Data Link Layer Flashcards
What is the responsibility of the data link layer?
data link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
What are the link layer services?
Framing: Encapsulate datagram into frame, adding header, trailer
Link access: Channel access if shared medium
Reliable delivery between adjacent nodes
Error detection and correction
Where is the link layer implemented
In NIC “adaptor” or a chip
Tell the sender and receivers side in the communication between hosts
sending side:
• encapsulates datagram in frame
• adds error checking bits, rdt, etc
receiving side
• looks for errors
• extracts datagram, passes to upper layer
what are the two types of link?
point-point and broadcast
what is multiple access protocol?
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
give examples of three MAC protocols?
random access, channel partitioning, taking turns
In FDMA, channels are partitioned by?
by frequency
In TDMA, channels are partitioned by?
by time
In Slotted Aloha, prob that given node has success in
a slot? How about N slots? What is its efficiency?
- p(1-p)^(N-1)
- Np(1-p)^(N-1)
- 0.37
In Pure Aloha, prob that given node has success in
a slot? How about N slots? What is its efficiency?
- p(1-p)^(2N-2)
- Np(1-p)^(2N-2)
- 0.18
Explain the concept of CSMA
CSMA: listen before transmit:
- if channel sensed idle: transmit entire frame
- if channel sensed busy, defer transmission
What is the difference between CSMA and CSMA/CD
Second has collision detection.
Tell the formula for CSMA/CD efficiency
1/(1 + 5t_prop/t_trans)
List taking turns protocols
Polling, token passing
