28-10-2010, 02:28 PM
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The Data Link Layer
Data Link Layer Issues
• Functions of the Data Link Layer
– Provide service interface to the network layer
– Dealing with transmission errors
– Regulating data flow: Slow receivers not swamped
by fast senders
• To accomplish these goals, the data link layer
encapsulates the packets into frames.
Framing
• To detect or correct errors in the raw bit stream
from the physical layer, the data link layer
breaks the bit stream up into discrete frames and
computes the checksum for each frame.
• Four methods can be used to mark the start and
end of each frame:
– Character count
– Flag bytes with byte stuffing
– Starting and ending flags, with bit stuffing
– Physical layer coding violations