The International Standards Organization (ISO) developed the seven-layer Open Systems Interconnection (OSI) telecommunications model in the early 1970s. Since then, the ISO-OSI model has become the standard used for most major data communications standards. The model breaks up information into seven stacked layers, the top of which is the most sophisticated applications layer and the bottom-most of which is the physical layer. Different devices, with different goals, work at different layers of the OSI model (Leibson 41).
Starting from the bottom up, the layers are: physical, data link, network, transport, session, presentation and application (Hasib 15). The top three layers (application, presentation and session) are application oriented, the middle layer (transport) is transitional, and the three lowest levels (network, data link and physical) are network dependent.
The three lowest layers are where fundamental communication activities take place. The physical layer is where raw streams of bits are sent back and forth. The data link layer sends messages out in frames with a character count specified in the header, and with an identifying special character at the end. Flow control at this layer synchronizes message activity, and the sender waits for permission from the receiver before transmitting. This is an inefficient method for sending large messages because of the permission aspect (Hasib 19).
The network layer is used for wide area networks, but is redundant for local area networks. This layer routes network connections, and establishes/releases those connections. This is also the first layer that supports a defined unified address (Hasib 18).
The transitional transport layer provides site-to-site communication.
Complex processes take place at the three topmost layers. The session layer provides process-to-process communication through buffering, and dialog synchronization such as synchr
...