Day -1 The OSI Models

What is it?

The Open Systems Interconnection (OSI) Reference Model 

• A conceptual framework showing us how data moves throughout a network.

 • Developed by the International Organization for Standardization (ISO) in 1977.

 It’s Purpose:

• Gives us a guide to understanding how networks operate.

It’s only a reference model, so don’t get wrapped up in the details. 

• Wasn’t implemented in the real world, TCP/IP is.

 

The OSI Model Stack The OSI Model breaks down the complex task of computer-to-computer network communications into seven layers. 

Upper Layers (Host Layers) • Handled by the host computer and performs application-specific functions, such as data formatting, encryption, and connection management. 

Lower Layers (Media Layers) • Provide network-specific functions, such as routing, addressing, and flow control.

1.Application Layer:

          It is responsible for providing an interface for the users to interact with application services or networking services.

Example: Web browser (HTTP) , Telnet, HTTPS, SMTP, POP etc.

Examples of networking services:

         Service           Port No

         HTTP              80

         FTP                 21

        SMTP              25

       Telnet               23

       TFTP                69

2. Presentation Layer:

 It is responsible for defining a standard format to the data.

 It deals with data presentation.

The Major functions described at this layer are:-

Encoding - Decoding

Example:

ASCII, EBCDIC (Text)

JPEG GIF, TIFF, (Graphics)

MIDI, WAV (Voice)

MPEG, DAT, AVI (Video)

Encryption- Decryption

Compression - Decompression

LLC (Logical Link Layer) : Manages the data link interface

Can detect some transmission errors using a Cyclic Redundancy Check (CRC). If the packet is bad the LLC will request the sender to resend that particular packet.

Encapsulation enables the network to route data from one device to another, ensuring that the data is properly transmitted and received.

Decapsulation reverses the process by removing the info, so a destination device can read the original data.

SNMP: The Simple Network Management Protocol is an application protocol to mange network devices. UDP port no 161/162

HTTPS: The Hyper text transfer protocol is an application protocol to access web pages from the web servers. Port no-443

FTP: The file transfer protocol is a standard communication protocol used to transfer computer files from one host to another host over a TCP based network.

Port no- 20 and 21

SMTP: The Simple Mail Transfer Protocol is an application protocol to sends mails. Port no -25