The extender forwards traffic between LANs transparent to higher network-layer protocols over distances that far exceed the limitations of standard Ethernet.
Copper-based Ethernet extenders must be used on unconditioned wire (without load coils), such as unused twisted pairs and alarm circuits.
If the buildings are within 100m, a normal Ethernet cable segment can be used, with due consideration of potential grounding problems between the locations.
Ordinary ADSL modems cannot be connected back-to-back, because the ATU-R (ADSL Termination Unit - Remote) units that are used by customers require specialized ATU-C (Central Office) support provided by phone company equipment, usually by a complex and expensive DSLAM (DSL access multiplexer).
However some symmetric digital subscriber line (SDSL) modems such as the SpeedStream 5851 can be connected back-to-back, allowing upload and download speeds of about 2 Mbit/s over substantial distances, using a simple twisted pair of wires.