Vehicular-2-X Communication

"Chapter 5 Routing (p. 67-68)

Routing refers to move a data packet from source to destination and if required the assignment of a path to the destination. In multi-hop regime routing means to forward packets that contain information through other vehicles [1]. This information refers to alerts about events that already happened, like local danger warnings and traffic flow information. If no vehicle is within the communication range a packet is stored and forwarded as soon as a new vehicle comes into reach. In multi-hop regime the information may be disseminated in two ways: to all surrounding nodes (multicast) or to the ones in a specific area (geocast). Nodes, by exchanging information about network links, compute the best path by which to route messages to other nodes.

Routing in highly mobile ad-hoc networks has to preserve the integrity of message information disseminated in the network while minimizing the number of propagations of each message. Different factors influence the message integrity, e.g. routing algorithm, environmental conditions (physical layer), but also intruder attacks (security). Several algorithms are presented below, and in order to compare them we introduce three vital parameters: PDR, latency and overhead. The PDR (Packet Delivery Ratio) [2, 3] represents the successfully received packets per sent packets. In theory, if no loss of messages occurs, this value should be 1.

In real tests this value is below 1 because in ad-hoc networks one big weakness is that route between a source and a destination is likely to have errors or even break during communication. It was demonstrated in [4] that the data packet loss rate can be decreased significantly by using relative local positions between nodes to discover routes and to make the routing decision for the ad-hoc network. The latency or end-to-end delay [5] represents the time delay from the source sending a packet to the destination receiving it. RTT (Round Trip Time) represents the latency plus time delay from destination back to source, also known as “ping”.

The routing overhead, which represents ratio of control data per payload data should be as low as possible to prevent excessive load of the network. The overhead can be reduced using location-based routing algorithms that may obtain absolute position from a positioning system such as Global Positioning System (GPS) or relative local position between nodes. Other important parameter in routing algorithms is the fault tolerance [6] that requires detection of and recovery from faults.

The above parameters can be unified under the so-called scalability effect. This effect [7–9] ensures adaptability and represents QoS (Quality of Service) impact (PDR, latency, overhead, fault tolerance, etc.) with the growth of network size. A scalable algorithm maintains its QoS parameters with the increase of the network. Routing with multi-hop position based regime is illustrated in Fig. 5.1. Different transmission strategies are possible: Either broadcast the message to all surrounding vehicles until the geocast region is reached, or create a path from source to the geocast region. Each transmission has its advantages and disadvantages. If a path is created, than less network load is required, but if the link is broken, information misses its target. If the flooding approach is used, then multiple paths exists. So, fault tolerance is provided, but also an increase of network load."