Linux Out of the Real World
Two pairs of processes want to communicate between the orbiting experiment and the ground side support computer. The main control process in the experiment communicates with a recorder/data display/remote control program on the support computer, using several different types of data (events-log, sensor readings, and control-parameter settings going down; control and meta-control commands going up). Another pair of processes mirror directories from the payload to ground side, in order to send data that was buffered before communications were established. Images from the frame grabber are sent using this method.
In this situation, it is natural to want a packet-switching, multiplexing communication system with an interface available to many processes. Since the channel is unreliable, you want some type of validation of received data. The usual networking code is not usable, since there are no packet-drivers for the interfaces NASA presents. In the interest of simplicity and code reusability, we chose to implement a modular user-space communication system.
We wanted the communications interface presented to our communicating processes to be identical on both machines (experiment and ground side support computer). Since these two machines need to talk to different hardware and software interfaces, we abstracted the NASA interfaces from the multiplexer. Between the multiplexer and NASA sits a process that performs the packetizing and unpacketizing they want us to do (possibly fragmenting and defragmenting multiplexer packets) and relays the data. The end result is that the payload and the ground side support computer can communicate in a UDP-like fashion. Multiplexer packets sent are guaranteed to arrive intact and correct or not at all. It's a miniature networking stack in user land.
When the old DOS version of the payload flew in 1996, we rented space in SpaceHab. SpaceHab is a privately owned company that rents a large volume in the Shuttle payload bay and some services (power, communications, etc.) from NASA, and then turns around and rents smaller quantities of volume and services to experimenters. The economic relationship between the three parties (NASA, SpaceHab, experimenter) in this situation defy comprehension by the author. Anyway, SpaceHab provides a significantly more functional communications interface, called the Serial Converter Unit (SCU). Sure, it's still a 9600 bps serial line, but the SCU has (angels sing) flow control.
Sebastian Kuzminsky is an undergraduate in Computer Science and Applied Mathematics at the University at Colorado at Boulder. If space flight work were not so fun and time consuming, he would have been a graduate by now. Questions about PGBA and other Bioserve payloads are welcome. He can be reached via e-mail at firstname.lastname@example.org.
Getting Started with DevOps - Including New Data on IT Performance from Puppet Labs 2015 State of DevOps Report
August 27, 2015
12:00 PM CDT
DevOps represents a profound change from the way most IT departments have traditionally worked: from siloed teams and high-anxiety releases to everyone collaborating on uneventful and more frequent releases of higher-quality code. It doesn't matter how large or small an organization is, or even whether it's historically slow moving or risk averse — there are ways to adopt DevOps sanely, and get measurable results in just weeks.
Free to Linux Journal readers.Register Now!
- Three More Lessons
- Django Models and Migrations
- August 2015 Issue of Linux Journal: Programming
- Hacking a Safe with Bash
- Secure Server Deployments in Hostile Territory, Part II
- The Controversy Behind Canonical's Intellectual Property Policy
- Huge Package Overhaul for Debian and Ubuntu
- Shashlik - a Tasty New Android Simulator
- Embed Linux in Monitoring and Control Systems
- General Relativity in Python