Disruption Tolerant Networking for Space Operations (DTN) - 07.29.14
ISS Science for Everyone
Science Objectives for Everyone
The Disruption Tolerant Networking (DTN) program is a step toward building a reliable Interplanetary Internet. The experiment establishes a long-term communications test bed on the International Space Station (ISS), which transmits test messages between the ISS and ground stations. Delay- and disruption-tolerant networks can improve electronic communications by storing data when a connection is interrupted, and forwarding it to its destination using relay stations.
Science Results for Everyone
University of Colorado at Boulder, BioServe Space Technologies, Boulder, CO, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration
March 2009 - March 2015
Previous ISS Missions
Expedition 18 is the first time DTN will be tested on the ISS.
- This research is needed to develop and operate the communications protocols that will enable Internet-like communications with space vehicles, remote planetary habitats, rover vehicles and support infrastructure on a planetary surface.
- Onboard (local) ISS , ISS-to-ground, and NASA ground communications networks will become DTN-enabled which is the key stepping stone to enabling the Interplanetary Internet.
- (1) DTN decreases mission operations control center labor costs via automated (unattended, lights-out) spacecraft/satellite/payload command and telemetry (C&T) transmission and receipt; (2) DTN decreases the need, and attendant infrastructure costs, for custom Mission or Payload Operations Control Centers, MOCS/POCCs, to interface with spacecraft/satellites/payloads; can use the Internet in cases where security concerns can be adequately addressed; (3) By providing a standards-based, publically available, space networking protocol the DTN network architecture decreases the labor costs required to provide bi-directional communications for spacecraft, satellite, and payload C&T systems; (4) 1.DTN significantly improves communication link utilization efficiency by ensuring telemetry is successfully received at the destination with the minimum amount of data transmission, DTN thereby optimizes RF transmit power requirements and decreases operational costs; (5) By providing a standardized, layered, network stack DTN promotes industry-standard application code reuse, with attendant cost reductions, by requiring proper stack modularity; (6) DTN gives earliest possible insight for improved situational awareness for critical ISS management functions, e.g., Inventory Management System (IMS) and Crew HealthCare System (CHeCS); (7) DTN enables improved data throughput during station handovers; maximize link utilization efficiency
The Office of Space Communications and Navigation (SCaN) at NASA Headquarters leads the Delay Tolerant Networking (DTN) investigation with the goal of advancing the maturity and heritage (space flight use) of the DTN communication protocols. Delay tolerant networks make use of store-and-forward techniques within the network in order to compensate for intermittent link connectivity. In the DTN the fundamental concept is an architecture based on Internet-independent middleware where protocols at all layers are used that best suit the operation within each environment, with a new overlay network protocol (bundle protocol) inserted between the applications and the locally optimized communications stacks. Many applications can benefit from the reliable delivery of messages in a disconnected network.
The internet, in contrast, is a connected network where internet protocols, most notably transmission control protocol/internet protocol (TCP/IP), are dependent upon (low) latencies of approximately milliseconds. This low latency, coupled with low bit error rates (BER), allows TCP to reliably transmit and receive acknowledgements for messages traversing the terrestrial Internet. One of the best examples of high latency, high BER links, with intermittent connectivity is that of space communications. One-way trip times, at the speed of light, from the Earth to the moon incurs a delay of 1.7 seconds; while one-way trip times to Mars incur a minimum delay of 8 minutes. The problem of latency for interplanetary links is exasperated with increased BER due to solar radiation. In addition, the celestial bodies are in constant motion, which can block the required line-of-sight between transmit and receive antennas, resulting in links that at best are only intermittently connected. Intermittent link connectivity is commonplace terrestrially as well. One example is the plethora of battery-powered mobile communications devices that go in and out of communication range to wired service interface points and are turned on and off at the users discretion.
Military applications in the DTN arena are substantial, allowing the retrieval of critical information in mobile battlefield scenarios using only intermittently connected network communications. For these types of applications, the delay tolerant protocol should transmit data segments across multiple-hop networks that consist of differing regional networks based on environmental network parameters (latency, loss, BER). This essentially implies that data from low-latency networks for which TCP may be suitable must also be forward across the long-haul interplanetary link. DTN achieves message reliability via employing custody transfer. The concept of custody transfer, where responsibility of some data segment (bundle or bundle fragment), migrates with the data segment as it progresses across a series of network hops is a fundamental strategy such that reliable delivery is accomplished on a hop-by-hop basis instead of an end-to-end basis which is impractical over high latency links. DTN is a set of protocols that act together to enable a standardized method of performing store and forward communications. DTN operates in two basic environments: low-propagation delay and high-propagation delay. In a low-propagation environment such as may occur in near-planetary or planetary surface environments, DTN bundle agents can utilize underlying Internet protocols that negotiate connectivity in real-time. In high-propagation delay environments such as deep space, DTN bundle agents must use other methods, such as some form of scheduling, to enable connectivity between the two agents. The convergence layer protocols provide the standard methods for transferring the bundles over various communications paths. The bundle agent discovery protocols are the equivalent to dynamic routing protocols in IP networks. To date the location of bundle agents, DTN agents, has been managed, analogous to static routing in internet protocol (IP) networks. The security protocols for DTN are important for the bundle protocol. The stressed environment of the underlying networks over which the bundle protocol will operate makes it important that the DTN be protected from unauthorized use, and this stressed environment poses unique challenges on the mechanisms needed to secure the bundle protocol. DTNs are likely to be deployed in organizationally heterogeneous environments where one does not control the entire network infrastructure. Furthermore, DTNs may very likely be deployed in environments where a portion of the network might become compromised, posing the usual security challenges related to confidentiality, integrity and availability.
The DTN protocol suite is still under active development. In addition to network security, research goals for the DTN activity will focus on testing and evolving important network services including naming and addressing, time synchronization, routing, network management and class of service. The DTN experiments on ISS consist of software which is to be placed on both Commercial Generic Bioprocessing Apparatus, CGBA-4 and CGBA-5, and then tested from a ground operations center. This software is not in any critical path of the CGBA operations and may be turned off at anytime. This software does not preclude the use of the CGBA units for other purposes or research support.
As NASA extends its reach to the Moon and beyond, a networked architecture such as DTN will be required to successfully complete these missions. The experiments that will be performed are designed to test the DTN protocol suite in an actual space environment, and to determine how well the protocols perform and what improvements may need to be made. The impact of the results of the research will help to advance the technical maturity of the DTN communications technology so that it is available for NASA use in both human and robotic Exploration missions.
A networked architecture such as DTN is required for future long-duration missions to the moon, asteroids and Mars. Information takes a long time to travel across interplanetary distances, and this can cause interruptions. Even with a good connection between Earth and space-based assets, radiation from the sun or other cosmic sources can interfere with radio signals. Experiments with DTN can help scientists develop Internet-like communications with space probes, rovers, and even remote planetary habitats.
Disruption-tolerant networks improve communications by ensuring no information is lost even when a connection is interrupted. Such networks can improve communication in remote areas, which could benefit the military, disaster-relief efforts, and people living in regions with limited communications infrastructure. The communications methods developed for DTN could be used for mobile phone-based pop-up networks, or MANETs.
DTN will require transmission and receipt of payload telemetry, which includes DTN protocol bundles, from ISS to the remote POCC in Boulder, CO. Automated transmission of uplink commands (ISS S-band uplink) to acknowledge receipt of transmitted DTN bundles at the Boulder POCC will also be needed. CGBA-4 and CGBA-5 are each allocated 500 kbps telemetry downlink.
The compact flash (CF) card swap activity will utilize procedures which have been written, trained and executed already in support of the CGBA investigations.
Jenkins A, Kuzminsky S, Gifford K, Pitts RL, Nichols K. Delay/Disruption-Tolerant Networking: Flight test results from the international space station. 2010 IEEE Aerospace Conference, Big Sky, MT; 2010 March 6-13
Ground Based Results Publications
BioServe Space Technologies
Delay Tolerant Networking Research Group
NASA Image: ISS002E5965 - Astronaut James Voss with CGBA (keypad of payload in upper right) on the ISS in 2001.
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NASA Image: S106E5181 - Astronaut Terrence W. Wilcutt, mission commander, works on the middeck of the Space Shuttle Atlantis performing a daily status check on the Commercial Generic Bioprocessing Apparatus (CGBA).
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