The Open Systems Interconnection model (OSI model) is a conceptual model for standardizing data communication functions.

OSI enables inter-operability of different communication systems with standard protocols. The model partitions a communication system into seven layers named:

Application, Presentation, Session, Transport,Network, Data Link, and Physical Layer.

7 layer OSI model Overview

1. Application Layer: Applications
2. Presentation Layer : formatting
3. Session layer : connections management and termination 
4. Transport layer :  port addressing and error checking, ACK on TCP
5. Network layer : routing  and ip addressing.
6. Data link layer : interface between network and physical layer
7. Physical layer : forwarding electrical signal to another end device (hub, repeater).

Examples of OSi 7 layer in Data Communications

Application - FTP, HTTP, HTTPS, SMTP, SSH, Telnet
Presentation - CSS, GIF, HTML, XML, JSON, S/MIME,
Session - RPC, SCP, NFS, PAP
Transport - NBF, TCP, UDP
Network - Pv4, IPv6, ICMP
Data link - IEEE 802.2, PPP, ATM
Physical - DSL, ISDN, USB

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PHYSICAL LAYER

• Transmission and reception of bit stream over a physical medium.
• Describes the electrical/optical, mechanical and functional interfaces to the physical medium
• Data encoding: modifies analog signal pattern to 1s and 0s
• frame synchronization.
• Transmits bits as electrical or optical signals

DATA LINK LAYER

• Provides error-free transfer of data frames from one node to another over the physical layer,
• Link establishment and termination between two nodes.
• Frame traffic control
• Frame sequencing: transmits/receives frames sequentially.
• Detects and recovers from errors that occur in the physical layer.
• Frame error checking: checks received frames for integrity.
• Determines when the node "has the right" to use the physical medium.

NETWORK LAYER

• The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions and, priority of service
• Routing frames among networks.
• Subnet traffic control
• Translates logical addresses into physical addresses.
• Subnet usage accounting to produce billing information.

TRANSPORT LAYER

• Ensures that messages are delivered error-free, in sequence, and with no losses or duplication.
• Message segmentation: accepts and splits the message into smaller units and passes to the network layer.
• The transport layer at the destination station reassembles the message.
• Message acknowledgment
• Message traffic control
• Session multiplexing: multiplexes several message streams

SESSION LAYER

• Allows session establishment between processes running on different stations.
• Session establishment, maintenance and termination
• allow processes to communicate over the network, performing security, name recognition, logging, and so on.
  
  

PRESENTATION LAYER

• Formats the data to be presented to the application layer.
• This layer translate data to a format used by the application layer from a common format and vice versa.
• Character code translation: for example, ASCII to EBCDIC.
• Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
• Data compression: reduces the number of bits that need to be transmitted on the network.
• Data encryption: encrypt data for security purposes. For example, password encryption.

APPLICATION LAYER

• Serves as the window for users to access network services. 
• Resource sharing and device redirection
• Remote file access
• Remote printer access
• Inter-process communication
• Network management
• Directory services
• Electronic messaging (such as mail)
• Network virtual terminals