Introduction to OSI 7 layer model for absolute Beginners

OSI 7 layer model is a newer network model compared to TCP/IP model. The OSI 7 layer model consists of the Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data Link, and Physical layers from top to bottom. Each layer is capable of servicing the layer above it and issues service requests to the layer below it. A layer is a collection of related functions.


From the diagram the data we can see that the data is transmitted from the application layer to the physical layer and data is received from physical layer to application layer.

Application Layer

The 7 level of the OSI model is Application Layer. It interfaces directly to the application. Sometimes presentation layer services application layer .HTTP, FTP are examples of protocols in the application layer.

Presentation Layer

It is the sixth level of the OSI 7 layer model. It accepts service requests from the application layer and responds to the request .It also issues service requests to the session layer. Different services like encryption (for security purposes), structuring of data using XML (extended markup language) are done in this layer.

Session Layer

It is the fifth level in this model. It responds to service requests from the presentation layer. Transport layer which is below it receives requests from this layer. The session layer helps in opening, closing and managing a session. The session would be between application processes which are at end-user side, i.e. a semi-permanent dialogue. Applications communicate in the form of requests and responses; session layer is responsible of maintaining session for such communications. This layer provides synchronization. Synchronization is the process where transmitter sends the data and at the same time receiver receives the data. It is very important in cases like web conferences, online TV applications where video and audio should be transmitted simultaneously.

Transport Layer

It is the fourth level of OSI 7 layer model where it responds to service requests from the application layer and will issue service requests to the network layer. Transfer of date will be in the form of packets in this layer which will have a header and a body. Header contains source IP and the destination IP. The body contains the data to be transferred.

TCP is an example of a protocol in this layer. Transport layer provides some services. They are

  • Transport layer is connection oriented.
  • Data reliability is present in this layer. That means the errors in data transfer is minimized. This is done by re-transmitting the erroneous packages.
  • Network congestion can be controlled.
  • The concept of ports in introduced in this layer. Ports can be used to operate different data transfers at the same time using a single machine.

Network Layer

The third layer in OSI 7 layer model is network layer. It is sometimes called as internet layer. It acts as an interface by responding to the service requests from transport layer and issuing them to the data link layer. The services like packet forwarding and packet routing are done in this layer. Forwarding is the process in which a packet is passed from one node to another whereas routing is the process in which the most appropriate path of packet transfer is selected.

Internet protocol (IP) is a protocol in this layer.

Data Link Layer

The second layer in the OSI 7 layer model is data link layer. It responds to the service requests from network layer and issues service requests to the physical layer as well. It transmits data between the adjacent network nodes in a wide area network and also between nodes on the same local area network segment. The data transfer between network entities in both functional and procedural means takes place in this layer and also the means to detect and correct the errors that may crop up in the Physical layer.

Following are the services provided by data layer.

  • Error Detection
  • Flow Control

Example of data link layer protocol is Ethernet.

Physical Layer

The first layer in the OSI 7 layer model of computer networking is physical layer. It carries out the services requested by data link layer. It is the basic network layer which provides the only means to transmit raw bits rather than sending packets over a physical data link connecting network nodes. The torrent of raw bits are grouped into code words or symbols and then converted into a physical signal which is then transmitted through a physical transmission medium. The type of network topology (which could be LAN, bus, star) is specified in this layer.