UNDERSTANDING OF 7 LAYERS OF OSI MODEL

1. Upper 3 Layers — Application-Level Communication

Layer 7 — Application

The Application layer provides network services to applications.

For example, when a browser wants to communicate with a web server, it can use HTTP/HTTPS.

Browser
   ↓
HTTP Request
   ↓
Application Layer

Examples of application-layer protocols include:

• HTTP/HTTPS
• DNS
• SMTP
• FTP

Layer 6 — Presentation

The Presentation layer is responsible for how data is represented.

Its common responsibilities include:

• Data translation/format conversion
• Encryption and decryption
• Compression and decompression

For example:

Application Data
       ↓
Presentation
       ↓
Encode / Encrypt / Compress
       ↓
Transport

So we can think of the Presentation layer as:

“How should the data be represented, secured, or compressed?”

Layer 5 — Session

The Session layer is responsible for establishing, managing, and terminating communication sessions between applications.

Think of it as managing a conversation:

Client                         Server

    ←── Establish Session ──→
    ←──── Communication ─────→
    ←──── Communication ─────→
    ←── Terminate Session ──→

It can also provide synchronization and checkpoints during long communications.

Upper 3 in simple terms

Application → What service do I need? Presentation → How should the data be represented? Session → How should the communication session be managed?

2. Middle 2 Layers — Host-to-Host Communication

The Transport and Network layers are responsible for communication between hosts, but they have different responsibilities.

Layer 4 — Transport

The Transport layer provides end-to-end communication between applications running on different hosts.

TCP provides features such as:

• Reliable delivery
• Retransmission of lost data
• Flow control
• Congestion control
• Port numbers for identifying applications

For example, if TCP detects that some data was lost, it can request/retransmit that data.

So:

Transport = End-to-end delivery

Layer 3 — Network

The Network layer is responsible for host-to-host delivery and routing.

Its main responsibilities include:

• Logical addressing using IP addresses
• Routing packets
• Forwarding packets through routers

For example:

Your Computer
      ↓
Router
      ↓
ISP Router
      ↓
Internet
      ↓
Server

Routers examine the destination IP address and forward packets toward the destination.

So:

Network = Routing and host-to-host packet delivery

Important distinction

Don’t confuse Transport and Network:

Transport → reliable end-to-end delivery Network → routing packets between networks

The Transport layer does not choose the path. The Network layer handles routing.

3. Lower 2 Layers — Node-to-Node / Local Delivery

The Data Link and Physical layers deal with communication over the local link.

Layer 2 — Data Link

The Data Link layer handles node-to-node delivery over a local network/link.

Its responsibilities include:

• Frames
• MAC addresses
• Local delivery
• Error detection
• Access to the transmission medium

For example:

Laptop → Wi-Fi Router

The communication between directly connected nodes is handled using the appropriate link-layer technology, such as Wi-Fi or Ethernet.

Layer 1 — Physical

The Physical layer deals with the actual transmission of bits.

It converts bits into physical signals such as:

• Electrical signals
• Radio signals
• Optical signals

For example:

Bits
 ↓
Physical signals
 ↓
Cable / Fiber / Radio

So:

Physical = transmitting raw bits through the physical medium