OSI

Question 1

What are the 7 layers of the OSI model ?

Answer 1

7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical

Question 2

What does OSI model stand for ?

Answer 2

Open Systems Interconnection Model

Question 3

Application Layer (Layer 7)

Answer 3

• Layer closest to the end user
• Interacts with software applications such as a web browser (doesn’t include the application itself, only the protocols that interact with the application)
• HTTP and HTTPS are layer 7 protocols

Question 4

Presentation Layer (Layer 6)

Answer 4

Data in the Application Layer is in “application format”. It needs to be translated into a different format to be sent over the network. The Presentation Layer’s job is to translate between application and network formats. One example is encryption of data as it is sent and decryption of data as it is recieved. It also translates between different application layer formats.

Question 5

Session Layer (Layer 5)

Answer 5

• Controls dialogues (sessions) between communicating hosts
• Establishes, manages, and terminates connections between the local application (ex: your browser) and the remote application (ex: YouTube)

Question 6

Transport Layer (Layer 4)

Answer 6

• Segments and Reassembles data for communications between end hosts
• Breaks large pieces of data into smaller segments which can be more easily sent over the network and are less likely to cause transmisison problems if errors occur
• Provides host to host communication

Data + L4 Header is called a ‘Segment’

Question 7

Network Layer (Layer 3)

Answer 7

• Provides connectivity between end hosts on different networks (outside of the LAN)
• Provides logical addressing (IP Addresses)
• Provides path selection between source and destination
• Routers operate at Layer 3

Data + L4 Header + L3 Header is called a ‘Packet’

Question 8

Data Link Layer (Layer 2)

Answer 8

• Provides node to node connectivity and data transfer (PC to switch, switch to router, etc)
• Defines how data is formatted for transmission over a physical medium
• Detects and possibly corrects Physical Layer errors
• Uses Layer 2 addressing
• Switches operate at Layer 2

L2 Trailer + Data + L4 Header + L3 Header + L2 Header is called a ‘Frame’

Question 9

Physical Layer (Layer 1)

Answer 9

• Defines physical characteristics of ther medium used to transfer data between devices
• Ex: voltage levels, max transmission distances, physical connectors, cable specifications, etc
• Digital bits are converted into electrical or radio (wired / wireless connections)

Question 10

This process of wrapping data in several layers of protocol information as its being sent over a network is called ?

Answer 10

Encapsulation

Question 11

Removing layers of protocol information, so a destination device can read the original data is called ?

Answer 11

Decapsulation

Question 12

What are the Protocol Data Units (PDUs) ?

Answer 12

Data = ‘Data’
Data + L4 Header = ‘Segment’
Data + L4 Header + L3 Header = ‘Packet’
L2 Trailer + Data + L4 Header + L3 Header + L2 Header = ‘Frame’
‘Bit’ (Layer 1)

Question 13

TCP/IP Suite

Answer 13

4 Application (OSI Layers 7-5)
3 Transport (OSI Layer 4)
2 Internet (OSI Layer 3)
1 Link (OSI Layer 2-1)