Understanding the OSI Model

Question 1

Open Systems Interconnection Reference Model

Answer 1

• 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

Question 2

Layer 1 - The Physical Layer

Answer 2

• 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

Question 3

Layer 2 - Data Link Layer

Answer 3

• 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

Question 4

Layer 3 - The Network Layer

Answer 4

• The “routing” layer
• Internet Protocol (IP)
• Fragments frames to traverse different networks

Question 5

What is IP Fragmentation?

Answer 5

• Fragments are always in multiples of 8 because of
the number of fragmentation offset bits in the IP
header

Question 6

Layer 4 - Transport Layer

Answer 6

• The “post office” layer
  
  • Parcels and letters

• TCP (Transmission Control Protocol) and
UDP (UserDatagram Protocol)

Question 7

Layer 5 - Session Layer

Answer 7

• Communication management between devices
  
  • Start, stop, restart
• Half-duplex, full-duplex
  
  • Control protocols, tunneling protocols

Question 8

Layer 6 - Presentation Layer

Answer 8

• Character encoding
• Application encryption
• Often combined with the Application Layer

Question 9

Layer 7 - Application Layer

Answer 9

• The layer we see - HTTP, FTP, DNS, POP3