103 lines
4.4 KiB
Markdown
103 lines
4.4 KiB
Markdown
# Vehicular Ad Hoc Networks
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- VANETs are a special type of mobile ad hoc network used to provide communication:
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- Between nearby vehicles (V2V)
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- Between vehicles on the road and fixed infrastructure on the roadside (V2I)
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- VANETs provide a complementary approach for intelligent transport systems (ITS) and are characterised by **high node mobility** and a limited degree of freedom in their mobility patterns.
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##### Categories of information
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1. Safety application information
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- e.g. information regarding an accident that has just occurred
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- the current conditions of the road
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2. Convenience application
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- traffic information
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- parking availability
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3. Commercial application for pleasure
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- games
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- real-time video relay
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### Why Do VANETs Need Different Protocols from MANETs?
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###### Large scale
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> All vehicles on the road are potential nodes in the VANET.
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###### Predictive Mobility
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> The nodes in a VANET cannot follow arbitrary directions. They have to stay on the road and cannot suddenly change direction.
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###### High Mobility
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> The network mobility in a VANET changes rapidly due to vehicular speeds.
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###### Partitioned Network
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> The range of wireless communication used in V2V networks is around 1 km, but vehicles can become disconnected. This can be thought of as many disconnected networks.
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The nodes in the VANET can move at **high speeds**, which **reduces transmission capacity**. This causes the following issues:
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- **Rapid changes in the network topology** because the state of connectivity between nodes is dynamically changing.
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- **Occasional disconnections due to low traffic density**. This keeps the nodes distant from each other and results in **link failure** that could last for a while.
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- **Node congestion**, a high traffic situation which affects protocol performance.
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### WAVE IEEE 802.11p
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WAVE - Wireless Access for Vehicular Environment
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- In WAVE, vehicles communicate with each other in a **hop-by-hop** manner
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- The area of coverage for the WAVE node is limited to 300 m-800 m
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- Beyond this range cars cannot communicate
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If there is dense traffic in the coverage region, **nodes become easily congested** because all nodes will be transmitting the same message to every other node.
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> To overcome the limitation of a restricted coverage region, DTNs were implemented using a **store-carry-forward paradigm**.
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>
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> With the store-carry-forward approach, a vehicle stores a message in a buffer and carries the message with it. When it comes into contact with another node, it forwards the message.
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>
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> - This introduced the idea of the **Vehicular Delay Tolerant Network (VDTN)** concept
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#### Vehicular Delay Tolerant Network (VDTN)
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VDTNs enable communication in the face of connectivity issues such as
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- long and variable delay
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- sparse and intermittent connectivity
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- high error rates
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- high latency
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- high asymmetric data rate
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Communication is made possible in the network when intermediate nodes become **custodians** of the message being transmitted and then forward the message only when an opportunity arises.
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###### Fixed DTN nodes
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- The stationary or relay nodes have store-and-forward capabilities and are located at **roadside intersections** (roadside units)
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- They allow mobile nodes that pass by to collect and leave data on them.
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- They contribute to increasing the frequency of node contacts and improve **delivery ratio** and **delivery delay**.
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## Categories of VANETs
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##### Pure cellular/WLAN
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> Pure cellular VANETs may use **fixed cellular gateways and WiMAX access points at road** intersections to gather information
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>
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> - Note that these roadside gateways may not be feasible due to the cost of infrastructure
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>
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> The information collected from sensors of a vehicle in the VANET can become valuable in notifying other nodes about the situation of the traffic in the network.
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##### Pure Ad-Hoc
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> Pure Ad-Hoc architecture is **not reliant** on infrastructure nodes
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>
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> In this architecture, nodes perform vehicle to vehicle (V2V) communication with each other.
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##### Hybrid
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> The hybrid category is a combination of the first two. It provides richer content and offers great **flexibility in the sharing of data**
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>
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> - Some vehicles with WLAN and cellular capabilities may be used as **gateways** and **mobile routers** so that vehicles with only WLAN capabilities can interact and communicate effectively with them via multi-hop links.
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