This report encompasses a comparison between existing radio based and wireless networks with underwater acoustic networks. The various challenges faced by underwater acoustic networks are analysed. The medium access control (MAC) protocol is a basic aspect for underwater acoustic networks (UWANs). Some of the problems are very small channel capacity, low channel quality and high dynamics of channel quality, slow signal propagation speed, and long propagation delays. As a result, various Medium Access Control strategies are used to counter attack these challenges. Still, underwater acoustic network is a hot research topic because it has many applications.
Underwater acoustic networks (UAN) are an evolving technology as they have wide range of applications. Also human underwater actions are growing rapidly because of which many sensors and different types of vehicles are expanding underwater. Underwater communication is capable of forming the Internet of Underwater Thing (IoUT).Radio frequency based networks are very important for earth like environment similarly underwater acoustic networks are a base to analyse the ocean environment.
Underwater acoustic communication networks basically examine a particular area of an ocean and then research it properly. A two-way acoustic link is formed between the machines placed in the ocean with the sensors placed outside the sea. It’s not possible to use radio waves in underwater as they do not forms communication over long-range therefore acoustic waves are used. Along with this they also have low absorption powers hence they are perfect to use in underwater acoustic networks. But there are many drawbacks of these networks and researches are made to resolve them. Few of the challenges faced are limited bandwidth which depends on two factors: range and frequency, the propagation speed is also low and it continuously oscillates. The large delays in transmission lead to low throughput. Under the water the battery’s power plays a key role and these networks are less power sufficient.
These networks examine and analyses a particular part of the ocean continuously. In radio channels, there is time-static multipath and hence the Doppler shifts and interference is very less while underwater acoustic signals are very much prone to these shifts. Underwater acoustic networks make us understand the complications faced in underwater environment. They also find the environmental frameworks inside the ocean.
Radio-frequency based wireless networks can be defined as networks that uses radio-frequency as a key element for communication. These networks have good frequency range which is defined between 500 KHz to 300 GHz band. Most of the wireless networks are based on radio-frequency like Wi-Fi, Bluetooth, radio and television system or satellite communication and many more. Because of the high frequency they have high propagation capacity. The propagation medium is only air and hence disruption of noise is more also there is a loss in amplitude. But these networks provide immunity from the noise. Antennas form a basic part of radio frequency signals and have a sending antenna and a receiving antenna. Radio frequency based networks can be described as follows,
Very few networks lie in this range and the devise which lies in this range were used years ago. Devices which use this frequency range are amateur television, cordless phones and ZigBee. Also few IEEE standards fall under this category.
WiMAX, GPS operates in this range along with microwave ovens. The ISM band also has a frequency of 2.4GHz.
Maximum attenuation and scattering is experienced in this range and the exposure to air is more. IEEE 802.11a, 802.11n and Wi-Fi falls under this category.
Radio frequency based wireless networks are preferred over underwater acoustic networks because,
Although radio-frequency based wireless networks have many advantages but along with that they have very less security. And in severe conditions they give low throughput.
2. Medium Access Protocols are constructed to reduce the challenges of underwater acoustic networks.
In ALOHA protocol, each time the nodes transfers a packet of data it can do it immediately without any delay. Whenever the packet is received at the other end, the receiver confirms that he has got the packet by sending an acknowledgement. The communication is said to be successful when the sender receives the acknowledgement. Even if there is heavy traffic ALOHA performs well and solves the problem of long propagation delays as it is a fast process.
Although handshaking is a complex protocol but it gives good throughput as it reduces the propagation delay. Also, before sending a data frame this protocol engages a request-to-send/clear-to-send frames. The basic advantage is that because of these frame all other interferences are blocked and hence the propagation delay is less. A particular node does not execute any other action until this on –doing transmission is not complete. It provides a propagation delay which is less than the length of the data frame and hence the throughput is good.
This protocol follows the concept of handshaking. It fixes the time which is wasted in setting the links for data frames by organising the actions of sensors. PCAP follows a concept which seems like it has a pipeline for transmission and hence the propagation delay is reduced. As a node involve itself I transmitting man other data frames rather than wasting its time waiting for signals to propagate. Basically, PCAP allows stations to schedule activities for multiple data frames, and hence reduces the time wasted in waiting for signals to travel over high-delay channels.
T-Lohi protocol catches and calculates the challenger nodes during the reservation and traffic-adaptive back off algorithm. Along with this it employs a wake up tone to reserve the data transmission. Every node contains a hardware wake-up tone detector; on this basis the nodes spend their minimal energy. It contains a reservation period which comprises of multiple contention slots. T-Lohi reserves the channels and carrier sensing so that the reservation result can be verified. T-Lohi is load stable and hence reduces the energy consumption and improves fairness of heavy traffic loads.
This protocol is particularly a type of master-slave protocol. ACME Net is established on scheduled transmission schedules which aim that the data packets can be transferred from slave to master node consecutively. This protocol uses the concept of CDMA in which the nodes are scheduled in a way that a group of data packets can be send and received simultaneously. These schedules are backed up with instructions to maintain the power levels and modulation rates at nodes.
In hybrid protocol, the whole network is divided into clusters and the transmission of these clusters I scheduled by TDMA algorithm. The nodes reserves the whole channel by a spreading technology and hence the channel efficiency is high. Along with TDMA at times it uses CDMA as well which is used to enable spatial reuse of slots throughout the network.
This scheduling protocol makes a use of scattered maximal independent set algorithm so that the maximum number of nodes which can transfer without collision in the same time interval can be determined.
This protocol is used to handle spatial unfairness which occurs due to distinct distances. SF-MAC accepts a receiver-based protocol without any information of distance availability. It guarantees the node by submitting a Receiver to Sender as soon as possible to transfer the data first. The receiver abducts the RTS frames to evaluate the earliest transmitter by the usage of potential transmission duration.
FERI MAC applies a receiver initiated handshaking protocol. The main aim of this protocol is to enhance the channel utilization and the energy efficiency. The collisions are avoided by RTR and ATS messages exchanging making it more effective in defending the receiver from packet collisions. This protocol also reduces the overhead of handshaking control messages by applying parallel reservation and packet train.
Conclusion
Underwater acoustic networks are an evolving technology and have wide range of applications. In comparison to radio frequency based wireless networks these networks have disadvantages like power limitations, bandwidth limitations along with long propagation delays. Therefore, MAC protocol strategies are implemented on these networks to overcome these problems.
Over the next decade, significant improvements are anticipated in the design and performance of UWA networks as more experience is gained through at-sea experiments and network simulation tools.
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