COMPUTER TIPS

 

          Open System Interconnection Model (OSI)

 

 

 

 

OSI model is a framework for data communication across network. It was developed by International Standard Organization (ISO) in 1970s. It is used for networking and to show how two different systems can communicate with each other without requiring changes to their underlying hardware and software models. OSI model has seven layers. Each layer performs its own defined function in data communication.  

Seven Layers of OSI model

• Physical layer

• Data link layer

• Network layer

• Transport layer

• Session layer

• Presentation layer

• Application layer

1. Physical layer: 

The Physical layer is responsible to transmit a stream of bits over a physical medium. The major duties of the physical are following:

•  Physical Characteristics of interfaces and media: The characteristics of the interface between the devices and the transmission media is defined by the Physical layer. It also defines the type of medium.

• Representation of bits: The data on physical layer consists of a stream of bits without any interpretation. For transmission this data should be encoded in electrical or optical signals. The physical layer encodes the data for efficient transmission.

• Data rate: The numbers of bits transmitted per second are also defined by the physical layer.

• Synchronization of bits: Synchronization of bits means that the sender and the receiver should be synchronized at the same bit rate and this job is also done by the physical layer.

2. Data link layer 

The data Link layer is responsible to transmit frames from one node to another without errors. The major duties of data link layer are following:

• Framing: The bits, which are received from the network layer, are divided into manageable data units called Frames, by the data link layer.

• Physical addressing: When frames are distributed to different systems on the network, a header is added to the frame to define the sender and receiver of the frame by the data link layer.

• Flow control: If the receiver data rate at the receiver end is less than the sender’s data rate then data link layer imposes a flow control mechanism to control the data rate.

• Error control: The data link layer controls the error by adding a Trailer to the end of the frame.

3. Network layer  

The network layer is responsible to deliver the packets of data from the source to the destination. The major duties of network layer are following:

• Logical addressing: The network layer adds a header to the packet coming from the upper layer plus the logical addresses of the sender and receiver.

• Routing: To create a large network, the connecting devices are used to route the packets to their destination.

4. Transport layer

The transport is responsible to deliver a message from one process to another. The major duties of transport layer are following:

• Port addressing: On computer several processes run at the same time. The transport layer, therefore, must add a header to plus a port address, to transfer the packet to the correct process on the computer.

• Segmentation and reassembly: A message is divided into segments and each segment has a sequence number. This number enables the transport layer to reassembles the message correctly and replace the packet that was lost in transmission.

• Connection control: The transport layer can be connectionless or connection oriented. A connection oriented transport layer first makes a connection with the transport layer at the destination and then deliver packets while a connectionless network layer deliver packets without making any connection.

 5. Session layer:

 The session layer is responsible to establish, maintain and synchronies the interaction between communicating systems. The major duties of session layer are following:

• Dialog control: This layer is the network-dialed controller. It allows the communication between two processes either in half duplex or full duplex mode.

• Synchronization Points: This layer adds synchronization points into a stream of bits. These points are checked after every transmission for efficient and error free transmission.

 6. Presentation layer:

 The presentation layer is responsible for the syntax and semantics of the information exchanged between two systems. The major duties of presentation layer are following:

• Translation: This layer translates the encoded information at both ends i.e. at sender and receiver.

• Encryption: This layer provides privacy to data. Encryption means that sender transform the original information into another form and send it over the network. Decryption at the receiver reverses the information.

• Compression: The number of bits can be reduced by the data compression. This is important in the transmission of multimedia such as text, audio and video.

 7. Application layer:

The application layer is responsible to provide services to the user. The major duties of application layer are following:

• Mail services: For email forwarding and storage this application is the basis.

• File transfer and access and management: This application allows a user to access and to retrieve files from a remote computer and to manage or control files in a remote computer locally.

• Directory services: For global information about various objects and services this application provides distributed databases.