A TDMA Module for Waterborne Communication with Focus on Clock Synchronization

University essay from Linköpings universitet/Programvara och systemLinköpings universitet/Tekniska högskolan

Author: Anders Persson; [2014]

Keywords: TDMA-Network; Waterborne;

Abstract:

This bachelor thesis has been carried out at the company Didamus which is located in Mjärdevi, Linköping. The company is currently developing a dive-console which aims to take the scuba diving experience to a whole new level and also to make scuba diving more secure.

An assembly of scuba divers that can communicate with each other during a dive session might be the difference between life and death. Many seas around the world have muddy water which means poor visibility. In each situation a computer providing a scuba diver with information about others connected to the network, hazardous accidents can possibly be avoided. 

The network itself consist of 10 nodes that need a network protocol which provides stability and reliability for every participant. The nodes themselves have a distributed responsibility to make the network reliable. The type of network implemented was a regular Time Division Multiple Access (TDMA) network where different nodes were given permission to access the medium in different instances of time. A global reference of time is always needed in a TDMA network to make it function properly. In a typical TDMA network a GPS-service gives each and every node information about the global time.  Unfortunately, GPS-services do not work well in water so a Master-Slave method was used instead. The master provides the rest of the nodes in the network with a global time reference. After a successful reception of a global time reference, the slave will be granted access to the network.

The communication between the nodes is based on ultrasonic waves propagating in the water. The velocity of ultrasonic waves in water is only 1500 meters per second, explained in Discovery of Sound in the Sea by University of Rhode Island, which is a relatively slow signal speed. With the slow velocity taken into account an efficient TDMA protocol was developed, to perform communication under water. 

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