A survey of urban vehicular sensing platforms
- 1. introduction
- 2. Background and related work
- 3. Vehicular sensing applications
- 4. V2V-based VSN platforms
- 5. Infrastructure-based VSN platforms
- 6. Conclusion
http://www.sciencedirect.com/science/article/pii/S1389128609002382
- Keywords
- Vehicular sensor networks, Survey, Comparative evaluation
Vehicular sensing where vehicles on the road continuously gather, process, and share location-relevant sensor data (e.g., road condition, traffic flow) is emerging as a new network paradigm for sensor information sharing in urban environments.
… advanced smartphone capabilities …
review the way sensor information is collected, stored and harvested using inter-vehicular communications (e.g., mobility-assist mobility-assisted dissemination and geographic storage), as well using the infrastructure (e.g., centralized and distributed storage in the wired Internet).
comparative study confirms that system performance is impacted by a variety of factors such as wireless access methods, mobility, user location, and popularity of the information.
1. introduction
Vehicular Ad Hoc Networks (VANETs) are acquiring commercial relevance … a brand new family of visionary services for vehicles, from entertainment applications to tourist/advertising information, from driver safety to opportunistic intermittent connectivity and Internet access
vehicular sensor networks (VSNs) are emerging as a new tool for effectively monitoring the physical world, especially in urban areas where a high concentration of vehicles equipped with onboard sensors is expected (see Fig. 1) [3,4]
- VSNs deployment scenario,
different from traditional wireless sensor network - vehicles usually exhibit constrained mobility patterns due to street layouts, junctions, and speed limitations.
- Vehicles not affected by strict energy constraints: Vehicles easily equipped with powerful processing units, wireless transmitters, and sensing devices even of some complexity, cost, and weight
- no strict limits on processing power and storage capabilities
In general, a vehicular sensor network (VSN) platform provides a means of collecting/processing/accessing sensor data.
Vehicles continuously collect sensor data from urban streets (e.g., images, accelerometer data, etc), which are then processed to search for information of interest (e.g., recognizing license plates, or inferring traffic patterns).
The architecture of information access in a vehicular sensor network is mainly dependent on the underlying wireless access methods in vehicular environments: V2V, V2I
- Vehicle-to-Vehicle (V2V)
- vehicles only equipped with inter-vehicular communications devices, sensor data has to be processed either locally or cooperatively
- V2I
- vehicles also equipped with a broadband wireless access method (2/3G and WiMax)
- sensor data sharing can be implemented over the Internet.
examine how vehicular mobility (e.g., speed, density, churning, location, etc.) influences the overall performance of the VSN platforms
2. Background and related work
key differences that distinguish the vehicular platform from the traditional mobile wireless ad hoc networks (MANETs)
2.1. Wireless access methods in vehicular environments
2.1.1. DSRC/WAVE
- Dedicated Short-Range Communication (DSRC)/ WAVE (Wireless Access in a Vehicular Environment)
- supports vehicle speed up to 120mph, default data rate of 6 Mbps
- enable operations related to the improvement of traffic flow, highway safety, and other Intelligent Transport System (ITS) applications
- DSRC operation modes
- Ad hoc mode characterized by distributed multi-hop networking (vehicle–vehicle)
- Infrastructure mode characterized by a centralized mobile single hop
network (vehicle-gateway).
depending on the deployment scenarios, gateways can be connected to one another or to the Internet
2.1.2. Cellular networks
evolving rapidly to support the ever increasing demands of mobile networking
- behavior of 3G services
- average RTT was consistently high (around 600ms) with high variance (σ = 350 ms; 2)
2.1.3. WiMAX/802.16e
2.1.4. WLAN
WiFi or WLAN can also support broadband wireless services.
2.1.5. Possible vehicular networking scenarios
If vehicles are only equipped with DSRC, we can have an infrastructure-free mode (V2V only), infrastructure mode (V2I), and mixed mode (V2V and V2I), as shown in Fig. 3a
2.2. Characteristics of vehicular network environments
nodes have significantly different characteristics and demands
- much higher power reserves
- support significantly heavier computing (and sensorial) components
vehicles ...orders of magnitude larger in size and weight
vehicular computers can be larger, more powerful, and equipped with extremely large storage (up to Terabytes of data), as well as powerful wireless transceivers capable of delivering wire-line data rates - Vehicles travel at speeds … — making sustained, consistent vehicle-to-vehicle communication difficult to maintain … tendencies to travel together or traffic patterns during commute hours, can help maintain connectivity across mobile vehicular groups
- Vehicles in a grid are always a few hops away from the
infrastructure (WiFi, cellular, satellite, etc.).
protocol and application design may depend on easy access to the Internet during ‘‘normal” operations.
2.3. Routing in vehicular networks
originate from prior ad hoc network architectures but have been extensively redesigned
2.3.1. Broadcasting
Safety related applications (e.g., forward/backward collision warnings, lane change assistance)
2.3.2. Unicast routing
MANET routing protocols
proactive routing (e.g., DSDV, OLSR) or reactive routing (e.g., AODV, DSR), geographic routing (e.g., GPSR), and hybrid geographic routing (e.g., Terminode)
cannot directly be used due to high mobility and non-uniform distribution of vehicles, which causes intermittent connectivity.
the carry-and-forward strategy … overcome intermittent connectivity
2.3.4. Infrastructure-assisted hybrid routing
- Overlay Location Service (OLS)
- maintains geographic locations of APs and mobile nodes, and allows mobile nodes to efficiently utilize geo-routing not only over the vehicular grid but also over the Internet.
OLS provides a "global" view of AP congestion levels, thus leveraging efficient use of communication resources.
2.4. Mobile sensing and sensor storage
Traditionally, sensor networks have been deployed in static environments, with application-specific monitoring tasks. Recently, opportunistic mobile sensor networks have emerged, which exploit existing devices and sensors, such as cameras in mobile phones.
3. Vehicular sensing applications
3.1. Street-level traffic flow estimation
Google Traffic: extremely limited (e.g., mainly highways) due to high installation and maintenance costs.
mobile sensor approach (use vehicles as sensors) greatly extends coverage, thus enabling street-level traffic flow estimation.
3.2. Proactive urban surveillance
vehicles continuously sense events from urban streets, maintain sensed
data in their local storage, autonomously process them
3.3. Vehicular safety warning services
safe navigation: forward collision warning and advisories to other vehicles about road perils
Due to the large RTT, it is difficult to implement time-critical real-time safety warning messaging
3.4. Ride quality monitoring
Ride quality is mainly measured by the pavement roughness of a road surface
expression of the surface irregularity
profiling roads using non-contact profiling devices mounted on the vehicles that use GPS, accelerometer/laser sensors [57].
- less expensive commodity sensors
- less accurate sensors
- a larger number of people can participate
4. V2V-based VSN platforms
FleaNet use mobility-assisted data dissemination to facilitate information access. VITP uses the concept of geographic storage; i.e., the sensor information is stored in an area where it is generated, and mobile users pull it by sending a request to the area of interest.
4.1. MobEyes: proactive urban monitoring services
challenge is to find a completely decentralized VSN solution, with low interference to other services, good scalability, and tolerance to disruption caused by mobility and attacks
4.2. Virtual information exchange bazaar
FleaNet [11] … people … communicate with each other as information traders and to efficiently find matches of interest.
- people
- Vehicles as well as static roadside Advertisement Stations (or Adstations)
- generate and propagate queries
- either user generated content using on-board sensors (e.g., images, video clips, etc.) or commercial advertisements (e.g., special offers)
5. Infrastructure-based VSN platforms
Internet-based VSN platforms, namely Senster [13] and CarTel [4].
5.1. Senster: A mobile platform for scalable vehicular sensing
a need for large scale “distributed” storage that facilitates information sharing among millions of mobile users via always-on 2/3G connections.
mobile-to-mobile overlay network of 2/3G users [22].
P2P connections between mobile devices … not always possible due to Network Address Translation (NAT).
NATing is a commonly used technique in the mobile operator’s domain due to lack of IP addresses [73].
... propose a two-tier sensor storage Senster that exploits the Internet infrastructure. We assume that users install Senster clients in both PCs/laptops and smart phones.
5.1.1. Senster protocols
5.1.1.1. SenterKBR
Key Based Routing. Senster uses Mercury DHT [76] for Key Based Routing
5.1.1.2. SensterDB
distributed database over SensterKBR.
- database can be extended to support stream data processing (e.g., periodic traffic information reporting).
- relax ACID requirements and perform best-effort (or approximate) query processing
5.2. CarTel: A distributed mobile sensor computing system
each vehicle gathers and processes sensor data locally … before delivering them to a central portal, where the data is stored in a database for further analysis and visualization.
CarTel … a simple query-oriented programming interface that can handle large amounts of heterogeneous data from sensors …
CarTel nodes rely on opportunistic wireless (e.g., Wi-Fi hotspots) connectivity to the Internet (for delay-tolerant data delivery).
CarTel applications run on an Internet portal that uses a delay tol- erant continuous query processor, called ICEDB …
5.2.1. Intermittently connected DB (ICEDB)
ICEDB has a server component (Internet portal side) and a client component (mobile side).
delivering data in FIFO order is suboptimal in bandwidth-constrained vehicular networks, ICEDB implements prioritization.
- local prioritization
- determines the priority within a given query buffer.
- global prioritization
- ICEDB clients send the summary results to the portal; and the portal applies a customized order function to order results and return the customized prioritization back to the clients.
5.2.2. Carry-and-forward network (CafNet)
CafNet is a general-purpose network stack for delay tolerant communications … messages across an intermittently connected network.
6. Conclusion
underlying vehicular wireless access methods (e.g., DSRC, 2/3G, mixture) mainly determine the VSN architecture, which can be classified as either V2V-based or infrastructure-based techniques
VSN system performance is mainly influenced by several factors, including: wireless access methods, vehicle mobility (density, speed, and churning), location of stationary users, and popularity of information.