Landmark routing in ad hoc networks with mobile backbones

A Mobile Ad Hoc Network (MANET) is usually assumed to be homogeneous, where each mobile node shares the same radio capacity. However, a homogeneous ad hoc network suffers from poor scalability. Recent research has demonstrated its performance bottleneck through both theoretical analysis and simulation experiments and testbed measurements. Poor scalability is due to the fact that in ad hoc networks, most bandwidth of a node is consumed by forwarding packets. This is further exacerbated by heavy routing overhead of ad hoc routing protocols when the network size is large. This will significantly affect several large scale ad hoc applications, such as in a digital battle field, where hundreds or even thousands of nodes must be supported. Building a physically hierarchical ad hoc network is a very promising way to achieve good scalability. In this paper, we present a design methodology to build a hierarchical large-scale ad hoc network using different types of radio capabilities at different layers. In such a structure, nodes are first dynamically grouped into multi-hop clusters. Each group elects a clusterhead to be a backbone node (BN). Then higher-level links are established to connect the BNs into a backbone network. Following this method recursively, a multilevel hierarchical network can be established. Three critical issues are addressed in this paper. We first analyze the optimal number of BNs for a layer in theory. Then, we propose a stable and light overhead clustering scheme to deploy the BNs. Finally LANMAR routing is extended to operate the physical hierarchy efficiently. We show that the Hierarchical LANMAR (H-LANMAR) can incorporate and efficiently utilize backbone links to reach remote destinations (thus reducing the hop distance). Simulation results using GloMoSim confirm that our proposed schemes achieve good performance. The key contributions of this paper are the methodology to select the backbone nodes and; the extension of LANMAR to make it work on the backbone network in a robust and efficient manner.