The Yam communication system has existed in some form or another since the founding of the Mongol Empire; however, it has evolved and changed significantly over the centuries.
Hordes (584AH - 683AH)Edit
In the original expansion of the Mongol Empire there was little need for speed in communication because there was very little distance to cross, and messages would generally travel around as part of the nomadic hordes. This communication system is not to be underestimated, though.
Many of the Mongol's enemies were amazed to discover that Mongol hordes could travel over 500 li per day.
Messangers (683AH - 1168AH)Edit
Messangers have been used the longest by the Mongol Empire and the system remained unchanged for over 485 years. The messanger system was introduced by Great Khan Temur in 683AH as the Mongol Empire began to expand to a point at which communication times became intolerable. Hordes have to sleep, rest, carry heavy equipment, move at a steady pace (as not to tire their mounts) and have other tasks to perform; this leaves a lot of room for improvement in making a faster messanging system.
Great Khan Temur had stables placed every 75 li on important roads between Khanates (the Silk Road between Great Khan and Ilkhanate being a prime example). This meant that messangers could swap their mounts every 75 li, and therefore constantly travel at full gallop, and also meant that they didn't need to carry any equipment or supplies, which would weigh down the mount. Similarly, when messagners tired they could simply stop at the next stable and transfer the message to another messanger; this way the message was constantly moving at full gallop.
This improved Yam system allowed messages to travel 1500 li (10 tu) per day. That's 3 times faster than before!
Semaphore Telegraph (1168AH - Present)EditThe Semaphore Telegraph is a fairly recent development that, despite being invented in 1168AH, took many years to implement. The Semaphore Telegraph was inspired by the old flag signals that were used by Noyan in battles to direct soldiers from far-off vantage points; a series of towers are built along the usual Yam routes so that each tower can see at least two others. Each tower has a tall pole sticking out of its top with two long, thin, sails; the positions of the sails are changed by pulleys and different positions represent different letters or phrases.
The building of these towers across the empire took many years, but the time for completion was shortened by the fact that many of the stables already used in the Yam system could be adapted to build Semaphore Telegraph towers. The project proved very expensive but was worth it for the dramatic reduction in communication times.
Messages could travel down a chain of towers 28.8 tu in an hour. That's almost 70 times faster than messangers could carry a message!