The lowest layer of our Internet Architecture is the Link layer. We call it the “lowest layer” because it is closest to the physical network media. Often the Link layer transmits data using a wire, a fiber optic cable, or a radio signal. A key element of the Link layer is that usually data can only be transmitted part of the way from the source computer to the destination computer. Wired Ethernet, WiFi, and the cellular phone network are examples of link layers that can transmit data about a kilometer. Fiber optic cables, particularly those under the oceans, can transmit data up to thousands of kilometers. Satellite links can also send data over long distances.
Regardless of the distance we can send the data, it is still traveling over a single link, and to reach the ultimate destination computer requires forwarding packets across multiple links. In this section we will look at how one of the most common link layers functions in some detail. WiFi is a great way to look at many issues that must be solved at the link layer.