OSI Model

Layers:
In 1974 ISO (International Organization of Standard) start to make a model of layers. In 1977 they finalized this model and named OSI (Open System Interconnection) Model.

OSI Model:
There are 7 layers in OSI Model.
7. Application Layer
6. Presentation Layer
5. Session Layer
4. Transport Layer
3. Network Layer
2. Data link Layer
1. Physical Layer

The work is start from Application layer. But the Counting of Layer is start from Physical layer. So Physical Layer is Layer-1.

To remember all layer easily, personally I remember APSeTNeDaP.
We know that different layer responsible for different job when we send data from one PC to another. Now we will see shortly what the certain job for each layer is. Later we will know details.

The Application layer is responsible for providing services to the user

The Presentation layer is responsible for translation, compression and encryption

The Session layer is responsible for establish, manage and terminate sessions.

The Transport layer is responsible for the delivery of a message from one process to another.

The Network layer is responsible for the delivery of individual packets from the source host to the destination host.

The Data link layer is responsible what it done in NIC such as modulation demodulation.

The Physical layer carries the data from one NIC to another NIC blindly.

If we compare what we have done with OSI Model, we found 3 layers. the layer with we work for sending data that is Application Layer, any job at NIC card is Data Link Layer and sends data through media in Physical Layer.

Different layer uses different protocols. The protocols of the various layers are together is called Protocol Stack. The Internet Protocol Stack consists of 4 Layers implemented from OSI Model. This are-

1. Process/Application Layer (combination of Application + Presentation + Session Layers of OSI Model)

2. Host to Host Layer (Transport)

3. Internet Layer (Network)

4. Network Access Layer (Data link + Physical)

TCP/IP protocol suite was generally designed to provide implementation of the networking stack from the Network Layer / Internet Layer (layer 3) and above. In most classical networks, TCP/IP doesn’t define any protocols operating at Network Access Layer (Data link + Physical). TCP/IP assumes that layer two functionally is provided by a WAN or LAN technology like Ethernet, Token Ring or IEEE 802.11. Among these technologies Ethernet is by far the dominant technology. We will learn later about this technology.

We have connected PC with cable, connector and NIC. Now I want to transfer data to my friend’s PC but my PC doesn’t know where to send. 
To find a house we need the house number, road number and area name. In this way PC also need numbering to find destination. What we can do? Numbering two PCs that both PCs can recognizes each other with that number. This number is known as IP address. For applying IP address there must be a rule, that means a protocol. The rules for IP address are defined in Internet Protocol (IP). IP Protocol works at Layer 3 (Network / Internet Access Layer). Later we will know details about IP Address and Internet Protocol. Now we will give number (IP) for PC are 192.168.1.1 and 192.158.1.2. And there is an option of subnet mask we will give each PC 255.255.255.0
Now each PC got an IP address by using Internet Protocol and data will send with source and destination address.

Source Address	Destination Address
Data

We solved 1^st problem of modulation and demodulation. But 2^nd problem arises when we send a big file across to the other PC. Cause Ethernet allows transferring data only 1500 bytes at a time. What can we do?

We can do one thing - divided data into pieces. Each pieces are called in networking world is Frame or packet. How we can make packets. So lets see how data makes packet or frame.

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