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Lab 4 — Satellites: handover and a failed link

Goal. See how a constellation's links change on a schedule, and how ant routing repairs a failure on a regular grid. Time. 60 min. Site. Campaign levels 5.1 and 5.3 (#campaign?open=all).

← Classroom pack · Answer key

Background (5 min)

In a low-Earth-orbit shell, each satellite links to four neighbours: two in its own orbital plane and one in each adjacent plane (the "+grid"). A ground station sees a satellite only while it is more than 25° above the horizon, then hands over to the next one. Unlike a MANET, these changes follow a schedule fixed by the orbits.

1. Predict (5 min)

  1. The teaching shell is 8 planes × 8 satellites, with a 120 s orbit. Will delivery between two ground stations be steady, random, or periodic? If periodic, with what period?
  2. In level 5.3, the three inter-satellite links the call uses fail at t = 40 s. What happens to delivery?

2. Run (25 min)

  1. Open the classroom link and play 5.1 First contact. Select a ground station, watch which satellite it talks to, and answer the quiz.
  2. Play 5.3 The failed link. Note the delivery shown every 10 s up to 120 s, and answer the quiz.

3. Explain (15 min)

  1. Delivery in 5.3 rises and falls on a cycle. Find the period from your notes. Why is it half the orbital period? Hint: think about the shell's symmetry.
  2. How long did the dip after the failure last? What did the ants do in that time?
  3. On a +grid, how many paths of equal hop count usually join two satellites? What does that mean for repair?

4. Compare with ns-3 (10 min)

The ns-3 ISL-grid failure cell (isl-grid.md) cuts a link mid-flow and times the reconvergence. AntHocNet reconverges in 0.111 s, at the oracle's floor. AODV takes 0.268 s, and OLSR 4.245 s. 1. Why is OLSR so much slower to reconverge here? 2. AntHocNet's detection time is 0 on every seed. Which mechanism makes it instant on an ISL? Hint: the interface goes down.