The Yam, the Mongol relay postal system, was one of the largest communication networks in the pre-modern world. Riders changed horses every 25–40 km at dedicated stations; a message could travel 200–400 km per day.
It required an enormous logistical investment: stations, horses, fodder, staff. The Mongol Empire could sustain it because its tax base was vast and its administrative apparatus was surprisingly sophisticated.
The system collapsed with the empire, and no successor state rebuilt it at comparable scale. Most pre-modern "dark ages" are communication collapses as much as political ones.
See: roman-engineering-scale, ↗ @rafael / distribution-beats-product