Towards efficient opportunistic communications: a hybrid approach Ranjana Pathak *† , Peizhao Hu † , Jadwiga Indulska *† , Marius Portmann *† and Wee Lum Tan † * The University of Queensland, School of Information Technology and Electrical Engineering † National ICT Australia (NICTA) Email: {Firstname.Lastname}@nicta.com.au Abstract—Wireless mesh networks are well-recognised by their self-organising properties. End-to-end routing protocols are pri- marily responsible for achieving these advanced features. However, wireless link failures can cause a route to be invalidated and subse- quently removed from the routing table in nodes along the path to destination. Once the route is not available, packets addressed for that destination will be dropped. To improve the packet delivery ratio of those end-to-end protocols, we propose a hybrid approach that integrates features of opportunistic protocols into traditional end-to-end routing protocols in mesh networks. The idea is to buffer packets for which there is no path to the destination and attempt to deliver these buffered packets when an alternative route is found or to pass them to neighbours who might eventually be able to establish a route to the destination. To demonstrate the concept, we present the AODV-OPP — an extension of the AODV protocol that uses opportunistic communication. AODV- OPP always prefers end-to-end route before attempting to send opportunistically to neighbours. Based on a number of systematic simulation scenarios, we observe that AODV-OPP consistently outperforms the original AODV, with a PDR gain greater than 8% most times and up to 45%. I. I NTRODUCTION As a result of the rapid growth in mobile device market, we see a significant growth in adoption of various wireless networking technologies. Among many promising wireless technologies, wireless mesh networks have been recommended as a complementary technology for offloading the ever increas- ing data traffic from cellular networks. In addition, around the world, wireless mesh networks have been deployed by the pub- lic safety sector as a way to establish essential communications for law enforcement personnel [1] and to provide city video surveillance [2]. Wireless mesh networks are recognised for their self- organising features - they respond to dynamic changes in network topology (due to device mobility or failure) and to dynamic changes in the channel quality. These networks are typically bundled with end-to-end routing protocols that are suitable for achieving those self-organising features. However, link failures are common in such networks due to mobility of wireless nodes and the ever increasing interference in dense networks. Link failures typically cause routing path to destina- tion to be removed from the routing table, and if a new path cannot be established packets are dropped. Another well known approach to communication in dynamic wireless networks is opportunistic communication in which packets are delivered to neighbouring nodes on encounter between mobile devices and packets travel in this manner hop by hop until they reach the destination. The performance of opportunistic protocols is much lower than end-to-end routing protocols if the end-to-end path from the source to the destination exists. In this paper, we explore a hybrid approach that integrates the store-and-forward functionalities of opportunistic commu- nication with the traditional end-to-end routing protocols as a means to improve the packet delivery ratio in highly mobile scenarios. In the literature, there exist solutions that attempt to combine the opportunistic and end-to-end protocols [3], [4], [5], [6], [7]. These solutions tend to switch over to the opportunistic communication paradigm for the lifetime of the packet flow when the packets are dropped due to link failures. In contrast, we propose a protocol in which the packets that would be dropped due to route failure are delivered opportunistically through the network until they reach a node that it is able to create an end-to-end path to the destination. Therefore the approach leverages the potential partial end-to-end routes that can be created in the mobile network. By doing so, the hybrid approach not only improves the packet delivery ratio of end- to-end routing protocols, but also improves the efficiency of opportunistic protocols. The proposed hybrid approach should be applicable to most end-to-end routing protocols (e.g., AODV, OLSR) due to the common similarities in most of these protocols. As an example, we present AODV-OPP, which is an extension of the AODV routing protocol with support for opportunistic communica- tions. We evaluate the performance of AODV-OPP through a set of systematic experiments in NS2 simulations. These experiments include (i) four validation tests, which demonstrate AODV-OPP behaves according to the design specifications; (ii) synthesis trace tests, which show AODV-OPP outperforms the original AODV across different network densities and connectivities; and (iii) realistic trace tests, which validate the observations we see from the controlled experiments and show that AODV-OPP consistently outperforms AODV. In addition, AODV-OPP does not introduce significant overhead in the case when no packets are dropped due to no route to destination. The remainder of the paper is organized as follows. Section II describes the general design principles of the hybrid approach. In Section III, we present AODV-OPP as an example of the proposed approach. The systematic evaluation of AODV-OPP 978-1-4244-9529-0/13/$31.00 ©2013 IEEE International Workshop on the Impact of Human Mobility in Pervasive Systems and Applications 2013, San Diego (18 March 2013) 255