Understanding the OSI Model

OSI - Open Systems Interconnection Reference Model
  • It's a guide (thus the term "model")
    • Don't get wrapped up in the details
  • This is not the OSI protocol suite
    • Most of the OSI protocols didn't catch on
  • There are unique protocols at every layer
  • You'll refer to this model for the rest of your career


Layer 1 - The Physical Layer
  • The physics of the network
    • Signaling, cabling, connectors
    • This layer isn't about protocols
  • You have a physical layer problem.
    • Fix your cabling, punch-downs, etc
    • Run loopback tests, test/replace cables, swap adapter cards
Layer 2 - Data Link Layer
  • The basic network "language"
    • The foundation of communication at the data link layer
    • Data link Control (DLC) protocols
      • MAC(Media Access Control) address on Ethernet
    • The "switching" layer
Layer 3 - The Network Layer
  • The "routing" layer
  • Internet Protocol(IP)
  • Fragments frames to traverse different networks
  • What is IP Fragmentation?
    • Fragments are always in multiples of 8 because of the number of fragmentation offset bits in the IP header
Layer 4 - Transport Layer
  • The "post office" layer
    • Parcels and letters
  • TCP (Transmission Control Protocol) and UDP (UserDatagram Protocol)
Layer 5 - Session Layer
  • Communication management between devices
    • Start, stop, restart
  • Half-duplex, full duplex
  • Control protocols, tunneling protocols
Layer 6 - Presentation Layer
  • Character encoding
  • Application encryption
  • Often combined with the Application Layer
Layer 7 - Application
  • The layer we see - HTTP, FTP, DNS, POP3
OSI Mnemonics
  • Please Do Not Trust Sales Person's Answers
  • All People Seem To Need Data Processing
  • Please Do Not Throw Sausage Pizza Away!


****
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data link Layer
Physical Layer
****

Comments

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