Data Link Layer
Error Detection and Correction
Network Layer
Transport Layer
Application Layer
Security
,Data Link Layer
Purpose of data link layer
What are the main purposes of the data link layer?
1 Error control - deal with bit flips and lost data
2 Channel allocation (contention) - deciding how to share medium
Byte stuffing
Pranav wants to send a frame of 423 bytes, which include header, payload, and trailer. Byte
stuffing is applied before transmission. How many bytes are transmitted at most to send the
frame?
Each byte might need an escape byte. Furthermore, there are flag bytes at the beginning
and end of the frame. Hence, the answer is 2 · 423 + 2 = 848
Sliding window protocol
Arya and Sansa use a sliding window protocol with k=4 for their communication. They
always send 180-byte frames and have a duplex channel that can send up to 766 bytes per
second in each direction. The latency from Arya to Sansa is 0.319 seconds, whereas the
latency from Sansa to Arya is 0.359 seconds. Assume that the channel is error- and
jitter-free, the acknowledgements are very short, and processing time can be ignored.
Furthermore, assume Arya always sends data when the protocol allows it. What is the
throughput in bytes?
First, compute the time to transmit one frame, which is 180/766 seconds. Second,
compute the time until the first ACK is received, which is 180/766 + 0.319 + 0.359.
Last, check whether that is less than the time to transmit 4 frames, i.e., 4 · 180/766
seconds. As the first ACK arrives before, the response is equal to the channel bandwidth
of 766 bytes per second
, Error Detection and Correction
Parity blocks
1 Compute the parity bits for the following data arranged in a parity block of 4 rows and 4
columns. Execute the computations first for even and then for odd parity
Even parity Odd parity
2 Gandalf has received the following parity block (even parity)
Has there been an error during transmission?
If so, what was the original block?
There was an error because the parity checks in row 2 and column 2 do not work out. Note
that an error is defined as one or multiple bit flips. We do not know for sure what the original
message was as multiple errors can produce this result. If there was only one bitflip, flipping
the bit in row 2 and column 2 corrects the error but without any information on the number of
bit flips, we cannot be sure we correct properly
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