Empirical study on spatial and temporal features for vehicular wireless communications
© Chen et al.; licensee Springer. 2014
Received: 1 January 2014
Accepted: 19 October 2014
Published: 3 November 2014
Static topology analysis is not sufficient for the dynamic vehicular ad hoc network. Understanding the evolving topology of vehicular ad hoc networkings (VANETs) caused by vehicle mobility is very important for routing protocol design and algorithm optimization. This paper explores the spatial and temporal features of vehicular network topologies based on two real taxi-trace datasets. The analysis results reveal that the whole topology of VANETs consists of a large number of small-sized connected components. Two quantitative metrics are proposed to measure the stability and location dependency of the connected components. When the communication range is greater than a threshold, a large proportion of vehicles will connect to the biggest connected component, which is relatively stable and covers the most part of the downtown region of the city. Based on the analytical results, we propose several design philosophies and new research issues for VANETs.
Vehicular ad hoc networking (VANET) is one kind of new technology supposed to provide innovative services for intelligent transportation systems. Equipped with dedicated short-range communications radios, vehicles can not only exchange messages directly with vicinity nodes but also communicate with other nodes through a number of intermediate nodes if all of them are connected. Since the nodes have the capability of communicating with each other, VANETs can provide a number of potential applications with highly diverse requirements. The three major classes of applications possible in VANETs are safety oriented, convenience oriented, and commercial oriented . Safety applications include immediate collision warning, forward obstacle detection and avoidance, emergency message dissemination, and so on. Convenience applications can provide route maps with real-time traffic jams and accident conditions to help drivers find the shortest path in terms of time consumption. Commercial applications can provide internet access as well as communications between passengers in cars in the same vicinity, allowing the passengers to surf the internet, watch online movies, and even play games.
To support information diffusion in VANETs, two kinds of network architectures are proposed. One is called vehicle-to-vehicle (V2V), in which vehicles can communicate when they are located in the communication range of each other. The other one is called vehicle-to-roadside (V2R) or vehicle-to-infrastructure (V2I) or vehicle-to-wayside (V2W), which is a cellular-like system. The roadside infrastructure is used as a basestation, and it may dominate the communication in its communication range. In V2V VANETs, vehicles are equal and information diffusion can be achieved by adopting MANET routing protocols. However, it is well known that moving vehicles result in a disconnected network topology ; thus, different kinds of carry-and-forward strategies [2–4] have been proposed to support intermittent connected networks. The carry-and-forward strategy is effective but not efficient, because the source might delay the forwarding until it meets the destination even though there is a routing path between them. It is shown that the packet forwarding delay caused by carry-and-forward can be several orders-of-magnitude longer than that caused by multi-hop forwarding over a connected network . Consequently, we should figure out when and where the vehicles are connected and can be reached for routing purpose. In V2R VANETs, the roadside infrastructure is usually more powerful than vehicles. To reduce system construction cost and improve communication efficiency, the deployment of roadside infrastructures should also be well studied. Intuitively speaking, the roadside infrastructures are better set up at those spots with poor V2V connectivity. However, moving vehicles bring a lot of uncertainty for infrastructure deployment. No one knows whether the well-connected vehicles will in turn be disconnected in the next minute, whether the connectivities of vehicles are depending on their location, and whether there is any difference between on-peak hours and off-peak hours. Therefore, connectivity distribution and time variance of network topology should be examined.
We find that by adopting a reasonable communication range, a large number of vehicles are connected as a main component of the whole network.
We propose a metric to measure the stability of the connected component for a dynamic topology. Based on this stability factor, we find that the biggest connected component of the VANET is relatively stable even though each individual vehicle keeps moving.
We analyze the location dependency of the biggest connected component. We find that the most part of the downtown region of the city can be covered by the connected component of the VANET.
According to the spatial and temporal features that we have found, we propose several design philosophies and new research issues for VANETs.
The rest of the paper is organized as follows: Section 2 briefly summarizes the related work, Section 3 describes the definitions used in spatial and temporal analysis for VANETs, and Section 4 introduces the two taxi-trace datasets and presents the spatial and temporal discoveries as well as their implications for VANET design and protocol optimization. Finally, Section 5 concludes the article.
2 Related work
With the broadcast feature of wireless channels, a VANET is always modeled by a unit disk graph, in which two vertices are connected if their distance is below a fixed threshold [6, 7]. By adopting this assumption, percolation theory can be used to analyze the connectivity of VANETs. For example, quantitative relationships among network connectivity, vehicle density, and transmission range are derived in . When considering more realistic constraints of VANETs such as non-uniform and non-Poisson distributions, or non-ideal environments with fading/shadowing impact, Miorandi et al.  proposed an equivalent GI|D|∞ queuing model to analyze the connectivity of one-dimensional ad hoc networks. Their results claim that no connectivity can be obtained in condition of channel randomness. Based on the equivalent GI|D|∞ model, the node isolation probability and the average size of connected components can be estimated in a one-dimensional case. For one-dimensional VANETs, Zhuang et al.  also derived the exact expression for the average size of the connected components and their size distribution. Furthermore, to analyze message propagation in two-dimensional traffic networks, the authors in  extended their model to calculate lattice-connectivity probability for all the blocks in a district. In real traffic, most vehicles are travelling in a co-directional way. Abuelela et al.  found that co-directional traffic is inherently partitioned into connected components and provided an analytical expression of the expected size of those connected components. By exploring the co-directional feature and the existence of connected components, the authors in  also designed an Opportunistic Packet Relaying protocol (OPERA) for packet delivery over disconnected VANETs.
In recent years, complex network- and network-science-related concepts  are widely adopted in many research domains. Monteiro et al.  decomposed the synthetic dynamical topology of VANETs into snapshots and calculated the macroparameters of the network such as the node degree distribution, the clustering coefficients, the average shortest path length, and so on for each topology snapshot. Based on the abstracted information from network science, a new efficient broadcasting protocol called UV-CAST has been proposed. To further explore the dynamics of VANETs in a completely new way, more analysis [13, 14] based on real and realistic simulated traces have been carried out in terms of many other kinds of complex network metrics. Literature  makes a thorough investigation of both microscale and macroscale metrics including node degree, lobby index, link duration, network diameter, closeness centrality, betweenness centrality, number of communities, clustering coefficients, and so on. Similar to , literature  conducts node-level and network-level analysis including node degree, network assortativity, betweenness centrality, and so on. Besides these, literature  also analyzes connected components’ dynamics when the number of vehicles and communication ranges vary.
Generally speaking, literatures [8–10] explore VANETs’ topology based on theoretical connectivity models. These models are useful in discussing the critical conditions or the connectivity boundary. But these theoretical models still rely on strong assumptions, which are very difficult to achieve in real cases. For example, the model in  requires a uniform distribution of the vehicles. However, it has been proved not to be true in real mobility scenarios . Literatures [11–14] provide new methods to explore VANETs’ topology based on network science models. Literatures  and  also analyze the evolving topology by addressing the time-variant size of the connected components. However, their investigation on the size of the connected components cannot correctly reveal the evolving topology, since the size of the connected components can remain the same while the vertices in the connected components might be completely changed. Different from the current work on VANETs’ topology analysis, we conduct our research work based on two real taxi traces and propose a new metric to measure the stability of connected components and evaluate the location dependency of the largest connected component of the VANET.
By adopting the UDG model, which is the intersection graph of equal-sized circles , VANET topology can be abstracted as a non-directional graph tagged with time stamps. For better explanation, we first introduce some annotations for the related definitions.
3.1 Network model
The traditional static graph model in describing a network is G = < V,E >, where V represents the nodes and E represents the edges between the nodes. However, a VANET is dynamic and the topology is evolving due to the movement of the vehicles. Therefore, the topology of VANET can be expressed by a time-stamped graph G (T) = < V,E,T >, where V represents all the vehicles, E represents the links between two vehicles, of which the Euclidean distance is smaller than the wireless communication range R, and T is the time stamps. In real cases, due to obstacles such as high buildings, two vehicles might not establish a link even though their distance is smaller than the wireless communication range. In this situation, every vehicle can explore its real neighbor list by exchanging beacon messages with its neighbors. Consequently, VANET topology can be figured out based on the neighbor lists of all vehicles.
3.2 Routing path
The length of the path is |V′ t |, which is n. Note that there are probably more than one path from v1 to v n in G(t). Therefore, the distance from v1 to v n is defined as the shortest path from v1 to v n in G(t). Meanwhile, there might be more than one shortest path from v1 to v n in G(t) as well.
3.3 Connected component
As mentioned in Section 1, packets are forwarded much faster in a multi-hop way than that in a carry-and-forward style if the source node and the destination node are in the same connected component. Therefore, the performance of the routing strategy can be greatly improved if there are enough stable connected components in VANETs. We will analyze both the number and the size of the connected components in Sections 4.2 and 4.3.
3.4 Connected component stability
It is obvious that 0 ≤ ζCC(t)≤ 1. CC(t) is more stable when the value of ζCC(t) is larger. The stability of the connected components will be discussed in Section 4.4.
3.5 Location dependency
It is obvious that . CC(T s ) is more location dependent when the value of is larger. The location dependency of the connected components will be discussed in Section 4.6.
4 Dataset analysis and implications
In this section, we will analyze the spatial and temporal features of VANETs by using the concept of connected component other than using the individual node. Based on the discoveries, we will also present our suggestions for network design and optimization.
4.1 Taxi-trace dataset
Taxi traces in
Taxi traces in
San Francisco, USA
30 to 60 s
Time stamp, latitude,
status, velocity, angle
91 MB compressed file
1.06 GB compressed file
To capture the evolving features of network topology, we use linear interpolation to generate consecutive time-synchronized coordinates with a coordinate-update frequency of 30 s. To find the difference between topologies both in on-peak hour and off-peak hour, we select two observation intervals. One is from 1:00 to 2:00 am and the other is from 7:00 to 8:00 am. To explore the variance according to different communication ranges, we choose six communication ranges from 100 to 600 m.
4.2 Size distribution of connected components
Although the whole topology of the VANET is broken into a large number of small-sized connected components, the biggest connected component can cover a large number of vehicles. As Figure 2 shows, the biggest connected component even contains up to 70% to 80% vehicles. Therefore, if the biggest connected component keeps stable, we can make use of this feature by keeping important information on the vehicles in the biggest connected component and design strategies to forward the information with the biggest connected component.
4.3 Size variance of the connected components
It is a piece of good news for routing protocol design that the biggest connected component in each snapshot covers a large number of vehicles. That means many vehicles might benefit from multi-hop forwarding since they get a high probability to be connected on the biggest connected component. However, this kind of multi-hop forwarding might be ceased in the next time stamp if the members in the connected component are changed. Thus, we need to further explore the topology evolvement of the biggest connected components to verify whether its vehicle members are stable during the dynamic process.
4.4 Stability of connected components
When the communication range is larger than 500 m, the averages of the stability factor in both the SF dataset and the SZ dataset are greater than 0.8. This means in average, over 80% of the vehicles in the biggest connected component will keep staying in the connected component in the consecutive time interval. If we define those vehicles that always stay in the biggest connected component as connected component core, we might improve the performance of the routing strategy if we select the connected component core as the packet forwarder, because these connected component core vehicles have more opportunities to forward packets by multi-hop routing. However, how to formally define and discover the connected component core from spatial and temporal datasets are still open problems.
4.5 Variance of connected components
It is useful to identify where the variation of the biggest connected component takes place when designing the routing protocol for VANETs. If we define those vehicles where variation takes place as connected component boundary, we might also improve the routing performance by not choosing the connected component boundary vehicles as forwarders. Again, either defining or detecting the connected component boundary are still unsolved problems.
4.6 Location dependency of connected components
The location dependency of the biggest connected component contributes to two aspects in network design. One benefit is that we do not need to deploy roadside infrastructure to the spots where the connected component forms, because the connectivity can be maintained by the connected component. The other benefit is that vehicles in the downtown region should use the multi-hop forwarding strategy rather than the carry-and-forward strategy, since vehicles in this region have a large probability to be connected to the biggest connected component.
In this paper, we analyzed the spatial and temporal dynamics of VANETs based on two real taxi-trace datasets. We found that the whole topology of VANETs consists of a large number of small-sized connected components; however, the biggest connected component among them contains a large proportion of vehicles. We also found that the connected component core in the biggest connected component is stable in the case where the communication range is large enough and the variation takes place at the connected component boundary. The performance of the routing protocol might be improved by using the information of the connected component core and connected component boundary. Furthermore, by adopting a reasonably communication range, the biggest connected component has the feature of location dependency, which is very useful to roadside infrastructure deployment and multi-hop packet forwarding. However, how to efficiently find the connected component core and detect the connected component boundary and how to design new network architecture and routing protocols are still open research problems and they are left as our future work.
This paper is partially funded by the National Natural Science Foundation of China under grant No. 61003304, 61105124, 61272485, 61379144 and China Scholarship Council under grant 2011611534, 201206115013. The authors would like to express great appreciation to the reviewers of the paper for their valuable comments on improving the quality of this paper.
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