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Intro to CS 9-10 / Week 11 / Wednesday
3/6
Week 11 · Systems and the Internet

Wednesday

Beacon Lab: The paper relay
// Packets, paths and a network that keeps working
⏱ about 20 min

Wednesday: Beacon Lab: The Paper Relay

A storm of dust has knocked out the link between Hub North and mission control. The plant team is waiting for tonight's readings.

"Our shortest path is gone," Comet says. "Do we just wait until it is fixed?"

Wren spreads the relay map across the table. "What do you notice? Hub North still has another link."

Nova hovers above Ridge Station. "A network with spare paths can bend without breaking," she says.

Comet grabs a stack of index cards. "Then let's play it out. Every card is a packet, and we are the routers."

Wren writes the first address. "To: Mission Control."

Your mission

You will run the relay network on paper, send a message as packets, break a link, and reroute.

Then you will decide how fault tolerant the relay really is.

  • Five sheets of paper, labeled Greenhouse, Hub North, Hub South, Ridge Station and Mission Control
  • Yarn, string or pencil lines to show the six links from Monday
  • Six index cards or paper scraps to use as packets
  • A pencil
Safety first
Use paper, string and cards only. Keep the string on the table or floor so nobody trips.
  1. Lay out the five station sheets and join them with the six links from Monday's table.
  2. Write a six-word Beacon message, one word per card. On each card add To, From and a number like 2 of 6.
  3. Shuffle the cards. Move each one from the greenhouse to mission control, one link at a time.
  4. At mission control, put the cards back in order using only the numbers.
  5. Now remove the link between Hub North and Mission Control. Send the cards again by another path.
  6. Finally, also remove the link between Ridge Station and Mission Control. Try once more.
  7. Record each round in the table below.
RoundLinks removedA path still exists?Path you used
1none
2Hub North to Mission Control
3Hub North to Mission Control, and Ridge Station to Mission Control
PREDICT BEFORE YOU PLAY
  • Read the question.
  • Tap your answer.
In round 2, which path still reaches mission control?
In round 3, can any packet reach mission control?
Packets took different paths in round 2. What keeps the message readable?

Redundancy and fault tolerance

Redundancy means including extra parts that can take over if other parts fail.

One way to build network redundancy is to have more than one path between any two connected devices.

If a device or link on the Internet fails, later data are sent by a different route, if one is possible.

A system that keeps working when parts fail is called fault tolerant. Redundancy often needs extra resources, but it gives that fault tolerance.

What the lab showsTrue or false?
Removing one link did not stop the message in round 2.?
The relay is fault tolerant for any two broken links.?
Extra links are an example of redundancy.?
Redundancy usually needs no extra resources at all.?
WHY THIS EXERCISETesting broken links shows exactly where a network is strong and where it is weak.
Extra parts that can take over when others fail are called ____.
A system that keeps working when parts fail is fault ____.
In round 3, which station has lost every one of its links? Type its name.
WHY THIS EXERCISEA device with only two links becomes a weak spot when both fail.
On paper, sketch the relay map with one new link that would keep round 3 working. Mark your new link clearly.

Great relay work. Tomorrow you will look inside the layers of a device.

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