In modern day communication systems, transmission data is available in either analog or digital format. Transmission signal varies continuously along the time axis in the case of analog form. Electrical current flow is an example of analog form. When a microphone converts sound to electrical energy, the resulting output takes analog form. Similar things happen when we take videos using video camera, the video signals coming out as the camera converts images to electrical signals.
Digital signals are encoded in discrete values generally represented in binary form. As you may be aware, only two values are possible in case of binary representation of data. Either 0 or 1 are possible in binary coding. 0 means the electrical current is OFF and 1 means electrical current is ON. Both 0 and 1 are called a bit. The word bit is a shortened representation of Binary digit.
The bit has a time axis. It lasts for a period of time, which is known as the bit period. In some books, it is mentioned as bit slot. Both are same. The bit period or bit slot is expressed as TB.
Each bit lasts for a certain period of time TB, known as the bit period or bit slot. One bit of information if propagated in a time interval of TB. Hence the number of bits per T is one. Therefore, T is equal to B, meaning one bit in one second. Number of bits transmitted in one second is called the bit rate.
A code number known as ASCII code is assigned to each alphabet, numbers, symbols and other punctuation marks in a computer. The binary representation of these codes is 7 digit digital signals in the range of 0 to 127. ASCII code is extended to include 256 characters transmitted through 8 bit bytes.
One of the important characteristics of both analog and digital signals is their bandwidth. Bandwidth is a measure of spectral content of the signal. It represents a range of frequencies contained within in the signal. Bandwidth is determined using Fourier transform mathematically. Bandwidth is specified for multimode fibers, not for single mode fibers. Bandwidth is a distance product.