OSI model [ Open Systems Interconnection model ] tutorials

  • The OSI model is the foundation for all communication that takes place between computers and other networking devices. The (ISO) International Organization for Standardization began developing the OSI model in 1974, after DOD (Department of Defense) developed and began using the TCP/IP (Transmission Control Protocol / Internet Protocol). TCP / IP is a suite of protocols that work together to provide communication. After many years of discussion, the OSI model was finally adopted in 1977. Today OSI model is used as the theoretical model for how communication takes place between devices. Though it is theoretical model, it defines the way in which devices interact and understanding the OSI model will help to design, implement and troubleshoot any network.

  • Each layer of the OSI model is independent from every other in its purpose and responsibilities. Each layer must do its own job, as well as provide service to layer above and below it. In this way model create framework. The OSI model provide a structure for software programmers and equipment manufacturers to develop products that will work together. This is known as interoperability.

 

 

Application

(Layer 7)

 

This layer provides a user interface. It supports application for communication over the network

SMTP (Simple Mail Transfer Protocol), HTTP (Hyper Text Transfer Protocol), FTP (File Transfer Protocol) work at this layer.

 

 

 

 

Presentation

(Layer 6)

 

 

It performs data Presentation, Compression and Encryption so that it is recognizable by the receiver. For most common computer native format is ASCII (American Standard Code for Information Interchange) while for IBM mainframe it would be EBCDIC (Extended Binary Coded Decimal Interchange Code)

 

 

 

Session

(Layer 5)

 

It Establishes connections, keeps different applications data separate, then terminates them after all the data has been sent. It send syn (Synchronization) packets to establish connection, while receiver send back ack acknowledgement .

 

 

 

 

 

Transport

(Layer 4)

 

data is converted into SEGMENT

 

Provides reliable transport method, Acknowledgement, error correction and retransmission of data when necessary. It makes sure that sender and receiver communicate at speed they both can handle called flow control i.e. 2 Ghz & 500 Mhz communicate @ 500 mhz.

 

 

Network

(Layer 3)

 

Segment converted into PACKET

 

It adds appropriate network addressing and delivery of packets called Datagrams. It provides logical addressing which router use for path determination.

 

 

Data Link

(Layer 2)

 

PACKET converted into FRAME

 

Combine packets into bytes and bytes into frames. It adds the MAC addresses to frames. It performs error detection not correction.

 

 

Physical

(Layer 1)

 

FRAME converted into BITS

 

Transmit data on the wire. Moves bits between devices. Specify voltage, wire speed and pin-out of cable. This layer is responsible for actual physical connection between devices. It may uses Coaxial, CAT, Fiber Optic cable or wireless for communication

 

 

Advantages of OSI model

  • It divides the network communication process into smaller and simpler components, thus aiding component development, design and troubleshooting.

  • It assists in data transfer between different hosts, i.e. it enables us to transfer data between UNIX to WINDOWS to MAC.

  • It allows multiple vendor development though standardization of network components.

  • It allows various types of network hardware and software to communicate.

 

DATA Encapsulation

  • When a host transmits data across a network to another device the data goes through encapsulation. It is wrapped with protocol information at each layer of the OSI mode. Each layer communicates only with same layer on the receiving device.

  • To communicate and exchange information, each layer attach header in front of data and also at trailer or end. When it reaches to receiving end, each layer then understands header and trailer and passes data to upper layer.

 

 

 

 

 

 

 

 

APPLICATION

 

 

 

 

 

 

 

 

PRESENTATION

 

 

 

 

 

Upper Layer Data

 

 

SESSION

 

 

 

 

Tcp. Header

Upper Layer Data

 

 

TRANSPORT

Segment

 

 

 

 

 

 

 

 

 

 

IP header

Upper Layer Data

 

 

NETWORK

Packet

 

 

 

 

 

 

 

 

 

Logic Link Header

UPPER LAYER DATA

Trailer

 

DATA LINK

Frame

 

 

 

 

 

 

 

 

Mac Add.

UPPER LAYER DATA

Trailer

 

PHYSICAL

Bits

Bits 11000111001111101111101111111

 

 

 

 

  • At a transmitting device data encapsulation method works like this.

 

Segment

 

 

 

 

 

 

 

Source

Port

Destination

Port

Data

 

 

 

 

 

 

 

 

 

 

 

Packet

 

 

 

 

Source

IP

Destination

IP

Protocol

Segment

 

 

 

 

 

 

 

 

 

 

 

Frame

 

Destination

MAC

Address

Source

MAC Add

Ether-

field

Packet

Trailer

 

 

 

 

 

 

 

 

 

 

Bits

 

Bits 11000111001111101111101111111


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