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Home/Blog/Articles/A beginner’s guide to the OSI Network Model

A beginner's guide to the OSI Network Model

1
Network Model Design Principles
2
Layer 1, the physical layer (L1)
3
Layer 2, the data link layer (L2)
4
Layer 3, the network layer (L3)
5
Layer 4, the transport layer (L4)
6
Layer 5 session layer (L5)
7
Layer 6, the data presentation layer (L6)
8
Layer 7, the application layer (L7)
9
Critique of the OSI Model
10
Conclusion: The Role of the OSI Model in Network Construction
The OSI model serves as a standard reference model. The full name of this model is "Open Systems Interconnection Basic Reference Model". The model comprises seven layers. Firstly, let's discuss general information before delving into specific details.

Network Model Design Principles

The OSI network model consists of seven layers, hierarchically arranged from highest to lowest. The highest layer is the seventh (Application), while the lowest is the first (Physical). Developed in the 1970s, the OSI model describes data network architecture and operational principles.

Data transmission involves a sender device, a receiver device, and data itself that needs transmitting and receiving. For most users, sending data is straightforward – they simply send it. The seven-layer OSI model meticulously details what occurs during data transmission and reception.

At the 7th level, information exists as data; at the 1st it exists as bits. The process in which information is sent and transitions from data to bits is known as encapsulation. The reverse process, where information received as bits at the first level is transformed into data at the seventh level, is termed decapsulation. At each of the seven layers, information is represented as Protocol Data Units (PDUs).

Consider this example: User 1 sends a picture that is processed as data at the seventh level; this data must traverse all the way down to the lowest (first) level, where it is presented as bits. This process is referred to as encapsulation. User computer 2 accepts bits that are to be transformed back into data. This reverse process is known as decapsulation. This article will explore what happens to information at each of the seven levels, including how and where bits are converted back into data.



Layer 1, the physical layer (L1)

Let’s begin at the lowest level. It handles the exchange of physical signals between physical devices, the so-called hardware. Computer hardware doesn’t understand images or their content; it only recognizes a picture as a series of zeros and ones, that is, bits.

Each layer has its own PDUs (Protocol Data Units), presented in a format that is comprehensible at that layer and possibly the next before being converted. Handling of raw data occurs only between levels 5 and 7.

Devices at the physical layer operate with bits. These are transmitted via cables (such as fiber optics) or wirelessly, for example, through Bluetooth, IRDA, Wi-Fi, GSM, 4G, etc.



Layer 2, the data link layer (L2)

When two users are on a network consisting only of two devices, it represents an ideal scenario. But what if there are more devices involved?

The second layer solves the problem of addressing during information transfer. The data link layer receives bits and converts them into frames. Here, the objective is to create frames with both sender and receiver addresses, and then transmit them across the network.

The link layer includes two sublayers: MAC and LLC. MAC (Media Access Control) assigns physical MAC addresses, while LLC (Logical Link Control) checks and corrects data, managing its transmission. For simplicity, we place LLC at the second level of the model, but strictly speaking, LLC spans between the first and second levels.

At OSI’s second layer is where network switches work, tasked with transmitting frames from one device to another using only physical MAC addresses.

The data link layer actively utilizes the ARP (Address Resolution Protocol). ARP maps 64-bit MAC addresses to 32-bit IP addresses and vice versa, facilitating both encapsulation and decapsulation of data.



Layer 3, the network layer (L3)

A new concept introduced at this level is routing. Third-level devices, routers, were developed for this task. Routers receive MAC addresses from previous layer switches and focus on routing data between devices while taking into account any potential network issues.



Layer 4, the transport layer (L4)

All seven of the OSI model’s layers can be categorized into two groups:

  • Media layers,
  • Host layers.

Media Layers (L1, L2, L3) handle information transmission (via wired or wireless means) and are utilized by network devices like switches and routers. Host Layers (L4, L5, L6, L7) are directly used on devices such as desktops or mobiles.

The fourth layer acts as an intermediary between Host Layers and Media Layers, more closely associated with the former. Its primary function is transporting packets. While transportation can lead to losses, some data types are more susceptible to loss than others. For instance, losing vowels in text can obscure meaning, whereas missing a few frames in a video stream barely impacts the end user. To transmit data highly sensitive to loss at the transport layer, the TCP protocol is employed to ensure the integrity of the information delivered.

For multimedia files, minor losses are less significant than delays. For data particularly sensitive to delays, the UDP protocol is used to facilitate communication without needing to establish a connection.

In TCP transmissions, data is segmented. A segment is a portion of a packet. When a data packet exceeding network capacity arrives, it is divided into segments of permissible size. Segmentation is also necessary in unreliable networks where there’s a high chance of losing large packets. In UDP data transmission, packets are divided into datagrams. A datagram is also a part of a packet but should not be confused with a segment.

The main difference between datagrams is autonomy. Each datagram contains all the necessary headers to reach the final destination, so they are network-independent and can be delivered by different routes and in different order. Loss of datagrams or segments results in fragmented pieces of data that cannot be correctly processed. 

The first four levels are specialized areas for network engineers.  They don’t encounter the last three as much because the fifth, sixth, and seventh are managed by developers.



Layer 5 session layer (L5)

Besides level 5, raw data is also used at levels six and seven.  The session layer ensures ongoing communication sessions are maintained.  The fifth layer provides a service to the following: it manages the interaction between applications, and allows to synchronize tasks, terminate sessions, and exchange information.

Session-level services are commonly used in environments requiring remote procedure calls to perform actions on remote computers or independent systems within a single device hosting multiple operating systems.

A typical operation at level five could be conducting a video call across a network. During video calls, the audio and video streams must be synchronized. Adding a third person to a two-person conversation turns it into a conference. The fifth level’s task is to ensure that participants can identify who is speaking.



Layer 6, the data presentation layer (L6)

This level’s task should be obvious from its name. Layer 6 manages protocol transformation and the encoding/decoding of data. Layer 6 also deals with displaying images (in JPEG, GIF, etc.) and video-audio formats (in MPEG, QuickTime). Additionally, it handles data encryption when protection is needed during transmission.



Layer 7, the application layer (L7)

The seventh layer is the layer of applications. The application layer serves as the user interface for the entire OSI model, interacting with other layers only minimally.

All services provided by the seventh layer from others are utilized to deliver data to the user. Protocols at Layer 7 do not need to handle routing or ensure data delivery, as these are managed by the previous six layers. The role of Layer 7 is to utilize its protocols to present data in a format understandable to the user.



Critique of the OSI Model

The seven-layer model was adopted as the ISO/IEC 7498 standard, which remains effective today, though it has its flaws. Key criticisms include poor timing, inadequate technology, delayed implementation, and unsuccessful policies.

The first drawback is poor timing. A disproportionate amount of time was spent developing the model without adequate focus on existing standards at that time. Consequently, the model is criticized for not reflecting reality. There’s some truth in these claims, as other companies were better prepared to work with the widely adopted TCP/IP model when OSI was introduced.

The second drawback is inadequate technology. The widespread adoption of the TCP/IP stack is often cited as evidence of OSI’s technological shortcomings. OSI protocols frequently duplicate each other, with functions unevenly distributed across layers, allowing similar tasks to be addressed at different levels. The original printed documentation of the architecture is one meter thick.

The division into seven layers was driven by political rather than technical reasons. In practice, it sometimes makes sense to omit Layers 5 and 6, and in rare cases, only the first 4 layers are necessary. 

Moreover, unlike TCP/IP, OSI has never been linked with UNIX. The OSI model failed to achieve widespread adoption because it was designed as a closed system, promoted by European telecom companies and the U.S. government. The TCP/IP protocol stack was initially open to all, enabling it to become popular among advocates of open-source software.

Although the primary issues with OSI architecture were political, its reputation suffered, preventing widespread adoption. Nevertheless, the OSI model is still commonly used in network technologies, especially in network switching.



Conclusion: The Role of the OSI Model in Network Construction

This article explores the principles behind the OSI network model. Each of the seven layers of the model has a specific function. In truth, the OSI architecture is more complex than described here. There are additional layers, such as the eighth layer, often referred to as the user himself.

As previously mentioned, if printed, the original documentation detailing all the network construction principles within this model would be one meter thick. However, companies actively use OSI as a benchmark. We have outlined only the basic structure in beginner-friendly terms.

The OSI model is utilized as a diagnostic tool in networking. If there’s a failure in the network, it’s much easier to pinpoint the problematic layer than to attempt rebuilding the entire network.

Understanding the network’s architecture simplifies both its construction and diagnosis. Just as a house cannot be built without a defined architecture, a network cannot be constructed without understanding the OSI model. When designing, it’s crucial to consider all aspects of development. It’s essential to account for how each layer interacts with others, the level of security provided, data encryption within the network, how much user growth the network can handle without failing, and whether the network can be transferred to another machine, among other factors. Each listed criterion corresponds to one of the seven level functions.

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