Networking Fundamentals​, Quick Notes – OSI Model

OSI Model

  • Open Systems Interconnection.
  • Introduced by by the International Standards Organization (ISO)in 1978.
  • Conceptual model for networking, divided into 7 organized layers.
  • Lays out standards for Interoperability between manufacturers.


Tip: OSI Model helps in understanding networking concepts layer-by-layer and can also be used as a foundation for step-by-step troubleshooting.

OSI Layers

Conceptual Flow in OSI Layer

PDU (Protocol Data Unit) is added at each layer referred to as “Encapsulation”; for example, when browsing the Internet – HTTP headers are added, then passed on to Transport Layer for TCP headers to be added, then passed on to Network Layer for IP headers to be added and so on. Similarly decapsulation happens at the destination.

  • Physical
    • Layer 1
    • Establishes and terminates connections.
    • Transmitting raw bits over a physical link.
    • Functional specifications for electrical, optical, radio waves, etc. defined in this layer.
    • Protocols in this layer: 1000BASE-SX, DSL, ISDN.
    • Unit of Measurement: Kbps, Mbps or Gbps.
  • Data-link
    • Layer 2
    • Manages delivery of frames between nodes within a LAN segment.
    • Devices at this layer: Network Interface Cards, Bridges and Switches.
    • Protocols in this layer: Ethernet, PPP, SLIP, Token Ring.
    • Unit of Measurement: Frames (Data Packet).
    • Sub-divided into:
      • Logical Link Control (LLC): Provides support to link Multiple protocols.
      • Media Access Control (MAC): Provides physical addressing Scheme.
      • LLC + MAC = Data Link Layer.
    • Network
      • Layer 3.
      • Packet forwarding and routing.
      • Protocols at this layer: IPv4, IPv6, IPX, RIP, OSPF.
      • Unit of Measurement: Packets or Datagram.
    • Transport
      • Layer 4.
      • Connection oriented communication.
      • Sequencing & reliable delivery of packets.
      • Flow Control & Multiplexing.
      • Protocols at this layer: TCP, UDP.
      • Unit of Measurement: S
    • Session
      • Layer 5.
      • Manage sessions between end-to-end application processes.
      • Establish, Maintain & Terminate connections between applications.
      • Protocols at this later: NetBIOS, PAP, PPTP, L2TP.
    • Presentation
      • Layer 6.
      • Data formats and delivery of information.
      • Encoding/decoding, encryption/decryption, compression/decompression of data.
      • Protocols at this later: ASCII, JPG, MIME, SSL, TLS.
    • Application
      • Layer 7.
      • Provides Interface and protocols required by users.
      • Process-to-Process communication between hosts on a network.
      • Protocols at this later: HTTP, FTP, SMTP, POP3, DNS, DHCP.

TCP / IP Model

  • Simplified 4 Layer Conceptual Model

OSI vs TCP/IP Model

Internet protocol suite (Mapped to TCP/IP Model)













Quiz 02

1. Acronym – ISO.

A. Internal Standards Organization B. International Standards Organization
C. Internet Standards Organization D. Intranet Standards Organization

2. Acronym – OSI.

A. Open Systems Internet B. Open Systems Intranet
C. Open Service Interconnect D. Open Systems Interconnection

3. _____ is the first layer of the OSI Model.

A. Transport B. Network C. Data-link D. Physical

4. _____ is the second layer of the OSI Model.

A. Presentation B. Session C. Transport D. Data-link

5. _____ is the third layer of the OSI Model.

A. Presentation B. Session C. Transport D. Network

6. _____ is the top-most layer of the OSI Model.

A. Application B. Presentation C. Session D. Transport

7. Sub-layers of data-link layer are:

A. Session B. MAC C. LLC D. Application

8. _____ layer defines the electrical and physical specification.

A. Transport B. Data-link C. Physical D. Both A & B

9. _____ layer handles physical addressing.

A. Physical B. Data-link C. Application D. Presentation

10. _____ layer handles logical addressing and routing.

A. Presentation B. Network C. Session D. Transport

11. _____ layer handles end-to-end communications between devices on a network.

A. Data-link B. Application C. Presentation D. Session

12. _____ layer deals with standards for data formats; encryption & compression.

A. Physical B. Data-link C. Presentation D. Application

13. Examples of layer 1 protocols:

A. DSL B. RS-232 C. 100BASE-TX D. All of the above

14. Examples of layer 2 protocols:

A. Ethernet B. PPP C. Token Ring D. All of the above

15. Examples of layer 3 protocols:

A. IP B. IPX C. ICMP D. All of the above

16. Examples of layer 4 protocols:

A. TCP B. UDP C. SCTP D. All of the above

17. Examples of layer 5 protocols:

A. PAP B. PPTP C. L2TP D. All of the above

18. Examples of layer 6 protocols:

A. ASCII B. MIDI C. SSL D. All of the above

19. Examples of layer 7 protocols:

A. HTTP B. POP3 C. DSL D. All of the above

20. Unit of measurement at Layer 1.

A. bits B. frames C. packets D. segments

21. Unit of measurement at Layer 2.

A. bits B. frames C. packets D. segments

22. Unit of measurement at Layer 3.

A. bits B. frames C. packets D. segments

23. Unit of measurement at Layer 4.

A. bits B. frames C. packets D. segments

24. ___ model has 4 layers.

A. OSI Model B. TCP/IP Model C. MIME Model D. Presentation Model

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