What Are OSI Layers?

• OSI Layers Definition

  OSI Layers Definition

  OSI layers, a fundamental part of the open systems interconnection model (OSI), operate in conjunction to transfer the information received from a particular device to another in a network.
• What is the OSI model?

  What is the OSI model?

  The OSI model, introduced in 1984 by the ISO, is the first globally recognized framework for standardizing network communication. It enlists the standard protocols or rules necessary for information exchange between two systems over a particular network as a conceptual model. It uses seven abstract OSI layers to split the network communication, each handling a specific task independently to ensure connectivity between communication agents.

  Although the OSI model doesn't have a significant role in modern TCP/IP-driven internet communication, it’s still helpful in conceptualizing network architecture and fixing network issues. IT teams can analyze the root cause of a network problem down to one specific abstract layer of this seven-layer model for more accurate troubleshooting. For resolving application-related issues, they can look to the upper layers of this framework, while lower layers are preferable for data transmission challenges.
• The 7 layers of OSI

  The 7 layers of OSI

  Outlined below are the 7 layers of OSI model:

  Application layer: The topmost layer among all OSI model layers that connect the user applications to the network. However, applications don’t reside on this layer; the protocol does. HTTP, FTP, and SMTP are typical examples of application layer protocols. As layer 7, it also identifies the communication parties and checks resource availability in the network.

  Presentation layer: This layer prepares data for layer 7 by checking its syntax and format against the standards. It also performs data translation, compression, and encryption to ensure communication parties understand the message accurately without encoding or formatting issues.

  Session layer: The session layer establishes and maintains the connection between communication agents during data exchange and ultimately terminates it after successful trade to avoid resource exhaustion. As layer 5, it also leverages checkpoints to prevent data transfer from scratch after interruption.

  Transport layer: This layer ensures reliable and accurate data exchange between the sender and the receiver node. Layer 4 also provides connection-oriented or connectionless communication with flow control and error control. After receiving data from the session layer, it divides this information into smaller chunks, called segments, and then transfers them. The transport layer on the receiving side aggregates these data segments into a whole message for the session layer to interpret.

  Network layer: Layer 3 handles inter-network communication by allowing devices from diverse networks to exchange data. It partitions the data segments received from layer 4 into packets on the sender's end and reassembles them again as packets on the receiver's end. Other notable functions of this layer include logical addressing, routing, and congestion control. Devices use IP addresses to identify themselves.

  Data link layer: This performs similar functions as layer 3 but within a network. In short, it manages intra-network communication. It divides the upper-layer packets into frames, making them suitable for transfer via physical wires in layer 1. Devices use MAC addresses to identify themselves.

  Physical layer: Layer 1 ensures a seamless physical connection between network nodes by enabling them to transfer and receive unstructured raw data in the form of 0s and 1s. It also performs line configuration and bit synchronization as the lowermost layer of the OSI model.
• Advantages of the OSI model

  Advantages of the OSI model

  The OSI reference model serves as a guiding framework for IT teams to build a robust and reliable network architecture by choosing appropriate networking hardware and software. It helps network admins accelerate troubleshooting by exploring the OSI model layers to pinpoint the exact cause of a network problem. This layered model also separates the services, interfaces, and protocols using different OSI layers, making it easy to understand and implement across various networks. The OSI layers explained in this model also help IT teams interpret the data exchange process followed by various network components.

  The OSI conceptual model is also vital for hardware and software vendors. It helps them manufacture universal products that follow most network protocols or standards for seamless communication in various networks. Besides product interoperability, it also allows them to specify network segments their device will support. Likewise, equipment vendors can use this framework to educate customers about the OSI model layers their devices are compatible with.
• How does data flow through the OSI model?

  How does data flow through the OSI model?

  For information exchange over a network, data must travel from the application to the physical layer of OSI on the sender's device. While on the receiver's end, it should flow in the reverse direction in the OSI layer stack. In the sending process, the information first reaches the application layer where it’s assigned a relevant protocol, such as SMTP, then forwarded to the presentation layer. Upon receiving the message, the presentation layer encrypts and compresses it if required and forwards it to the session layer to open a communication session. After this, the message reaches the transportation layer from which the process of data fragmentation begins. The data is broken into segments, then packets, and ultimately into frames at the transportation, network, and data link layer, respectively. Finally, the frames at the data link layer are forwarded to the physical layer for further transmission via physical cables in a bitstream of 1s and 0s.

  The receiver's device collects the bitstream via a physical medium, such as Wi-Fi and Ethernet cables. Then, the data travels through OSI layers again on this device but in the reverse direction towards the application layer. This iteration aggregates the data chunks received as a bitstream into frames, then packets, then segments, and ultimately into a human-readable message displayed on the recipient's device.
• Differences between OSI and TCP/IP

  Differences between OSI and TCP/IP

  TCP/IP, also known as the internet protocol suite, is more flexible and older than the OSI reference model. This model underpinning the modern internet architecture has fewer layers than the OSI framework to keep the network communication straightforward. TCP/IP has four layers: application, transport, internet, and network access layer. Its application layer handles all the functions of layers 5, 6, and 7 of the OSI model, whereas the network access layer operates as both Layer 1 and 2. However, the internet layer of TCP/IP is considered less reliable than the OSI data link layer as it doesn't perform sequencing and acknowledgment functions. It delegates these tasks to the transport layer.

  Other significant differences:

  • The TCP/IP model tackles specific networking issues through a unique set of internet standards and protocols. In contrast, OSI is a protocol-independent, common framework to address all sorts of network communication challenges.
  • In the OSI model, there’s a clear separation between interfaces, protocols, and services at each level, unlike TCP/IP.
  • Applications mostly rely on the OSI model's physical, data link, and network layer for data transfer over a network. In contrast, apps use all layers of TCP/IP for information exchange.
• Benefits of OSI layer 2 and 3 network mapping

  Benefits of OSI layer 2 and 3 network mapping

  Network mapping helps IT teams visualize the network architecture, including the linkages or dependencies of various components in a network. Similarly, OSI layer 2 and 3 network maps help IT teams discover and understand the interconnections between devices at these layers, including switch-to-switch and switch-to-router port links. Businesses can use automated tools to map, monitor, and visualize their network structure effectively. These network mapping tools can help track changes across the network through dynamically updated topology maps. IT admins can also schedule network scans to get timely alerts for events like network topology changes and device configuration errors using these tools. Such software can perform multi-tier network discovery to produce comprehensive, integrated network diagrams of OSI layer 2 and 3 with in-depth device details. IT teams can also export these topology maps into PDF, PNG, and other formats using automated tools.