# Vehicular Ad Hoc Networks

- VANETs are a special type of mobile ad hoc network used to provide communication:
    - Between nearby vehicles (V2V)
    - Between vehicles on the road and fixed infrastructure on the roadside (V2I)
- 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.

##### Categories of information

1. Safety application information
    - e.g. information regarding an accident that has just occurred
    - the current conditions of the road
2. Convenience application
    - traffic information
    - parking availability
3. Commercial application for pleasure
    - games
    - real-time video relay

### Why Do VANETs Need Different Protocols from MANETs?

###### Large scale

> All vehicles on the road are potential nodes in the VANET.

###### Predictive Mobility

> The nodes in a VANET cannot follow arbitrary directions. They have to stay on the road and cannot suddenly change direction.

###### High Mobility

> The network mobility in a VANET changes rapidly due to vehicular speeds.

###### Partitioned Network

> 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.

The nodes in the VANET can move at **high speeds**, which **reduces transmission capacity**. This causes the following issues:

- **Rapid changes in the network topology** because the state of connectivity between nodes is dynamically changing.

- **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.

- **Node congestion**, a high traffic situation which affects protocol performance.

### WAVE IEEE 802.11p

WAVE - Wireless Access for Vehicular Environment

- In WAVE, vehicles communicate with each other in a **hop-by-hop** manner
- The area of coverage for the WAVE node is limited to 300 m-800 m
    - Beyond this range cars cannot communicate

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.

> To overcome the limitation of a restricted coverage region, DTNs were implemented using a **store-carry-forward paradigm**.
>
> 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.
>
> - This introduced the idea of the **Vehicular Delay Tolerant Network (VDTN)** concept

#### Vehicular Delay Tolerant Network (VDTN)

VDTNs enable communication in the face of connectivity issues such as

- long and variable delay
- sparse and intermittent connectivity
- high error rates
- high latency
- high asymmetric data rate

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.

###### Fixed DTN nodes

- The stationary or relay nodes have store-and-forward capabilities and are located at **roadside intersections** (roadside units)
- They allow mobile nodes that pass by to collect and leave data on them.
- They contribute to increasing the frequency of node contacts and improve **delivery ratio** and **delivery delay**.

![img](img/f.png)

## Categories of VANETs

##### Pure cellular/WLAN

> Pure cellular VANETs may use **fixed cellular gateways and WiMAX access points at road** intersections to gather information
>
> - Note that these roadside gateways may not be feasible due to the cost of infrastructure
>
> 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.

##### Pure Ad-Hoc

> Pure Ad-Hoc architecture is **not reliant** on infrastructure nodes
>
> In this architecture, nodes perform vehicle to vehicle (V2V) communication with each other.

##### Hybrid

> The hybrid category is a combination of the first two. It provides richer content and offers great **flexibility in the sharing of data**
>
> - 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.