The western Han had two types of armies: the standing army and the frontier army. The standing army consists of an elite cadre of professionals and conscripts that are there for a total of 2 years (1 year training, 1 year in their units). The frontier armies, on the other hand, are fully professional due to their expeditionary nature. Reinforcements from allied nomadic tribes serve to bolster their numbers (similar to Ceasar's campaign in Gaul). The other difference is that the frontier armies tend to consists entirely of cavalry, due to the large distance they cover. These frontier troops closely resemble the horse archer nomads they were fighting.
At the beginning of the Eastern Han, around 24 AD, conscription was entirely abolished. Conscription, itself, was a throwback to the warring states period where large standing armies were required. With the threat coming entirely from the nomads, there was no need to have a system requiring large standing armies. In addition, the Han court were very suspicious of the militarily trained peasantry that can become potential rebels, as clearly proven by Wang Mang's usurpation of the Han throne.
The Eastern Han reached its zenith around the late 1st century AD. It had a professional military unit that are backed by barbarian auxiliary units (in similar fashion to what Rome was doing). These were the military units that were sent west towards Bactria, Sogdiana, and Parthia.
Unfortunately, we currently have no documents that detailed the Han army in the same way the Roman army was described. We have no mention of their small unit tactics. The best sources we have is the vague descriptions from era historians, who were schooled in the pacifist confucian tradition and achaeology.
Han infantry showing their large shields.
Guarding the tomb of Zhu Yafu, a general who served under Emperor Jing, these terra-cotta soldiers stand roughly 20 inches (50 centimeters) tall and compose part of a miniature army 3,000 strong.
The same sheild in bronze, taken from the terrocota army of the Qin.
