Open System Interconnection Model (OSI Model) is a 7 layered architecture structure to transmit the data from one point to another point.
It is basically a reference model that interprets how the data from one application get passed to another application via a physical medium.
Every layer of the model is independent of the other tasks of each layer can be performed independently.
Thus, this model interprets the functionalities of data transfer and networking.
Classification of the OSI Model
The OSI Model is classified into 7 layers as mentioned below:
- Physical layer
- Data Link layer
- Network layer
- Transport layer
- Session layer
- Presentation layer
- Application layer
1. Physical Layer (Layer 1)
The layer on the OSI model, responsible for the real-time connection between devices is the Physical Layer. It transmits the information in the form of
bits from one node point to another. This layer defines the connectors, cables, and all the physical networking equipment.
The physical medium acts as a platform or medium to transfer the bits from one node to another. Basically, it establishes the connection and transfers the bits and then the connection is deactivated. Further, the received signals get converted in the form of 0’s and 1’s and are passed as input to the Data Link Layer.
Features of the Physical layer
- It determines the path for the connection of two or more devices in general.
- The Physical layer defines the mode of signal for the purpose of transmission of bits.
- This layer enables Bit Synchronization by providing a clock which controls the sender and the receiver.
- Controls the rate of bit transmission.
- It defines the topology of the connected nodes/devices.
Devices associated with Physical Layer
2. Data Link Layer (Layer 2)
The Data Link layer receives the input from the Physical layer in the form of 0’s and 1’s. Further, the Data Link layer on the OSI model is responsible for the error-free transfer of data from one node to another node.
Most importantly, the Data Link layer provides a unique identification address to every device in the local network.
Understanding the structure of Frame
In this layer, the
raw data(bit stream) received from the Physical layer gets converted into
packets, also known as
Frames. Then, a header and a trailer are added to the frame which contains the source and destination address of the frame.
Data Link layer is divided into two sub-layers:
- Logical Link Control layer: It transfers the packets to the Network layer of the receiving application.
- Media Access Control layer: It serves as a link between the Logical Link Control layer and the physical layer of the receiving network.
Moreover, the Data Link layer is responsible for forwarding the frames to the Network layer.
Features of Data Link layer
- Physical Addressing: To send the packet to the right system, the Data Link layer provides the physical address i.e. MAC address of the sender and the receiver system in the header bit of the frames created.
- Flow Control: It controls the quantity of data being sent to the receiver.
- Error Control: This mechanism helps detect the errors in the frame and re-transmits the loss of damaged frames ahead.
- Access Control: This mechanism is responsible for determining the device which has control over a particular channel at that point of time.
Devices associated with the Data Link layer
- Network Cards
3. Network Layer (Layer 3)
The Network Layer transmits the data from one network to another network. It creates a logical connection between devices.
It is responsible for determining the best suitable path for the transfer of data. The Network layer in the OSI model receives the data from the upper layers and converts them into
packets. Further, routes the packets through the best optimal path.
Features of Network Layer
- Routing of packets: It finds the best route through multiple paths and transfers the packets over the network.
- IPv4 and IPv6 network protocols are used to manage network traffic.
- Logical Addressing: The logical address i.e.
IP addressof the sender and the receiver is placed in the header of the packet to determine each device in the network.
Devices associated with Network Layer
4. Transport Layer (Layer 4)
The Transport layer accepts the packets from the Network layer and sends it to the session layer. It is responsible for the end-to-end delivery of the data. The packets received from the Network layer are here referred to as Segments.
In the Transport Layer, the acknowledgment of the data transfer is provided. It encapsulates the sender’s and receiver’s port number to header of the Segmented data and forwards it to the Network layer.
Services provided by Transport layer:
- Connection-oriented Service: It is a reliable source of communication between the devices. In this service, the data transfer takes place in three phases: Establishment of connection, Data transfer, and Termination of the connection. Further, after the data transfer, it sends an acknowledgment to the sender regarding the successful data transfer.
- Connection-less oriented Service: It is the fastest mode of data transfer service. Here, the data transfer happens directly. No acknowledgment is sent to the sender regarding the transfer of the packets.
Functions of Transport Layer
- Segmentation and Assembling of data: At the sender’s end, it accepts the data from the session layer and divides it into small units. The transport layer of the receiver is responsible for assembling the units together.
- Service – Point Addressing of data: In the Transport layer, the port number of the sender and the receiver added to the header of the Segment helps to transfer the data to the correct process.
- Connection Control: In this layer, the data is transferred either in a connection-oriented or connection-less format.
- Flow Control from end-to-end of the system application.
- Error Control from end-to-end of the system application.
Protocols associated with Transport layer
- Transmission Control Protocol (TCP): If the data is sent using the TCP protocol, then it divided into smaller units called segments. The segments follow multiple routes to reach the receiver’s end. At the receiver’s end, the TCP protocol reassembles the segments. It is more reliable, since, it establishes, maintains and terminates the connection between the sender and the receiver.
- User Datagram Protocol (UDP): In this protocol, the data transfer takes place without any acknowledgment. Thus, it happens to be an unreliable protocol.
5. Session Layer (Layer 5)
The Session layer ensures
secure connection at a particular platform/application system.
It enables us to
establish a connection and maintains a particular session of the user. It also authenticates the user for security among the communicating devices.
Functions of Session Layer
- Synchronization: While transmitting data between devices, the Session layer adds checkpoints between them. If an error raises during the transmission of data, then the transmission re-starts again from that particular checkpoint.
- Dialogue Control: It creates a dialogue box between two processes and thus builds communication between the communicating devices.
Protocols of the Session Layer
- DNS – Domain Name Server
- LDAP – Lightweight Directory Access Protocol
- NetBIOS – Network Basic Input/Output System
6. Presentation Layer (Layer 6)
The Presentation Layer takes the input or sends decoded output from and to the Application Layer. It
manipulates the data according to the necessary format and then forwards the data over the network.
Thus, it is also known as the Data Translator layer. The Presentation Layer mainly focuses on the
semantics of the transmitting data.
Functions of the Presentation Layer
- Encryption/Decryption: This mechanism is needed to serve the purpose of the security of data.
Encryptiontranslates the data from the readable format to another form and forwards it over the network. At the receiver’s end, the encrypted data is then again converted into the readable form, known as
- Data Compression: The Presentation layer is responsible for compressing the data i.e. it reduces and compresses the data thus, reducing the number of bits to be transferred across the networking devices.
- Translation: This layer is responsible for the conversion of data from one format to another.
7. Application Layer (Layer 7)
The Application layer serves as a platform for the user or operating system to interact with the data on a high level. It produces the data to be forwarded onto the network and acts as a window for the applications to produce and display the data over the network.
Functions of Application Layer
- E-mail Services: This layer provides the platform for the transfer of mail over the network.
- File transfer and management of data: It enables the user to access and retrieve the files from the remote devices.
Applications associated with the Application layer
- Any desktop or mobile application such as Facebook, Skype, etc.
Thus, in this article, we have covered the OSI Model’s 7 layers and have dived into the working and role of each layer in the transfer of data right from the data manipulation at the remote system to the conversion of data into bits.