The Data Link Layer

Question 1

Roles of the data link layer

Answer 1

Framing: delimiting data units flowing on the channel
Error handling: detecting and correcting errors introduced by the physical layer
Flow control: prevents a fast sender from out-pacing a slow receiver

Question 2

Frame

Answer 2

transmitted data element

Question 3

Framing

Answer 3

Sender assembles/breaks an incoming sequence of packets (from network layer) into frames
Reciever does opposite

Question 4

Error handling

Answer 4

Whenever signals are transmitted over long distances, the signal weakens, and noise is introduced. The receive cannot resolve the signal levels. This introduces transmission errors.

Question 5

Redundant data

Answer 5

Added to the message to allow: detection and correction at destination, detection and subsequent retransmission

Question 6

Flow control

Answer 6

These protocols control the sending of transmission frames at a faster pace than they can be accepted.
e.g. simple stop and wait protocol
These protocols can also be equipped with error control mechanisms to ensure that all frames are eventually delivered e.g. ARQ protocols