OSI stands for (Open Systems Interconnection). IIt is a 7-layer architecture, with each layer having specific functionality. All these 7 layers work to transmit the data from one person to another across the globe.
This model serves as the standard for network communications, introduced in 1983, and later adopted by ISO as an international standard in 1984. It helps us visualize and communicate how networks operate, as well as isolate and troubleshoot networking issues.
7 Layers of OSI Model
We’ll explain the 7 OSI layers model from ‘top to bottom,’ starting from the application layer to the physical layer.
Application Layer
Application Layer is the first layer of the OSI model. This layer is used by end-user software or Network applications such as web browsers and email clients. This layer uses multiple protocols using different software to send and receive information and present meaningful data to users.
Protocols: HTTP, FTP, SMTP, POP3, Telnet, POP, DNS, DHCP, SNMP
Presentation Layer
Presentation Layer is responsible for encryption and decryption. It compresses and decompresses the size of data for faster data connection and makes sure that data is received correctly on the other end. Data could be lossy or lossless. It is helpful in real-time video and audio confession.
Protocols: SSL, MPEG, ASCH, TLS, WEP, PKI
Session Layer
Session Layer is responsible for creating or opening the terminal and connecting two different devices. It performs two functions Authentication and Authorization. This layer can also create checkpoints during a data transfer. if the session is interrupted, devices can resume data transfer from the last checkpoint. It also helps in reconnecting the broken signals.
Protocols: RPC, NetBIOS, SAP, Apple Talk, Gateway
Transport Layer
The transport layer accepts data from the session layer and breaks it into “segments” on the transmitting end. This layer has 3 important roles.
Segmentation: Data received from the session layer is divided into small frames and each segment contains a source, destination, and a sequence number that helps to resemble all the packets in order.
Flow Control: It controls the amount of data being transmitted.
Example: There is one server and a smartphone and the server transmits data with 100mbps while the smartphone is able to send and receive data up to 10mbps. So in this case the server reduces the bandwidth size to maintain consistency.
Error Control: If some data doesn’t arrive at the destination then it sends a request to the server. Each packet contains checksums that help to find correct data.
Protocol: TCP, UDP
Network Layer
Networking Layers break the segments into network packets and reassemble the packets on the receiving end. Here we do routing and design data which is about to be sent. We’ll take the help of IP (Internet Protocol) to send data to its destination.
Logical Addressing: In this network, the layer provides an IP address to each packet (segment) of data that helps to reach data from source to destination.
Path Destination: Select the best possible route to send and receive data.
Protocols: IPV5, IPV6, ICMP, IPSEC, ARP, MPLS, RIP, OSPR, IGERP
Data Link Layer
It helps in mapping and casting. In this layer, bits are encoded and decoded in the frame. It is responsible for establishing and terminating the connection between two physically connected devices.
Logical link layer: It deals with error and flow control.
MAC: It deals with how to share links in the point-to-point or Broadcast process. It contains a 48-bit address.
Protocols: RAPA, PPP, Frame Relay, ATM, Fiber Cable
Physical Layer
It is responsible for the actual physical connection between two devices. It sends bits from one computer to another computer. Here the data converts from Digital signals into analogue signals and vice-versa and we call it Modulator and De-Modulator. Here we use connectors like RJ45 and RJ47.
Protocols: RS232, 100BaseTX, ISDN, 11,
OSI Layer in a nutshell
| Layers | Layer Name | Responsibility | Information Form (Data Unit) | Device |
|---|---|---|---|---|
| 1 | Application Layer | Provides a user interface Application access network services. | Message | PC’s (Personal Computer), Phones, Servers Gateways and Firewalls |
| 2 | Presentation Layer | Present data in a usable format Encryption/Decryption Compress and Decompress data | Message | Gateway |
| 3 | Session Layer | Establish, manage and terminate session Synchronization data transfer with checkpoints. | Message | Gateway |
| 4 | Transport Layer | Provide reliable process to process and error delivery. Error Checking & Data Segmentation. | Segment | Firewall, Gateway |
| 5 | Network Layer | Move packets from source to destination. | Packet | Router |
| 6 | Data Link Layer | Organise bits into frames. | Frame | Switch, Bridge, NIC |
| 7 | Physical Layer | Transmit RAW bits over the physical medium. | Bits | Hub, Repeater, Modem, Cables |
Advantages of the OSI Model
- Each OSI Layer serves a specific subnet or task: Each layer in the OSI model has a defined role and responsibility in network communication.
- Establish connections with different types of computers: This is typically a function of the Transport Layer (Layer 4), which manages end-to-end communication.
- Supports multiple routing protocols: Network Layer (Layer 3) is responsible for routing and choosing the best path for data packets.
- Provide common reference points between network professionals: The OSI model offers a common framework and terminology for network professionals.
- Interoperable technology between networks and devices: The OSI model promotes interoperability by standardizing communication protocols.
- Supports connectionless and connection-oriented services: This distinction between connectionless (e.g., UDP) and connection-oriented (e.g., TCP) services is relevant to the Transport Layer.
- Easily standardize router, switch, motherboard, and other hardware: The OSI model helps standardize networking protocols and interfaces, making it easier to design and manufacture networking hardware.
