The OSI Model: A Guide to the Seven Layers

The OSI (Open Systems Interconnection) model is a conceptual model that describes the communication functions of a telecommunication or computer network. The model consists of seven layers, each of which performs a specific set of functions in the process of transmitting data over a network.

In this tech journal, we will go through the seven layers of the OSI model, explaining the devices used at each level and the actions performed in each layer.

Layer 7: Application

The application layer is the layer that interacts directly with the end user or application. At this layer, network applications such as email clients, web browsers, and file transfer programs initiate communication and exchange data. Common protocols used at this layer include HTTP, FTP, and SMTP.

Devices used at this layer include:

• End-user devices such as desktop computers, laptops, and mobile devices
• Servers that host applications and data

Actions performed at this layer include:

• Encoding and formatting of data to be transmitted
• Presentation of data to the user or application
• Session management, including authentication and data synchronization

Layer 6: Presentation

The presentation layer is responsible for the representation and formatting of data. At this layer, data is translated into a standardized format that can be understood by the receiving system. Common protocols used at this layer include SSL, JPEG, and MPEG.

Devices used at this layer include:

• End-user devices such as desktop computers, laptops, and mobile devices
• Servers that host applications and data

Actions performed at this layer include:

• Data compression and encryption
• Translation of data formats between different systems
• Formatting of data for presentation to the user or application

Layer 5: Session

The session layer is responsible for establishing, managing, and terminating connections between devices. At this layer, sessions are established and maintained, and protocols are defined for checkpointing, suspending, and resuming data exchange.

Devices used at this layer include:

• End-user devices such as desktop computers, laptops, and mobile devices
• Servers that host applications and data

Actions performed at this layer include:

• Establishment, maintenance, and termination of sessions between devices
• Checkpointing, suspending, and resuming of data exchange
• Session encryption and authentication

Layer 4: Transport

The transport layer is responsible for end-to-end error-free delivery of data. At this layer, data is segmented into smaller pieces, and protocols are defined for error detection and correction, flow control, and congestion control. Common protocols used at this layer include TCP and UDP.

Devices used at this layer include:

• Routers and switches
• End-user devices such as desktop computers, laptops, and mobile devices

Actions performed at this layer include:

• Segmentation of data into smaller pieces for transmission
• Error detection and correction
• Flow control and congestion control

Layer 3: Network

The network layer is responsible for addressing and routing data between devices. At this layer, data is divided into packets and addressed for delivery. Common protocols used at this layer include IP, ICMP, and ARP.

Devices used at this layer include:

• Routers and switches
• End-user devices such as desktop computers, laptops, and mobile devices

Actions performed at this layer include:

• Addressing and routing of data between devices
• Packet fragmentation and reassembly
• Network congestion control

Layer 2: Data Link

The data link layer is responsible for reliable data transfer between adjacent network nodes. At this layer, data is framed and addressed for transmission over the physical layer. Common protocols used at this layer include Ethernet and Wi-Fi.

Devices used at this layer include:

• Network interface cards (NICs)
• Switches
• Bridges

Actions performed at this layer include:

• Framing and addressing of data packets for transmission
• Error detection and correction
• Flow control and access control

Layer 1: Physical

The physical layer is responsible for the physical transmission of data over a network. At this layer, data is transmitted as raw bits over the physical medium, such as copper wire, fiber optic cable, or radio waves.

Devices used at this layer include:

• Network interface cards (NICs)
• Repeaters
• Hubs

Actions performed at this layer include:

• Transmission and reception of raw bit streams
• Encoding and decoding of digital signals into physical signals
• Establishment and termination of physical connections