This paper investigates a three-tier hierarchical hybrid wireless network designed to provide significant improvements in system capacity and performance relative to conventional "flat" ad hoc and two-tier "hybrid" networking approaches. The analytical results illustrate that in a three-tier hierarchical network with nAaccess points (AP), nFdual-radio forwarding nodes (FN), and nMmobile nodes (MN), linear scaling of low-tier capacity can be achieved when the low-tier transmission range satisfies rL= O(1/radic(nF)); linear scaling of high-tier capacity can be approached in the scaling regime nA= Omega(radic(nF)) and nA= O(nF/log nF), when the high-tier transmission range satisfies rH= O(1/radic(nA)). The scaling properties of the three-tier hierarchical ad hoc network are studied using ns-2 based system simulation models. In particular, we investigate the impact of relative node densities and traffic pattern on the scalability of the network. The simulation results are consistent with the analysis, and demonstrate that system throughput can be scaled by using the right proportions of FN's and AP's, making it possible to design high-capacity, low-cost mesh networks with a moderate number of radio FN's and only a few wired AP's.

Ad-hoc wireless networks with multi-rate radios (such as 802.11a, b, g) require a new class of MAC/PHY aware metrics that take into account factors such as physical-layer link speed and MAC-layer channel congestion. Conventional "layer 3" ad-hoc routing algorithms typically make routing decisions based on the minimum hop-count (MH). Use of the MH metric leads to the selection of paths with few hops, but one or more of these hops may turn out to be low-speed radio links due to adaptive rate selection at the physical layer We investigate a new cross-layer routing metric that takes into account both physical layer link speed and estimated channel congestion, thus aiming to minimize end-to-end delay that includes both transmission and access times. The proposed "PARMA" routing metric thus helps to spread the traffic across the "good links and nodes" in the network, increasing network capacity and reducing packet loss and delay. The paper presents the design and implementation of the proposed PARMA metric for proactive ad-hoc routing protocols, such as DSDV. DSDV modifications for incorporating the MAC/PHY aware metric into an ns-2 simulation model are given. Simulation results for typical multi-rate 802.11 ad-hoc network scenarios show that the proposed cross-layer PHY/MAC aware metric achieves significantly higher network throughput and decreases network congestion by selecting paths with high bit-rate links, while also avoiding areas of MAC congestion.

This paper describes the design and implementation of a novel 802.11-based self-organizing hierarchical ad-hoc wireless network (SOHAN), and presents some initial experimental results obtained from a proof-of-concept prototype. The proposed network has a three-tier hierarchy consisting of low-power mobile nodes (MNs) at the lowest layer, forwarding nodes (FNs) with higher power and multi-hop routing capability at the middle layer, and wired access points (APs) without power constraints at the highest layer. Specifics of new protocols used for bootstrapping, node discovery, and multi-hop routing are presented, and the overall operation of the complete hierarchical ad-hoc network is explained. A prototype implementation of the SOHAN network is outlined in terms of major hardware and software components, and initial experimental results are given.

A novel self-organizing hierarchical architecture is proposed for improving the performance and scalability properties of ad hoc wireless networks. This paper presents the results of a simulation study of the performance and throughput capacity of a specific three-tier hierarchical ad hoc network with 802.11 radios, forwarding nodes and access points. The performance of the proposed hierarchical network is evaluated for two well-known classes of ad hoc routing protocols: dynamic source routing (DSR) and ad hoc on-demand distance vector (AODV), and compared with that of a conventional "flat" ad hoc network. The results for an example sensor network scenario have shown significant capacity increases with the hierarchical architecture for both DSR and AODV cases. Modifications to ad hoc routing metrics for energy efficiency are also considered. The scalability properties of the three-tier hierarchy are studied further in terms of the achievable system capacity as a function of the relative densities of sensor nodes, forwarding nodes and access points. It is shown that the capacity of the three-tier hierarchical network scales well when the number of forwarding nodes and access points are increased in the right proportions.