• Review:
– Hamming distance of a code.
– Relation between the Hamming distance (N) of
a code and the number of errors that can be
detected/corrected?
– Error correction code and error detection code,
which one is more efficient?
– How do sender and receiver work when the
CRC code is used to do error detection?
– How to generate (and verify) CRC code:
• E.g: G(x) = x^2+1, data = 100001, final message=?
• Example of a data link layer protocol:
HDLC (High-level Data link Control).
• Frame:
01111110 Address Control Data Checksum 01111110
8
8
8
>=0
16
8
• Framing: bit oriented framing with bit stuffing
• Error detection: CRC code with CRC-CCITT as the
generator polynomial.
• Flow control and Error control: Sliding window
protocol with 3 bits sequence number and
piggybacking.
• Sequence number in the control field.
• Control frames to do line maintenance.
• Three kind of frames:
– information frame
control field:
0
seq
3
P/F
1
next
3
– Supervisory frame
control field 1 0 Type P/F
next
Type: ACK, NACK, RECEIVE NOT READY,
SELECTIVE REJECT.
– Unnumbered
• use a control frame, or sending unacknowledged
data.
• Control field: 1 1 Type P/F Modifier