OSI Model

Question 1

What’s the OSI Model?

Answer 1

Open Systems Interconnection Reference Model

Question 2

Layer 7

Answer 2

Application Layer

Question 3

Layer 1

Answer 3

Physical Layer

Question 4

Layer 6

Answer 4

Presentation Layer

Question 5

Layer 2

Answer 5

Data Link Layer

Question 6

Layer 5

Answer 6

Session Layer

Question 7

Layer 3

Answer 7

Network Layer

Question 8

Layer 4

Answer 8

Transport Layer

Question 9

What relation does the OSI Model have with the OSI Protocol Suite?

Answer 9

The OSI model was based around the protocols in the OSI Protocol Suite; however, most of them didn’t catch on. Nowadays we mainly use the TCP/IP protocol.

Question 10

What’s a good mnemonic for memorizing the 7 Layers of the OSI model?

Answer 10

All
People
Seem
To
Need
Data
Processing

Question 11

Physical Layer

Answer 11

Layer 1
- physics of the network (no protocols)
- Signaling, cabling, fiber, connectors
- “physical layer problem”
- Fix cabling, punch-downs, connectors
- Run loopback tests, test/replace cables, swap adapter cards

Question 12

Data Link Layer

Answer 12

Layer 2
- 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, also referred to as Layer 2 addresses because they correlate back to DLC protocols
- The “Switching” layer
- Frame, MAC Address, Extended Unique Identifier (EUI-48, EUI-64), Switch

Question 13

Network Layer

Answer 13

Layer 3
- The “routing” layer
- layer associated with Internet Protocol (IP) addresses
- Fragments frames to traverse different types of networks
- e.g., Ethernet -> WAN -> Ethernet, this occurs at Layer 3
- Router, Packet

Question 14

Transport Layer

Answer 14

Layer 4
- The “post office” layer (how and where)
- Transmission Control Protocol (TCP) segment and User Datagram Protocol (UDP) datagram
- port numbers
- e.g., accessing a large webpage where it can’t be sent via a single frame and is split into multiple pieces and put back together on the other side

Question 15

Session Layer

Answer 15

Layer 5
- Communication management between devices
- Start, Stop, Restart between endpoints
- Control protocols, tunneling protocols

Question 16

Presentation Layer

Answer 16

Layer 6
- Character Encoding
- Application Encryption (SSL/TLS)
- Often Combined with the Application Layer (Layer 7) b/c functionality is so closely associated with the ability to use these applications

Question 17

Application Layer

Answer 17

Layer 7
- The Layer we see
- HTTP, FTP, DNS, POP3

Question 18

Packet Capture Application

Answer 18

• Wireshark
  
  • Summary View, w/ ea. Line showing a frame going across the network
  • detail view, each part of the communication broken up into the OSI layers for a frame
  • 5,6,7 layer info grouped together
  • Hexadecimal representation of the data

Question 19

ex. Google Mail describe via OSI Model

Answer 19

• Layer 7: https://www.mail.google.com
• Layer 6: SSL encryption
• Layer 5: Link the presentation to the transport
• Layer 4: TCP encapsulation, Port 443
• Layer 3: IP encapsulation, Ethernet frames
• Layer 2: Ethernet
• Layer 1: Physical electrical signals

Question 20

PDU

Answer 20

• Protocol Data Unit
• getting data from one part of the network to the other relies on PDUs
• or Transmission Units
  
  • a different group of data at different OSI layers
• Ethernet operates on a frame of data
  
  • it doesn’t care what’s inside, sends everything within the ethernet frame across (encapsulation)
• IP operates on a packet of data
  
  • inside is TCP data, or UDP data, or some other type but IP doesn’t care
• TCP header or UDP header
  
  • TCP segment
  • UDP datagram

Question 21

Encapsulation and Decapsulation