Both ends of the network cable need to be installed with lightning protection devices, because if an induced lightning surge occurs on the network cable, the equipment at both ends of the network cable may be damaged. If only one end of the network lightning protection device is installed, only this end can be protected. The equipment at one end cannot be protected.
For typical applications such as outdoor surveillance cameras, a network cable connects the camera and the switch (optical transceiver). If only one network lightning protection device is installed here, the switch's network port can only be protected against lightning.
If you want to protect both ends, you need to connect a network lightning protection device in series at both ends, and the output end of the lightning protection device faces the switch and the camera.