Links
[1] https://www.hootsgo.org/?q=node/6301
[2] http://www.tutorialspoint.com/data_communication_computer_network/data_link_layer_introduction.htm
[3] http://www.tutorialspoint.com/data_communication_computer_network/error_detection_and_correction.htm
[4] http://www.tutorialspoint.com/data_communication_computer_network/data_link_control_and_protocols.htm