Engineering Tutorials

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.