• It is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. 
• Multiplexing is done using a device called Multiplexer (MUX) that combine n input lines to generate one output line i.e. (many to one).
• At the receiving end a device called Demultiplexer (DEMUX) is used that separate signal into its component signals i.e. one input and several outputs (one to many).

Advantages of Multiplexing : -

• More than one signals can be sent over single medium or link
• Effective use of the bandwidth of medium