Have you ever wondered what exactly happens behind the scenes when you send an email or download a file from the internet? The process of getting information like emails, web pages, and files from a computer on one network to another computer often feels magical. However, complex communication is occurring every step of the way to make it happen.
Understanding models like the OSI (Open Systems Interconnection) model helps shed light on the “magic” of network communication. Even if you don’t work in IT, grasping networking concepts goes a long way in using technology more effectively.
In this beginner’s guide, we’ll strip away the complexity and provide a friendly explanation of the OSI model works. You’ll finish this article with a solid grasp of:
- What the OSI model is
- Its 7 layers and what each one does
- How it allows devices to communicate
- Why it still matters even in modern networks
Let’s get cracking!
A Little History First…
Before diving into the details of the OSI model, it‘s helpful to understand a bit of background.
The OSI model was created in the late 1970s by an international organization called the ISO (International Organization for Standardization). At the time, many vendors like IBM, DEC, and Xerox were creating computer networking technologies. However, these vendors‘ products were largely incompatible with each other.
The ISO aimed to create a standard reference model for computer networking to enable interoperability between various vendors and technologies. This reference model became known as the Open Systems Interconnection (OSI) model.
The official OSI model with 7 layers was published in 1984. While modern networks like the internet do not strictly follow the OSI model, it nonetheless provides a handy conceptual framework for understanding distinct steps in network communication.
Now let’s explore what exactly this 7 layer cake consists of!
The 7 Layers of the OSI Model
The OSI model consists of 7 hierarchical layers, with each layer building upon the layer below it. Here’s a quick overview from the highest to lowest level:
Layer 7 – Application Layer: Interface between network communication and software applications. Includes protocols like HTTP, SMTP, FTP.
Layer 6 – Presentation Layer: Translates and encrypts data into standardized formats.
Layer 5 – Session Layer: Manages communication sessions and coordinates dialogues between source and destination.
Layer 4 – Transport Layer: Transmits data using transmission protocols and performs error checking.
Layer 3 – Network Layer: Routes and forwards data packets from source to destination.
Layer 2 – Data Link Layer: Frames data packets and provides node-to-node data transfer.
Layer 1 – Physical Layer: Transmits raw bit streams over physical hardware like cable, fiber optic, WiFi.
Here is a diagram summarizing the OSI model‘s layers:
[Insert OSI model diagram with 7 layers]Let‘s now walk through what happens at each layer, using the example of sending an email from your computer to a recipient‘s email server over the public internet.
Layer 7 – Application Layer
The application layer serves as the interface between the software application you’re using and underlying network communication. Protocols at this layer include HTTP (for web browsing), SMTP (for email), and FTP (for file transfer).
In our email example, the email client software on your computer uses SMTP to communicate with the email server. The application layer adds header information like sender, receiver, subject line, mail body, attachments to the message.
Layer 6 – Presentation Layer
The presentation layer is responsible for preparing the data for transmission over the network. It performs data translation, compression, and encryption:
-
Data translation: Converts data into standardized formats that network layers can understand. For example, ASCII text, JPEG images, MPEG video.
-
Data compression: Reduces the number of bits needed to represent data. This saves bandwidth and speeds transmission.
-
Encryption: Scrambles data using encryption algorithms to prevent unauthorized access. Common protocols like SSL and TLS operate at this layer.
In our email example, the presentation layer would translate text and attachments into standardized formats. It would also encrypt the communication using TLS to prevent eavesdropping of sensitive email content.
Layer 5 – Session Layer
The session layer coordinates communication and maintains sessions between endpoints. It establishes, manages and terminates connections between local and remote applications.
Functions of session layer include:
- Session establishment: Initializes communication channel between source and destination
- Syncing: Inserts checkpoints to ensure reliably delivery of data streams
- Session termination: Closes communication channel when data transmission is complete
For our email, the session layer sets up and maintains a communication session between your email client and the mail server until the message transfer finishes.
Layer 4 – Transport Layer
The transport layer manages end-to-end transmission of data from source to destination host. Key responsibilities include:
- Segmentation: Divides data streams into segments called packets or datagrams.
- Error checking: Verifies correct packet delivery using checksums, ACK/NACK signals, retries, etc.
- Congestion control: Manages data traffic to prevent overloading resources.
Common transport layer protocols are TCP and UDP. TCP provides reliable transmissions using sequence numbers, retries, and error checking. UDP offers simpler connectionless communication without guarantees.
For sending email, TCP is used at transport layer to enable reliable point-to-point message delivery. After breaking the email data into packets, the transport layer adds source and destination port number and performs error checking.
Layer 3 – Network Layer
The network layer, also called IP layer, handles routing and forwarding of data packets across multiple networks. Key aspects are:
- Logical addressing: Encapsulates packets with logical source and destination addresses like IP addresses. This enables communication between hosts that are not physically connected.
- Routing: Determines network routes for forwarding data packets via various physical links.
- Packet forwarding: Transmits outgoing packet links and receives incoming packets.
In our email example, the network layer determines the route over the internet to transmit TCP packet segments from your computer to recipient‘s email server IP.
Layer 2 – Data Link Layer
The data link layer establishes communication between physically connected nodes on the same local network. Responsibilities are:
- Framing: Structures raw bit streams into frames or packets
- Physical addressing: Adds physical hardware addresses like MAC addresses for source and destination hosts
- Error detection: Detects errors from physical transmission medium
For computers on the same LAN, the data link layer handles delivery of frames between their MAC addresses. It helps move packet segments from network layer within that local network.
Layer 1 – Physical Layer
The physical layer deals with physical devices and electrical/optical signals for binary data communication. This includes:
- Cables: Twisted pair copper wire, fiber optic cable, etc.
- Wireless media: Radio waves, microwaves, infrared waves.
- Network hardware: Hubs, repeaters, network adapters, modems.
At this layer, the physical medium and hardware carry the raw bit stream from computer to router onto the internet cable backbone. Electrical pulses or light signals convey email‘s 1s and 0s!
OSI Model vs. TCP/IP
The OSI model provides a solid theoretical basis to understand networking layers. However, modern networks are more closely aligned with the TCP/IP model consisting of four layers:
- Application (HTTP, SMTP etc.)
- Transport (TCP, UDP)
- Internet (IP and routing)
- Link layer
The TCP/IP model led to the interconnected network of networks we know today as the internet. It combines the physical, data link and network layers of OSI into one "Internet" layer.
Here‘s a comparison of the two models:
[Insert table comparing OSI and TCP/IP layers]The OSI model is considered more of an idealized, conceptual framework. The TCP/IP model evolved from practical implementations used in the internet and most internal business networks.
However, the OSI model still provides value as pedagogical tool to systematically understand the modular functions of networking communication.
Why the OSI Model Still Matters
Given it was created 35+ years ago, is the OSI model still relevant in modern networking?
The answer is yes. Here’s why the OSI abstraction continues providing value:
⛓️ Modularity: Divides networking tasks into independent layers for flexibility. New protocols and technologies can be deployed by changing specific layers rather than entire stack.
🔬Troubleshooting: Isolates issues and failures to individual OSI layers. If something breaks, you know specifically which layer to fix.
🎯 Single reference model: Universal way to discuss network architecture and design. Vendors must conform systems to standard interfaces between layers.
In addition, while TCP/IP dominates business networks and the internet, OSI is still widely used in industrial, telecom, and distributed computing applications. Understanding OSI principles helps you grasp modern networking topology concepts as well.
Summing Up the OSI Model
We‘ve covered a ton of ground explaining the OSI model layers! Here is a summary diagram of the model:
[Insert summary diagram of 7 OSI layers]To recap, here are answers to some common OSI-related questions:
⛓️ What does OSI stand for? Open Systems Interconnection
🕵🏻♂️ Who developed the OSI model? Developed by the International Organization for Standardization (ISO) in the late 1970s.
📆 When was the OSI model established? Initial concept work began in the 70s, but first official specification published in 1984
💻 Is the OSI model still used today? Yes, it still provides a solid conceptual foundation for network communication across various technologies.
I hope this article helped demystify some of the “magic” behind networking communication! Let me know in the comments if you have any other questions. Understanding these fundamental concepts goes a long way in leveraging technology more effectively as a user and IT professional.
