Modulation
Most big and small businesses that use T1 lines, integrated T1 providers, voice T1 and T3 should be familiar with basic telecommunication terms such as modulation. Modulation is the addition of information to an electronic or optical signal carrier. It can be applied to direct current by turning it on and off, to alternating current, and to optical signals. Modulation is kind of like a blanket waving used in smoke signal transmission if you were to think of the carrier being a steady stream of smoke. Morse code was invented for telegraphy and is still used in amateur radio. It uses a binary digital code similar to the code used by modern computers. For most of radio and telecommunication today, the carrier is alternating current in a given range of frequencies.
Common modulation methods include: Frequency modulation (FM), in which
the frequency of the carrier waveform is varied in small but meaningful
amounts. Amplitude modulation (AM), in which the voltage applied to the
carrier is varied over time. Phase modulation (PM), in which the natural
flow of the alternating current waveform is delayed temporarily.
These are also known as continuous modulation methods to distinguish them
from pulse code modulation (PCM), which is used to encode both digital and
analog information in a binary way. Radio and television broadcast stations
use AM or FM. Most two-way radios use FM as well, although some use a mode
known as single sideband (SSB).
Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM) are other
types of modulation less commonly used. Optical signals are modulated by
applying an electromagnetic current to vary the intensity of a laser beam.
Large amounts of information can be transferred in one amount of time by
dividing the bandwidth of a signal carrier so that more than one modulated
signal is sent on the same carrier. Known as multiplexing, the carrier is
sometimes referred to as a channel and each separate signal carried on it
is called a subchannel. The device that puts the separate signals on the
carrier and takes them off received transmissions is a multiplexer.
Common types of multiplexing include frequency-division
multiplexing (FDM) and time-division multiplexing (TDM). FDM is usually
used for analog communication and divides the main frequency of the carrier
into separate subchannels, each with its own frequency band within the overall
bandwidth. TDM is used for digital communication
and divides the main signal into time-slots, with each time-slot carrying
a separate signal.
A computer with an online or Internet connection
that connects over a regular analog phone line includes a modem. This term
is derived by combining beginning letters from the words modulator and demodulator.
In a modem, the modulation process involves the conversion of the digital
computer signals (high and low, or logic 1 and 0 states) to analog audio-frequency
(AF) tones.
