OSI / TCP

 What is OSI?

The open system interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal function of a communication system by partitioning it into abstraction layers. The model is a product of the open system interconnection project at the international organization for standardization (ISO), maintained by the identification ISO/IEC 7498-1. 

A model groups communication functions into seven logical layers.

OSI(Open System Interconnect)

Application

Presentation

Session

Transport

Network

Data link

Physical

We study about all layers of OSI one by one:-
(1)Application –This layer provides the user communication that creates information & data.
     Example: - FTP protocol

(2)Presentation –This layer considered with the format of data exchange between the end systems.

(3)Session –This layer allows user a different machine to create session between in them. This layer also binds the different streams like audio signal and video signal.    

(4)Transport –This layer considered with end to end communication message. It receives the data from session layer and divides it into smaller units called as message. These messages are passed on to network layer and then these messages are passed down to upper layer.

(5)Network –This layer considered to routing of packets to correct destination.

(6)Data Link –This layer considered transmission of error free data in the form of terms.

(7)Physical layer –This layer considered with transmission on raw bits over the communication link.  

How two PCs are connected on network.

OSI – Open System Interconnect
Application-----------------------	---------------------------------------	Application
Presentation---------------------	---------------------------------------	Presentation
Session-----------------------------	---------------------------------------	Session
Transport-------------------------	---------------------------------------	Transport
Network---------------------------	--------------Network -----------	Network
Data link--------------------------	---------------Data link----------	Data link
Physical---------------------------	--------------Physical------------	Physical

What is TCP?

Transmission control protocol. The TCP is one of the core protocols of the internet protocol suit, and is so common that the entire suit is often called TCP/IP. TCP provides reliable, order and error-check delivery of a stream of octets between programs running on computers connected to a local area network, intranet or the public internet. It resides at the transport layer. Web browser use TCP when they connect to servers on the World Wide Web, and it is used to deliver email and transfer files from one location to another. HTTP, HTTPS, SMTP, POP3, IMAP, SSH, FTP, Telnet and a variety of other protocols are typically encapsulated in TCP. Applications that do not require the reliability of a TCP connection may instead use the connection less user Data-gram Protocol (UDP), which emphasis low-overhead operation and reduced latency rather than error checking and delivery validation.

It is also divided into four layers.

TCP
Application	HTTP , SMTP    DNS , RIP.
Transport	TCP(Reliable) , UDP(Unreliable).
Internet	IPv4 ,  IPv6.
Network Interface	Ethernet , Wireless , Frame relay , ATM.

Layer 2:-Switches work on layer 2 devices. It supports local area network.

Layer 3:-Routers work on layer 3 devices. It supports world wide web.