Protocol Gateways Speed Data over Satellite

Friday, October 12, 2001
Despite the critical role of the maritime industry in laying the fiber that connects the world to the Internet, the maritime industry itself relies almost exclusively on satellites for ship-to-shore and ship-to-ship connectivity. However, the networking protocols used to transfer data between computers do not operate efficiently over satellite links. This is especially true at the higher data rates that are starting to become more prevalent for ship-based connectivity, but even at low bandwidths, the noisy links typical of mobile satellite systems prevent the efficient use of expensive satellite time.

Fortunately, a new class of products known as protocol gateways can overcome these limitations to enhance link performance and efficiency and help bring the benefits of Internet and corporate connectivity to the maritime community.

TCP/IP over Satellite Transmission Control Protocol/Internet Protocol (TCP/IP) are the protocols used to transfer data over the Internet and most corporate networks. Specifically, TCP provides a reliable transfer mechanism over an unreliable IP network. Nearly all data networking applications, including web, e-mail, file sharing, database access, logistics, inventory and scheduling applications, and remote connectivity use TCP to transfer data over the network.

TCP includes a variety of mechanisms to ensure the reliable transfer of data and the fair and safe utilization of bandwidth resources. Unfortunately, these algorithms, which are effective over terrestrial networks, interact with satellite link conditions and place severe restrictions on the rate at which data can be transmitted over satellite links.

The most obvious limitations are created by the satellite latency. The half-second delay caused by bouncing a signal off a satellite orbiting at a height of 22,300 miles above the earth has the effect of limiting download speeds for most users to a maximum of 128 Kbps per connection, regardless of how much bandwidth is available. Other factors, especially the high data loss rates typical of mobile satellite systems affects throughput rates and reduces link efficiency.

Protocol Gateways It is important to note that these throughput and efficiency limitations are not due to the satellite conditions themselves, but to TCP's reaction to those conditions. This makes it possible for protocol gateways to overcome these limitations by replacing TCP with a protocol optimized for satellite conditions. Protocol gateways operate by intercepting TCP traffic, transmitting the data over the satellite link using a satellite-optimized protocol, then establishing a new TCP connection on the other side of the satellite link for compatibility with the terrestrial infrastructure.

Unlike TCP, which is designed as a general protocol and optimized for terrestrial networks, a satellite protocol can maximize performance and efficiency for satellite conditions. Because protocol gateways communicate with the client and server using standard TCP, these systems can be designed to be entirely transparent to the end user without requiring any changes or tuning to end nodes.

Protocol Gateway Benefits To users, link enhancement provides faster downloads for their e-mail, file transfers, web access and other networking applications both for Internet usage and internal corporate communications. For operators, efficient link utilization helps cut costs. Testing conducted by LCDC Telecom, a Paris-based reseller and integrator of satellite systems, demonstrated that using a 64 Kbps Inmarsat M4 terminal, protocol gateways can cut the time required to complete a file download by over 70%.

For a ship-to-shore, single satellite hop from the M4 terminal to an Inmarsat land earth station on a clean link, the protocol gateway reduced the time to transfer a large file from two hours and seven minutes to one hour and thirty-nine minutes, a 28% improvement. For a ship-to-ship double satellite hop on a noisy link, the protocol gateway cut the transfer time from nearly nine hours to under two and half hours. Similar tests for e-mail downloads show a 43% increase in download speeds and the equivalent reduction in satellite time usage. Government and military organizations which use satellite networks for national security, diplomatic, scientific, and humanitarian purposes also depend heavily on protocol gateways. The U.S. Coast Guard Academy tested a protocol gateway for their Large Cutter project utilizing the 64 Kbps Inmarsat-B service for ship-to-shore communications. For file transfers, they found an improvement from 43 Kbps to 60 Kbps, an increase of 40%. Turning on data compression functionality integrated with the protocol gateway, throughput increased further to 121 Kbps, an improvement of nearly three times over the unenhanced link.

In addition to faster downloads, protocol gateways offer considerable cost savings to link operators by more efficiently utilizing expensive satellite bandwidth. With satellite time costing up to $18 per minute for a 64 Kbps channel, the ability of protocol gateways to reduce download times can lead to large cost savings. Because of their ability to enhance the user experience while reducing satellite usage costs, protocol gateways are quickly being adopted throughout the satellite industry. Within the maritime community, corporate, military, and passenger needs for Internet and internal corporate network connectivity are leading to an explosion in satellite bandwidth utilization and protocol gateways are quickly becoming a fixture of these networks.

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