OSI Model

OSI Layers

Layer 7 – Application layer

  • Provide network service to the end host’s application.
    •  Eg: Email, FTP, Telnet.

Layer 6 – Presentation layer

  • Ensure that data can be understood between two end hosts.
    • Eg: ASCII character encoding.

Layer 5 – Session layer

  • Manages session between end hosts.
    • Eg: Web server tracking its open connections to web browser clients.

Layer 4 – Transport layer

  • Breaks up data between sender and receiver into small parts called segmentation.
  • Establish end to end connectivity.
  • Can ensure reliable delivery.
    • Error detection and re-transmission.
  • Typical layer 4 protocol
    • Transmission control protocol (TCP)
    • User datagram protocol (UDP)

Layer 3 – Network layer

  • Provide connectivity and path selection. Eg: routing.
  • Define local addressing / logical addressing. Eg: Ipv4, Ipv6.
  • Typical layer 3 devices and protocols
    • Routing, Ipv4,Ipv6.

Layer 2 – Data link layer

  • Defines data format for transmission. Eg: Ethernet, Frame Relay, PPP(Point to Point Protocol).
  • Controls access to physical media. Eg: Ethernet CSMA/CD
  • Defines physical addressing. Eg: Ehternet MAC address.
  • Typical layer 2 devices and protocols
    • Bridges, Switches, Wireless Access Point, Ethernet.

Layer 1 – Physical layer

  • Defines physical media properties
    • Electrical functions.
    • Physical data rates.
    • Physical connectors.
    • Cable distances.
    • Optical wavelength.
    • Wireless frequency.
  • Typical layer 1 devices
    • Cat5 cables, Fiber optical cable, Repeaters, Hubs.

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