Data Communications

Topics: Modulation, Data transmission, Phase-shift keying Pages: 21 (6424 words) Published: April 18, 2008
Encoding methods and Modulation schemes
Computers use digital signalling, digitals signals are represented by the base 2 numbering system “Binary”. Binary bits are shown as 1 or 0, where 1 means ON and 0 means OFF. In a network where digital devices exist, digital signalling will be used. Dependant upon the encoding method used, a high voltage on a communications medium can represent a logical 1 and a low voltage of can represent a logical 0. Different methods of encoding exist these are used for different types of communication between electrical devices. Distortion, noise, and cross talk on a cabling medium are factors that prevent the accuracy of transmitted data to be intact. For these reasons different encoding methods exist. An example is when 2 wires are used to transmit music data to a speaker Digital signals don’t always have to be carried over to the receiving end by electricity, light can also be used for digital communication. Fibre Optics use light to transmit data through optical fibre within the cable. The strength of the light ray can also be a determining factor to what logic the ray represents, either 0 or 1. Digital signals are complex waveforms that can be described as a discontinuous waveform having a finite range of levels. You can recognise a digital signal by the wave created, if the wave keeps going up and down in straight lines and forms squares and rectangles these are digital signals. Digital signals assume two states (logic 0 and logic 1). Where the signal changes from one state to another, the signal can be said to be discontinuous. As the signal state changes from 0 to 1, a finite time is required for the voltage to increase to the maximum value, and a corresponding time is required for the voltage to fall when the signal state changes from 1 to 0. Each voltage pulse in a digital signal is a signal element. Encoding each data bit into signal elements transmits binary data. There are many encoding schemes that go with digital signalling. The encoding scheme is simply the way in which the signalling elements are used to represent data bits.

Below I will be listing different encoding methods that exist. Below is a diagram showing the digital waveform of an NRZ-L (Non-Return-to Zero- Level) encoding method. NRZ-L is a variation of the NRZ encoding method. The NRZ (Non-Return-to Zero) encoding method is a form of digital transmission. Binary 1’s and 0’s are represented by low and high level states on the line. These states are formed on the line by specific and constant DC (Direct-current) voltages that are transmitted from the NIC (Network interface card). The difference between NRZ and NRZ-L is that, NRZ does not consider the first data bit to be transmitted as a polarity change whereas NRZ-L does.

NRZ logic can be represented by more negative or less positive voltages or vice versa. Example diagrams are shown below. Logic 0 = +5.0 volts
Logic 1 = +0.5 volts
Logic 0 = 0.0 volts
Logic 1 = -3.0 volts

Below is a diagram of the NRZ-I (Non-Return-to Zero- Inverted) encoding method. NRZ-I is another variation of the NRZ encoding method. The NRZI (Non-Return-to Zero-Inverted) encoding method is also a form of digital transmission. Binary 1’s and 0’s are represented by changes in level and constant states on the line. For example a binary 0 is encoded as no change in the level of voltage, whereas a binary 1 is encoded when the level changes from the previous constant state. A binary 1 can be encoded either when the voltage drops or increases on the line. This encoding method is used with FDDI and USB.

Logic 0 = Constant voltage over time
Logic 1 = Drop or rise in voltage
There are many other types of encoding methods, a few of them are Manchester encoding, Pseudoternary encoding, Bipolar – AMI etc. Manchester encoding encodes a binary 1 by the change of a voltage from a low level state...
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