RTT · Round-Trip Time
The there-and-back network delay; it sets the interactive-inference latency budget and, between sites, decides whether a training run can remain synchronous at all.
Current numbers
~1,000–1,500 kmpractical sub-50 ms inference reach (50 ms RTT ⇒ ~5,000 km propagation-only; switching/queuing/compute overhead and the ~1.3–1.5× fiber route factor cut it to practical reach)
1,024max accelerators per UALink 200G 1.0 domain; <1 µs RTT on <4 m reach, 200 Gb/s/lane
~5 µs/kmone-way fiber propagation floor (SMF-28, ~4.9 µs/km); ~10 ms RTT per 1,000 km — the latency physics scale-across cannot engineer around