Does the Backlog Fit the Contact Window?

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

Does the Backlog Fit the Contact Window?

Two elephant collars brush past each other for just 30 seconds, and at 250 kbps that contact can move 937.5 KB — yet the backlog waiting to cross is only 250 KB, a collar’s own 50 KB five-day log plus about 200 KB of relayed data. The whole store-carry-forward promise rests on that one inequality. This audit re-derives the capacity, payload, and transfer time and asks whether the backlog really fits the contact window, and by how safe a margin.

Companion to the chapter Real-World DTN Deployments — every number here comes from that chapter.

Ada: The whole store-carry-forward promise here rests on one inequality: the buffered payload must fit inside a brief encounter. This chapter’s contact-window rule is C = R x t, and it claims a 30-second collar meeting at 250 kbps moves 937.5 KB while the payload needs only 250 KB. Let me check both, using the chapter’s decimal convention throughout (1 kbps = 1000 bit/s, 1 KB = 1000 bytes).

  • Contact capacity: 250,000 bit/s x 30 s = 7,500,000 bits = 937,500 bytes = 937.5 KB.
  • Payload to move: own five-day backlog 10 KB/day x 5 = 50 KB, plus about 200 KB of relayed data, so 50 + 200 = 250 KB.
  • Transfer time: 250 KB x 8 bit/byte / 250,000 bit/s = 2,000,000 / 250,000 = 8.0 s.
  • Contact margin: 30 - 8 = 22 s left for setup, handshake, and retries.
  • Window used: 250 / 937.5 = 0.2667 = 26.7% of the capacity a full 30-second contact offers.
  • Short-contact stress test: even a 10-second brush carries 250,000 x 10 / 8 = 312,500 bytes = 312.5 KB, still clearing the 250 KB payload.

The audit confirms the design claim and shows why it is robust rather than lucky: the backlog consumes about a quarter of the window, so a contact cut to a third of its planned length still delivers everything. In DTN sizing the margin, not the headline capacity, is what turns a chance encounter into a dependable delivery opportunity.

Every number above is taken from the chapter’s own material and re-derived step by step.