ISO OSI and TCP/IP Networking Models Explained

The ISO OSI Reference Model

Created in 1984 by ISO, the model consists of seven layers:

• Layer 1: Physical Layer – Global connections, transmitting raw bits, no buffering.
• Layer 2: Data Link Layer – Secured channels, reducing connection errors, structuring, and transmission controls.
• Layer 3: Network Layer – Multiple connectivity, chosen quality, and transmission systems.
• Layer 4: Transport Layer – Quality selection, recurring connections, user-to-user connection, and transparent data transport.
• Layer 5: Session Layer – Manages sessions between users and machines.
• Layer 6: Presentation Layer – Acts as the translator.
• Layer 7: Application Layer – Includes protocols like HTTP.

TCP/IP Reference Model

The TCP/IP model is the standard family of internet protocols:

• TCP: Connection-oriented transmission between machines.
• UDP: Connectionless protocol.
• Hybrid: Combines OSI and TCP/IP models.
• Network Standardization: Managed by organizations like ISO and IAB.

Physical Layer and Data Transmission

The Physical Layer is responsible for transmitting raw bits.

• Data: Representation of thoughts via formal methodology.
• Signals: Physical representation of data.
• Twisted Pair: Short distance, cost-effective, 100 MHz.
• Coaxial Cable: Longer distance, more expensive, telephone long-distance connections, 1 GHz.
• Fiber Optics: Long distance, 10,000 GHz, cheaper than coaxial, used for intercontinental connections.
• Multiplexing: Several senders utilize the same medium simultaneously.

Data Link Layer: Securing Transmission

This layer focuses on securing the transmission medium.

• ALOHA: Simple model for concurrent access.
• CSMA/CD: Listen before sending; if the medium is free, start sending. If a collision occurs during transmission, stop sending.
• Errors: Single-bit errors, bundle errors, and synchronization errors.
• Error Effects: Data errors and recording errors.
• Detection and Correction: Methods include parity checks and CRC.
• Stop and Wait: The receiver must confirm reception; the sender waits before continuing. For packet loss, the sender uses time monitoring.

Network Layer: Routing

Tasked with end-to-end and point-to-point interconnection.

• Interconnecting Networks:
  • Repeater: For similar networks.
  • Bridge: For different media with different access methods.
  • Router: Switching between networks.
  • Internetworking: A router switching between different subnetworks (Ethernet, WLAN, etc.).
• Mechanisms: Length adjustment and routing.
• Internet Protocol (IP): Features include no fixed connection and no flow control.
• IP Address: 32-bit addressing (2^32).

Transport Layer and Application Services

• Addressing: Identification of TCP service ports.
• DNS: Translates binary numbers into human-readable addresses.
• Email: Asynchronous communication; a major influence on modern business processes.
• FTP: Exchanges data between two hosts.
• URL: Unique addressing for websites.
• HTTP: Based on TCP; uses commands like GET, HEAD, and POST.
• HTML: The standard language for websites.