The OSI Model or the Open Systems Interconnection Model is a conceptual framework which describes the functions of a networking system. It is used for the transfer of data over a network which moves through different layers. The seven layers are:
1. Physical Layer
It is the bottom-most or the first layer of the OSI Model.
It comprises the raw data which is further transmitted to the higher layers of the structure.
Preparing the physical devices in the network and accepting the received data for transmission.
The termination of connection between two nodes of a network also takes place at this stage.
This layer converts the digital bits into electrical, radio, or optical signals.
Examples of hardware in the physical layer are network adapters, ethernet, repeaters, networking hubs, etc.
2. Data Link Layer
Access to get the data is achieved at this layer.
It breaks the input data into frames which makes analysing the data easier.
Ensures that the data received is free of any errors.
It controls the flow of data in the stipulated time duration and along with a set speed of transmission.
The data is sent to the next layer in the form of packets which are then reviewed for further processing.
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The data link layer is subdivided into two types of sublayers:
- Media Access Control (MAC) layer- It is responsible for controlling how device in a network gain access to medium and permits to transmit data.
- Logical link control layer- This layer is responsible for identity and encapsulating network-layer protocols and allows you to find the error.
3. Network Layer
It acts as a network controller.
Transferring of variable data from one node to another, connected in a network, takes place at this layer.
Each node has a specific address and the network layer ensures that the data is sent to its destination address.
The data is sent in the form of fragments which are then connected to each other once the processing is done.
4. Transport Layer
The delivery of data packets is managed by the transport layer.
It manages the flow of data, segmentation and desegmentation and error control.
There are five classes of the transport protocol, starting from 0 and continuing till 4 (TP0 to TP4).
Fragmentation and reassembly of data packets occur that this stage.
5. Session Layer
The connection between the computers connected in a network is managed at this layer.
Establishment, management and termination between the remote and local application takes place here.
Authentication and authorisation happen at this layer.
This layer can also terminate or end any session or transmission which is complete.
6. Presentation Layer
The data is converted into the syntax or semantics which an application understands.
Before passing on the data any further, the data is formatted at this stage.
Functions including compression, encryption, compatible character code set, etc. are also done at this layer of the model.
It serves as a data translator for the network.
7. Application Layer
The interaction with the user or the user application takes place at this stage.
When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.
Advantages of the OSI Model
It helps to standardize router, switch, motherboard, and other hardware.
Reduces complexity and standardizes interfaces.
Facilitates modular engineering.
Helps to ensure interoperable technology.
Helps to accelerate the evolution.
Protocols can be replaced by new protocols when technology changes.
Provide support for connection-oriented services as well as connectionless service.
It is a standard model in computer networking.
Supports connectionless and connection-oriented services.
Offers flexibility to adapt to various types of protocols.
Disadvantages of the OSI Model
Fitting of protocols is a tedious task.
We can only use it as a reference model.
Doesn’t define any specific protocol.
In the OSI network layer model, some services are duplicated in many layers such as the transport and data link layers.
Layers can’t work in parallel as each layer need to wait to obtain data from the previous layer.
Summary
| Layer | Name | Function | Protocols |
|---|---|---|---|
| Layer 7 | Application | To allow access to network resources. | SMTP, HTTP, FTP, POP3, SNMP |
| Layer 6 | Presentation | To translate, encrypt and compress data. | MPEG, ASCH, SSL, TLS |
| Layer 5 | Session | To establish, manage, and terminate the session | NetBIOS, SAP |
| Layer 4 | Transport | The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine. | TCP, UDP |
| Layer 3 | Network | To provide internetworking. To move packets from source to destination | IPV5, IPV6, ICMP, IPSEC, ARP, MPLS. |
| Layer 2 | Data Link | To organize bits into frames. To provide hop-to-hop delivery | RAPA, PPP, Frame Relay, ATM, Fiber Cable, etc. |
| Layer 1 | Physical | To transmit bits over a medium. To provide mechanical and electrical specifications | RS232, 100BaseTX, ISDN, 11. |
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