OSI Model

Question 1

What does OSI stand for?

Answer 1

Open Systems Interconnection model (OSI Model) - how data is moved across a network.

Question 2

What is layer 5 of the OSI model?

Answer 2

Session layer

Question 3

What is layer 2 of the OSI model?

Answer 3

Data Control/Data Link layer (MAC address routing layer).

Question 4

What is layer 6 of the OSI model?

Answer 4

Presentation layer.

Question 5

What is the top layer/layer 7 of the OSI model?

Answer 5

Application Layer.

Question 6

What is the bottom layer/layer 1 of the OSI model?

Answer 6

Physical layer - cabling, connectors, electrical signals - loopback tests, cable tests etc.

Question 7

What are all 7 layers of the OSI model working from the bottom up?

Answer 7

1. Physical
2. Data link control (DLC) - switching layer via MAC address, frames/EUI addresses
3. Network - routers and smart switches - PACKET routing via IP
4. Transport (TCP/UDP IP) - post office layer - gets traffic from one side of network to the other
5. Session - start,stop,restarting sessions - VPN
6. Presentation
7. Application

Question 8

What layer do smart switches exist at?

Answer 8

Layer 3 (because they route packets based on IP)

Question 9

What layer do dumb switches work at of the OSI model?

Answer 9

Layer 2 switches - because they route based on MAC address (good for small networks)

Question 10

If there is a problem with an IP address or subnet mask configuration at what layer of the OSI are you looking?

Answer 10

Layer 3 - network layer encapsulates IP addressing and routing/subnetting.

Question 11

What is the role of the presentation layer (Layer 6)?

Answer 11

The presentation layer makes data human readable, encryption/decryption. Closely linked/tied to the Application layer (7). Presentation preparation layer.

Question 12

When looking at e.g. a webpage what layer of the OSI model are we seeing?

Answer 12

Layer 7 - the application layer is what you see.
HTTPS, pop3, etc protocols operate at this layer.

Question 13

At what level are data/IP PACKETS routed?

Answer 13

Layer 3 - the network layer is where data packets/IP packets are exchanged.

Question 14

What layer has TCP segments and UDP datagrams?

Answer 14

Layer 4 - Transport Layer has TCP segments and UDP datagrams.

Question 15

At which layer would you find tunnelling and control protocols.

Answer 15

Layer 5 is the session layer where you’d find control protocols and tunnelling protocols.

Question 16

Where would you find SSL/TLS encryption in the OSI model?

Answer 16

Layer 6 - Presentation layer deals with all things encryption/decryption like Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols.

Question 17

What layer is the handshake layer?

Answer 17

Session Layer (layer 5) = the handshake layer, where dialogue between devices begins to allow persistent/synchronised connections between them. As well as where connections/sessions get closed down.

Question 18

What are the 5 names of data at each stage (remember application, presentation and session layer are merged into one layer) of the OSI model?

Answer 18

Data Stream = data at the application, presentation and session (merged) layer is called a Data Stream.

Segment (TCP)/Datagram (UDP) = data at the Transport layer is either a datagram (UDP) or a segment (TCP)

Packet = IP packet = network layer = routed by IP

Frame = MAC address, data link layer

Bits = Physical layer, electrical signal/electrical oscillation/high/low voltage levels signalling a sequence of bits/1/0.

Don’t Stop Pouring Free Beer

Data Stream → Segment/Datagram → Packet → Frame → Bits.

Question 19

Encapsulation Process 7 -> 1

Source system encapsulates data stream within a TCP or UDP Segment.

The segment is then encapsulated inside a Packet/IP packet.

Packet is then encapsulated inside a Frame (MAC address and error-checking added)

The frame is then CONVERTED to bits (electrical signals/light pulses for fiber/radio waves for wireless) and transmitted across the network medium to the destination system which then decapsulates (1 -> 7) it in the reverse order to how it was encapsulated. Ending up with the human readable application data.

Answer 19

Data Stream → Segment/Datagram → Packet → Frame → Bits.

Question 20

What is full duplex transmission?

Answer 20

Send and receive data at the same time (concurrently) - Full duplex

E.g. switches.

Question 21

What’s half duplex?

Answer 21

One machine sends while one receives, can’t both send at the same time (when they try collisions occur and runts (tiny file fragments) are left that congest the network.

E.g. hubs.

Question 22

What is simplex transmission?

Answer 22

One way only communication. No return path for acknowledgement of receipt or reply/response.

E.g. a television transmission

Question 23

What is a collision domain?

Answer 23

A hub is a collision domain whereby traffic going into multiple ports of a device can collide thus returning fragmented data and slowing down the network as well as leading to packet loss.