Telephone Company Data CircuitsAs with any network, a WAN requires a medium to transmit data from "here" to "there". By definition, this medium must be able to transmit long distances. The medium could be a long copper or fiber optic cable, it could be a radio or satellite transmission, or it could be something really strange like a laser relay or a carrier pigeon (See RFC 1149).
However, even the simplest of these is more complex than we might imagine when travelling over long distances. For instance, a copper cable has a significant signal loss over distance, there is a significant cost to laying the cable, a significant number of permits or right-of-ways must be obtained, and the cable must be maintained against physical damage.
Most of us choose to buy our WAN medium from telephone companies, who specialize in this sort of thing. They save a significant amount of money by doing everything in large quantities. We benefit from those cost savings, and the scope of our WAN project is significantly reduced.
Telephone companies offer a range of services to handle the needs of a WAN. This section will attempt to describe those most commonly used by an ISP and show how they fit into the puzzle.