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Line drivers are boxes which more or less boost the signal. They make it possible to exceed the distance limit of a certain interface. The way they accomplish this is multiple.
Are all examples of line drivers. To exceed e.g. the distance of a RS232 interface, which is 15 meters, you could convert it into RS422 and go 1200 meters.
Another possibility is that the conversion of a known interface is not into another known one, but one a manufacturer has invented himself. This way the product is not a real interface converter, nor is it a modem. These products are actually the real line drivers. You always need two boxes. One on the local side and the other on the remote side. Inbetween can be copper or fiber.
These line drivers excist in various types (even if do not take the speed they can handle into account.
These devices draw the power from the interface. The benefit is that you don't have a bulky power supply and that you can use them all over the world without worrying about the power outlet. The withdraw is that an interface has to provide enough power to make the line driver work.
Often there is also the problem of having not the right pins. When a line driver e.g. draws it's power from a RS232 interface pins 4 and 5 and you only have a 3 wire cable (meaning pins 2, 3, 7) your line driver will not work.
These units have an external or internal power supply. These power supplies are directly connected to a wall outlet providing 230 Vac or 115 Vac to the unit. This makes them country specific. An US version can not be used in Europe and vice versa. The benefit is that these units are always powered correctly, this in contrary to the non-powered units.2-wire
Between a local and remote line driver you only need two wires. Most of the time this has to be UTP (Unshielded Twisted Pair) although line drivers exist that can handle ordinary telephone wire.4-wire
4-wire line drivers are mostly cheaper than 2-wire units. The reason for this is that with four wires two can be used for transmit and two can be used for receive. With a 2-wire unit full duplex is more difficult.Fiber
What goes for two and four copper wire line drivers goes exactly the same for duplex and simplex fiber. It is possible to use just one fiber or a fiber pair. The reason why people use fiber line drivers instead of copper based drivers is because fiber is immune for EMI/RFI. This makes fiber line drivers suitable for very long distances and outdoor use.