OSI Model Flashcards
What does OSI stand for?
Open Systems Interconnection model (OSI Model) - how data is moved across a network.
What is layer 5 of the OSI model?
Session layer
What is layer 2 of the OSI model?
Data Control/Data Link layer (MAC address routing layer).
What is layer 6 of the OSI model?
Presentation layer.
What is the top layer/layer 7 of the OSI model?
Application Layer.
What is the bottom layer/layer 1 of the OSI model?
Physical layer - cabling, connectors, electrical signals - loopback tests, cable tests etc.
What are all 7 layers of the OSI model working from the bottom up?
- Physical
- Data link control (DLC) - switching layer via MAC address, frames/EUI addresses
- Network - routers and smart switches - PACKET routing via IP
- Transport (TCP/UDP IP) - post office layer - gets traffic from one side of network to the other
- Session - start,stop,restarting sessions - VPN
- Presentation
- Application
What layer do smart switches exist at?
Layer 3 (because they route packets based on IP)
What layer do dumb switches work at of the OSI model?
Layer 2 switches - because they route based on MAC address (good for small networks)
If there is a problem with an IP address or subnet mask configuration at what layer of the OSI are you looking?
Layer 3 - network layer encapsulates IP addressing and routing/subnetting.
What is the role of the presentation layer (Layer 6)?
The presentation layer makes data human readable, encryption/decryption. Closely linked/tied to the Application layer (7). Presentation preparation layer.
When looking at e.g. a webpage what layer of the OSI model are we seeing?
Layer 7 - the application layer is what you see.
HTTPS, pop3, etc protocols operate at this layer.
At what level are data/IP PACKETS routed?
Layer 3 - the network layer is where data packets/IP packets are exchanged.
What layer has TCP segments and UDP datagrams?
Layer 4 - Transport Layer has TCP segments and UDP datagrams.
At which layer would you find tunnelling and control protocols.
Layer 5 is the session layer where you’d find control protocols and tunnelling protocols.
Where would you find SSL/TLS encryption in the OSI model?
Layer 6 - Presentation layer deals with all things encryption/decryption like Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols.
What layer is the handshake layer?
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.
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?
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.
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.
Data Stream → Segment/Datagram → Packet → Frame → Bits.
What is full duplex transmission?
Send and receive data at the same time (concurrently) - Full duplex
E.g. switches.
What’s half duplex?
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.
What is simplex transmission?
One way only communication. No return path for acknowledgement of receipt or reply/response.
E.g. a television transmission
What is a collision domain?
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.
