Link Layer Flashcards
What does the Link Layer handle?
Transmission of frames of physical medium.
What is the MAC protocol?
Media Access Control Protocol - Manages access to and from physical media.
What is ARP?
Address Resoluion Protocol - resolves destination MAC address requests.
Why do packets get dropped in the Link Layer?
Congestion
WHat are the Link Layer models and when are they used?
Connectionless, no ACK - low error rate like ethernet.
Connectionless, acks - high error rate, like WIFI, needs delivery assurance.
Connection-oriented + ACKS - Long delay, unreliable links like satelites.
How are ACKs handled? (3)
Stop-and-wait Automatic Repeat and Request (ARQ): Send frame, get ack, send next frame.
Improved ARQ with pipeline: Send multiple frames with sequence numbers, don’t wait for acks, if an ack isn’t received for a frame, go back to that frame and retransmit from there.
Selective-Repeat-Arq - Similar to above, but only retransmit frame with no ack.
How are errors checked in the link layer? (2)
Parity bit.
Checksum (compare sender and receivers)
What happens when receiving a Link Layer message?
The message is unwrapped to get the IP datagram to pass up to the network layer.
How are messages forwarded?
IP datagram is encapsulated into frame, MTU added.
What does the MTU do?
Defines how much data can be transmitted from a link at one time.
WHat is a MAC address (+structure)?
Physical address for network hardware.
48-bit: 24-bit vendor ID, 24-bit assigned by vendor (usually sequential)
What is an ethernet frame?
48-bit source and destination MAC addresses.
MTU
802.1Q tag for optional priority and VLAN id.
What do ethernet switches do?
Forward frames to ports.
What is smart forwarding?
When an ethernet switch learns which MAC address goes to which port.
How does the ethernet switch support VLANs and QoS?
VLAN: Forwards according to VLAN id, no need to rewire hte network.
QoS : Uses frame priority.
