OSI MODEL

OSI MODEL: OSI stands for “Open Systems Interconnection”. Main aim of OSI is to describe how data will be transmitted between two points on a telecommunication. OSI is of seven layers and they are divided into two parts

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data Link
7. Physical

Application, Presentation and Session comes under Application Set

Transport, Network, Data link and Physical comes under Transport Set

See this diagram for Quick Reference

Layer Responsible for interacting with OS while sending mails

1) APPLICATION LAYER: This is the layer responsible to communicate with operating systems whenever user choses to send e-mails, files or documents.

Functions:

• File accessing
• Directory Services
• Inter-process communication
• Network Management
• Directory Services
• Mailing System (EMAIL)

Protocols Used: FTP, HTTP, SMTP, DNS, NFS, TELNET

File Transfer System (FTP): Used for transferring files over network

Hyper Text Transfer Protocol (HTTP): Allows the servers and browsers to interact each other

Simple Mail Transfer Protocol (SMTP): Used for transferring emails between servers

Domain Name System (DNS): Translates the domain and host names to IP Address

TELNET: Terminal emulation program which connects PS to the server on the network

Network File System (NFS): A system designed by SUN Microsystems for accessing files on different computers

Layer responsible for converting the data understandable by other layers

2) PRESENTATION LAYER: Presentation layer is responsible for converting the data provided by application layer, which is understandable by remaining Layers. We can say this is the Translator of the network.

Presentation Layer is also called as “Syntax Layer”

Functions:

• Code Translation (ASCII to human readable)
• Data Conversion
• Data Compression
• Data Encryption

Protocols Used: ASCII, EBCDIC, MIDI, MPEG, JPEG

American Standard Code for Information Interchange (ASCII): Represents the English Alphabets as Numbers

Extended Binary Coded Decimal Interchange Code (EBCDIC): An 8-bit code by IBM for representing the English Alphabets as Numbers

Musical Instrument Device Interface (MIDI): A computer instrument which controls note events, timing events, pitch bends and pedal information.

Layer responsible for Authentication Process

3) SESSION LAYER: Session is nothing but allowing the connections between the applications

Functions:

Establishing, managing, terminating the connections between applications is the responsibility of session layer

It performs some of the actions like Logon, Name Reorganization for security over the network

Protocols Used: SQL and RPC

Layer Responsible for Data Transmission

4) TRANSPORT LAYER: It is responsible for transferring the data between the devices. It ensures that whether the message is delivered without any error or duplication.

Functions:

Message Acknowledgement: Delivers a message whether message is sent or not

Message Segmentation: Divides the message into segments while sending

Error Recovery: It checks whether the message is transferred completely without any errors

Flow Control

Protocols Used: TCP and UDP

Layer Responsible for sending data to right place

5) NETWORK LAYER: It is responsible for sending the data in right path to right destination using routing, virtual circuits and switching technologies.

Functions:

• Routing and forwarding
• Addressing
• Internetworking
• Error Handling
• Congestion Control
• Packet Sequencing

Protocols Used: IP, ICMP, ARP and PING

Layer Responsible for sending packets in the form of bits

6) DATA LINK LAYER:

In this layer, Data is sent in the form of Frames

Data link layer sends the coded and decoded data into bits over Medium Access Control (MAC) layer and Logical Link Layer (LLC) layer.

Functions:

Synchronization

Flow Control

Error Recovery

Protocols Used: IEEE 802.2, 802.3, 802.5

7) PHYSICAL LAYER: This is the lowest layer in OSI which is responsible for transmitting and receiving the raw data for higher layers. Some of the physical characteristics are Connections, timing and voltage levels

Protocols Used: IEEE 802.2 and 802.5