Logical Link Control
(cont.)
ä Error Control Schemes:
ä used to increase reliability. Complete reliability is not
possible, even internal circuits are not completely
reliable
ä must deal with (at the data link layer) duplication,
distortion, deletion, and insertion of messages
ä work by increasing redundancy of message in some
well-defined way; consistency of received message is
used to assess the reliability of its information
ä types of error control schemes:
ä forward error control: uses error-correcting codes
(such as Reed-Solomon or Hamming codes) to send
enough information to allow receiver to detect and
correct errors
ä feedback error control: uses error-detecting codes
(such as parity bits, CRC) to allow receiver to detect
but not to correct errors. Requires retransmission for
correction
ä feedback error control typically superior to forward
error control (but there are exceptions....)