What is the symbol of wavelength ?

This is the usage:

AC power line frequency

What is the frequency?

It determines the maximum speed data pulses the medium can handle.

Who invented the telephone?

It converts the electrical signal to a suitable signal for transmission.

It is an electronic method for transmitting information from one place to another

It is a two way communication system where both parties can talk and listen simultaneously .

It is the recipient of information or intelligence.