Open System Interconnection (OSI) Reference Model

A network reference model serves as a blueprint, dictating how network communication should occur. The OSI model was the first true network model, and consisted of seven layers. The Open Systems Interconnection (OSI) model was developed in the 1970’s and formalized in 1983 by the International Organization for Standardization (ISO). The OSI Model consists of seven layers, each corresponding to a particular network function:

 OSI_Model
Application layer consists of protocols that focus on process-to-process communication across an IP network and provides a firm communication interface and end-user services.It is also called User layer because it provides interaction to User. Example: SMTP, DHCP, FTP, HTTP, HTTPS etc
PDU format is DATA.
Presentation layer is used to present data to the application layer in an accurate, well-defined and standardized format. It provides proper format to the file. Encryption & Decryption and Compression & Decompression is done. Example : Text, Images, Videos, Audio etc.
PDU format is DATA.
Session layer is the fifth layer and provides the mechanism for opening, closing and managing a session between end-user application processes. It creates different session for different session ID with the help of Port number. Port no. is of 16 bit (0-65535) value. Port are of 2 types. Well known ports(0-1023) & Open ports (1024-65535). Example: FTP-80, DNS-53, DHCP-67/68.
PDU format is DATA.
The transport layer is the fourth layer in the open system interconnection model responsible for end-to-end communication over a network. Transport layer ensures the reliable arrival of messages and provides error checking mechanisms and data flow controls. It provides transparent transfer of data between end systems using the services of the network. There are 2 Services in Transport layer i.e TCP and UDP
1. TCP stands for Transmission Control Protocol, It is Connection Oriented, Slow but Reliable transport protocol.
2. UDP stands for User Datagram Protocol, It is Connection Less, fast but Unreliable transport protocol.

PDU format is SEGMENT.
The network layer is the third level of the Open Systems Interconnection ModelOSI Model and the layer that provides data routing paths for network communication. Data is transferred in the form of packets via logical network paths in an ordered format controlled by the network layer. L3 is responsible for Best path selection between Source to Destination. Example: EIGRP, OSPF, VRRP, PIM, ICMP etc
PDU format is PACKET.
The data link layer is the second layer of the OSI model & It is most reliable node to node delivery of data. It forms frames from the packets that are received from network layer and gives it to physical layer. Data-link layer is responsible for implementation of point-to-point flow and error control mechanism. Flow Control. Example, CDP, ARP, PPP, FDDI etc
It has a 2 sub layer: LLC sublayer & MAC sublayer.
1. LLC-Logical Link Control is the upper sublayer of L2 and it performs Error control, Framing, Flow control & responsible for identifying different protocol logicall.
2. MAC is the lower sublayer of L2 and it frames data received from the upper layer and passes them to the physical layer. It also defines how to transmit data on physical layer. Example CSMA/CD.

PDU format is FRAME.
The physical layer is the lower layer of the OSI Model and it deals with bit-level transmission between different devices. Example: Cables, NIC, Hub, Repeater etc.
PDU format is BITS<

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