notes/docs/lectures/acn/03_VANET_and_DTN.md

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.

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.