OSI Model and Layers Description

OSI MODEL

Open System Interconnection also known as an OSI model is a networking Framework to implement protocols in 7 layers. The control is passed from the 7 layers starting from layer 7 to layer 1.

The layers of an OSI model are as follows:

• Layer 7- Application Layer
• Layer 6- Presentation Layer
• Layer 5- Session Layer
• Layer 4- Transport Layer
• Layer 3- Network Layer 
• Layer 2- Data link Layer
• Layer 1- Physical Layer

OSI LAYER MODEL

Layer 7 (The Application layer )

 

The application layer is the layer that helps the user to communicate with the computer. In this layer,the communication partners and quality of services are identified. Protocols have been defined for applications to communicate with other applications. For e.g SMTP(Simple Mail Transfer Protocol) is a protocol present in the application Layer that helps us to send E-mail between different E-mail applications. 

Other example includes WWW(World Wide Web) ,FTP ,EDI ,Telnet ,NFS e.t.c.

Layer 6 (The Presentation layer )

The Presentation Layer also known as the Syntax layer is like a translator. It contains all the coding and conversion functions which is need for communication. When we send data from our Application layer to another computer's Application Layer we need to make sure that the other computer understands our data. That's where the presentation Layer does it's work. The tasks which are involved in this layer are data compression ,decompression ,encryption and decryption. Standards Are also involve in multimedia operations too.

Now take for example if a data is sent in EBCDIC to a computer that can understand ASCII, The presentation layer then converts the EBCDIC into ASCII. Now the standards which are involve in multimedia are PICT ,TIFF ,JPEG ,MPEG ,MIDI etc.

Layer 5 (The Session layer )

The main purpose of the session Layer is to open , manage, closing, tearing down sessions which are created between end user applications .It helps to co-ordinate communication between systems and also organize the communication in three different modes:

1.   Simplex

2.   Half-duplex

3.   Full-duplex

Following are the examples for session layer

• Structured Query Language (SQL)
• Network File System (NFS)
• Remote Procedure Call (RPC)
• X window

and the list goes on....

Layer 4 (The Transport layer )

The main function of the Transport Layer is to make segments or  packets of Large amount of Data that is being used for communication. Data integrity is ensured by this layer by maintaining flow control so that reliable data is transported between the systems.

The segments that are delivered are acknowledged back to the sender upon there reception. The segments are sequenced back into proper order upon the arrival of their destination. If any segment is not acknowledged it will be retransmitted back to the sender. This type of transmission helps us to avoid overloading ,congestion and data loss.

The process of the transport layer starts with both host application programs begin by notifying each other that the connection is going to be initiated. Then both operating systems sends messages over the network confirming that the transfer is approved and both sides are ready.the synchronization gets complete, the connection is established and the data transfer begins.

Layer 3 (The Network layer )

The Network layer is responsible for giving identity or addressing the data. The main purpose of the network layer is that it holds the responsibility for transporting traffic between devices that are not locally attached. Routers are the main device that does the routing service in an internetwork.

The network layer uses an IP(Internet Protocol) address or logical address for routing. IP address is a unique address in data networks. We cannot assign IP's manually to our network. An organization known as INTERNIC (InterNetwork Information Center) is primary responsible  responsible to assign IP addresses to our network via ISP(Internet Service Provider). An IP Address is a 32-bit address that looks like this 

192.168.0.1

10.0.0.1 

 Layer 2 (The Data Link layer )

The data link layer main purpose is to ensure that the messages are delivered to the proper devices and also to translate messages from the network layer into bits so that it can be transmit to the physical layer(Layer 1). This Layer formats the message into data frames so that it could add a header which contains the hardware destination and also the source address. The address which the data link provide is the MAC (media access control) address because a computer can understand a MAC address not the IP address.

A MAC address is a 48 bit address. From the 48 bit address the 24 is given to the vendor code and the rest is the serial code. The MAC address cannot be changed by us. The responsibility for providing the MAC address is given to IEEE (Institute of Electrical and Electronics Engineers).

Note:If  you ever want to check your computer MAC address, go to Command prompt and type " ipconfig/all ".

Layer 1 (The Physical layer )

The responsibility of the physical layer is to send and receive bits which is in the form of 0 and 1. The physical layer communicates directly with the real communication media. The Physical layer specifications specify the electrical, mechanical, procedural and functional requirements for activating, maintaining, and deactivating a physical link between end systems.

The transmission is done through a device called HUB. Hubs are multiple port repeater which do not look at any traffic as they enter and are transmitted out to the other parts of the physical media.