The OSI Model: Comprehensive Notes
Open Systems Interconnection Reference Model
1. History & Origin (UGC NET Highlights)
- Full Name: Open Systems Interconnection Reference Model.
- Founding Organisation: International Organization for Standardization (ISO) & CCITT (now ITU-T).
- Key Architect / Founder: Hubert Zimmermann (French software engineer). Zimmermann proposed the initial architecture in Washington, D.C., in Feb 1978.
- Important Dates:
- 1978: Initial proposal by Zimmermann.
- 1983: ISO and CCITT documents merged.
- 1984: Officially published as standard ISO 7498.
- Purpose: To create a standard allowing different computer systems (e.g., IBM, DEC) to communicate.
"Please Do Not Throw Sausage Pizza Away"
(Physical, Data Link, Network, Transport, Session, Presentation, Application)
The 7 Layers: Functions, Devices & Protocols
Layer 7: Application Layer
Function: Provides network services directly to the end-user. It is the interface between the software application and the network.
- Key Responsibilities: File transfer, Email services, Remote login, Directory services.
- PDU (Protocol Data Unit): Data
- Protocols: HTTP, FTP, SMTP, DNS, Telnet, POP3.
- Devices: Gateway, PC, Phones.
Layer 6: Presentation Layer (Translation Layer)
Function: Ensures data is in a usable format. It acts as the network's "translator."
- Key Responsibilities:
- Translation: ASCII to EBCDIC.
- Encryption/Decryption: SSL/TLS.
- Compression: Reducing file size (JPEG, MP3). - PDU: Data
- Protocols: SSL, TLS, JPEG, MPEG, GIF.
Layer 5: Session Layer
Function: Establishes, manages, and terminates connections (sessions) between local and remote applications.
- Key Responsibilities: Dialog control (Simplex, Half-duplex, Full-duplex), Synchronization (adding checkpoints to data streams).
- PDU: Data
- Protocols: NetBIOS, RPC, PAP.
Layer 4: Transport Layer (Heart of OSI)
Function: Ensures reliable, transparent transfer of data between end systems (End-to-End delivery).
- Key Responsibilities: Segmentation and Reassembly, Flow Control, Error Control, Connection-oriented (TCP) vs Connectionless (UDP) transmission.
- PDU: Segments (TCP) / Datagrams (UDP)
- Protocols: TCP, UDP.
- Devices: Firewalls.
Layer 3: Network Layer
Function: Responsible for Logical Addressing and Routing (finding the best path) across different networks.
- Key Responsibilities: IP Addressing (Source & Destination IP), Routing.
- PDU: Packets
- Protocols: IP (IPv4/IPv6), ICMP, IPsec.
- Devices: Router, Layer 3 Switch.
Layer 2: Data Link Layer (DLL)
Function: Responsible for Node-to-Node delivery and physical addressing (MAC).
- Sub-layers: LLC (Logical Link Control) and MAC (Media Access Control).
- Key Responsibilities: Framing, Physical Addressing (MAC Address), Error detection (Trailer/CRC), Flow Control.
- PDU: Frames
- Protocols: Ethernet, PPP, Switch protocols.
- Devices: Switch, Bridge, NIC.
Layer 1: Physical Layer
Function: Transmits raw unstructured data over a physical medium. It deals with mechanical, electrical, and procedural specifications.
- Key Responsibilities: Bit synchronization, Bit rate control, Physical topologies (Bus, Star), Transmission mode (Simplex/Duplex).
- PDU: Bits
- Hardware: Cables (Fiber/Coax), Hubs, Repeaters, Modem.
Prepared for UGC NET / LIS Aspirants