Developed by International Organization for Standardization or ISO, Open Systems Interconnection model or OSI model is a critical building block in networking. It helps in troubleshooting and understanding networks because of the layered approach that it follows; the various layers being:
(i) Physical layer (i) Data Link layer (ii) Network layer (iii) Transport layer (iv) Session layer (v) Presentation layer (vi) Application layer
These layers are given bottom to up as following below.
- Provides standards and interoperability.
- Split development(a person working in layer 3 need not be concerned with layer 7).
- Quicker development (as each layer is independent of the other, development in an OSI model is faster as compared to the old proprietary models).
OSI model was used for connectionless protocols like CLNS and CLMNP; but with the advent of TCP (connection oriented protocol) a new model; i.e., TCP/IP model came into play. In this model, the Application, Presentation and Session layers of OSI model were combined to form the Application layer in the TCP/IP model and the Datalink and Physical layers in the OSI model were combined to form the Network access layer in the TCP/IP model and the Internet layer in the TCP/IP model was the equivalent of Network layer in OSI model.
In the real world, we use a mix of both the OSI model and the TCP/IP model, called the Hybrid model. In the Hybrid model, the Application layer is a combination of layer 7, layer 6 and layer 5 of OSI model (similar to TCP/IP model). The remaining layers (layer 1, 2, 3 and 4) are the same as the OSI model.
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