A system and method for the transmission of optical signals via optical fibers in a building provided with a system of conduits for electric wires. The optical fibers are ducted through the same conduits as the electric wires. The optical fibers are fitted to the same wall or ceiling terminals as the electric wires. The terminals can comprise connectors to which the optical fibers are fitted, or optical transceivers. The terminals can also comprise electrically supplied modules for amplifying, processing, routing or converting the optical signals. Monitoring, controlling, processing etc. of the optical signals can be done in a central element, in the proximity of a central module for monitoring etc. the electric energy in the building.