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Unveiling the Layers: A Journey Through the OSI Model

Unveiling the Layers: A Journey Through the OSI Model

A beginner-friendly guide using restaurant analogies

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Abhijeet Sinha
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8 min read

TLDR:

OSI-Open Systems Interconnection-is a seven-layer model that standardizes the way devices communicate over a network. This facilitates interoperability and provides for modular design; network communication is conducted in terms of these layers:

  • Application: Provides network services directly to users; it includes, for example, browsing the web, e-mail, and similar examples.

  • Presentation: Handles data formatting, encryption, and compression for compatibility.

  • Session: Manages sessions, including setup, maintenance, and termination.

  • Transportation: Reliable data transfer by segmentation, flow control, and error control is done with the help of TCP/UDP.

  • Network: It decides on which path the data will travel using IP addresses and routing.

  • Data Link: Detects errors, provides MAC addressing, and framing of data in a network segment.

  • Physical: Transfers raw bits over cables, wireless signals, or other physical media.

Each of them has a different role, yet all work in harmony to provide a smooth process of communication. The OSI model provides invaluable assistance during troubleshooting, design, and understanding networks.

Introduction:

  • The OSI (Open Systems Interconnection) model is a conceptual model for understanding and implementing network communication. It was created by the International Organization for Standardization (ISO) in 1984 to standardize networking protocols and enable interoperability between differing systems.

  • The OSI model consists of seven layers, where each layer plays a unique role in the process of communication. The hierarchical layers of abstraction move from bottom to top from the physical layer (hardware-related) to the application layer (user-related). So, let us explore each layer in depth.

Fun Fact: The name "OSI" was almost "SIO" (Standard International Organization), but they realized it would sound too much like a disappointed sigh!

Application Layer:

Application layer is closest to the end user and this is where the user interacts with the application. It receives user data is generated and from where the request originates and acts as a portal to provide network services to application.

  • HTTP/HTTPS: The backbone of web browsing

  • SMTP/POP3/IMAP: Your email's best friends

  • FTP: The moving company of the internet

  • DNS: The internet's phone book

  • IDS/IPS (Intrusion Detection/Prevention Systems)

  • Application Proxy Servers

  • Application Gateways

Real-Life Analogy:

Think of a restaurant menu. The customer, interact with the menu or application layer to request dishes or services. The menu translates customer needs in question-terms of hunger-into specific items that the kitchen, or network, understands.

Presentation Layer: The Universal Translator

Presentation layer also known as Translation Layer is responsible for transforming the data such that it can be used and understood by Application Layer. This layer is responsible for handling tasks like data-encryption to protect during transmission, data-compression to reduce the size of data for efficient transmission, data-translation - converting data fomats (e.g. EBCDIC to ASCII))

  • SSL/TLS (Secure Sockets Layer/Transport Layer Security): Encrypts data for secure transmission

  • JPEG, GIF, PNG: Handles image formatting and compression

  • ASCII, EBCDIC: Manages character encoding and data conversion

  • MPEG: Handles video and audio compression and encoding

Network Devices operating on Presentation Layer:

  • SSL/TLS Accelerators

  • Encryption/Decryption Devices

  • Protocol Converters

Real-Life Analogy:

The waiter places your order and makes sure that the kitchen gets it right: If you ordered "medium rare" - a specific format request - the waiter ensures the chef cooks it to order. Encryption locks a delivery box so that when it arrives you can be certain that it is not tampered with.

Session Layer: The Digital Traffic Controller

Session Layer is responsible for managing ,controlling and terminating the communication between two applications. This layer makes sure that the data exchange happens in an organised and synchronised manner.

  • RPC (Remote Procedure Call): Enables programs to execute procedures on remote systems

  • PAP (Password Authentication Protocol): Provides basic password-based authentication

  • SIP (Session Initiation Protocol): Manages multimedia communication sessions

Network Devices operating on Session Layer:

  • Session Border Controllers

  • NetBIOS Gateways

  • Authentication Servers

Real-Life Analogy:

It's like saying it's a session, like making a reservation at a restaurant. It's like the host seats you, sets up your table for you, and it's a session. If you need to pause, say take a phone call, then it keeps your table reserved.

Transport Layer: The Quality Assurance Department

Transport Layer is the layer where it gets decided how the data will be transferred over the network in a reliable manner using TCP(Transimission Control Manner) or using UDP(User Datagram Protocol) in a speedy manner based on the Application layer protocols like HTTP/s. Depending on transport protocol selected the larger data set is divided into smaller data chunks/segments. This is the layer where source and destination port number headers are also assigned to the data segments.

  • TCP (Transmission Control Protocol):

    • Ensures all data arrives correctly.

    • Handles lost or corrupted packets.

    • Perfect for important stuff like emails and web browsing.

  • UDP (User Datagram Protocol):

    • Based on fire and forget mechanism, offers fast, connectionless data transmission without guarantee.

    • Perfect for streaming and gaming where speed matters more than perfection

    • The "we'll send it and hope for the best" protocol.

Network Devices operating on Transport Layer:

  • Transport Layer Firewalls

  • Load Balancers (Transport layer)

  • Stateful Packet Inspection Firewalls

  • Protocol Analyzers

Real-Life Analogy:

The kitchen prepares food in portions that are easy to deliver and assures its quality. In case something goes wrong, for example, they made a mistake in the order, they will remake it, and that is how it works with the recovery from an error. More specifically, TCP is a waiter who assures the order is perfect. UDP on the other side is like a buffet where you serve yourself fast; no checks are performed.

Network Layer: The Global GPS

Network Layer is responsible for transmission of data over the network. It is responsible for finding the shortest path to transmit the data segments to reach it’s destination over the network, also adds source and destination IP address headers to the data chunks and after this the segments are referred as packets(for TCP) or datagrams(for UDP).

Network Engineer Joke: "Why did the UDP packet cross the road? Because it didn't care if it made it to the other side!"

  • IP→Internet Protocol (IPv4 or IPv6): Routes packets between networks

  • ICMP (Internet Control Message Protocol): Reports errors and provides network diagnostics

  • IGMP (Internet Group Management Protocol): Manages multicast group memberships

Network Devices operating on Network Layer:

  • Routers

  • Multilayer Switches

  • Network Layer Firewalls

Real-Life Analogy:

The delivery service (e.g., Swiggy/Zomato) decides the best route to deliver food. Logical addressing is the delivery address (IP address) that the customer provides. The routing process is the driver choosing the most efficient path to customer’s location.

Data-Link layer is responsible for reliable delivery of frames between directly connected nodes and handles local addressing. This layer also adds source and destination MAC address headers to packets/datagrams and then these packets/datagrams are referred as Frames.

  • Ethernet: Enables data transmission between devices on a local network

  • PPP (Point-to-Point Protocol): Establishes direct connections between two network nodes

  • HDLC (High-Level Data Link Control): Provides error control for transmissions

  • MAC (Media Access Control): Controls hardware addressing and channel access

  • Switches

  • Bridges

  • Network Interface Cards (NICs)

  • Wireless Access Points (WAPs)

Real-Life Analogy:

Within the restaurant, the food is placed on trays (frames) for delivery to the table. Each tray has a table number (MAC address) so the waiter knows where it goes. If the tray is dropped (error), the food is replaced.

Physical Layer: The Digital Highway

Physical Layer, the bottom layer of OSI model is responsible for converting data into physical signals(electrical signal, light pulses or radio waves) over the physical medium.

  • DSL (Digital Subscriber Line): Transmits digital data over telephone lines

  • USB (Universal Serial Bus): Defines physical connection and power delivery standards

  • Bluetooth: Specifies wireless communication between devices over short distances

Network Devices operating on Application Layer:

  • Hubs

  • Repeaters

  • Network Cables

  • Fiber Modems

  • DSL Modems

Real-Life Analogy:

This is the infrastructure—tables, chairs, plates, and utensils in a restaurant. It’s the physical medium that makes the dining experience possible. Similarly, the physical layer consists of cables, switches, and wireless signals allowing network communication to take place.

Conclusion:

The OSI model is not just a concept; it's an indispensable resource for network communication. The OSI model simplifies the way we think about data flow and interoperability between diverse systems and technologies by breaking down complex processes into seven distinct layers.

Happy networking, and don't forget the classic network engineer's mnemonic: "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application)!

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