OSI Model

Question 1

OSI Model

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

Question 5

Layer 4

Transport Layer

Answer 5

• The “post office” layer
• Parcels and letters
• TCP (Transmission Control Protocol) and
UDP (UserDatagram Protocol)

Question 6

Layer 5

Session Layer

Answer 6

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

Question 7

Layer 6

Presentation Layer

Answer 7

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

Question 8

Layer 7

Application Layer

Answer 8

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