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 email@example.com.
- Free Today: September Issue of Linux Journal (Retail value: $5.99)
- The Tiny Internet Project, Part I
- Bitcoin on Amazon! Sort of...
- Download "Linux Management with Red Hat Satellite: Measuring Business Impact and ROI"
- Android Browser Security--What You Haven't Been Told
- Returning Values from Bash Functions
- Nativ Disc
- Epiq Solutions' Sidekiq M.2
- Securing the Programmer
Pick up any e-commerce web or mobile app today, and you’ll be holding a mashup of interconnected applications and services from a variety of different providers. For instance, when you connect to Amazon’s e-commerce app, cookies, tags and pixels that are monitored by solutions like Exact Target, BazaarVoice, Bing, Shopzilla, Liveramp and Google Tag Manager track every action you take. You’re presented with special offers and coupons based on your viewing and buying patterns. If you find something you want for your birthday, a third party manages your wish list, which you can share through multiple social- media outlets or email to a friend. When you select something to buy, you find yourself presented with similar items as kind suggestions. And when you finally check out, you’re offered the ability to pay with promo codes, gifts cards, PayPal or a variety of credit cards.Get the Guide