Understanding OSI

OSI model is also known as Open Systems Interconnection model is a standard in networking to create other standards of networking domain. You can take it as a framework to create networking standards.

When we talk about OSI model, a basic diagram as shown below comes to our mind which tells about various layers that are included in the OSI model.

basics_osimodel

Understanding of OSI model really helps in troubleshooting the network issues. Each layer of OSI model holds significant importance and defines its own set of protocols. Various devices being used in the networking domain may fall under one of the layers of OSI model.

Knowledge of OSI model helps to identify as in which stage of network transmission an issue occurs. When dealing with layers, you usually call them with numbers i.e. layer 1, layer 2 etc.

Layer Number Layer name To remember in Order
Layer 1 Physical layer Please
Layer 2 Data Link layer Do
Layer 3 Network layer Not
Layer 4 Transport Layer Throw
Layer 5 Session Layer Sauce
Layer 6 Presentation Layer Pizza
Layer 7 Application Layer Away

 

Today in this tutorial we are going to understand the OSI model and its layers with some practical examples. To start with consider below-mentioned diagram.

network_OSI

Let’s say I am using a banking application and I want to transfer 50 bucks to some other account. In that case, let’s see how this all process takes place and how different layers are involved in the same.

Layer 7 (Application Layer):- All applications that are running on the system and interact with the network comes under the layer 7 for example for example if you are playing counter strike game, then it comes underlayer 7.In the same way, the banking app that I am using to do the transaction falls under the layer 7.It interacts with the internet using some API’s which are themselves taken care by the OS on which this app is running.

Layer 6(Presentation layer):- Before sending the data on to the network, it shall be well formatted as per the standards defined for that data. For example, if I am trying to send an image may be jpeg, png or whatever format defined for the images. Such type of thing is handled by the presentation layer. This thing is also managed by OS by its own.

Layer 5(Session layer):- Session layer is also managed at OS level, here user sessions are managed. In banking application as soon as I log in, my user session is created. This functionality is part of the session layer.

From layer 7 to 5 all the things are handled at OS level but in next layers, we will now deal with the network. The information of transferring 50 bucks has left the system and now we will see how this info will reach the end server.

Here comes the role of layer 4 (Transport layer): At this layer following decision is made.

  • Shall transmission be reliable or non-reliable(TCP vs UDP)
  • What is the source port and destination port

Layer 3(Network Layer): Once the decision to above question and some other homework is done, Things goes to the Network layer, here information regarding logical address is provided. In our banking app example, the information regarding IP of the end server is provided at this layer. Devices like routers fall into this category.

Layer 2(Data Link layer): So now at this time info regarding the transfer of 50 bucks has reached the Router B of the example diagram shown above. Now there may be several systems communicating with router B. So router B needs to decide as to where send the packet so work is done on retrieving the mac addresses of the devices. So at this layer work is done on physical addresses i.e. MAC addresses.

Layer 1(Physical layer):  Now info has reached the server but it needs to read the same and computer understands only binary. So at this layer dealing is done with electric signals and information is converted into binary format. Modems are the layer 1 devices which do the job. So now info regarding 50 bucks transfer has reached the server and stored on the server.

To conclude, OSI model is only a reference model and may not always be followed when working in networking domain. It is a conceptual framework so we can better understand complex interactions that are happening in the networks. As using this OSI model, I can easily understand how data is being transferred into the server from app taking into account various complex protocols and processes.

 

 

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