The Open Systems Interconnection model is standarized model that using layer
to graphical description which is divided by 7 layer.

Let’s think step by step when we try to send a package to friend house.

1. choose the present that we send.(layer 7)
   
2. wrap the thing.(layer 6)
   
3. put it inside the box.(layer 5)
   
4. mark the address on the box.(layer 4)
   
5. deliver the box to a warehouse.(layer 3)
   
6. choose the transportation to deliver.(layer 2)
   
7. start delivery on the road.(layer 1)
   

From now, as perspective of package.

1. package is delivered on the road.(layer 1)
   
2. Use the transportation.(layer 2)
   
3. deliver from warehouse to friend’s house.(layer 3)
   
4. friend get the box.(layer 4)
   
5. unpack the box.(layer 5)
   
6. unpack the cover.(layer 6)
   
7. Get the present.(layer 7)
   

So the layer 1 is Physical layer. the unit of data is bit.

In the above example, it would be a road.
physical layer transfers the data to electric signal vice versa.
when we did not turn on the device or connect the cable, this layer has a problem.
This layer include Wireless frequency, cable type, voltage and some other physical factor.
Represented devices are Hub, Repeater, Cable.

the layer 2 is Data link layer. the unit of data is frame(bunch of bit).

Data link layer transfers the data to other device without error.
This layer divided by two sublayer, MAX and LLC.
MAC(Media Access Control) is used to find specific device to send a data.
LLC(Logical Link Control) is used to connect the device logically and maintain the connection.
Represented devices are Bridge, Switch.

the layer 3 is Network layer. the unit of data is packet.

Network link sends a packet using fastest way.
This layer does not care about error because of above layer will take care about error.
The objective of this layer is that find a fastest way to send a packet.
We are using IP address in this layer.
Represented devices are Router.

the layer 4 is Transport layer. the unit of data is segment.

Transport layer takes care about flow control and error recovery.
It measn this layer ensure complete data transfer.
There are two famous protocol in this layer, TCP(Transmission Control Protocol)
, UDP(User Datagram Protocol). we will take a look later.
Represented device is Gateway.

the layer 5 is Session layer. the unit of data is message.

Session layer is similar to transport layer. This layer manage, establish and terminate
connections between application. There are three type of connection
duplex(ex. radio), half duplex(ex. walkie talkie) and full duplex(ex. cell phone).
This layer has responsibility to establish/termaninate TCP/UDP session.

the layer 6 is Presentation layer. the unit of data is messsage.

Presentation layer represents the translation of application format to network vice versa.
JPG, GIF, JPEG are good examples of this area. This layer present data to appication or network.

the layer 7 is Application layer. the unit of data is message.

Application layer supports application and user processes.
This layer focus on the appication, directly communicate with user.
HTTP, FTP, Telnet and WWW browser are good example for this layer.

Higher level layer take care about lowel level layer.
interconnection on high level layer impacts on lowel lever layer.
If Data create on application, every layer attach the information on the data to send properly.

There are two important reason to use this model. Standard/Learning tool.
For standard, even though this is abstract concept, it could explain lots of interconnection.
Global Standard can reduce the cost and make easily people communicate about it.
For Learning tool, you can learn when you chat with your friend via messenger.
You can understand how it works and concept of interconnection.

Actually nowadays OSI 7 layer model is quiet old one, so TCP/IP model is used a lot.

You can see how the layer match to the TCP/IP model.
TCP/IP model focus on abstarcting TCP/IP protocol.
like OSI 7 layer, every layer take their data. dont’ take care about other layer’s data.

It might be still abstract. Please leave some comments to how do you clarify those concept.

Thank you.