–3]. Queries for data with a particular attribute will be sent directly to the relevant node(s) instead of performing flooding throughout the network, thereby data-centric storage enables efficient data dissemination/access.
In data-centric storage of wireless networks, wireless devices that collect the data are called collector nodes. Whereas the data can be stored on other nodes, called storage nodes [3–5], based on their attributes or types. Most existing data-centric storage models can only deal with static wireless networks. However, with the increasing deployment of wireless devices, there are emerging pervasive applications that rely on the mobility of wireless device. Two representative examples are: ( ) sensors are used for animal migration tracking, and ( ) wireless devices are equipped with police officers to monitor their daily patrol routes, collect crime information by areas, and record corresponding law enforcement actions. In these two scenarios, efficient data retrieval can be achieved if the data-centric storage is enabled, that is, the data is stored by the types of animals, by the activities performed by the animals, or by the tasks that are carried out by the police officers. The challenge is to design schemes that can support data-centric storage when all the nodes are moving around. In this paper, we consider a fully distributed network, in which there is no node playing the sole role as storage; each node can act as both the collector and storage node. For instance, a wireless node, playing the role of a collector node, can collect data of more than one type, but usually it only stores one type of data and transfers the rest of data of other data types to other nodes, which are the storage nodes corresponding to this collector node. Further, to reduce the communication overhead, the storage nodes are picked from the neighborhood, that is, the nodes in the transmission range, of the collector node.
However, in mobile wireless networks, it is possible that both storage and collector nodes move in a broad area, which brings the possibility that the storage nodes that are currently in the neighborhood of the collector nodes may move far away and cannot be easily reached in the future. Thus, when a user sends queries to the collector nodes, the queries need to be redirected to those storage nodes with much communication overhead. Therefore, it is desirable that the collector nodes migrate their data to the storage nodes that not only possess similar data types but also highly likely to travel with them in the future. We define this kind of storage nodes as near likely nodes, which are the nodes that are in the neighborhood (i.e., near) and carry the same type of data that needs to be stored (i.e., likely). In this paper, we propose mechanisms to predict near likely nodes for data-centric mobile wireless networks to achieve efficient data storage and retrieval. More specifically, we propose PARIS, a fully distributed neighborhood prediction framework based on reinforcement learning techniques that utilize past node trajectory information to determine the best content distributee for the future. We first define a probability-based neighborhood prediction model. We then propose two approaches, namely point-based and traced-based, that predict the future neighborhood based on the correlations of the past trajectories. Moreover, we develop WINTER (WINdow adjusTment with Expanding and shRinking) algorithm, which can perform adaptive adjustment during runtime and improve the prediction accuracy by using the reinforcement learning technique. In addition, a probability-based metric is developed to measure the accuracy of prediction. Our approach of data transfer based on neighbor prediction helps to reduce communication overhead and consequently the overall energy consumption during data retrieval because the storage nodes most likely move together.
To evaluate the effectiveness and efficiency of our scheme, we conducted experiments using mobile wireless networks simulated based on a city environment and its vicinity in Germany [6, 7]. By examining two representative scenarios, walking scenario, and vehicular driving scenario, our experimental results show high-prediction accuracy and low-computational time when using PARIS, thereby providing strong evidence of the effectiveness of using data-centric approach through the prediction of near likely nodes in mobile wireless applications.
The rest of the paper is organized as follows. We place our work in the context of the related research in Section 2. In Section 3, we provide an overview of our problem and formulate our probability-based neighborhood prediction model. We next discuss the likelihood of neighborhood by presenting our two prediction approaches and the new metric for measuring accuracy prediction in Section 4. Further, we present the protocols of data transfer and data retrieval and our adaptive accuracy adjustment using reinforcement learning under the PARIS framework in Section 5. We present the experimental evaluation of our approach in Section 6. Finally, we conclude our work in Section 7.
2. Related Work
There has been active work on data-centric storage in sensor networks. In addition to the approaches of global data storage in which the wireless device data is aggregated to be stored at external central servers, algorithms of local information processing , and wide-area data dissemination [9, 10] are proposed. Reference  used signal processing techniques to collaborate among local nodes for information processing. References [9, 10] proposed directed diffusion algorithms that implement in-network aggregation and allow nodes to access data by name across wireless networks. Further, recent work is more focused on data-centric storage [1–3, 5], where the data is stored decentralized by attributes and types. Reference  achieved data-centric storage based on the GPSR routing algorithm and an efficient peer-to-peer lookup system. Reference  developed schemes for resilient data-centric storage from the viewpoint of energy savings and scalability in wireless networks. Whereas the security and privacy concerns in data-centric storage are addressed in . Most of these current works only deal with static sensor networks. In this paper, we study data-centric storage in mobile wireless networks.
To detect mobility of wireless nodes,  used received signal strength in wireless LAN to detect wireless device mobility. Reference  determined mobility from GSM traces using different metrics. In  signal variance is used with Hidden Markov Model (HMM) to eliminate oscillations between the static and mobile states for mobility detection. Further,  proposed to use correlation coefficients on RSSI traces to detect wireless devices that are moving together.
The works that are most closely related to ours are [15–17]. A user-centric approach was proposed in  for colocation prediction that is used for media sharing based on repeating similar journeys in the urban transportation environment. Unlike , our approach does not require repeated trajectory patterns, and thus is more generic and can be applied to a broad array of pervasive applications involving mobile devices. References [16, 17] addressed the detection of nodes of similar mobility patterns in group caching in MANET. However, these works do not support fully distributed models. Further, their work focused on current neighbors, not the prediction of future ones. Our work is novel in that we utilize the past node trajectories to predict the future co-movement of nodes for data-centric storage in mobile environments.
3. Problem Overview
In this section, we first present our assumptions. We then provide an overview of PARIS and define our probability model for neighborhood prediction.
Mobility. Wireless device are moving, randomly or in some pattern, in a well-defined area, though the nodes are not aware of their moving patterns, if there is any. There are no predefined trajectories for each node. However, we assume that there exists a comovement pattern within nodes, that is, group of nodes may travel together to common destinations. For example, a group of tourists in New York City may travel to visit the Metropolitan Museum together and they use their mobile phones to take pictures, shoot videos, and write multimedia blogs on the way.
Location-Aware. We assume that the nodes know their physical locations at all time points during moving. It is a reasonable assumption because in many cases the data is useful only if the location of its source is known. For example, knowing that a crime occurred, which requires a law enforcement action, but without knowing where it occurred is useless. Localization of the mobile nodes can be achieved through the use of GPS or some other approximates but less burdensome localization algorithms [18, 19].
Neighborhood. Each node has a short communication range and can communicate only with nodes within its transmission range. We call the nodes in the transmission range the neighbors. Mobility of wireless devices may result in the change of the neighborhood. However, we assume that for every node, it has a stable neighborhood within a period of time. For example, police officers who carry out the same tasks are kept in neighborhood while they are on duty.
Data-centric storage. We assume that the storage is data-centric, that is, the particular node that stores a given data object is determined by the object's type such as event type [1, 2]. Hence, all data with the same type will be stored at the same node (not necessarily the collector node), so that the subsequent data retrieval requests could be efficiently directed. In particular, we propose to transfer data of the same type to a node's near likely nodes. The subsequent data queries will reach a collector node first through routing protocols for mobile wireless networks  and will then be redirected to the corresponding storage nodes.
3.2. Overview of PARIS
Data-centric approaches provide low-communication overhead and efficient search, however applying data-centric mechanisms to mobile environments brings new challenges. Since mobility of nodes can change the reachability of nodes and consequently affect the routing decision and long-term storage capability, data-centric storage in mobile wireless networks must take mobility into consideration. In mobile wireless networks, when a node stores its data on other nodes [1, 2], it is desirable that the chosen nodes are in the neighborhood in the future, so that when there come the requests for the data, the collector node can efficiently redirect the requests to its near likely nodes that store the data in its neighborhood.
In PARIS, we study how to store and retrieve data efficiently by making use of neighborhood predication for data-centric storage in mobile wireless networks. In addition, PARIS can be easily extended to help in load balancing when a node exceeds its storage. The main logical components in PARIS are on-demand data transfer, runtime update of near likely node (for efficient data retrieval), and adaptive adjustment through reinforcement learning. By on-demand data transfer, each collector node calculates its near likely node only when it needs to transfer its data to another node. During data retrieval, the collector node is responsible to redirect the corresponding queries to its near likely node. If at a later time, the near likely node that carries the data from a collector node is moving out of the neighborhood of the collector node, the collector node will run the neighborhood prediction process again and perform a runtime update to transfer the data from the storage node to its current near likely node. As it is only performed at certain time points, the on-demand data transfer mechanism reduces both the communication overhead and energy consumption in the neighborhood prediction procedure. Additionally, the WINTER algorithm based on the reinforcement learning technique is developed to adaptively improve the accuracy of neighborhood prediction in each prediction round. The details of PARIS will be presented in Section 5.
3.3. Probability Model
Given the time window , a collector node and its neighbor nodes, if needs to store its data on its near likely node, then from its neighbor nodes that have available storage and store the same type of data that needs to be transferred from , picks the node that is of the maximum . Our model can be easily extended to choose nodes that are of top- .
4. Neighborhood Likelihood
In this section, we first explain how to compute the neighbor probability . We then propose two approaches, namely, trace-based and point-based, to calculate the direction probability . We next develop a new metric that measures the prediction accuracy.
4.1. Neighbor Probability
4.2. Direction Probability
where ( , resp.) and ( , resp.) are the mean and standard deviation of and . The value ranges from 1 to +1. Correlation +1/ 1 means that there is a perfect positive/negative linear relationship between and . In Figure 1, the high value 0.96 for both the dimension and the dimension shows high correlation between the coordinates of two nodes that are moving together.
Further, to measure the direction probability, we develop two schemes, point-based and trace-based, based on the Pearson correlation coefficient. These two schemes consider both spatial and temporal changes of nodes in mobile environments.
4.2.1. Point-Based Scheme
This approach utilizes the moving direction of the node and at each time point within a time window to determine whether two nodes are moving together. The key idea is that the collector node computes the moving directions of the neighbor nodes at all time points in the time window and measures the Pearson correlation coefficients of the moving directions.
4.2.2. Trace-Based Scheme
4.3. Measurement of Accuracy of Neighborhood Prediction
One challenge of data-centric mobile wireless networks is the efficiency of data retrieval, which highly depends on the accuracy of neighborhood prediction results. Wrong prediction results may cause data to be stored on unreachable nodes and thus incur expensive communication overhead and consume more energy. Therefore, it is necessary to measure the accuracy of neighborhood prediction and evaluate the effectiveness of our prediction schemes. In this section, we present our new metric Prediction Accuracy in measuring neighborhood prediction accuracy.
In Prediction Accuracy metric, the time points are split into two time windows, past and future . The window of past is used as the "training set" to predict the near likely nodes, whereas the window of future , is used as the "test set" to verify the accuracy of the prediction. We choose nodes, denoted as , as the "test participants". Our accuracy measurement consists of two steps.
Step 1 (Training).
For each node in , we find its near likely node that is of the maximum in the time window . For nodes, we collect such neighbor nodes and put their into a vector . Thus consists of probability values.
Step 2 (Testing).
For each near likely neighbor from Step 1, we calculate its of the window and store in a vector , which is also a set of probability values. Our measurement of accuracy is based on the distance of and . The smaller the distance is, the more accurate the prediction result will be.
The nodes in ( , resp.) are the ones whose probabilities are increasing (decreasing, resp.). We measure CDF of both and . Intuitively, the closer the distributions of and to the value 0 are, the more accurate the prediction is.
5. Framework of PARIS for Data Transfer and Data Retrieval
In this section, we describe the three main logical components in PARIS framework, on-demand data transfer, runtime update of near likely nodes, and adaptive adjustment through reinforcement learning.
5.1. On-Demand Data Transfer
Following the data-centric requirement, the collector node picks the neighbor nodes that have not only sufficient storage but also the matching type of data that will be transferred.
A collector node sends a request to all the nodes in its neighborhood. The request consists of the inquiry of the allowed data type, the size of the available storage, and the trajectory of the next time points in the time window . The neighbor nodes reply the request of with proper information.
The collector node sends its data to its near likely node , and updates its data track table. The data track table consists of entries in the format of , with each entry used for tracking which node the data is stored on, so that when there is a user query for the data, node can efficiently redirect the query. The is the node identity of the near likely node that stores data with index in the data track table.
5.2. Runtime Update of Near Likely Node
Given the fact that the estimated near likely node has a belief probability to be in the neighborhood in the future, it is possible that when a data query arrives at a future time point, the near likely node has already moved out of the neighborhood of the collector node. This will increase the communication overhead in order to locate the "previous" near likely node for data retrieval. In order to minimize the communication overhead, it is desirable to always keep the transferred data in the neighborhood of the collector node in a mobile wireless network environment.
A node may be identified as a near likely node for more than one collector nodes. In PARIS, the near likely node is stateless, whereas the collector nodes keep a data track table to maintain the data transfer information. The advantage of the runtime update of near likely node is that the data is stored on either the collector node itself or its near likely nodes. Thereby no flooding messages are needed during data retrieval, and thus reduce the overall communication overhead.
5.3. Adaptive Adjustment by Reinforcement Learning
Although runtime update always keeps data close, it may incur expensive energy consumption and increased communication overhead if the update is frequent. The reason for such frequent update is the prediction of near likely neighbors that is not accurate enough. As shown in Section 4.3, the prediction accuracy is affected by the configuration of time windows that are used to collect the past trajectories of a node. Time windows that are too small cannot capture the correct neighborhood and cause inaccurate neighbor prediction, while time windows that are too large will consume more energy on each neighboring nodes for collecting trajectory traces and increase the communication overhead when sending the trajectory traces to the collector node. Therefore, the appropriate time window will allow PARIS to be effective for neighborhood prediction.
To improve the neighbor prediction accuracy, we adaptively adjust the time windows by applying the reinforcement learning mechanism from the beginning of the whole procedure. Reinforcement learning is a machine learning technique that deals with sequential control problems .
After a sequence of expansions and shrinkages, it is possible that different collector nodes have time windows of different sizes.
The pseudocode that implements WINTER is shown in Algorithm 1.
Algorithm 1: The WINTER algorithm.
6. Experimental Evaluation
In this section, we describe our experimental methodology and present the results that evaluate the effectiveness of our approaches.
The 100 nodes are randomly chosen from 600 nodes. We observed that the average number of neighbors increases from a few nodes to around 14 nodes as the study time moves along, indicating that groups of nodes are gradually formed and traveling together to the similar destinations. The vehicular driving scenario has the similar trend as the walking scenario. This is in line with our co-movement assumption. Thus, these datasets are suitable for our neighborhood prediction study.
We will utilize the following performance metrics to evaluate the effectiveness of PARIS in terms of prediction of near likely nodes.
As described in Section 4.3, the Prediction Accuracy metric measures the statistical characteristics of neighborhood prediction based on the Cumulative Distribution Function (CDF) of the difference of the future probability to the past probability of the near likely node on top of the KL-divergence. We split our study time to a past time window for prediction and a future time window to evaluate our prediction. In the following discussion, we use the percentage of study time as the measurement of window size.
We investigate the impact of different window sizes of the past as well as the future on the prediction accuracy using both point-based and trace-based schemes.
By measuring the time that each scheme needs to provide the prediction results, we evaluate the feasibility of applying these schemes to nodes that usually have limited computational power and memory. The Time Performance metric helps to benchmark our approaches in the simulation environment and further indicates the possibility to implement them in real wireless device.
For the walking speed scenario, we observed small KL-divergence values that are always less than 0.5. This is encouraging as the smaller KL-divergence values indicate that the distribution of the belief probability in the future is close to the distribution of that in the past. Further, as shown in Figures 5(a) and 5(b), when fixing the time window of the future, 0.2 and 0.4 of the total study time, respectively, as the size of the past time window increases, the KL-divergence value presents an overall decreasing trend for the point-based scheme. This means that by using the point-based scheme, the larger the past window size, the more accurate the prediction of near likely node can become. However, for the trace-based scheme, we observed that the KL-divergence value fluctuates. This is interesting since it shows that for the trace-based scheme, simply increasing the past window size does not increase the accuracy, which indicates that we need both expansion and shrinkage for adaptive adjustment of window sizes.
On the other hand, when fixing the time window of the past, 0.2 and 0.4 of the total study time, respectively, as presented in Figures 6(a) and 6(b), we observed an increasing trend of the KL-divergence value for both schemes as the window size of future is increasing when the average number of neighbors is 5, indicating that the near likely node may gradually move away from the collector node when the future is long enough.
We also investigate the neighbor prediction accuracy in our proposed mechanism for the vehicular driving scenario. Figures 5(c) and 6(c) present the neighborhood prediction accuracy in the vehicular-driving scenario. First, similar to the walking scenario, the values of KL-divergence are less than 0.5, which indicates that our scheme obtains accurate prediction accuracy in the vehicular driving scenario as in the walking scenario. We also observed similar changing trend as the result of walking scenario.
In particular, as shown in Figure 5(c), when fixing the future window size to 0.4 of the total study time, as the size of the past window size increases, the KL-divergence value presents an overall decreasing trend, which is similar to the trend in Figure 5(b). While fixing the past window size to 0.4 as shown in Figure 6(c), we also observed similar increasing trend and KL-divergence value as in Figure 6(b). Further, the increased amount of the KL-divergence values is always small (around 0.05). These results indicate that our proposed schemes are appropriate for different mobility scenarios.
In general, we found that the KL-divergence values of trace-based scheme is smaller than those using point-based scheme for both walking and vehicular driving scenarios. Moreover, for the walking scenario, we observed similar results when the average number of neighbors increases to 15 and 45. Due to space limitation, the results are omitted. Therefore, the trace-based scheme has better prediction accuracy than the point-based scheme.
We observed similar behavior for both large data set and small data set as the KL-divergence value presents an obvious decreasing trend when increasing the past window size and decreasing the future window size simultaneously. Furthermore, the KL-divergence values are smaller for the large data set. This is because there are more nodes in the large dataset, which form larger neighborhood and thereby provides better prediction result. In the sequel, due to the space limit, we will only present the results obtained from the small data set.
Cumulative Distribution Function (CDF)
This shows that in terms of neighborhood prediction accuracy, the trace-based scheme outperforms the point-based scheme, which is inline with the results obtained from the KL-divergence.
We observed that the point-based scheme runs at about two times faster than the trace-based scheme constantly under different average number of neighbors and various window sizes. This is because the trace-based scheme needs to calculate correlation coefficients for both and dimensions, whereas the point-based scheme only calculates the correlation coefficient for gradient. Further, the time measurements of the large data set are also in the order of milliseconds as shown in Figure 12(b). This indicates that even when a node has large number of neighbors, our schemes can efficiently predict the near likely nodes.
Next, we measure the communication overhead incurred by collecting the trajectory information from a collector's neighbors. Let us consider the transmission packets of 512 bytes . We assume that each trajectory record consists of a pair of ( ) coordinates and a timestamp, each of float type. In other words, each trajectory record consists of 12 bytes. Therefore, one transmission packet can contain at most 42 trajectories. If one node records its trajectory every seconds, then the trajectory information of seconds can be stored in packets.
Moreover, we assume that for every seconds, to apply the neighborhood prediction mechanism, the collector node needs to collect the trajectory of seconds from its neighbors. Therefore, there are packets transmitted to the collector node from its neighbors. Assume the collector node needs to transfer its data of size to the storage node within these seconds. Then the size of the transmission packets needed for collecting trajectory information is of the data sent to the identified near likely node by the collector node. We assume that is less than 1.
Based on our analysis, we can see that both the packet size and the percentage of transmission packets are not affected by the moving speed of the mobile nodes. Thus, the communication overhead incurred from the trajectory information exchange in our approach is not sensitive to the mobility model.
Furthermore, we realized that there exists a tradeoff between the communication overhead and the frequency of data update. The higher frequency the data is updated, the higher prediction accuracy may be achieved, however, higher communication overhead can occur. We note that in our scheme the data update is performed on-demand, and thus the frequency of data update can be configured.
In summary, our experimental evaluation in prediction accuracy, time performance, and communication overhead is highly encouraging as they clearly indicate that our prediction schemes of near likely nodes can not only effectively but also efficiently perform future neighborhood prediction. Our results also point out that there is a tradeoff between the prediction accuracy and the time efficiency when choosing prediction schemes—the scheme that provides better prediction accuracy runs slower.
The development of data-centric networks has enabled efficient data dissemination and access when the increasing large volume of data is spread across the networks. New challenges arise when there is a demand of implementing data-centric approaches in mobile wireless applications. In this paper, we proposed PARIS, a fully distributed framework based on reinforcement learning technique for data-centric storage in mobile wireless networks. PARIS is based on neighborhood prediction and utilizes the past node trajectory information to predict the near likely node that stores the same type of data and will most likely to remain in the neighborhood in the near future. These near likely nodes are chosen as the content distributee so that the later data retrieval is only needed in the neighborhood and is thus more efficient in terms of communication overhead and energy consumption. We proposed two schemes to predict the future neighborhood, point-based and trace-based. We derived a probability-based metric to measure the accuracy of prediction. Further, we developed WINTER (WINdow adjusTment with Expanding and shRinking) algorithm to adaptively improve the prediction accuracy using the reinforcement learning technique. Additionally, we derived a probability-based metric to measure the accuracy of prediction. Our results using simulation data generated from mobile wireless networks in a city environment show that our prediction schemes of near likely nodes can both effectively as well as efficiently perform future neighborhood prediction.
There are several avenues for future work. Since it is possible that multiple collector nodes choose the same nodes as the near likely nodes, it is interesting to study how to balance the load of the "popular" near likely nodes with others based on data types. Further, as energy-efficiency being an important feature of wireless networks, we want to quantify the energy consumption model in PARIS.
This paper was supported in part by NSF Grant CNS-0954020. The preliminary results have been published in "Prediction of Near Likely Nodes in Data-Centric Mobile Wireless Networks"  in MILCOM 2009.
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