OSI Model

OSI Model

OSI stands for Open Systems Interconnection Model

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.

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: Ethernet MAC address.

Typical layer 2 devices and protocols

  • Bridges
  • Switches
  • Wireless Access Point
  • Ethernet

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 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 5 – Session layer

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

Layer 6 – Presentation layer

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

Layer 7 – Application layer

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

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