Which OSI layer cannot be protected by traditional firewalls?

The physical layer of the OSI model is responsible for the actual physical connection between devices, including the transmission of raw bits over a physical medium like cables or radio waves. Since traditional firewalls operate mainly at higher layers of the OSI model—such as the network, transport, and application layers—they focus on filtering traffic, managing sessions, and controlling applications.

Traditional firewalls analyze data packets based on protocol rules, source and destination IP addresses, and application-level information, but they do not have the ability to manage or protect the physical transmission of data itself. Issues like electromagnetic interference, physical access to the hardware, and damage to transmission media fall outside the capabilities of firewalls, which is why the physical layer cannot be protected by them.

This distinction highlights the importance of implementing additional security measures specific to physical security, such as secure facilities, surveillance, and hardware redundancy, which cannot be addressed through the traditional firewall's functionality.