Difference between OSI and TCP/IP Model

What is OSI model?

• Before going into OSI model e ill learn hitory of networking. In history there were two companies whose computer were used i.e, DECnet solution and IBM solutions. 
• Suppose there is company who uses these two computers and they want to transfer data from DECnet to IBM computer then they don't support and information is not sent i.e, different computer having different operating system doesn't share information. 
• So to resolve this problem ISO(International Organization for Standardization) in 1984 developed OSI(Open System Interconnection) model based on layered approach.
• To understand the meaning of layered approach consider example, A company is divided into many department to continue smooth work.Like this to continue smooth communication different layers are defined for different purpose. o it is called layered approach. 
• In OSI model there are seven different layers. The main purpose of OSI model is how data and network information are communicated from one computer to other computer. Below figure show the OSI reference model architecture. 
• OSI model is divided into three parts i.e, upper layer, middle layer, lowermost layer. In upper layer there are application, presentation, session layers. The middle layer is transport layer and the lowermost layers are network, datalink, physical layer.
• Consider example of communication between two computers i.e, computer 1 and computer 2 as shown in below figure.
• If computer 1 send data to computer 2 it goes through application layer of computer 1 to application layer of computer 2. Through application layer it goes to presentation, session, transport, network, datalink, physical layer.

Application Layer

• Application layer is one of the topmost layer of OSI model. 
• Application layer is the actual interface between the computer screen and the real world person.
• Application layer is used by network application such as Chrome, Firefox mozilla, Skype, Outlook etc. So application layer provide services to network applications with the help of protocol to perform user activities.
• The interface we see on the computer screen is produced due to application layer protocol. 
• The protocol used in application layer of OSI model are HTTP, HTTPS, FTP, NFS, FMTP, DHCP, SNMP, TELNET, POP3, IRC, NNTP.
• These protocols provide various functions such a file transfer through protocol FTP, email through SMTP, web surfing through HTTP and HTTPS, virtual terminals through Telnet etc.

Presentation Layer 

• From the application layer data is transferred to presentation layer. In this translation, data compression, encryption, decryption takes place.
• As the name presentation layer it present the data in various format i.e, translation, data compression, encryption, decryption.
• Translation- Application layer send data to presentation layer in the form of ASCII(American Standard Code for Information Interchange) i.e, in the form of characters and numbers.But computer doesnot understand ASCII format so it convert ASCII into binary format i.e, EBCDIC(Extended Binary Coded Decimal Interchange Code) in the form of zero and one.
• Data compression- Before data translation the presentation layer reduces the number of bits that are used to represent the original data. This bit reduction process is called data compression.i.e, it reduces the size of file so that it can be received at destination in less time.
• Encryption- To keep data safe it is encrypted i.e, in coded format which is known by the person who is sharing data and the person who is receiving data to enhance the security of data.Encryption process is used by the person who is transferring data and receiver used decryption to transfer the data in original format.
• The protocol used in presentation layer is SSL(Secure Sockets Layer) to perform encryption and decryption.

Session Layer

• Session layer helps us in setting and managing connection enables to send and receive data.
• Session layer use API(Application Programming Interface) to allow applications on different computer to communicate with eachother. eg- NetBIOS(Network Basic Input/Output System). 
• The authentication process takes place before establishing the connection between sever and computer i.e, it asks the user name and password, if it matches then a connection is established between computer and the server.
• After authentication authorization process takes place. In this server will check that if you have permission to access the file otherwise it will display that "you are not authorized to this file or page".

Transport Layer

• The layer below the session layer is transport layer. The data in transport layer is present in the form of segments.
• The transport layer perform three functions i.e, segmentation, flow control, error control.
• Transport layer is responsible for end to end data transmission. 
• Segmentation-The data received from session layer is divided into small data units called segment.Each segment contain source port and sequence number. Port number direct to correct application and sequence number reassembles the data in correct sequence to send correct data to receiver.
• Flow Control-In this amount of data transmitted is controlled by transport layer. Ex- consider a computer and tablet want to communicate then computer sent data to tablet with data transmission speed of 80 Mbps and tablet data receiving speed is 40 Mbps the using transport layer tablet send information to transport layer of computer to reduce its speed to 40 Mbps so that proper data transmission takes place without any loss.
•  Error Control-Consider the communication between computer and mobile, if mobile doesnot receive any data then it will generate automatic repeat request. Like this it will receive all data using error control.A checksum is added to each data segment to find the missing data.
• The protocols used in transport layer are Transmission control protocol(TCP) and User Datagram Protocol.(UDP) 
• The transport layer perform two types of services i.e,  Connection oriented transmission done by TCP and connection less transmission is done by UDP.
• The data transmission is faster in UDP then TCP i.e, UDP doesnot provide feedback and TCP provide feedback due to this data transmission speed is less in TCP as compared to UDP. TCP ex- www, email, ftp. UDP ex- Video games, TFTP, Voice over IP.

Network Layer

• The data unit in the form of segments in transport layer is transferred to network layer and is converted into packets. 
• The main functions of network layer is Logical addressing, Routing, Path determination. 
• IP addressing i.e, IPv4 and IPv6 done in network layer is called logical addressing.Every computer has its unique internet protocol(IP) address.
• Network layer assign IP address to each computer segment to form a IP packet. IP address is used to transfer data to particular address without any data loss.
• Routing is a method to transferring data from source to destination based on logical addressing of network layer.  
• A computer can be connected to internet server in number of ways. So to find best possible path to transfer data from source to destination is done by path determination.
• The protocol used to determine best path are OSPF(Open Shortest Path First), BGP(Border Gateway Protocol), IS-IS(Intermediate system to Intermediate System).

Datalink Layer

• The packet from network layer is transferred to datalink layer and is converted into frames.
• There are two types addressing i.e, Logical addressing and physical addressing. Logical addressing is done in network layer and the physical addressing is done in datalink layer.
• MAC(Media Access Control) is used for physical addressing in datalink layer to form a frame.
• MAC address are 6 byte(48 bit) in length and is manufactured into every network interface card.
• Datalink layer provide access to media for higher layers of OSI model and also control how data is placed and received from the media.

Physical Layer   

• Physical layer converts data bits in signal and transmit over the local media i.e, electric signal in case of copper and LAN cable, light signal in optical fiber cable and radio signal in case of air.
• So signal generated by physical layer depend on type of media used to connect two devices. 
• At the receiver receive signals and converts bits and passed to datalink layer as a frame. Likethis frame is decapsulated in further layer and moved to application layer.
• Application layer protocol make visible message sent by sender to receiver on computer screen.

What is TCP/IP Model?

• In 1957 Department of Defence(DOD) of America has developed TCP model in cold war reaction to Soviet's launching of Sputnik.
• There were two protocols used communication i.e, TCP and IP. Later in 1983 both the protocols combined to form TCP/IP model for reliable communication without data loss.
• The transmission control protocol/ Internet protocol (TCP/IP) was designed by department of defence (DOD) to ensure and preserve data integrity as well as to maintain communication in wars. 
• The DOD model is a condensed version of OSI model that contains four layers instead of seven layer in OSI model.
• OSI model upper most layer are application, presentation, session layer but in TCP/IP model all these three uppermost layer are combined to form Process or application layer.
• Next the transport layer and network layer are as it is in both layer i.e, two different layer.
• Later the datalink and physical layer of OSI model are combined to form Link Layer in TCP/IP model.
• The main difference between OSI and TCP/IP model is data can be lost in OSI model but in TCP/IP model data cannot lost. So TCP/IP is much more reliable than OSI model.

Difference Between OSI and TCP/IP Model