# Marine Internet for Internetworking in Oceans: A Tutorial
## 1 Introduction  
Ever-increasing human activities in oceans require handy, reliable and cost-effective high-speed network access not only on the water surface but also underwater. Although terrestrial Internet can be easily accessed almost any time in populated lands via various types of terminals, this is not yet the case in oceans mainly due to huge differences between terrestrial and marine environments. Oceans account for more than 71% of the Earth's surface with 3.62 \(\times\) 10\({}^{8}\) km\({}^{2}\) and an average depth of 3682 as well as a coastline of 356,000 km, containing about 1.3 billion cubic kilometers of saltwater. These features make it extremely difficult and costly to deploy network infrastructures therein. Although satellite Internet is accessible in most areas of the oceans, the services are not cost-effective due to high costs in construction, launching and operation of satellites [1]. Furthermore, many underwater sensors/devices have been deployed in oceans, but it is difficult for them alone to construct an Internet of Underwater Things (IoUT) [2]. This is because the current underwater wireless network can only cover small areas at high speed due to difficulties in underwater networking [3; 4] caused by the physical limitations of underwater communication media, i.e., acoustic wave [5; 6; 7] and blue/green laser [8] (see Section 3). Now, how to provide cost-effective network services to users in oceans is a challenging issue under research mainly following two tracks. The first one is to improve the cost-effectiveness of satellite services [9] not only for the users in oceans but also for the global users. Typical satellite services available today include those provided by Inmarsat and Iridium, whereas their cost-effectiveness cannot be comparable with terrestrial ones. As mentioned in [10], data transfer cost can be 30 times more expensive with approximately $300-2000 per month on satellite cost. Due to high radio frequencies used by satellites, their communication quality is affected by moisture and various formsof precipitation frequently present in marine environments [9], while radio communication cannot cover underwater networks either (More discussion can be found in Section 4.2). The second one is non-satellite solutions, such as exploiting mobile communication systems (e.g., [11; 12]) or wireless ad hoc networks (e.g., [13; 14; 15]) in oceans. However, too small coverage relative to the ocean surface and network reliability are two difficult issues (see Sections 4.3 and 4.6). Although there are several different proposals trying to address the above issue, they are common in the sense that each of them addresses the above issues separately and cannot provide an effective solution to the problem alone. For example, shore-ship and ship-ship communication often termed in \"maritime communication and network\" [10; 16; 17; 18] do not address underwater networking issues. Actually, there are also some well-established or under developing communication systems running in oceans, such as maritime radio, underwater acoustic networks (UANs), etc., which is discussed in Section 4. Exploiting these systems collaboratively to provide cost-effective network services to users in oceans is the basic idea of marine Internet, proposed about six years ago [1]. In this case, the above-mentioned issue becomes how to interconnect these systems efficiently on real-time. Thus, a large-scale cooperative wireless networking structure mainly based on maritime radio and mobile communication systems as well as satellites are proposed by jointly using wireless ad hoc networks in [19; 20]. However, marine Internet cannot be a simple extension of the terrestrial Internet due to the reasons mentioned above. It is still one of the less developed fields for networking in oceans, and more research efforts and investigations are needed to foster its development.