OSI Model (Open System Interconnection)

The OSI model is an conceptual model that is used to characterize and standardize the internal functions of the communication by dividing it into different layers. OSI model is based on Seven Layers showing that how networks work. OSI model shows that how data/ packets travel through one end device/ computer to another model also give the simplest way to troubleshoot the issues occurred on networks.

Seven Layers of OSI Model are 

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

Physical Layer

Physical Layer performs the task of encoding binary data into electrical, optical or other signals which are suitable for transmission over the network media used to perform transmission.

Function of Physical Layer

• Physical Layer defines the specification of the data connection both electrical and physical.
• Physical layer defines relationship between the end device and the medium (UTP/STP, Fiber-Optic or Wireless) on which data traveled.
• Physical Layer also defines the suitable protocol to perform that communication.
• Physical Layer defines the mode of transmission which may be simplex, Half Duplex or Full Duplex.
• Physical Layer defines the topology of the network.

Data Link Layer

In IEE 802 Local Area Standards Data link layer is categorize into Two Sb-layers.

• Media Access Control (MAC) Layer
• Logical Link Control (LLC) Layer

Media Access Control (MAC) Layer

MAC Layer is responsible for controlling the access of computers/ end devices to the network and transmission of data over the network.

Logical Link Control (LLC) Layer

LLC Layer is responsible for checking errors occurs during the data transmission and synchronization.

Functions of Data Link Layer

• Data Link Layer establishes and terminates the logical link between the two nodes.
• Data Link Layer tells the nodes while transmitting to "back-off" when frame buffer is not available.
• Data Link Layer transmits and receives the frames sequentially.
• Data Link Layer is responsible for frame acknowledgment. If there any error occurs over the physical layer it is the responsibility of data link layer to detect that error and recover that error by re-transmitting the frames whose acknowledgment does't received.
• Data Link Layer also handles the frame duplication.
• Data Link Layer also determines when the computer/ end device has the right to access and use the physical medium.

Network Layer

Network Layer is responsible to provide suitable inter-network routing to transmit packets form source host to remote destination host.

Functions of Network Layer

• Network Layer routes the frames from one network to another networks.
• Network Layer translates the logical addresses and names into physical addresses to perform routing.

Transport Layer

Transport Layer provides the services which are essential to gain quality of services and are not provides by the lower layers (Physical, Data Link and Network Layer).

Functions of Transport Layer

• Transport Layer receives the message from Session Layer and divides it into smaller units if they are not already small enough. After dividing into smaller parts transport layer send that units towards the network layer.
• Transport Layer provides the reliable end to end connection and message delivery with error free acknowledgments.
• Transport Layer also tells the station of transmissions to back-off if message buffer is not available.

Session Layer

Session Layer gives the mechanisms by which message dialogue between end devices/ computers can be established, controlled and broken down.

Functions of Session Layer

• Session Layer establishes, maintain and terminate the two application processes working on different machines.
• Session Layer allows the processes to communicate over the network, provide security, recognize names and so on.

Presentation Layer

Presentation Layer provides the appropriate syntax to allow different type of end devices/ computers to communicate with each other.

Functions of Presentation Layer

• Presentation Layer translate codes i.e., ASCII to EBCDIC.
• Presentation Layer performs data conversion such as bit order and many more.
• Presentation Layer compresses the data that is used to be transferred over the network.
• Presentation Layer encrypts the data to secure the data.

Application Layer

Application Layer is the interface between the network and running application on the end device/ computer so that it can access the resource over the network which it wants to access.

Functions of Application Layer

• Share resources and redirection of devices
• Access remote files over the network
• Management of the network
• Virtual network terminals