What is the situation of the Silk Road between the 1st century? BC and the 1st century. ap.?

What are the great empires on the land part of the Silk Road?
Roman Empire
Parthian Empire
Kushan Empire
Han Empire

What wealth is transported and where does it come from?

The main resources come from the Orient

Silk (For the Romans following the Battle of Carrhes? from China)
Lapis lazuli (for blue paint (mosaic, wall paint)
Horses (AC for China)

The Silk Road and the kingdoms that control it are constantly attacked by nomads from the northern steppes. For this reason caravans must protect themselves against these attacks.

Later, during the Islamic era, caravanserais were set up to protect them. But empires have an interest in protecting these trade routes which earn them customs taxes and allow traders to work.

Further south, the spice and herbs route passes through Arabia and heads to India by sea.