Introduction
Data or information can be stored in two ways, analog and digital. For a computer to use that data is must be in discrete digital form. Like data, signals can also be in analog and digital form. To transmit data digitally it needs to be first converted to digital form.Digital-to-digital conversion
This section explains how to convert digital data into digital signals. It can be done in two ways, line coding and block coding. For all communications, line coding is necessary whereas block coding is optional.Line Coding
The process for converting digital data into digital signal is said to be Line Coding. Digital data is found in digital format, which is binary bits. It is represented (stored) internally as series of 1s and 0s.Uni-Polar Encoding
Unipolar encoding schemes uses single voltage level to represent data. In this case, to represent binary 1 high voltage is transmitted and to represent 0 no voltage is transmitted. It is also called Unipolar-Non-return-to-zero, because there’s no rest condition i.e. it either represents 1 or 0.Polar Encoding
Polar encoding schemes multiple voltage levels are used to represent binary values. Polar encodings are available in four types:Polar-NRZ (Non-return to zero)
It uses two different voltage levels to represent binary values, generally positive voltage represents 1 and negative value represents 0. It is also NRZ because there’s no rest condition.
NRZ scheme has two variants: NRZ-L and NRZ-I.
[Image: NRZ-L and NRZ-I] RZ (Return to zero)
Problem with NRZ was the receiver cannot conclude when a bit ended and when the next bit is started, in case when sender and receiver’s clock are not synchronized.
[Image: Return-to-Zero Encoding] Manchester
This encoding scheme is a combination of RZ and NRZ-L. Bit time is divided into two halves. It transitions at the middle of the bit and changes phase when a different bit is encountered.Differential Manchester
This encoding scheme is a combination of RZ and NRZ-I. It also transitions at the middle of the bit but changes phase only when 1 is encountered.
Bipolar Encoding
Bipolar encoding uses three voltage levels, positive, negative and zero. Zero voltage represents binary 0 and bit 1 is represented by altering positive and negative voltages.Block Coding
To ensure accuracy of data frame received, redundant bits are used. For example, in even parity one parity bit is added to make the count of 1s in the frame even. This way the original number of bits are increased. It is called Block Coding.Block coding is represented by slash notation, mB/nB, that is m-bit block is substituted with n-bit block where n > m. Block coding involves three steps: division, substitution and combination.
After block coding is done it is line coded for transmission.
Analog-to-digital conversion
Microphones creates analog voice and camera creates analog videos, which here in our case is treated is analog data. To transmit this analog data over digital signals we need an analog to digital conversion.Analog data is wave form continuous stream of data whereas digital data is discrete. To convert analog wave into digital data we use Pulse Code Modulation.
Pulse Code Modulation is one of the most commonly used method to convert analog data into digital form. It involves three steps: Sampling, Quantization and Encoding.
Sampling
Quantization
Encoding
Transmission Modes
How data is to be transferred between to computer is decided by the transmission mode they are using. Binary data i.e. 1s and 0s can be sent in two different modes: Parallel and Serial.Parallel Transmission
Serial Transmission
In serial transmission, bits are sent one after another in a queue manner. Serial transmission requires only one communication channel as oppose parallel transmission where communication lines depends upon bit word length.Asynchronous Serial Transmission
It is named so because there’s no importance of timing. Data-bits have specific pattern and helps receiver recognize when the actual data bits start and where it ends. For example, a 0 is prefixed on every data byte and one or more 1s added at the end.Two continuous data-frames (bytes) may have gap between them.
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