Links
[1] https://www.hootsgo.org/?q=node/6300
[2] http://www.tutorialspoint.com/data_communication_computer_network/physical_layer_introduction.htm
[3] http://www.tutorialspoint.com/data_communication_computer_network/digital_transmission.htm
[4] http://www.tutorialspoint.com/data_communication_computer_network/transmission_media.htm
[5] http://www.tutorialspoint.com/data_communication_computer_network/wireless_transmission.htm
[6] http://www.tutorialspoint.com/data_communication_computer_network/analog_transmission.htm
[7] http://www.tutorialspoint.com/data_communication_computer_network/physical_layer_multiplexing.htm
[8] http://www.tutorialspoint.com/data_communication_computer_network/physical_layer_switching.htm