OSI Reference Model

→ Stands for Open System Interconnectioin Model.
→ Developed by the International Organization for Standardizatin(ISO) that "provides a common basis for the coordination of standards development for the purpose of system interconnection."
→ It defines how computer transfer data from computer to another computer in network.

Layer architecture

Layer	Data Unit	Function
7. Application	APDU,  Message	Human-computer interaction layer, where applications can access the network services
6. Presentation	PPDU, Fragment, Data	Ensures that data is in usable format and is where data encryption occurs
5. Session	Session	Maintains connections and is responsible for controlling ports and sessions
4. Transport	Segment, Datagram	Transmits data using transmission protocol including TCP and UDP
3. Network	Packet	Decides which physical path the data will take
2. Data Link	Frame	Defines the format of data on the network
1. Physical	Bit, Signal	Transmits raw bit stream over the physical medium

7. Application Layer

→ Serves as the interface between the end-user application and network services
→ Data unit in application layer is in the form of Application Protocol Data Unit (APDU), Message.
→ Function of application layer include resource sharing, remote file access and network management.
→ Protocol used in application laryer are HTTP, DHCP, FTP, SMTP, POP3, TELNET, etc. [HTTP: for web surfing, FTP: used to transfer files, SMTP: for Email, TELNET: for virtual termination]

6. Presentation Layer

→ Also known as Syntax layer
→ Data unit in presentation layer is in the form of Presentation Protocol Data Unit(PPDU), fragment, data.
→ Responsible for data translation, character encoding, data compression, encryption and decryption
→ Protocol used in presentation layer are SSL/TLS, XDR, MIME

5. Session Layer

→ Manages and controls the connection between computer.
→ Data units in session layer is in the form of Session
→ Responsible for session management, Authentication, authorization and synchronization
→ Proctocol used are TCP, RTP, PPTP

4. Transport Layer

→ Provides end to end communication services for applications.
→ Data unit in transport layer is in the form of segment, datagram.
→ Ensures complete data transfer, flow control, error control connection and connection less transmission.
→ Protocol used are TCP, UDP, SLTP, DCCP

3. Network Layer

→ Responsible for data routing, forwarding and logical addressing
→ Data units is in the form of packets
→ Determines path of data to reach destination.
→ Protocol used are IP, ICMP, RIP

2. Data Link Layer

→ Responsible for node to node data transfer and error detection and correction
→ Data unit is in the form of frame.
→ Ensure that data is transmitted to the correct device and manages MAC addressing
→ Protocol used are PPP, SLIP

1. Physical Layer

→ Responsible for physical connection between devices
→ Data unit is in the form of bit or signal
→ Function include the modulation, bit synchronization and transmission of raw binary data over physical medium
→ Protocol used are TCP/IP