computer science

SpaceWire

SpaceWire is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand SpaceWire rather than just read about it. In short: SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency (ESA) in collaboration with international space agencies including NASA, JAXA, and RKA.

Key takeaways

SpaceWire belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SpaceWire to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SpaceWire from memory before moving on to harder problems.

Reference excerpt

SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency (ESA) in collaboration with international space agencies including NASA, JAXA, and RKA. Within a SpaceWire network the nodes are connected through low-cost, low-latency, full-duplex, point-to-point serial links, and packet switching wormhole routing routers.

Architecture

Physical layer SpaceWire's modulation and data formats generally follow the data strobe encoding - differential ended signaling (DS-DE) part of the IEEE Std 1355-1995. SpaceWire utilizes asynchronous communication and allows speeds between 2 Mbit/s and 200 Mbit/s, with an initial signalling rate of 10 Mbit/. . DS-DE is well-favored because it describes modulation, bit formats, routing, flow control, and error detection in hardware, with little need for software. SpaceWire also has very low error rates, deterministic system behavior, and relatively simple digital electronics. SpaceWire replaced old PECL differential drivers in the physical layer of IEEE 1355 DS-DE by low-voltage differential signaling (LVDS). SpaceWire also proposes the use of space-qualified 9-pin connectors. SpaceWire and IEEE 1355 DS-DE allows for a wider set of speeds for data transmission, and some new features for automatic failover. The failover features let data find alternate routes, so a spacecraft can have multiple data busses, and be made fault-tolerant. SpaceWire also allows the propagation of time interrupts over SpaceWire links, eliminating the need for separate time discretes.

Link layer Each transferred character starts with a parity bit and a data-control flag bit. If data-control flag is a 0-bit, an 8-bit LSB character follows. Otherwise one of the control codes, including end of packet (EOP).

Network layer The network data frames look as follows:

One or more address bytes are used for the routing. Addresses are either physical ones (0–31), or logical ones. The difference is that the physical addresses are deleted from the frame header during routing - which is used for hop-based routing (based on path specified in the frame itself). Logical addresses may be deleted as well, depending on the router configuration.

Interconnection The hardware devices may be connected either directly, or via a SpaceWire router. In the former case, usually pairs of devices are used to guarantee a fail-safe operation, which is handled by the software. A SpaceWire router is usually a crossbar switch-type device, operating in wormhole switching mode. This also may limit the speed of the communication to the lowest common speed. The routing decisions are based on the programmed routing table and the initial incoming frame contents.

Uses SpaceWire is used all around the globe. Its use began primarily in ESA projects, but it is currently used by NASA, JAXA, RKA, and many other organizations and companies. Some NASA projects using it include the James Webb Space Telescope, Swift's Burst Alert Telescope, the Lunar Reconnaissance Orbiter, LCROSS, the Geostationary Operational Environmental Satellite (GOES-R), and the SCaN Testbed, previously known as the Communications, Navigation, and Networking Reconfigurable Testbed (CoNNeCT). It has also been selected by the United States Department of Defense for Operationally Responsive Space. SpaceWire initiatives are being coordinated between several space agencies in the frame of Consultative Committee for Space Data Systems (CCSDS) in order to extend its communication model to the network and transport layers of the OSI model. SpaceWire supports highly fault-tolerant networks and systems, which is one reason for its popularity.

Protocols The ESA has a draft specification in place for the Protocol ID. The following Protocol ID's have been assigned in ECSS-E-ST-50-51C:

References

Other sources ECSS-E-ST-50-12C - SpaceWire - Links, nodes, routers, and networks, ESA-ESTEC. ECSS-E-50-12A (SUPERSEDED only document number has been changed to ECSS-E-ST-50-12C) SpaceWire - Nodes, links, and networks, ESA-ESTEC. ECSS-E-ST-50-11C Draft 1.3 "Space engineering - SpaceWire protocols"

External links Official website

Worked examples

Example 1 — a first encounter with SpaceWire

Start with the simplest possible case. Write down what SpaceWire claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SpaceWire before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SpaceWire ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SpaceWire

In research: SpaceWire appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SpaceWire in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SpaceWire is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer buses, ECSS standards, Fault-tolerant computer systems, so understanding it makes those chapters shorter.
In everyday life: Look for SpaceWire outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study SpaceWire in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SpaceWire means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SpaceWire out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SpaceWire in simple terms?

Why does SpaceWire matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SpaceWire?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SpaceWire.