# Vehicular Ad Hoc Networks

* VANETs are a special type of Mobile Ad Hoc network which is used to
  * provide communication between vehicles that are nearby (V2V)
  * between vehicles on the road and fixed infrastructures on the roadside (V2I)
* VANETs provide complementary approach for intelligent transport system (ITS) and are characterised by **high node mobility** and the limited degree of freedom in the 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 to MANETs

###### Large scale

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

###### Predictive Mobility

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

###### High Mobility

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

###### Partitioned Network

> The ranges of wireless communication used in V2V networks is near 1 km but vehicles can get disconnected. Can be thought of 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 to **link failure** that could last for awhile.

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

### WAVE IEEE 802.11p

WAVE - Wireless Access for Vehicular Environment

* In WAVE vehicles communicate in a **hop by hop** manner with each other
* The area of coverage for the WAVE node is limited to 300m-800m
  * 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 restricted coverage region, the use of DTNs was implemented which uses 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 a opportunity arises.

###### Fixed DTN nodes

* The stationary or relay nodes have store and forward capabilities and are located at **road-side intersections** (road side 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](/lectures/acn/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 these road side gateways may not be feasible due to 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 a 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.