one-way fiber propagation floor (SMF-28, ~4.9 µs/km); ~10 ms RTT per 1,000 km: the floor for standard solid-core fiber, while hollow-core routes run about a third lower
~5 µs/kmobserved
| Value kind | observed — Reported measurements, counts, and specifications keep the precision and scope stated by their source; an exact specification is not treated as a range. |
|---|---|
| Scope | Standard solid-core single-mode fiber (group index ~1.47). Hollow-core fiber guides light through air at roughly 3.4 µs/km; Microsoft has run it on some Azure routes since 2023 (Azure Networking blog, 2025-09-23), but it is a route-specific build, not something to assume on leased fiber. |
| As of | 2025 |
| Source | M2 Optics |
| Review | checking…review by 2026-11-23 · standard cadence |
| Recorded changes | last 2026-07-14 · 2 revisions tracked |
| Claim id | one-way-fiber-propagation-floor-smf-28-4-9-s-km |
Where the guide uses it
← Full numbers register — every date-stamped figure in the guide, with revision history.