Evaluation of the IEEE 802.16 Mesh MAC for Multihop Inter-ship Communications

Maritime Networking: Bringing Internet to the Sea

“…A similar investigation in [19] shows that, given a transmission range of 10 km, ships are connected most of the time; (c) among the stationary ships within a time duration of one minute, 60% do not observe any changes, and 40% observe less than four changes, to their respective neighboring ships; (d) among the mobile ships within a time duration of one minute, 40% do not observe any changes, 50% observe up to 7 changes, and 10% observe more than 7 changes, to their respective neighboring ships; (e) most traffic is attributed to a single-hop or multi-hop ship-to-shore communication between a ship and a shore station, while a single-hop or multi-hop communication between two ships is rare.…”

“…(a) low bandwidth (e.g., INMARSAT GAN and INMARSAT BGAN provide data rates of up to 64 kbps and 432 kbps per satellite link, respectively); (b) high propagation delay (typically a round trip time of approximately 600 ms [17]); (c) high packet error rate (typically ranging from 10 −3 to 10 −1 ) [18]; (d) high cost because of the large initial investment on satellite-related equipment (e.g., stabilizers for onboard antennas) [6], satellite launching cost, and data transfer cost (e.g., approximately US$13.75 per minute for a voice service [15] (or 30 times more expensive [19]) and approximately US$300 to US$2000 per month on satellite cost [14]), and so satellite communication is either not installed [3] or used for exchanging a large volume of data, and data can be compressed before transmission [12]; (e) low stability with long duration and high frequency of link breakages; (f) lack of coverage in certain areas, such as fjords, ports, and some polar regions, particularly relatively high latitude areas (e.g., Europe and US) where a small elevation angle can cause frequent link breakages. Secondly, terrestrial line of sight (LOS) radio that provides direct ship-to-ship and ship-to-shore communications (see Figure 3b).…”

“…Firstly, dynamic link quality and bandwidth constrained. The availability and quality of a link can be affected by two main variations: a) long-term variations, including different levels of path loss caused by weather condition and season (e.g., sea surface salinity and temperature) that affect water vapor concentration above the sea surface; and b) short-term variations, including sea-surface interference (e.g., two-ray, reflection, scattering, refraction, pulse noise, and Doppler shift), random ship location and ship-to-ship distance (e.g., the direction of a sea wave, which changes with time and location, affects the direction of a ship-to-ship connection, the antenna direction, and the direction of arrival of signal [19]), as well as the availability and unavailability of LOS due to wave occlusions. Wave occlusion occurs when the random and rough movement of the sea surface generates waves of sufficient height blocking a link between two nodes as shown in Figure 5.…”

“…Secondly, dynamic network topology. In general: (a) ships have lower mobility (e.g., ships take longer time to make a turn, accelerate and decelerate [19]) whereby this can reduce link breakages caused by ships moving beyond their transmission ranges; (b) ships are either moving in the same direction or small groups (e.g., two to five ships involved in a search and rescue mission); (c) ships have traffic pattern characterized by ship movement trajectories, rather than pedestrians and cars; (d) ships have traffic pattern that does not change with time of day [14]; (e) there is a long distance between two moving ships at sea (e.g., around 15 km to 25 km apart, with their relative distance changes randomly within 5 km from their respective base positions); (f) ships have different node densities (or the number of nodes in an area) in different areas (e.g., higher node density among ships anchored at ports and surroundings, and lower node density among ships at sea) whereby a higher node density can increase the number of candidate routes during route discovery and maintenance, although this can increase routing overhead and network interference; (g) ships are heterogeneous with different communication capabilities, such as satellite communication (e.g., 64 kbps and 432 kbps satellite links), and terrestrial LOS (e.g., HF, BLOS, ELOS, and UHF/VHF LOS). The ship traffic can be characterized by the inter-arrival time and the speed of ships.…”

“…This is because increasing the transmission power increases the number of disjoint routes and achieves a connected network as a result of increased node density [26] and the number of available links, while decreasing the transmission power reduces energy consumption and routing overhead. For instance, some temporary network disconnections may lead to a significant reduction in energy consumption [19], [26]; (e) addressing the effects of low node density (e.g., limited contact opportunities in a network with low node density); (f) addressing the effects of environmental factors on hop counts (e.g., hop count increases when channel quality and node density reduce). Meanwhile, investigation can also be made on the deployment of the shore stations, particularly on the number and the locations, as well as channel frequency for achieving an enhanced network performance.…”