The presence of asymmetric links is a common and non-negligible phenomenon in many ad-hoc networks, including MANETs and sensor networks. Asymmetry is caused by node mobility, heterogeneous radio technologies, and irregularities in radio ranges and packet loss patterns. Most existing ad-hoc routing protocols either assume fully symmetric networks or simply ignore any asymmetric links. In the first case, route discovery can fail when the symmetry assumption does not hold true, e.g., many reactive routing protocols rely on a two-phase communication process, where the same path is used to communicate between a sender and a receiver. If a single link on this path is asymmetric, the route establishment may fail. In the second case, asymmetric links are identified and explicitly ignored in the route establishment phase. This can lead to route discovery failure if there is no symmetric path between a sender and a receiver or it can lead to less than optimal routes. This document provides an overview of routing protocols that explicitly consider asymmetric links in the route discovery phase and introduces robust mechanisms that bypass asymmetric links to ensure successful route establishment.
Mobility Prediction issues and Challenges in Vehicular Ad-Hoc Networks