How to Fine Tune the Route Update Parameters of a Mobile Sink Routing Protocol Can Tunca, Sinan Isik * , M. Yunus Donmez † , Cem Ersoy NETLAB, Computer Networks Research Laboratory Computer Engineering Department, Bogazici University * Mathematics Department, Bogazici University † Netas Telecommunications A.S. E-mail: {can.tunca,isiks,yunus.donmez,ersoy}@boun.edu.tr Abstract—Mobile sinks are proposed as a possible solution to the hotspot problem, which leads to the early death of the nodes close to the sink, due to the concentration of data traffic towards the sinks. However, the route updates employed by the source nodes to determine the position of the sinks introduce an overhead in terms of data reporting delays and energy consumptions. On the other hand, decreased frequency of route updates causes stale sink position information to be utilized for data packets which results in suboptimal data dissemination paths. In this paper, we investigate this inherent trade-off and provide an insight on how to fine tune the route update parameters of a mobile sink routing protocol. For this purpose, we define a system employing a simple routing protocol and determine the optimal operating points from the results obtained via simulations, under varying degrees of route updates and sink mobility. I. I NTRODUCTION In wireless sensor networks (WSNs) with static sinks, the nodes close to the sinks are likely to deplete their battery supplies before other nodes due to the intersection of multi-hop routes and concentration of data traffic towards the sinks. The mobile sinks are proposed and explored as a possible solution to this problem [1], [2]. Mobile sinks implicitly provide load balancing without extra effort [3]. The hotspots around the sink change as the sink moves, and the increased energy drainage around the sink spread through the network which helps achieving uniform energy consumption and extends the network lifetime. The advantages of mobile sinks do however come at a cost. Advertising the changing position of the sink freshly across the network is not trivial. Source nodes with sensor data have to acquire the sink position information to disseminate their data. Hence mobile sink routing protocols aim to advertise the sink position information to source nodes with low overhead and provide reliable data delivery. The most naive approach at mobile sink routing is flooding the sink’s position periodically across the network. However, such an approach would introduce an immense overhead since every node in the network frequently transmits and receives broadcasts. A better approach is to utilize a virtual structure composed of a subset of the network that acquires the fresh sink position information while the source nodes retrieve it from this structure. These protocols are referred to as hier- archical two-tier routing protocols. The network is organized into two tiers: the second-tier nodes constructing the virtual structure and the first tier (regular) nodes. The most prominent hierarchical two-tier protocols include Grid-Based Energy-Efficient Routing (GBEER) [4], Hierarchi- cal Cluster-based Data Dissemination (HCDD) [5], Dynamic Directed Backbone (DDB) [6] and Line-Based Data Dissem- ination (LBDD) [7]. These protocols respectively utilize a rectangular grid, a clustered tree, a backbone, and a line strip as the second-tier structures. The structures defined by these protocols have different properties and shapes; however, similar underlying sink position advertisement mechanisms are employed. The basis of these mechanisms is the frequent advertisement of the mobile sink’s fresh position to the second- tier structure, and the request/response mechanism to enable source nodes to acquire the fresh sink position information from the second-tier structure whenever needed. Once the sink position information is retrieved by a source node, it can directly disseminate its data to the sink by greedy geographic routing, assuming that the sensor nodes are position-aware. We refer to the acquiry of sink position information from the second-tier structure by the source nodes as route update. The source nodes become aware of the topological changes due to the sink’s mobility by utilizing a request/response mechanism, and thus performing the route update. Route updates cause an overhead in terms of data reporting delays and energy consumptions. The need to wait for a re- sponse to a sink position request introduces extra delays, while the dissemination of request/response packets require extra transmissions which lead to increased energy consumption across the network. Especially in scenarios where all nodes in the network periodically generate and report sensor data, the overhead becomes even larger since requests are generated by the whole network. In order to decrease the overhead of route updates, the acquired sink position information is remembered for a speci- fied duration and used for disseminating multiple data packets. Hence the frequency of route updates is reduced. However, due to the sink’s mobility, the freshness of the acquired sink position information decays in time. Disseminating data using stale sink position information leads to longer, suboptimal routes which cause inefficiencies in terms of reporting delays and energy consumptions. Therefore, the overheads of route updates and the suboptimal paths due to the decay of sink position’s freshness pose a trade-off. 978-1-4799-3083-8/14/$31.00 c  2014 IEEE 2249