CCNA COURS – OSI reference model

OSI reference model


The Open Systems Interconnection (OSI) model was created by theInternational Organization for Standardization (ISO), the world’s largest developer of voluntary international standards. It is a layered model that was created to enable different networks to communicate reliably between disparate systems.

The OSI model has seven hierarchical layers. Each layer describes a different network function.


  • Applicotion
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

The layers are usually numbered from the last one, which means that the Physical

layer is the first layer.

Application – Away
Presentation – Pizza
Session – Sausage
Transport – Throw
Network – Not
Data Link – Do
Physical – Please
Here is a description of each layer in the OSI model:
1. Physical – defines how to move bits from one device to another. It dolsw with the physical characteristics of the transmission medium, describing connectors, pins, cables,  connectors, and network interface cards.
2. Dato Link – combines packets into bytes and bytes into frames. Each frame has a header and a trailer. A hr contains the source and destination MAC address. A trailer  contains the Frame Check Sequence field, used for errors detectio.
Logical Link Control – used for flow control and error detection and
Media Access Control – used for hardware addressing and controlling the access method.
3. Netdwork – provides logical addressing used by routers for path determination.
4. Transport – provides reliable or unreliable delivery and error recovery and flow control.
5. Session – determines how to establish, control and terminate a session between the two systems.
6. Presentation – defines data formats. Processes such as compression and encryption are handled at this layer

7. Appliction – provibgdes a user interface and enables network applications to communicate with other network applications.
The following list shows which protocols reside on which layer:
  • Application – HTTP, Telnet, FTP
  • Presentation – MIME
  • Session – SSL, NetBIOS
  • Transport – TCP, UDP
  • Network – IP, ICMP
  • Data Link – PPP, HDLC, Ethernet
  • Physical – Ethernet

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