On the use of interwoven order of oncoming packets for reliable underwater acoustic data transfer

Physical properties of hydroacoustic communication channels differ essentially from those of conventional terrestrial radio channels, videlicet the channel is characterized by the long propagation delays, limited bandwidth, extremely complex and quickly varying reverberation, and half duplex nature of underwater acoustic modems. This motivates a substantial redesign of the algorithms and techniques of the underwater data transfer concerning both physical and data link layers, especially concerning the development of underwater acoustic sensor networks. In this paper, the task of point-to-point data transfer is addressed. Since the task seems to be simple, it captures the most important characteristics of the hydroacoustic channel and is the most practical case for the use of underwater acoustic modems nowadays. Conventional protocols of reliable data transfer suffer from the long propagation delays of acoustic signals. The protocols spend the major amount of time waiting for packets with acknowledgments. This results in a poor efficiency of the channel utilization. The use of more complex solutions, namely the packet train transmission or rate-less coding, allows one to improve the efficiency of the data transfer and to reduce the energy consumption significantly. But, in the case of packet train transmission, the optimal number of packets in a train should have quite a high value. Moreover, in the case of the transmission of a packet sequence coded with rate-less code, the feedback channel is either excluded or used to send a “stop” message from the receiver side after the transmission of a big number of packets. This results in a partial or complete elimination of the feedback channel, blocking the remote side from oncoming data transfer. The elimination of the feedback channel excludes a possibility to send urgent data from the receiver side, as well as a possibility for the combination of an underwater acoustic modem with some other devices using the same bandwidth, for instance, position tracking devices. Moreover, this excludes a possibility of the parameter adaptation on a physical layer during the data transmission, which is necessary for the efficient data transmission for given channel parameters. The described restriction can be lifted with the use of the interwoven order of oncoming packets. The idea is to measure the propagation delay between communicating devices during handshaking and then to send the data packets continuously one after another, by interrupting only to accept the control packet from the opposite side at the estimated time of arrival. The propagation delay change tracking during the data transfer is also possible and highly important for the communication between moving stations. This makes the data transfer protocol fully propagation-delay-tolerant and returns the feedback channel, which can be used to deliver acknowledgments and other urgent control data from the receiver side. In the present paper, the efficiency of conventional methods and advanced methods of data transfer using the interwoven order of packets are compared. In this analysis, the task of data transfer is divided into two subtasks. The first one is to send a file, namely the a priori specified amount of data, and the second is to send a byte stream, where the amount of data to transfer is not known by the modem initially.