Telephony is the technology that enables the transmission of voice communication over a distance. It encompasses the various methods and devices used to make and receive phone calls, including traditional landline phones, mobile phones, and internet-based telephony services.

Multiplexing 

It is a technique that combines multiple data streams into a single transmission channel. The device that performs multiplexing is called a multiplexer (MUX). Multiplexing typically follows a many-to-one approach, meaning that multiple input lines are combined into a single output line.

Demultiplexing 

It is the reverse process of multiplexing. It separates a single transmission channel into its original data streams. The device that performs demultiplexing is called a demultiplexer (DEMUX). Demultiplexing typically follows a one-to-many approach, meaning that a single input line is divided into multiple output lines.

Wavelength division multiplexing (WDM) is a technique that combines multiple optical signals onto a single fiber optic cable by using different wavelengths of laser light. WDM allows for bidirectional communication over a single fiber strand, significantly increasing the transmission capacity of fiber optic networks.

Categories of WDM-

• Course WDM (CWDM)

• Dense WDM (DWDM)

Time Division Multiplexing (TDM) is a transmission technique that divides a communication channel into multiple time slots, each assigned to a different signal. Unlike Frequency Division Multiplexing (FDM), TDM signals operate at the same frequency but at different times, allowing for parallel transmission.

Types of TDM-

1. Synchronous TDM

2. Asynchronous (or statistical) TDM

Frequency Division Multiplexing (FDM) is a technique that transmits multiple signals simultaneously using different frequency bands. Each signal is assigned a specific frequency range to prevent interference. Guard bands are used to further isolate adjacent channels. FDM is commonly used in radio and TV broadcasting.

Application of FDM-

• In the first generation of mobile phones, FDM was used.

• The use of FDM in television broadcasting

• FDM is used to broadcast FM and AM radio frequencies.