OSI layer theory

 

What is OSI layer?

As per “geeksforgeeks” website: OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International Organization of Standardization‘, in the year 1984. It is 7 layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe.

Layers of OSI:

1. Physical Layer

2. Data Link Layer

3. Network Layer

4. Transport Layer

5. Session Layer

6. Presentation Layer

7. Application Layer

At receiver side Physical layer will be the first layer and data link layer will be second layer and so on

 But at sender side Application layer will be the first layer and Presentation layer will be the second layer

We can remember the layers name by below line:

Please do not touch Stephen pet animal (PDNTSPA)

Each work first letter start with each layer name:

1. Physical Layer:

This layer is the first layer at receiver side and last layer at sender side, as the name indicate it is responsible for actual physical connection with the device.

It converts the signal in bits and sends by physical medium

This layer converts the digital bits into electrical, radio, or optical signals

Responsibility of physical layer:

Topology management: The topology like Bus topology, Star topology etc are managed by physical layer.

Data Flow control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.

Synchronization: It is responsible for bit synchronization

Device use at physical layer: Cables, Hubs

Protocols used at this layer: The major protocols used by this layer include Bluetooth, PON, OTN, DSL, IEEE.802.11, IEEE.802.3, L431 and TIA 449

Data Link Layer:

• The main use of data link layer is to control error. Ensures that the data received is free of any errors, and to do that it also monitors flow control of packets. It sends data as per acknowledgement of receiver and vise versa.

 

Data link layer has two parts:

1. Media Access control MAC

2. Logic Link control LLC

Ø The packet received by Data link layer divided into frames.

Ø After framing it add MAC address on header of each frame it is also called physical addressing

Ø It encapsulates Sender and Receiver’s MAC address in the header

Ø It uses ARP (Address Resolution Protocol) protocol to get receiver MAC address.

Ø Switch & Bridge are Data Link Layer devices

Protocol used: ARP, CSLIP, HDLC,

Network Layer:

It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available

It uses sender and receiver IP address are added to its header for routing

It provide logical addressing for routing address

Device used in network layer is routers. 

Protocols: Routing Protocols, IP, ICMP

Transport layer:

Transport layer is responsible for segmentation and flow control error to ensure proper data transmission. Each segment has its header which contains basic information which helps in reassembly of segments. It also adds Source and Destination port numbers in its header and forwards the segmented data to the Network Layer. 

Transport Layer is called as Heart of OSI model.

It provides connection oriented connection in such connection below steps take place:

1. Stable a connection  

2. Transfer data in segments and do acknowledgement

3. Once sender get acknowledgement it disconnect the connection

Protocols: TCP (TCP stands for transmission control protocol)

, IP, UDP, DCCP and SCTP

 Session layer:

This layer is responsible for the establishment of connection, maintenance of sessions, authentication, and also ensures security. It is end to end layer which stabiles connection and disconnect only when data get transfer and session layer get confirmation for same

It also provide logical ports for data transfer

It support communication between two device by half and full duplex model

PPTP, SAP, L2TP and NetBIOS

Presentation layer:

This layer is responsible for Translation, Encryption/ Decryption, Compression:

We can understand this with an example.

Suppose a user send a message then he send message by any application like Gmail, facebook etc that application work on Application layer but when we send message it get encrypted before it send to someone else this encryption take place at presentation layer.

Or suppose we receive any file (for ex. MP3 file) when we try to open that we get option to open in VLC media this selection option take place at presentation layer.

And we open file in an application this application work at application layer.

Protocols: XDR, TLS, SSL and MIME

Application layer:

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications

Ex: Application – Browsers, Skype Messenger, etc. 

Protocols: HTTP, SMTP, DHCP, FTP, Telnet, SNMP and SMPP.