Although two nodes are physically connected by a transmission medium such as cable or air, we need to remember that the data-link layer controls how the medium is used. We can have a data-link layer that uses the whole capacity of the medium; we can also have a data-link layer that uses only part of the capacity of the link. In other words, we
can have a point-to-point link or a broadcast link. In a point-to-point link, the link is
dedicated to the two devices; in a broadcast link, the link is shared between several
pairs of devices. For example, when two friends use the traditional home phones to
chat, they are using a point-to-point link; when the same two friends use their cellular
phones, they are using a broadcast link (the air is shared among many cell phone users).