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The 7 OSI Network Layers Explained

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The Open Systems Interconnection (OSI) network model defines a conceptual framework for communications between computer systems. the model is to ISO standards that identifies seven fundamental network layers, from physical hardware to high-level software applications.

Each layer in the model handles a specific network function. The standard helps administrators visualize networks, isolate problems, and understand use cases for new technologies. Many network equipment vendors advertise the OSI layer that their products are designed to fit into.

OSI was adopted as an international standard in 1984. It remains relevant today despite changes in network implementation that have occurred since it was first published. Cloud, edge, and IoT can all be accommodated within the model.

Diagram showing the 7 OSI network layers

In this article, we will explain each of the seven OSI layers separately. We’ll start from the lowest level, labeled Layer 1.

1. Physical layers

All networks start with the physical equipment. This layer encapsulates the hardware involved in communications, such as switches and cables. The data is transferred as a stream of binary digits, 0 or 1, that the hardware prepares from the input it has received. The physical layer specifies the electrical signals that are used to encode the data over the wire, such as a 5-volt pulse to indicate a binary “1.”

Errors in the physical layer tend to cause data not to be transferred at all. There could be a break in the connection due to a missing plug or incorrect power supply. Problems can also arise when two components disagree on the physical encoding of data values. For wireless connections, a weak signal can cause bits to be dropped during transmission.

The second layer of the model deals with communication between two devices that are directly connected to each other on the same network. It is responsible for establishing a link that allows the exchange of data through an agreed protocol. Many network switches work at Layer 2.

The data link layer will eventually pass bits to the physical layer. Because it sits on top of hardware, the data link layer can perform basic error detection and correction in response to physical transfer problems. There are two sublayers that define these responsibilities: Logical link control (LLC) which handles frame synchronization and error detection, and media access control (MAC) which uses MAC addresses to restrict how devices acquire permission to transfer data.

3. Network Layer

The network layer is the first level that supports data transfer between two separately maintained networks. It is redundant in situations where all of your devices exist on the same network.

Data arriving at the network layer from higher levels is first divided into packets suitable for transmission. Packets received from the remote network in response are reassembled into usable data.

The network layer is where several important protocols first meet. These include IP (to determine the route to a destination), ICMP, routing, and virtual LAN. Together, these mechanisms facilitate communications between networks with a familiar degree of ease of use. However, operations at this level are not necessarily reliable: messages are not required to succeed and may not necessarily be retrieved.

4. Transport layer

The transport layer provides high-level abstractions to coordinate data transfers between devices. The transport controllers determine where the data will be sent and the rate at which it should be transferred.

Layer 4 is where TCP and UDP are implemented, providing the port numbers that allow devices to expose multiple communication channels. As a result, load balancing is often placed at Layer 4, allowing traffic to be routed between ports on a target device.

Transport mechanisms are expected to ensure successful communication. Strict error controls are applied to recover from packet loss and retry failed transfers. Flow control is applied so that the sender does not overwhelm the remote device by sending data faster than the available bandwidth allows.

5. Session Layer

Layer 5 creates continuous communication sessions between two devices. Sessions are used to negotiate new connections, agree on their duration, and gracefully close the connection once the data exchange is complete. This layer ensures that sessions remain open long enough to transfer all the data that is sent.

Checkpoint control is another responsibility layer 5 has. Sessions can define checkpoints to facilitate progress updates and resumable transmissions. A new checkpoint can be set every few megabytes for a file upload, allowing the sender to continue from a particular point if the transfer is interrupted.

Many important protocols operate at Layer 5, including authentication and login technologies such as LDAP and NetBIOS. These establish semi-permanent communication channels to manage an end user session on a specific device.

6. Presentation Layer

The presentation layer handles data preparation for the application layer that comes next in the model. Once the data has been formed from the hardware, through the data link and transport, it is almost ready to be consumed by higher-level components. The presentation layer completes the process by performing any formatting that may be necessary.

Decryption, decryption, and decompression are three common operations found at this level. The presentation layer processes the received data into formats that can eventually be used by a client application. Similarly, the output data is reformatted into compressed and encrypted structures that are suitable for network transmission.

TLS is an important technology that is part of the presentation layer. Certificate verification and data decryption are handled before requests reach the network client, allowing the information to be consumed with confidence that it is authentic.

7. Application Layer

The application layer is the top of the stack. Represents the functionality perceived by end users of the network. Applications in the OSI model provide a convenient end-to-end interface to facilitate complete data transfers, without having to think about hardware, data links, sessions, and compression.

Despite its name, this layer is not related to client-side software, such as your web browser or email client. An application in OSI terms is a protocol that deals with the complete communication of complex data across layers 1-6.

HTTP, FTP, DHCP, DNS and SSH all exist in application layers. These are high-level mechanisms that allow direct transfers of user data between a source device and a remote server. You only need a minimal understanding of how the other layers work.

Summary

The seven OSI layers describe the transfer of data across computer networks. Understanding the roles and responsibilities of each layer can help you identify the source of problems and assess the intended use case for new components.

OSI is an abstract model that does not map directly to the specific networking implementations in common use today. As an example, the TCP/IP The protocol works on its own simpler system of four layers: network access, Internet, transport, and application. thesis abstract and absorb the equivalent OSI layers: The application layer covers OSI L5 to L7, while L1 and L2 are combined in the network access concept of TCP/IP.

OSI remains applicable despite its lack of direct real-world application. It has been around for so long that it is widely understood among managers of all backgrounds. Its relatively high level of abstraction has also ensured that it remains relevant in the face of new networking paradigms, many of which have targeted Layer 3 and above. Knowledge of the seven layers and their responsibilities can still help you appreciate the flow of data through a network while uncovering integration opportunities for new components.


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