What Is The OSI Model? Definition, Layers, and Importance
The OSI model specifies rules that describe how devices communicate in a networking environment.
The open systems interconnection (OSI) model is defined as a standard model used to describe the flow of information from one computing device to another operating in a networking environment. This article explains the OSI model in detail, its seven layers, and its importance in today’s networked world.
Table of Contents
What Is The OSI Model?
The open systems interconnection (OSI) model refers to a standard model used to describe the flow of information from one computing device to another operating in a networking environment. The model defines a set of rules and requirements for data communication and interoperability between different devices, products, and software in a network infrastructure. The OSI model is split into seven fundamental layers (bottom to top): Physical, Data Link, Network, Transport, Session, Presentation, and Application.
In 1984, the International Organization for Standardization (ISO) published the OSI framework to standardize network design and equipment manufacturing principles. Until OSI emerged, network architecture lacked the standard protocols necessary for effective data communication and design infrastructure.
As such, network administrators found installing, configuring, and setting up new equipment in existing networks challenging. Moreover, integrating such devices with outside networks seemed even more complex. With the arrival of the OSI reference model, administrators were able to design efficient network infrastructure where the equipment was capable of communicating with other universal networks.
Why does the OSI model matter?
Establishing standard communication protocols is key to formalizing communication across internal and external networks such as the cloud or the internet. Such standardization facilitates faster communication between devices and networks, irrespective of the data resides, where it is sent, or from where it is received.
The OSI model allows equipment manufacturers to define their standards and protocols while maintaining interconnectivity with other manufacturers. Moreover, the OSI model is key to troubleshooting network devices. When a networking unit fails, or an application goes down and cannot communicate with the rest of the network, the OSI model is handy as it allows administrators to zero in on the specific OSI layer and troubleshoot the failed component. Thus, the conceptual OSI framework is crucial for designing, manufacturing, and troubleshooting network technology.
How does the OSI model facilitate data communication?
Let’s take an example of an email application to understand better how data flows through the OSI model. You must note that the data flows from layer 7 to layer 1 at the sender side, while from layer 1 to layer 7 at the recipient’s side.
When an individual sends an email, it is sent to the presentation layer (layer 6) with the help of a standard outgoing protocol (SMTP protocol). At layer 6, the email message is compressed and forwarded to the session layer (layer 5). The session layer then establishes communication between the sender and the outgoing server, thereby starting the communication session.
The session layer then sends the message to the transport layer (layer 4), which performs data segmentation. The segmented message is further sent to the network layer (layer 3), which breaks down the segments into data packets. These data packets are forwarded to the data link layer (layer 2) where data packets are further divided into frames. These frames are then sent to the physical layer (layer 1) where the received data is converted into zeros and ones. These bit streams are then sent through the network via cables or network connections.
As the email message reaches the recipient’s device, the above process is reversed, implying that data flow starts at the physical layer and ends at the application layer. Thus, the above bit streams of zeros and ones are converted into the original email message that is finally available on the recipient’s email. When the recipient responds to this mail thread, the process is repeated with the data flow starting from layer 7 to layer 1. In this way, the OSI model facilitates communication between network components.
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OSI Model Layers
Each layer in the OSI model performs a defined function essential to maintain smooth data flow in a network. It communicates and works with layers above and below it to allow physical and virtual data communication across a networking architecture. Let’s understand each layer in greater detail. We start with the uppermost layer 7 and move to layer 1.
7. Application layer
The application layer is the topmost layer in the OSI model. The layer establishes communication between the application on the network and the end user using it by defining the protocols for successful user interaction. An excellent example of this layer is that of web browsers.
Application layer protocols allow the software to direct data flow and present it to the user. Some of the known protocols include Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP), and File Transfer Protocol (FTP).
Key functions:
- The application layer provides user interfaces (UI) that are key to user interaction
- Supports a variety of applications such as e-mail and remote file transfer
In summary, layer 7 ensures effective communication between applications on different computing systems and networks.
6. Presentation layer
The presentation layer is often referred to as a syntax or translation layer as it translates the application data into a network format. This layer also encrypts and decrypts data before transmitting it over the network. For instance, layer 6 encrypts data from the application and decrypts it at the recipient’s end, ensuring secure data transmission. Moreover, this layer is known to compress data received from layer 7 to reduce the overall size of the data transferred.
Key functions:
- Performs data translation based on the application’s data semantics
- Encrypts and decrypts sensitive data transferred over communication channels
- Performs data compression to reduce the number of bits in exchanged data
In summary, layer 6 ensures that the communicated information is in the desired format as required by the receiving application.
5. Session layer
The session layer establishes a communication session between communicating entities. The session is maintained at a sufficient time interval to ensure efficient data transmission and avoid wasting computing resources.
This OSI layer is also responsible for data synchronization to maintain smooth data flow. This implies that in situations where large volumes of data are sent at once, layer 5 can break down the data into smaller chunks by adding checkpoints.
For example, let’s say you want to send a 500-page document to another person. In this case, this layer can add checkpoints at 50 or 100 pages. This is in case a document transfer is interrupted due to network or system failure. Once the system failure issue is resolved, the document transfer resumes from the last checkpoint. Such a system saves time by not restarting the file transfer from the beginning.
Key functions:
- Opens maintains, and closes communication sessions
- Enables data synchronization by adding checkpoints to data streams
In summary, layer 5 establishes, maintains, synchronizes, and terminates sessions between end-user applications.
4. Transport layer
The transport layer allows safe message transfer between the sender and the receiver. It divides the data received from the layer above into smaller segments. It also reassembles the data at the receiver side to allow the session layer to read it.
Layer 4 performs two critical functions: flow control and error control. Flow control implies regulating data transfer speeds. It ensures that the communicating device with a good network connection does not send data at higher rates, which is difficult for devices with slower connections to handle. Error control refers to the error-checking functionality to ensure the completeness of data. In incomplete data cases, this layer requests the system to resend the incomplete data.
Examples of transport layer protocols include transmission control protocol (TCP) and user datagram protocol (UDP).
Key functions:
- Ensures completeness of each message exchanged between source and destination
- Maintains proper data transmission through flow control and error control
- Performs data segmentation and reassembling of data
In summary, layer 4 is responsible for transmitting an entire message from a sender application to a receiver application.
3. Network layer
The network layer enables the communication between multiple networks. It receives data segments from the layer above, further broken down into smaller packets at the sender side. On the receiver side, this layer reassembles the data together.
The network layer also handles routing functionality, wherein the data transmission is accomplished by choosing the best possible route or path that connects different networks and ensures efficient data transfer. This network layer uses internet protocol (IP) for data delivery.
Key functions:
- Handles routing to recognize suitable routes from sender to receiver
- Performs logical addressing that assigns unique names to each device operating over the network
In summary, layer 3 is responsible for dividing segmented data into network packets, reassembling them at the recipient’s side, and identifying the shortest yet most suitable and secure path for transmitting data packets.
2. Data link layer
The data link layer transmits data between two nodes that are directly connected or are operating over the same network architecture. Typically, this layer takes data packets from layer 3 and breaks them down into frames before sending them to the destination.
Layer 2 is divided into two sub-layers: media access control (MAC) and logical link control (LLC). The MAC layer encapsulates data frames transmitted through the network connecting media such as wires or cables. In situations where such data transmission fails, LLC helps manage packet retransmission.
The well-known data link layer protocol includes the Address Resolution Protocol (ARP) that translates IP addresses to MAC addresses to establish communication between systems whose addresses vary in bit length (32 bits vs. 48 bits).
Key functions:
- Detects damaged or lost frames and retransmits them
- Performs framing where data received from layer 3 is further subdivided into smaller units called frames
- Updates headers of created frames by adding the MAC address of the sending device and receiving device
In summary, layer 2 is responsible for setting up and terminating physical connections between participating network nodes.
1. Physical layer
The last OSI layer is the physical layer that manages physical hardware and network components such as cables, switches, or routers that transmit data.
In the context of data, layer 1 transmits data in the form of ones and zeros. Technically, this layer picks up bits from the sender end, encodes them into a signal, sends the signal over the network, and decodes the signal at the receiver end. Thus, without layer 1, communicating data bits across network devices through physical media is not possible.
Key functions:
- Synchronizes data bits
- Enables modulation (conversion of a signal from one form to another for data transmission)
- Defines data transmission rate (bits/sec)
- Outlines the arrangement of network devices across different network topologies such as bus, tree, star, or mesh topology
- Defines transmission modes such as simple or half-duplex mode
In summary, layer 1 is responsible for transmitting data bits of 0s and 1s between network systems via electrical, mechanical, or procedural interfaces.
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Importance of OSI Model
IT professionals have relied on the OSI model for decades to understand and resolve networking problems that occur during the networking process. Let’s dive deeper into why the OSI model is essential even today.
1. Handles networking vulnerabilities & security issues
The design of the OSI model facilitates the breakdown of the communication system into seven distinct layers. These layers allow the isolation of networking issues at their origin. Thus, one can address networking vulnerabilities and security issues such as communication failures, cabling problems, or bad routers based on the OSI layers they impact without disturbing the entire OSI framework. The OSI model allows the freedom to focus on securing, optimizing, and troubleshooting each layer independently of the other.
2. Suitable for cloud-first environments
Most businesses and tech companies have already adopted the cloud-first policy to enable IT modernization. Information security is following suit as it is now changing to the cloud-first approach. However, despite the move, the OSI framework remains relevant even today as it helps detect security threats throughout an organization’s tech stack.
3. Helps create an inventory of physical assets, security resources, & applications
OSI layers allow you to classify your firm’s physical assets and create an inventory for the applications your employees often use. The model gives you a better understanding of where the company’s data resides, irrespective of whether it is an on-premise or cloud environment. Such data allows you to refine your information security policy further. Thus, you can invest in appropriate security solutions for your firm once you have the data visibility and knowledge of the OSI layers responsible for handling sensitive data.
Let’s say you have stored your organization’s critical data in SaaS services. In this case, simple monitoring and safeguarding data through an endpoint manager may be risky. API-based data discovery solutions can prove to be beneficial. Your company can invest its resources and money in more API-based solutions designed to monitor several such cloud services seamlessly.
The OSI model allows the inventory of security resources and assets. As such, companies migrating to the cloud for the first time can use the OSI framework as it can assist them in determining the specific security concerns that cloud adoption may bring.
4. Allows updates to standard OSI models
Regarding securing the cloud infrastructure, several tech companies have customized and updated their OSI models to make the layers more operationally functional for different cloud services, such as IaaS. Considering the above case and the flexible nature of the OSI system, it is worthwhile to decide whether you intend to design and modify the OSI model to enhance the security of your networking environment.
5. Beneficial for equipment manufacturers
The OSI model is also crucial for hardware manufacturers as it allows them to create their own devices having unique configurations that can communicate over any network, irrespective of their hardware specifications.
Thus, considering all these factors, it would be safe to say that the OSI model is highly relevant even today and may continue to be so over the coming decades.
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Takeaway
The OSI model plays a crucial role in standardizing a networking system and assists in troubleshooting several networking problems. It is a helpful framework for equipment manufacturers that enables them to create products that can communicate and interact with any software, irrespective of its physical makeup.
The OSI model enhances the interoperability between devices used by end users, thereby ensuring smooth communication between different computer networks. Advanced standards such as TCP/IP may replace OSI one day; however, network administrators may continue to use the OSI model to safeguard their computing systems.
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