top of page

OSI Model Explained



INTRODUCTION

The Open Systems Interconnection (OSI) Model is a 7-layer framework that describes how computer networks function and connect to one another. There are 7 layers in the OSI model, each with its own set of protocols and functions. Each layer is designed in a modular formation, meaning that each layer has as little to do with the other layers as possible. This is a highly convenient feature because if a problem occurs in one layer, the issue can be isolated and will not largely affect the function of other layers. According to Mike Meyers' standard CompTIA Network+ Certification 7th edition book, (a manual that is vital to learning the comprehensive subject of networking), the OSI model is important for two main reasons:

1. It is used to diagnose and figure out problems in a network according to each layer.

2. It provides a common language and protocols for network techs to use to describe specific network functions.

There are a variety of ways to remember all 7 layers, such as "Please Do Not Throw Sausage Pizza Away" (forwards) or "All Pros Search Top Notch Donut Places" (backwards). Each layer is uniquely different from all other layers, so it is important to know which layer is which along with their identifying characteristics. In this post, we will be covering all the layers of the OSI model in detail.

LAYER 1- PHYSICAL

The first layer in the OSI model is the physical layer, and all it does is move data from one place to another. The Physical layer deals in the media of bits (0s and 1s). Layer 1 is known for copper cabling, radio waves, fiber optics, hubs, or anything else that involves the movement of data.

Key Words: bits, cabling, radio waves, hub/central boxes

LAYER 2- DATA LINK

The second layer of the OSI model is the Data Link layer. A common rule of recognition is that if a device deals with a MAC address, it probably belongs to the second layer. The Data Link layer has two main jobs:

  1. Logical Link Control (LLC)- talks to the operating system of a computer and handles multiple protocols.

  2. Media Access Control (MAC)- creates and addresses MAC addresses (both the destination and sending system's address).

A Media Access Control (MAC) address is a physical address that is burned into the ROM of a computer's NIC (Network Interface Card). Switches belong to Layer 2 of the OSI model because they use MAC addresses to send copies of the message frame to the port associated with the destination MAC address. This process ultimately moves the data in frames from the physical layer to the network layer.

Key Words: frames, switches, MAC addresses, NICs

LAYER 3- NETWORK

The Network layer creates and addresses packets to move from one network to another. The Internet Protocol (IP) is the primary protocol for the Network layer. Using IP, each piece of data gets addressed to its proper destination on the network. Each device on the network is assigned an IP address; an 8-bit logical address ranging from 0-255 that is used to forward data. Routers also exist at this OSI layer, for they use IP addresses to connect networks together.

Example IP address: 173.30.2.0

Key Words: packets, IP, routers

LAYER 4- TRANSPORT

The Transport layer is known for the segmentation and reassembly of data. Segmentation involves taking apart the data piece by piece in order to send it to the receiving system in smaller packets. This allows for easier transmission of data instead of sending it all in one massive packet. If one large packet was going to be sent, it would take much longer to send than segmenting the packet into smaller pieces. After the packet is received, the receiving system starts organizing the packet pieces into the correct order, initializing any errors of transmission at the same time.

Key Words: Segments

LAYER 5- SESSION


The Session layer is responsible for the talking between systems and connection-handling. It is in control of all computer sessions and communication: opening sessions, closing sessions, initializing sessions in real-time, etc.

LAYER 6- PRESENTATION


The Presentation layer of the OSI model is responsible for translating all data from the lower layers into a format that is acceptable for the Application layer to interpret. It is also responsible for the encryption and decryption of data.

Key Words: TSL/SSL

LAYER 7- APPLICATION


The final layer of the OSI model is the Application layer, which incorporates the source code of all applications visible on the network. The Application Layer is like the user interface layer of the OSI model, in which the user can visibly see this part of the network and interact with it.

Key Words: Applications (Telnet, FTP, Email clients, Web Browsers, etc.)


WHY CYBER PROFESSIONALS NEED TO KNOW THE OSI MODEL


Regardless if you are in the cyber industry or not, the OSI model should be memorized by all individuals in the IT industry mainly because of its relevance and importance to all technology as we know it. Specifically, as a cyber or infosec professional, the OSI model is useful for multiple reasons:

  • All 7 layers are the foundation of network security

The OSI model provides an important foundation to network security, which for any organization, is definitely a big deal. Network security is the set of protocols and procedures to protect networks and ensure the confidentiality, integrity, and availability (CIA triad) of networks. By fully understanding all the layers of the OSI model, a security professional is more capable of managing their network security needs.

  • Understanding the different kinds of attacks that can occur at each layer

Cyber professionals need to understand all layers of the OSI model so that they can predict what kinds of attacks may occur at each layer. Depending on the possible attack, security professionals then can implement the appropriate measures to prevent and detect any incidents.


  • The future of cloud computing!

Cloud computing has become the future of hardware, software, storage, multitasking, etc. (you name it), in the current technology era of today, and will continue to grow within the next 100 years. With its cost-effective approach and expansive capabilities, many organizations are switching to the cloud for its numerous benefits. The OSI model has been adapted to managing cloud infrastructure and addressing cloud computing needs. Below is an overview of Cisco's OSI model for the cloud:



  • Helps you maintain security standards

Knowing each layer's function and recognizing their correct operation is one way of maintaining your organization's network security standards. A cyber professional is more likely to identify any problems existing in a certain layer.

CONCLUSION

The OSI model is a key part of understanding the network communication process, as well as a highly important and useful part of the tech world. Besides its many applications, the OSI model is a great place for beginners to start learning about computing and troubleshooting networks.


SOURCES

bottom of page