Following is the diagram of my company's network and the infrastructure consists of

• Domain Controller
• Additional Domain Controller
• Local Web Server
• Proxy Server
• 100 workstations located in 8 different departments.
• 9 16-port unmanaged switches (Model: D-Link DES-1016D). one for each department and one for main connection to the servers
• A class C network of 192.168.1.x    IP scheme
• Star topology

How can i imporve this network?