This document explains only the OSI-model in a VERY basic way, so it's easy to understand for newbies. If you want to understand the OSI-model completely you'll have to read some more tutorials, RFC's and books that you can find on the internet. Well in the past, communications between computers from different vendors were difficult, because they used different protocols and data-formats. Physical connections. . So there is a huge difference between the Physical layer and the Application layer.
Binary data is encoded into impulses suitable for transmission over the network medium.
The primary task of the network layer is to provide the routing functionality by which packets can be sent across the boundaries of the local network segment to a destination that may be located on an adjacent network or on one thousands of miles away. IP, Internet Protocol, is mostly used for this task. Another protocol on the Network layer is IPX, Internet Packet eXchange. These protocols have been imported at the Netware Operating System. The Transport layer is the centre of the OSI-model.