Abstract
The underwater acoustical communication channel is a harsh environment, making it tough to transmit data error free. Due to high absorption of electromagnetic energy, neither radio nor optics are suited for long range transmissions under water. Despite high attenuation in the upper frequency range, a great deal of noise in the lower end, limited bandwidth, and high propagation delay, acoustics is the most suited way to carry information underwater. The problems stated above makes achieving a robust and reliable network hard and tough requirements are set to the protocols. Because of the long and varying propagation delay, the focus of this thesis has been to improve a network protocol with focus to increase packet delivery ratio (PDR). The changes were implemented, and to verify the changes, simulations were done using measurement based Look up Tables supplied by the RACUN (Robust Acoustic Communication in Underwater Networks) project. There were also investigated if the improvements to the protocol can out of the box solve other scenarios than the focus scenario, like a Postman-scenario using an AUV (Autonomous Underwater Vehicle) to gather and deliver packets. The protocol s ability to survive topology changes was also simulated. The thesis focuses on simulations of long- range networks (10 - 20 nautical miles from source to sink). Initial simulations indicated that adding retransmissions to Dflood (the network protocol), PDR increased significantly in some cases, but only slightly increased in other. In the latter cases, the original Dflood protocol performed rather well. Overall, with the implementation of the improvements to Dflood, the protocol performed better in terms of PDR. But realization of a Postman scenario on the network layer using Dflood, was not a complete success. Improvements of Dflood protocol implemented in this thesis have been added to the RACUN software framework and will be applied during the RACUN Sea Trial in May 2014.