📙 OSI Model

📙 OSI Model#

OSI_Model_v1.svg

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OSI Model Layers

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes the functions of a communication system into seven distinct layers, each with specific responsibilities and associated protocols.

The layers, from the bottom (physical) to the top (application), are as follows:

  • Layer 1: Physical Layer

    • Responsible for transmitting raw bits over a physical medium.

    • It defines the mechanical, electrical, and functional specifications for the transmission medium.

    • Protocols and standards at this layer include RS-232, 100BaseTX, ISDN, RS-449, SONET/SDH, DSL, Wi-Fi radios, and Ethernet physical standards.

  • Layer 2: Data Link Layer

    • Organizes bits into frames and provides hop-to-hop delivery between directly connected nodes.

    • It handles frame synchronization, flow control, error detection, and access control.

    • Protocols include Ethernet (IEEE 802.3), VLAN (IEEE 802.1Q), PPP, HDLC, Frame Relay, ATM, IEEE 802.2, and MAC sublayer protocols.

  • Layer 3: Network Layer

    • Manages the routing and forwarding of packets from source to destination across different networks.

    • It is responsible for logical addressing and path determination.

    • Key protocols include IP (IPv4, IPv6), ICMP, IGMP, ARP, IPsec, OSPF, BGP, and IS-IS.

  • Layer 4: Transport Layer

    • Ensures reliable end-to-end data transfer between processes on source and destination machines.

    • It manages segmentation, flow control, error recovery, and data integrity.

    • Protocols include TCP (connection-oriented), UDP (connectionless), SCTP, DCCP, QUIC, and SPX.

  • Layer 5: Session Layer

    • Establishes, manages, and terminates communication sessions between applications.

    • It handles session synchronization, checkpointing, and recovery.

    • Protocols include NetBIOS, RPC, PPTP, SAP, and Named Pipe.

  • Layer 6: Presentation Layer

    • Translates, encrypts, and compresses data to ensure compatibility between different systems.

    • It handles data formatting, syntax, and encryption.

    • Protocols include SSL/TLS, MIME, JPEG, ASCII/EBCDIC conversion, ASN.1, and XDR.

  • Layer 7: Application Layer

    • Provides network services directly to end-user applications.

    • It supports user-level protocols for tasks like web browsing, email, and file transfer.

    • Protocols include HTTP, HTTPS, SMTP, POP3, FTP, DNS, SNMP, DHCP, SSH, Telnet, NFS, and NTP.