1 AODVH: Ad hoc On-demand Distance Vector Routing for Hybrid Nodes Yasmin Jahir 1 , Mohammed Atiquzzaman 2 , Hazem Refai 1 , Peter G. LoPresti 3 1 School of Electrical and Computer Engineering, University of Oklahoma, Norman, OK. 2 School of Computer Science, University of Oklahoma, Norman, OK. 3 Department of Electrical Engineering, The University of Tulsa, Tulsa, OK. Email: {yasmin,atiq}@ou.edu Abstract—Ad hoc networks are characterized by nodes which are usually connected by low data rate Radio Frequency (RF) links when compared to optical links. RF links make it challeng- ing for transmitting video and running high bandwidth appli- cations over ad hoc networks. The objective of this paper is to develop a novel routing protocol for Ad hoc networks consisting of hybrid nodes using high bandwidth Free Space Optical (FSO) and RF links, with FSO being the primary link and RF as backup in case of FSO link failures. Our protocol, called Ad hoc On- demand Distance Vector hybrid (AODVH), computes multiple “FSO only” paths to ensure high data rate communication, and uses “hybrid paths” consisting of FSO and RF links as backups. AODVH can be used to design a rapidly deployable, reliable and high bandwidth communication infrastructure for re-establishing communication following a disaster. We evaluate the performance of AODVH using ns-2 simulations and compare with three other Ad hoc routing protocols. Results show that AODVH performs better in terms of packet loss, end-to-end delay, overhead, packet delivery ratio, route discovery frequency and throughput when compared to the other three protocols. I. I NTRODUCTION A rapidly deployable, reliable and high bandwidth network is needed during natural or man made disasters resulting in breakdown of telecommunications infrastructure [1]. We propose such a network architecture, called Disaster Area Wireless Network (DAWN), consisting of helium-filled bal- loons carrying routers. The balloons form an ad hoc network up in the sky using Free Space Optical (FSO) and Radio Frequency (RF) links. An ad hoc network is characterized by a multihop, tem- porary network consisting of a set of nodes that do not rely on any preexisting infrastructure support [2] and usually com- municate with each other using RF links. RF links have low bandwidth and, therefore, are challenging for high bandwidth applications such as multimedia transmission. A Free Space Optical (FSO) link has significantly higher bandwidth and lower error rate [1] over RF links. However, link unavailability remains a big challenge for FSO links due to environmental phenomenon such as absorption, scattering (fog) and shimmer [3]. Because of this intermittent character- istic of FSO links, it is not always possible to maintain “FSO” links. Hybrid nodes consisting of FSO and RF links, with RF This work was supported by National Science Foundation grant no. 0725801. links serving as backup in case of FSO link unavailability, can be used to overcome the above limitation of FSO links. The challenge, however, is to design multipath routing protocols which will take into account the varying characteristics of FSO and RF links. Multiple paths increase fault tolerance and reduce control message overhead [4], thus improving the effective bandwidth among communicating pairs. Our objective is to develop a multipath on-demand distance vector routing protocol for hybrid nodes, called multipath Ad hoc On-demand Distance Vector Hybrid (AODVH). AODVH differs from other Ad Hoc routing protocols (DSR [5], OLSR [6], AODV [7], AOMDV [8], AODVM [9], etc.) mainly in two aspects: • Current single path Ad Hoc routing protocols require considerable time to find new routes in case of link failures, and are thus unsuitable for DAWN where link failure is frequent with hybrid nodes having intermittent FSO links; • Current multipath protocols do not support routing over hybrid links (FSO and RF) with different link character- istics. Existing multipath Ad hoc routing algorithms [2], [8], [9] are extensions of unipath routing protocols viz. DSR [5] and AODV [7]. Lee et al. [2] developed a split multipath routing protocol with maximally disjoint paths for homogeneous RF nodes. Marina et al. [8] developed a loop free and link-disjoint multipath routing protocol (AOMDV). Zhenqiang et al. [9] proposed AODVM to discover multiple node-disjoint paths to achieve reliability in path setup for homogeneous RF nodes. The above [2], [8], [9] are based on homogeneous nodes and hence are not suitable for our hybrid nodes having heteroge- neous link characteristics. Our proposed protocol (AODVH), differs from other multipath routing protocols because it con- siders hybrid nodes with heterogeneous link characteristics. Bilgi et al. [10] and Yuksel et al. [11] proposed nodes with FSO links only, and hence differ from our requirements of hybrid nodes. The main contribution of this paper is to develop a novel multipath Ad Hoc routing protocol for computing multiple “FSO only” paths to ensure faster communication among hybrid nodes using FSO and RF links in DAWN. We have simulated AODVH in ns-2 and compared the results with unipath AODV [7] and two multipath protocols