Future space networks will be a combination of private industry, government, and academia that will be adaptable and extensible to accommodate dynamic nodes. The Communication Technology Division’s Network Emulation Laboratory (NEL) at the NASA Glenn Research Center is emulating different architectures, based on Space Communication Architecture Working Group studies, and technologies to determine how they will impact future net-centric architectures and communications. The emulations revolve around a dynamic channel emulator (CE), which provides dynamic space-based data link characteristics during the execution of the scenarios.
The channel emulator with two channels configured using virtual interfaces. NIC, network interface card.
The CE is a realistic, flexible platform that functions as a Level 2 link-layer 802.3 bridge, which allows it to accommodate a number of protocols in addition to the Internet Protocol. To apply emulated network attributes, the CE configures the queuing discipline (qdisc) on the egress queue of each interface, where packet delay, loss, corruption, and duplications are set. The CE also incorporates the 802.1q virtual (tag-based Virtual Local Area Network (VLAN)) interface, which creates virtual interfaces to permit the partitioning of a single pair of networking interfaces into many that represent physical devices on a satellite. The current version is distributed on a Knoppix LiveCD to simplify the installation and setup procedures.
The CE currently provides the following capabilities:
Delay: The quantity of time that a packet should be held at the CE before it is transmitted to the receiver; for each time interval, the delay represents a static quantity. The CE can also introduce a dynamic delay, which changes with respect to time (i.e., the delay is static at each time interval). Dynamic delays permit the emulation of an object approaching or receding from another object, such as the Crew Exploration Vehicle approaching the Moon. Delays have been validated for the Moon (3.4-sec round trip) and Mars (20-min round trip).
Jitter: The CE supports jitter, which is a variable amount of time that the packet is held at the CE before it is transmitted to the receiver; the additional delta time is added to the delay. Jitter can result in packet reordering, since each packet is held for a random amount of time; and conversely, when jitter is not implemented, the emulator should only produce a constant delay.
Stream mangling—packet corruption (bit errors), loss, and duplication: The CE can flip a random bit in a random packet at a configurable statistical rate that is sent to the receiver in that condition to force it to compensate for the error. The emulator must also be able to drop packets at a configurable rate, forcing the receiver to detect the missing packet by relying on packets already received.
Maximum throughput: Currently, the CE can support a maximum throughput of 622 Mbits/sec. Since potential throughput is primarily a hardware limitation, the bandwidth delay product dictates how much memory will be required to achieve the maximum throughput.
Midscenario attribute changes: The CE can process dynamic midscenario changes, where objects are moving and getting closer or farther apart, gaining or losing visibility, or changing weather conditions, without disrupting active data streams.
Remote configurability: The CE provides remote configuration through simple remote shell sessions and a programmatic interface. The CE supports SSH (Secure Shell) and Web access for direct user interaction and Simple Object Access Protocol (SOAP) and Web Services Description Language (WSDL) for programmatic functions.
Data collection: The CE uses the Linux ulog facility to collect packet headers and counters for both real-time and post-scenario analysis and visualization.
One-way communication: The CE can mimic one-way communications by ignoring packets that are directed to it from the receiver’s side. It is not uncommon for a receiver in space to be unable, by design, to reply back to a sender.
Serial connections: The CE provides a unique ability to connect to a satellite modem via an RS-422 serial port rather than an Ethernet connection.
Find out more about the research of Glenn’s Satellite Networks and Architectures Branch: http://ctd.grc.nasa.gov/organization/branches/snab/snab.html
Glenn contact:
Richard A. Slywczak, 216-433-3493, Richard.A.Slywczak@nasa.gov
Authors:
Richard A. Slywczak and Thaddeus J. Kollar
Headquarters program office:
Exploration Systems Mission Directorate
Programs/projects:
Crew Exploration Vehicle, Life Support & Habitation, Next Generation Launch Technology, Return to Flight, Vehicle Systems, Satellite Missions
Last updated: December 14, 2007
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