• The Physical Layer transforms data into bits that are sent across the physical media.
• The Data Link layer determines access to the network media in terms of frames. Its Media Access Control (MAC) sub layer is responsible for physical addressing.
• The Network Layer routes data through a large network.
• The Transport Layer provides end-to-end, reliable connections, often in terms of segments [Book02].
• The Session Layer allows users to establish connections using intelligently chosen names in packets.
• The Presentation Layer negotiates data exchange formats, also in terms of packets.
• Finally, the Application Layer provides the interface between the user's application and the network through messages.
OSI Model
• OSI model is just a guideline for protocol design, not the actual protocols
• Not all layers are always used
o Internet uses only five layers
• Some layers may be combined together
o Top three layers are normally combined into one layer .
Why a OSI model?
• Idea was originally to get a message across different networks.
• By layering, each layer performs a separate function. Makes changes and modifications easier. Change of lower layers does not affect higher layers as long as their interfaces are the same.
• Higher layers deal more with end-to-end communications, user services and applications .
• Lowest three layers deal primarily with the details of data transmission in networks.
• Each layer offers certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.
The Open Systems Interconnect (OSI), established in 1984 by the ISO (International Standards Organization), divides network functions into seven layers: Physical, Data Link, Network, Transport, Session, Presentation and Application Protocol.
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