## 3 Simulation Results and Discussion  
In this paper, we use NS2 (Network Simulator version 2) as the simulator. NS2 [13] is an object-oriented simulation tool and it is a discrete event simulator with a virtual clock, that drives all simulations by discrete events. In underwater sensor networks, since the environment is totally different from the land scenario, the channel quality is also worse than in traditional wireless sensor networks. The difference compared with traditional WSNs is that acoustic communications become the physical layer technology in underwater networks and the available bandwidth is severely limited when applying underwater acoustic channels. The underwater channel is severely impaired due to multi-path interference and fading. Propagation delay is another challenging issue. The delay time in underwater environments is five orders of magnitude higher than in radio frequency (RF) terrestrial channels and it varies. This fact may lead to unsuccessfully transmission between nodes due to the uncertain delay. When we perform a simulation to evaluate the performance of the proposed routing protocol, we also consider the channel quality issue. In such a case, we implement our routing protocol on the NS-2 simulation platform. Since the NS-2 platform has fully considered the underwater environment, the simulation results are very close to the real underwater sensor networks. In order to understand the impact of channel contention and the interaction of multiple flows in the networks, the underwater acoustic channel model had been implemented in NS-2. The NS-2 implementation includes the propagation time model, the bandwidth-distance relationship and the attenuation and signal-to-noise ratio for three-dimensional underwater sensor networks. The existing well designed routing protocols for mobile wireless sensor network are hard to implement due to the unique characteristics of underwater sensor networks. First of all, radio does not work well in water because of its rapid attenuation. Thus acoustic communications are usually adopted in underwater environments. Acoustic channels often feature low bandwidths and long propagation delays. Thus a routing protocol with long end-to-end delays or high bandwidth requirements is not a good choice. Secondly, most nodes in a UWSN can move passively with water currents (except that some gateway nodes are fixed on the water surface or anchored to the bottom), resulting in a highly dynamic network topology. To handle dynamic networks, existing routing protocols for land-based sensor networks need to update routing information periodically, which introduces significant communication overhead. Thirdly, like land-based sensor nodes, underwater sensor nodes are usually powered by batteries, which are even harder to recharge or replace in harsh underwater environments. Thus, energy efficiency is another important concern for UWSN routing. There are three categories of dynamic routing protocols. Proactive routing protocols maintain the route periodically in order to create a fresh enough route for data transmission. However, periodically route maintenance decreases available limited bandwidth. Reactive routing protocols are on demand. They build routes between nodes only as desired by source nodes. Some research has tried to strike a balance between proactive and reactive routing as a hybrid mode. Conventional proactive routing protocols rely on systematic flooding for route discovery and maintenance, potentially causing excessive energy consumption and collisions. Geographic routing protocols need help from a globe positioning system (GPS). These protocols are not designed for underwater sensor networks. In an underwater sensor network scenario, general 3D geographic routing is preferable as it is stateless. However, geographic routing requires online mode, distributed localization of mobile sensors which is expensive and takes a long time to converge.