Some general terminology is now in order. Links that can be used in both directions
at the same time, like a two-lane road, are called full-duplex links. In
contrast, links that can be used in either direction, but only one way at a time, like
a single-track railroad line. are called half-duplex links. A third category consists
of links that allow traffic in only one direction, like a one-way street. They
are called simplex links.