[Day 3] 2. OSI Model

OSI Model Overview:

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO) in the late 1970s.
It is a standard for network communication, providing a framework to understand and implement different communication protocols.

Role of ISO in Standardization:

The ISO is responsible for defining various global standards, including those for weights, volume (e.g., liter), distance (e.g., kilometer), length (e.g., meter), time (e.g., seconds), and more.
ISO certification is given to companies that meet certain quality standards, ensuring they follow standardized processes.

Why Standardization is Important?

Standardization is crucial for ensuring flawless communication between different networks and devices.
Without these standards, communication across diverse systems and networks could become inconsistent or error-prone, making global connectivity difficult.

7 Layers

1. Physical Layer

Function: Transmits raw bitstreams over a physical medium.
Protocols: RS-232, ISDN (Integrated Services Digital Network), 100BaseTX, Ethernet standards.
Devices: Hubs, repeaters, network cables (Ethernet cables, fiber optics), and modems.

2. Data Link Layer

Function: Provides node-to-node data transfer and handles error correction from the physical layer.
Protocols:
- Ethernet (IEEE 802.3)
- Point-to-Point Protocol (PPP)
- Frame Relay
- ATM (Asynchronous Transfer Mode)
Devices: Switches, bridges, and network interface cards (NICs).

3. Network Layer

Function: Manages packet forwarding, routing, and addressing across multiple networks.
Protocols:
- Internet Protocol (IPv4 and IPv6)
- Internet Control Message Protocol (ICMP)
- Address Resolution Protocol (ARP)
- Multiprotocol Label Switching (MPLS)
Devices: Routers and layer 3 switches.

4. Transport Layer

Function: Ensures reliable or unreliable delivery of messages between hosts.
Protocols:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
Devices: Firewalls (which can operate at this layer for traffic filtering).

5. Session Layer

Function: Manages sessions between applications, establishing, maintaining, and terminating connections.
Protocols:
- NetBIOS
- Remote Procedure Call (RPC)
- Point-to-Point Tunneling Protocol (PPTP)
Devices: Gateways that manage sessions.

6. Presentation Layer

Function: Translates data formats for the application layer; handles encryption and compression.
Protocols:
- Secure Sockets Layer (SSL)
- Transport Layer Security (TLS)
- JPEG, MPEG for media formats.
Devices: None specifically; this layer is often implemented in software.

7. Application Layer

Function: Interfaces directly with end-user applications to provide network services.
Protocols:
- Hypertext Transfer Protocol (HTTP)
- Simple Mail Transfer Protocol (SMTP)
- File Transfer Protocol (FTP)
- Domain Name System (DNS)
Devices: Web servers, email servers, and application servers.

What exactly happens?

Sender’s Process: As data moves down through the layers, it is encapsulated into different forms:
- Data at the Application Layer is encapsulated into a Segment at the Transport Layer.
- The Segment is then encapsulated into a Packet at the Network Layer.
- The Packet becomes a Frame at the Data Link Layer.
- Finally, it is transmitted as Bits at the Physical Layer.
Receiver’s Process: As data moves up through the layers, it is decapsulated:
- Bits are received at the Physical Layer and converted back into a Frame at the Data Link Layer.
- The Frame is then de-encapsulated into a Packet at the Network Layer.
- The Packet is further de-encapsulated into a Segment at the Transport Layer.
- Finally, the original Data is extracted at the Application Layer.

This encapsulation and decapsulation process ensures that data can be properly packaged for transmission and then accurately reconstructed upon receipt. Each layer adds or removes specific headers and trailers to facilitate this communication effectively.