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Vehicular ad hoc networks (VANETs) present a multilevel feature when vehicles move in multilevel scenarios, such as viaducts, tunnels, and ramps. Different from city and highway scenarios, complicated node distribution and transmission conditions make the routing decision more challenging in the multilevel environment. To address the issue, this paper investigates geographic routing protocols for the multilevel scenario of VANETs. We first reveal the impacts of the multilevel feature on network characteristics by an outdoor transmission experiment and a stochastic analysis. In particular, the measured data and analysis results show that the wireless transmission range dramatically degrades, which deteriorates the connectivity probability and the performance of geographic routing protocols in the network. Motivated by these results, we propose a multilevel-scenario-oriented greedy opportunity routing protocol (M-GOR). In the protocol, we present a calculation method for the connectivity probability and a greedy opportunity forwarding (GOF) algorithm to respond to the impacts of the multilevel structure. Both the analysis and simulation results demonstrate benefits of M-GOR in terms of the average hop count and delivery ratio.
Zhu et al. (Tue,) studied this question.