Scalability and Performance Evaluation of Hierarchical Hybrid Wireless Networks

This paper considers the problem of scaling ad hoc wireless networks now being applied to urban mesh and sensor network scenarios. Previous results have shown that the inherent scaling problems of a multihop ldquoflatrdquo ad hoc wireless network can be improved by a ldquohybrid networkrdquo with an appropriate proportion of radio nodes with wired network connections. In this work, we generalize the system model to a hierarchical hybrid wireless network with three tiers of radio nodes: low-power end-user mobile nodes (MNs) at the lowest tier, higher power radio forwarding nodes (FNs) that support multihop routing at intermediate level, and wired access points (APs) at the highest level. Scalability properties of the proposed three-tier hierarchical hybrid wireless network are analyzed, leading to an identification of the proportion of FNs and APs as well as transmission range required for linear increase in end-user throughput. In particular, it is shown analytically that in a three-tier hierarchical network with nA APs, nF FNs, and nM MNs, the low-tier capacity increases linearly with nF, and the high-tier capacity increases linearly with nA when nA = Omega(radic{nF}) and n _A = O(nF). This analytical result is validated via ns-2 simulations for an example dense network scenario, and the model is used to study scaling behavior and performance as a function of key parameters such as AP and FN node densities for different traffic patterns and bandwidth allocation at each tier of the network.