So you are studying networking – and then you get to read about the OSI conceptual framework which explains how network communication happens…

Well, I don’t know about you but when I was studying this stuff, my head would start spinning very quickly!

So I thought I would make this blog post trying to put the whole idea slightly lighter, while still keeping the essence of it. So this is not a comprehensive analysis of the OSI Model. I was rather thinking this post could be complementary to what you understand already (or think you do).

The OSI model provides a framework for network communications and therefore, it provides a set of rules and guidelines which apply to any protocol stack – TCP/IP, Novell NetWare, Appletalk, etc.

With scalability in mind, the OSI model defines seven communication layers. In reality, each layer represents a set of functions (aka. protocols) implemented within the respective protocol stack towards achieving a common objective. For example:

screenshot338

Some devices, would only implement protocols up to Layer 2 – such as Switches; other devices implement functions up to Layer 3 – such as Routers; Operating Systems protocols up to Layer 7.

Do note that all layers implement more than one protocol – for example, SMTP (for sending emails), FTP (File transfer), HTTP (web) – all belong to the Application Layer 7.

Tip for developers/coders: When you think of layers, associate them to Libraries. 

The OSI model was later simplified into what is referred to as the TCP/IP Model. Since it’s simpler (4 layers only), going forward, I will refer to the latter – the concepts remain the same.

Below is a conceptual diagram of both models.

osi-model

In the diagram, I am also showing how the actual data is referred to depending on the current layer – Frames, bits, packets, segments and data.

Sending an eMail …

In a very simplistic example, let’s see what happens when we send an email.

  1. Data is encapsulated into a special format defined by the SMTP protocol at the APPLICATION layer
  2. OS passes processing to the TRANSPORT layer / port 25) – which is also going to do its “magic” and encapsulates the Segment into a format defined by the TCP protocol
  3. Control is given to the INTERNET layer which, once again, will encapsulate the data into Packets, as defined by the IP protocol
  4. At the NETWORK ACCESS layer, data is converted into frames and later, into bits, to further be converted into electrical signals
  5. Decapsulation begins … At the remote end, the network card connected to the wire receives electrical signals and converts them back into bitsonce all relevant bits are received, the frame is then revealed
  6. The frame is passed to the INTERNET layer – here the frame is now decapsulated and the IP packet is revealed
  7. The packet reaches the TRANSPORT layer where it is further decapsulated revealing the segment
  8. At the APPLICATION layer, the segment is one last time decapsulated and the email content is revealed to the email recipient

It is important to understand that layers communicate horizontally – i.e. when communication starts at Layer3, it will always terminate at Layer 3 – never above, never below. In practice, this really means that two protocols communicate together in a server-client fashion.

 


Thank you,
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