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Friday 2 March 2018

dtn,Reliable Solar System Internet Connection

Disruption Tolerant Networking

Reliable Solar System Internet Connection

Identifier for Disruption Tolerant Network
Communicating from Earth to any spacecraft is a complex challenge, largely due to the extreme distances involved. When data are transmitted and received across thousands and even millions of miles, the delay and potential for disruption or data loss is significant. Delay/Disruption Tolerant Networking (DTN) is NASA’s solution to reliable internetworking for space missions.
The moon is about 250 thousand miles away and Mars is 140 million miles away on average. To communicate across these vast distances, NASA manages three communication networks consisting of distributed ground stations and space relay satellites for data transmission and reception that support both NASA and non-NASA missions. These are the Deep Space Network (DSN), the Near Earth Network (NEN), and the Space Network (SN).
Concept image depicting a single-hope relay date communications network
NASA missions have used direct or single relay communication, but future missions will require Internet-like communication.
For previous missions from low-Earth orbit to deep space, NASA has used point-to-point (direct) or single relay links to communicate with spacecraft; this operates much like the phone system by directly connecting two communication nodes. While this approach has been successful for previous missions, future exploration concepts will introduce much more complex communication needs, with data transfer between many nodes. These transmissions will need to operate like the Internet here on Earth – involving multiple hops via relay spacecraft and other intermediate nodes, creating the foundation for a Solar System Internet (SSI).
Like the terrestrial Internet, the SSI will offer users a well-defined, standardized platform upon which to build a wide variety of applications by accessing end-to-end network services. The SSI will utilize the Delay/Disruption Tolerant Networking (DTN) protocol suite, which can be used in any scenario, including those with longer light times or frequent link disruptions, where conventional Internet Protocols (IP) fail.

How DTN Works

Concept image depicting the Disruption Tolerant Network
The Disruption Tolerant Network protocols will enable the Solar System Internet, allowing data to be stored in nodes until transmission is successful.
DTN is a computer networking model and a system of rules for transmitting information, often referred to as a protocol suite, that extends the terrestrial Internet capabilities into the challenging communication environments in space where the conventional Internet does not work well. These environments are typically subject to frequent disruptions, links that are limited to one direction, possibly long delays and high error rates.
The DTN protocol suite can operate in tandem with the terrestrial IP suite or it can operate independently. DTN provides assured delivery of data using automatic store-and-forward mechanisms. Each data packet that is received is forwarded immediately if possible, but stored for future transmission if forwarding is not currently possible but is expected to be possible in the future. As a result, only the next hop needs to be available when using DTN.
The DTN suite also contains network management, security, routing and quality-of-service capabilities, which are similar to the capabilities provided by the terrestrial Internet suite.
Even though DTN was developed with space applications in mind, the benefits hold true for terrestrial applications where frequent disruptions and high-error rates are common. Some examples include disaster response and wireless sensor networks.
  • Improved Operations and Situational Awareness: The DTN store-and-forward mechanism along with automatic retransmission provides more insight into events during communication outages that occur as result of relay or ground station handovers and poor atmospheric conditions, and significantly reduces the need to schedule ground stations to send or receive data, which can sometimes require up to five days of planning before a transmission takes place.
  • Interoperability and Reuse: A standardized DTN protocol suite enables interoperability of ground stations and spacecraft operated by any space agency or private entity with space assets. It also allows NASA to use the same communication protocols for future missions (low-Earth orbit, near-Earth orbit or deep space).
  • Space Link Efficiency, Utilization and Robustness: DTN enables more reliable and efficient data transmissions resulting in more usable bandwidth. DTN also improves link reliability by having multiple network paths and assets for potential communication hops.
  • Security: The DTN Bundle Protocol Security allows for integrity checks, authentication and encryption, even on links where not previously used.
  • Quality-of-Service: The DTN protocol suite allows for many priority levels to be set for different data types, ensuring that the most important data is received ahead of less important data.

The Future of DTN

The DTN protocol suite is being developed by the NASA Advanced Exploration Systems (AES) DTN Project. The AES DTN team is also supporting the standardization of DTN by both the Consultative Committee for Space Data Systems (CCSDS) and the Internet Engineering Task Force (IETF) and all of the DTN protocols will be open international standards, supported by open-source software. Several DTN implementations exist and are publicly available, including NASA’s Interplanetary Overlay Network (ION) implementation.
This animation shows how our traditional Internet Protocols (IP) can be disrupted and cause delays or data losses, then shows how the Delay/Disruption Tolerant Network (DTN) allows data to be transmitted even when there is poor connectivity.
Credits: NASA
In May 2016, the International Space Station (ISS) implemented an institutional DTN service to support payloads.  The ISS institutional DTN implementation greatly enhances the reliability of the payload science data transmissions and reduces operational overhead and planning, and provides an architecture to support future mission support applications, while advancing DTN technology to support the future SSI.
Several other NASA missions have used DTN, such as the Deep Impact Networking (DINET) experiment, the Earth Observing Mission 1 (EO-1) and the Lunar Laser Communication Demonstration (LLCD), and more are looking to use DTN in the future.  As a result, the AES DTN team is working with the Space Communications and Navigation (SCaN) program to get DTN integrated with NASA’s communication networks, including the DSN, NEN and SN, to support future missions.  The AES DTN team is also working with the InterPlanetary Networking Special Interest Group (IPNSIG) to help make the SSI a reality.
More about Disruption Tolerant Networking
DTN TechPort Page
June 2016 - New Solar System Internet Technology Debuts on the International Space Station
December 2013 - Disruption Tolerant Networking Experiments with Optical Communications
Additional Resources:
from https://www.nasa.gov/content/dtn
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美国国家航空航天局（NASA）的国际空间站‘可容忍延迟网络’（Disruption Tolerant Networking ) 
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