The underwater acoustic channel features long and variable propagation delay, high bit error rate and limited bandwidth. Moreover, underwater mobile networks consisting of Autonomous Underwater Vehicles (AUVs) for surveillance and monitoring applications often suffer from intermittent connectivity as nodes move around the area of operations. Therefore, this scenario is appropriately modeled as a Delay-Tolerant Network (DTN). In this paper, we investigate two classes of DTN routing protocols: Spray-and-Wait (SNW) and the Resource Allocation Protocol for Intentional DTN (RAPID), together with a standard flooding protocol for underwater mobile networks, and their performance is analyzed in terms of packet delivery ratio, average end-to-end delivery delay and throughput, in different load conditions across various node mobility scenarios. Our results show that RAPID performs better than other DTN routing protocols in terms of packet delivery ratio at higher load conditions irrespective of node mobility, whereas the Binary version of SNW has the best performance in terms of average delivery delay.

Delay-Tolerant Networks (DTNs) are sparse dynamic wireless networks, where most of the time a complete end-to-end path from the source to the destination does not exist. There are many real networks that follow this model, for example, military networks, vehicular ad-hoc networks (VANETs), wildlife tracking sensor networks, etc. In this context, conventional mobile ad-hoc routing schemes would fail, because they try to establish complete end-to-end paths, before any data sent. Therefore, performance analysis of different DTN routing mechanisms plays an important role in understanding the design of DTNs that encourages one to choose proper routing protocol for a particular scenario. This paper investigates the performance of replication-based DTN routing protocols, namely Epidemic, Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET), MaxProp, Resource Allocation Protocol for Intentional DTN (RAPID), Binary Spray-and-Wait (B-SNW), and Spray-and-Focus (SNF) in DTN scenario against varying number of mobile nodes for three mobility models, namely Random Walk (RW), Random Direction (RD) and Shortest Path Map Based (SPMB) movement. Performance has been evaluated and analyzed using Opportunistic Network Environment (ONE) simulator by considering three metrics: delivery probability, average latency and overhead ratio. Simulation results show that the investigated DTN routing protocols, in general, exhibit better performance in the SPMB movement model than other movement models, i.e., RW and RD, because they yield maximum message delivery probability, minimum overhead ratio (for B-SNW and SNF), and minimum average latency.

In Delay-Tolerant Network (DTN), there exists a lack of direct path from source to destination, and it is also featured by extremely high bit error rate, unwanted delay, limited resource, etc. For such network, we have investigated the effects of buffer size and mobility models on the optimization of the number of message copies for multi-copy routing protocols such as Spray-and-Focus (SNF) and Spray-and-Wait (SNW). This investigation is essential for providing us with the insight on the number of message copies that would provide better delivery, lower latency and lower overhead considering the effects of buffer size and mobility models in scalable delay-tolerant networks. In this investigation, we have considered three performance metrics, namely delivery probability, average latency, and overhead ratio. Simulation is done using Opportunistic Network Environment (ONE) simulator, which is designed basically for evaluating the performance of DTN routing strategies. Here we have considered three mobility models, namely Shortest Path Map Based (SPMB) movement, Random Walk, and Random Direction. Simulation results show that for increasing buffer size and number of nodes on these considered mobility models for a particular value of number of message copies, indicated by L, SNF routing exhibits satisfactory performance, especially in the case of SPMB movement using only L = 2 copies. Since our purpose is to provide a satisfactory performance, i.e., higher delivery, lower latency and lower overhead, SNW routing shows overall good performance using L = 10 copies. Therefore, it would be a good optimization using Shortest Path Map Based movement model using L = 2 copies for SNF routing, and L = 10 copies for SNW in scalable DTN scenario, where there may be a high possibility of varying buffer size with number of mobile nodes.

Intermittently Connected Mobile Networks (ICMNs) are sparse wireless networks where most of the time an end-to-end path does not exist from source to destination. These networks are characterized by opportunistic connectivity, long and variable delay, asymmetric data rate and high error rate. Hence, they can be appropriately modeled as Delay-Tolerant Networks (DTNs). In this paper, the performance of replication-based DTN routing protocols, namely Epidemic, PRoPHET, MaxProp, RAPID, Binary Spray-and-Wait, and Spray-and-Focus has been investigated in an ICMN scenario against varying message generation rates and number of mobile nodes, respectively. The performance is analyzed on the basis of three metrics: Delivery Probability, Average Latency, and Overhead Ratio. Opportunistic Network Environment (ONE) simulator is used as the simulation tool for evaluating these performance metrics. The outcome of this work shows that for the ICMN scenario and given simulation setting, the best DTN routing technique is Spray-and-Focus, whereas Epidemic routing exhibits the worst performance in terms of all the metrics considered here.