Self study on Analog and Digital Transmisssion

Analog and Digital Transmission

There are a number of differences between analog and digital transmission, and it is

important to understand how conversions between analog and digital occur. Let's look first

at the older form of transmission, analog.

Analog Transmission

An analog wave form (or signal) is characterized by being continuously variable along

amplitude and frequency. In the case of telephony, for instance, when you speak into a

handset, there are changes in the air pressure around your mouth. Those changes in air

pressure fall onto the handset, where they are amplified and then converted into current, or

voltage fluctuations. Those fluctuations in current are an analog of the actual voice

pattern—hence the use of the term analog to describe these signals.

 

Analog facilities have limited bandwidth, which means they cannot support high-speed data.

Another characteristic of analog is that noise is accumulated as the signal traverses the

network. As the signal moves across the distance, it loses power and becomes impaired by

factors such as moisture in the cable, dirt on a contact, and critters chewing on the cable

somewhere in the network. By the time the signal arrives at the amplifier, it is not only

attenuated, it is also impaired and noisy. One of the problems with a basic amplifier is that it

is a dumb device. All it knows how to do is to add power, so it takes a weak and impaired

signal, adds power to it, and brings it back up to its original power level. But along with an

increased signal, the amplifier passes along an increased noise level. So in an analog

network, each time a signal goes through an amplifier, it accumulates noise. After you mix

together coffee and cream, you can no longer separate them. The same concept applies in

analog networks: After you mix the signal and the noise, you can no longer separate the

two, and, as a result, you end up with very high error rates

Digital Transmission

 

Digital transmission is quite different from analog transmission. For one thing, the signal is

much simpler. Rather than being a continuously variable wave form, it is a series of discrete

pulses, representing one bits and zero bits. Each computer uses a coding

scheme that defines what combinations of ones and zeros constitute all the characters in a

character set (that is, lowercase letters, uppercase letters, punctuation marks, digits,

keyboard control functions).