Lesson 2: OSI Reference Model

The Layered Model | Physical & Data Link Layers | Network Layer | Transport, Session, Presentation, and Application Layers

Why a Layered Network Model.........?

That’s essentially the same thing that goes in networking with the OSI model. This image illustrates the model.

So, why use a layered network model in the first place? Well, a layered network model does a number of things. It reduces the complexity of the problems from one large one to seven smaller ones. It allows the standardization of interfaces among devices. It also facilitates modular engineering so engineers can work on one layer of the network model without being concerned with what happens at another layer. This modularity both accelerates evolution of technology and finally teaching and learning by dividing the complexity of internetworking into discrete, more easily learned operation subsets.
Note that a layered model does not define or constrain an implementation; it provides a framework. Implementations, therefore, do not conform to the OSI reference model, but they do conform to the standards developed from the OSI reference model principles.

Devices Function at Layers

Let’s put this in some context. You are already familiar with different networking devices such as hubs, switches, and routers. Each of these devices operate at a different level of the OSI Model.
NIC cards receive information from upper level applications and properly package data for transmission on to the network media. Essentially, NIC cards live at the lower four layers of the OSI Model.
Hubs, whether Ethernet, or FDDI, live at the physical layer. They are only concerned with passing bits from one station to other connected stations on the network. They do not filter any traffic.
Bridges and switches on the other hand, will filter traffic and build bridging and switching tables in order to keep track of what device is connected to what port.
Routers, or the technology of routing, lives at layer 3.
These are the layers people are referring to when they speak of “layer 2” or “layer 3” devices.
Let’s take a closer look at the model.

Host Layers & Media Layers

Host Layers :-

The upper four layers, Application, Presentation, Session, and Transport, are responsible for accurate data delivery between computers. The tasks or functions of these upper four layers must “interoperate” with the upper four layers in the system being communicated with.

Media Layers :-

The lower three layers – Network, Data Link and Physical -- are called the media layers. The media layers are responsible for seeing that the information does indeed arrive at the destination for which it was intended.

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