Open Systems Interconnection (OSI) Model

Introduction

Let Suppose when we connect the two or more computers together for communication , if the all computers are using same protocols then they would be communicate to each other. but if the communication protocols are different all of commuters then communication would not be there.Then ISO (International Organization for Standardization) has decided to make this Model that is a standard for protocols which makes for Networking. All Networking Protocols makers have to follow this OSI model standard, at the time of making protocols.So We can say that Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems.

• It divides the communications processes into seven layers.
•  Each layer both performs specific functions to support the layers above it and offers services to the layers below it.
•  The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.
  All Seven layers are given below:-

1.  Application Layer ------------ Layer 7

2.  Presentation Layer-------------Layer 6

3.  Session Layer ------------------Layer 5

4.  Transport Layer----------------Layer 4

5.  Network Layer-----------------Layer 3

6.  Data Link Layer---------------Layer 2

7.  Physical Layer-----------------Layer 1

Without OSI & With OSI

The main benefits of the OSI model include the following:

1. Helps users understand the big picture of networking and understand how hardware and software elements function together and understand new technologies as they are developed
2. Makes troubleshooting easiest, Because of every Layer Define the Data flow status in the Network.
3. Any Vendor can explain the hardware properties in better manner.

 All Layers Brief Explanation is Following Below:-

Layer-7 

This Layer provide a network interface to users, so that they could reach to the Data and can direct access. (exp. FTP, Telnet, HTTP, etc.)

Layer-6

This layer is responsible for presenting the data in the required format which may include:
Code Formatting, Encryption, Compression

Layer-5

This layer establishes, manages, and terminates sessions between two communicating hosts.Creates Virtual Circuit,Coordinates communication between system, Organize their communication by offering three different modes

1. Simplex
2. Half Duplex
3. Full Duplex

Layer-4

This layer breaks up the data from the sending host and then reassembles it in the receiver.It also is used to insure reliable data transport across the network.Can be reliable or unreliable.Sequencing, Acknowledgment, if error found in transmission then do Retransmission again, and

Flow Control

PDU - Segments

Exp. TCP, UDP,

Layer-3

Sometimes referred to as the “Cisco Layer”.End to End Delivery, Provide logical addressing that routers use for path determination, Segments are encapsulated. Internetwork Communication, Packet forwarding to destination, Packet Filtering, Makes “Best Path Determination” according to dynamic protocol ,Fragmentation.

PDU – Packets
Router works on this layer. and IP is also layer 3 protocol.


Layer-2

Performs Physical Addressing, This layer provides reliable transit of data across a physical link.Combines bits into bytes and bytes into frames ,Access to media using MAC address, Error detection, not correction, LLC ( Logical Link Control ) performs Link establishment and MAC Performs Access method.
PDU - Frames
Switch works on this layer.
Layer-1

This is the physical media through which the data, represented as electronic signals, is sent  from the source host to the destination host.,Move bits between devices,Encoding
PDU - Bits
Hub, Connector, work on layer 1.

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