Back Ground Theory Nrz Non Return To Zero Computer Science Essay

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The Basic interest in the study of analogue communication system is the masking of the transmitted signal by additive noise .This effect is most conveniently analysed by considering the special properties of the noise waveform .This situation when compared in the digital signal is entirely different since we provide fixed signal levels. When these levels are obscured by the noise the receiver is required to decide which of the level represents noise .If the receiver makes any incorrect decision the result can be catastrophic. Digital communication implies that information is somehow expressed in terms of numbers or digits. One complication of digital communication, among others, is that it always requires some sort of coding and decoding processes. That is to say, some organized relationship (the code) must be established between each item of information to be transmitted, and the group of digits which will be sent as a signal to represent it. The code must be chosen in such a way that, when the digits are received, they enable the receiver to decode the information, i.e. to find out from the digits (using the rules of the code) what the information was.

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For Over a Distance of greater than a few hundred meters the transmission of pulse type signal at reasonably high bit rates becomes quiet difficult .Frequently some form of modulation technique has to be employed. In many countries the transmission of information outside the premises of an organization must be by means of licensed carriers (British Telecom or Cable and wireless).Here there are safety requirements and generally the customer apparatus has to be isolated from the lines by means of transformers and capacitors.

A line pair will pass pulse ,but due to inherent characteristics of the transmission line the received pulse rate increases .This is due to the capacitance and inductance of the transmission line ,the higher frequency being transmitted more than the lower frequencies ,the phase shift ,or time delay of the signal component at different frequencies will also change with frequency and can cause severe distortions to pulses. In many practical systems the transmitter and receiver have to be isolated from the line, so they are coupled to the line via Transformer. Transformer blocks DC, greatly attenuate frequencies below a given lower cutoff frequencies and also attenuate frequencies above a given upper cut off frequency. A transformer is therefore a band pass filter.

A sequence of pulse signal can be analyzed as containing frequencies at the fundamental repetition rate(this may approach dc if the rate is instantaneously low)and will range well above multiple harmonics of the fastest repetition rate (i.e when the sequence of pulse as 10101010…)

AIM AND OBJECTIVE: The main aim of this experiment is to investigate the typical data signals and some modulation methods used for transmitting data over a long distance communication channel. The purpose or the objective is to investigate the followings and also to see the behavior of the noise on the original data transmitted signal.

Investigate the signal that occours in typical data communication systems.

Investigate how data signal can be conveyed over typical links.

Investigate the effect of circuit upon the signal ; and

Investigate the effect of noise upon the signal and system performance.

The Aim of the second experiment is to show the interference or noise provided is less than half the pulse height and it has little effect upon the reproduction of the pulse sequence also to show how parity codes canbe used to indicate when a digit in a code word is in the error.

Objective: Determine how errors in digital data transmissions are detected using parity and also how errors in synchronous digital data transmissions are corrected using Hamming Code

BACK GROUND THEORY: NRZ (non-return-to-zero):

Non-return-to-zero (NRZ) line code is a binary code in which "1s" are represented by one significant condition (usually a positive voltage) and "0s" are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition as shown is Fig. 1.

Fig. 1: NRZ

RZ (return-to-zero) :

It is defined as a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by voltage pulses having certain characteristics. The signal state is determined by the voltage during the first half of each data binary digit . The signal returns to a resting state (called zero) during the second half of each bit. The resting state is usually zero volts, although it does not have to be. In positive-logic RZ, the low state is represented by the more negative or less positive voltage, and the high state is represented by the less negative or more positive voltage.

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Many practical data system employ Frequency shift Keying (FSK), Amplitude shift Keying (ASK) or Phase shift Keying (PSK) or some derivative of the latter two systems, in order to transmit data signal over band limited channels, E. g telephone links and radio system.

Data systems are usually generate signal which in the terminals are TTL compatible i.e 0 Volts represent a zero and +5 represent one .These signals are grouped for transmission into words ,being a serial sequence of digits .For low speed asynchronous system 7 to 11 bits are transmitted in a burst of digit ,one after the other, and then there is perhaps a pause .Higher speed synchronous system transmit a start sequence of the bit for synchronization and then a very long stream of bits at a regular rate .In both system the information being transmitted as a reasonably random sequence of zeros and ones ,Overall there are almost equal number of ones and zeros.

Some communication data terminals equipment (DTE) employ Non return to zero NRZ pulse outputs, others use return to zero (RTZ) outputs. For the same bit rate the latter requires twice bandwidth, however it does have a significant frequency component at the maximum bit rate .This component frequency is often used in clock rate extension at receivers and repeaters. Signals which are if this form where they extend down to zero frequency,or near to zero ,are often referred to as BASEBAND signal .Baseband signal can only be transmitted over directly connected wire pairs for relatively short distance (i.e terminal to computer within a campus) .If the signal is modified so that over a short period the DC Value is Zero (i.e equal energy exist either side of zero) then it is possible to transmit the pulse through transformers and over limited distance. This may be achieved by making 0 be -V and 1 +V a bipolar signal ,or making alternate 1s -V and +V.The latter system is employed in PCM transmission .Signal which are modulated onto a carrier are referred to as CARRIER BONE signal .The Carrier is changed by the pulse in either amplitude, Frequency or Phase.

Effect of Noise:

Noise can block, distort, or change/interfere with the meaning of a message in both human and electronic communication. The effect of noise on a digital signal is shown in below figure. special advantage of digital communication is that it allows a signal which has been corrupted, to be regenerated with the corruption removed.

EXPERIMENTAL SET UP:

This experiment consist of the following two set up to find

1. BASIC DIGITAL SIGNAL.

2. NOISE ON A DIGITAL SIGNAL.

1.BASIC DIGITAL DESIGN : The Purpose of these experiment is to investigate the following signals.

Investigate the signal that occours in typical data communication systems.

Investigate how data signal can be conveyed over typical links.

Investigate the effect of circuit upon the signal ; and

Investigate the effect of noise upon the signal and system performance.

Equipments Required :

The equipments consist of a number of modules produced by FEEDBACK LTD .It uses the Following Modules

DCS297-A Data source

DCS297-B Data formulate

DCS297-F Data clock Regenator

DCS297-G Data clock Circuits

DCS297-H Data Reciver

DCS297-M Power supply.

Function Generator

Oscilloscope.

Connections : To investigate the meaning of Bit Clock, Word Clock, Data clock, Parity etc:

1 .Connect the module DCS297-A to the power supply DCS297-M and switch .Set the following switches on module A. Data source to Mid Position ,Format to its Lower position (8 Bits).connect the clock input to the 160 KHz clock output on the left of the module as shown in the following figure 2.

2. Connect each of the oscilloscope (CRO) channel to the 80 KHz Bit clock and the other to the Word clock output .Connect the external trigger of the CRO to the word clock word clock and set the CRO to external trigger. The Following output is observed (both of them must be DC coupled).

3. Remove the CRO Lead to the word clock and reconnect to the NRZ data output as shown in Fig 2. View the Pattern of bits. This will depend upon the settings of the eight black push button switches towards the bottom of the module .Set some of these to a suitable pattern(LED on indicating a 1 and off as 0).Check the sequence of 0s and 1s on the CRO. The pattern will be continuously repeated as the word (the setting of the switch)is being viewed cyclically. View the word clock and NRZ data outputs.

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The various outputs are as shown below.

FIG 2.

MEASUREMENT AND RESULTS:

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2.Noise on a Digital signal:

To show that interference or noise .Provided it is less than half the pulse height will have little effect upon the reproduction of the pulse sequence.

To show how a parity code can be used to indicate when a digit in a code word is in error.

Equipments Required :

The equipments consist of a number of modules produced by FEEDBACK LTD .It uses the Following Modules

DCS297-A Data source

DCS297-B Data formulate

DCS297-F Data clock Regenator

DCS297-G Data clock Circuits

DCS297-H Data Reciver

DCS297-M Power supply.

Function Generator

Oscilloscope.

Connections:

1.To the NRZ signal from the data source is added a signal from the generator .This is performed by feeding the signal Via two Kilo Ohm Resistor attached to the summing amplifier(data squarer) on the data clock generator (Module DCS297 F).A schematic is shown below.

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2.connect the NRZ output of Data source (A)via 2K ohm resistor to data squarer input (F),Connect 80 KHz Bit clock of (A) to bit clock of (F) and Word Clock (A) to Word Clock(F) and connect Data squarer output (F) to NRZ data Input. Function generator output via 2K ohm resistor to data squarer input on the data clock regenerator (Module F).

3. Switch the Function Generator and set it to give an output of 100 Hz at 1 Volt sinusoidal peak to peak.

4. set the lower bias control on the data squarer to the mid position .This should slice the incoming signal in half given clear pulses .View the signal input of the data squarer (Module F) with one CRO input and the squarer output with the other .With a suitable pattern set on the code word buttons (Module A) see how the squarer cleans up the pulses pattern. Note that the data word set on the data receiver is same as the data source. Sketch the two wave forms. Vary the amplitude of the Function generator output and note the effect it has upon reproduction of the pulses and the patterns received on the data receiver. Vary the Bias control on the data squarer to obtain the optimum result.

MEASUREMENT AND RESULTS:

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5. Change the Frequency of the function generator to approximately 200KHz and note the effect now upon the width of the square pulse when the pulse input and the sinusoidal signal are of approximately the same amplitude .

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Error Detection and correction:

5. set the format switch on module A to its central position .The right hand switch is now designated the parity bit (green LED) and you will not be able to set it by pressing the switch .By pressing the appropriate bit patterns in the remaining 7 bits the LED in bit position 8 should now make for the total for the word an even number of 1 bits .Ensure that function generator output is turned down to a low level.Set the right hand switch on the data receiver (Module H) to its central position 7 data and parity .The parity light here is green and should be lit when ever the parity LED on the transmitter is lit.An error can ne simulated in one of the eight bits by pressing one if the black buttons in the data receiver.

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ODD PARIYT BIT ON MODULE A ODD PARIYT BIT ON MODULE H

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EVEN PARITY ON MODULE A EVEN PARITY ON MODULE H.

6. Set the format switch on the data source (module A) to its top most position.Press the left hand (MSB)bit position and hold it down .After one second ,the display panel will show four bits lit in red and green .The red bits can be changed but the green parity bits are the result of parity cheching routine ,the first three green parity bit are sets according to th arrows indicated above the switches ,the foutth is a straight parity on the previous 7 bits.check out the code arrangement and write down some of the combination that occurs by setting the red bits.

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Error Bit On Receiver Error Free Bit On Receiver.

ANALYSIS OF THE EXPERIMENT:

The main analysis of this experiment is to study the behaviour of noise over the data which is transmitted over a channel and how the transmitted signal gets effected with that noise .Noise is one the impairments of the digital communication. In this experiment the noise which is generated from the function generator and the data signal which is given from the data source. Both the data signal and the noise are transmitted over a data square circuit which is also called as slicer circuit and the output found of this circuit is effected with the high frequency component of the input data signal. The slicer circuit has a threshold level which is used to indicate the level of the transmitted signal. The output of the slicer circuit consist of bias control which has some resistivity which controls or slice the high frequency component of the data input as we increase the bias it removes the high frequency since the signal to noise is moderate, for very low noise say 10K HZ the Bias circuit or slice the the high frequency component as shown in the above figures. But at high noise say 100K Hz the noise with the data signal is more and we cannot able to remove the high frequency components as the Signal to noise ratio is high which means that it completely transmit the noise signal rather than the Original transmitted data signal.

The signal to noise ratio is defined as the signal power to noise power which mathmaticlly defines as

SNR= Signal power/Noise power.

If this ratio is high which means high signal strength so it is easy to retrive the original transmiited signal .Whereas if the overall ratio is less which means that the noise and the received signal is not the original transmitted signal .

Error Detection and correction:

The occurrence of a data bit error in a serial stream of digital data is an infrequent occurrence.Even less frequent is the experience of numerous errors within the transmission ofa single message. Usually if a number of errors occur then it can be presumed that either significant interference occurred effecting the transmission line or that there is a major failure in the communications path. Largely because of the extremely low bit-error rates in data transmissions, most error detection methods and algorithms are designed to address the detection or correction of a single bit error. However, as we shall soon see, many of these methods will also detect multiple errors. Error correction, though, will remain a one-bit error concern. Probably the most common and oldest method of error detection is the use of parity. While parity is used in both asynchronous and synchronous data streams, it seems to find greater use in low-speed asynchronous transmission applications, however, its use is not exclusive to this.

Parity Error Detection:

Parity works by adding an additional bit to each character word transmitted. The state of this bit is determined by a combination of factors, the first of which is the type of parity system employed. The two types are even and odd parity. The second factor is the number of logic 1 bits in the data character. In an even parity system, the parity bit is set to a low state if the number of logic 1s in the data word is even. If the count is odd, then the parity bit is set high. For an odd parity system, the state of the parity bit is reversed. For an odd count, the bit is set low, and for an even count, it is set high. To detect data errors, each character word that is sent has a parity bit computed for it and appended after the last bit of each character is sent . At the receiving site, parity bits are recalculated for each received character. The parity bits sent with each character are compared to the parity bits the receiver computes. If their states do not match, then an error has occurred. If the states do match, then the character may be error free.

Parity error-detection (parity) and error-correction (VRC) techniques based on the odd or even count of logic 1's in a transmitted character.

Error Correction:

Error detection is an acceptable method of handling data errors in lan-based networks because retransmission of most messages result in a short delay and a little extra use of bandwidth resources. Imagine a satellite orbiting around Jupiter or Saturn, transmitting critical visual data as binary stream information. The time it takes for those transmissions to reach Earth is measured in hours. During this time, the satellite has adjusted its orbit and is soaring across new territory and sending additional data. Correcting errors in these messages cannot be done by retransmission. A request for that retransmission takes as long to get to the satellite as the original message took to get to Earth. Then consider the time it would take to retransmit the message. What would the satellite do with new data, reach it while it tries to handle the retransmitting of old data? The memory needed to hold the old data in case it would need to be resent is astronomical to say the least. Instead, an error-correcting method such as the Hamming code is used so that errors can be corrected as they are detected.

Hamming Code:

For synchronous data streams, a error-correcting process called Hamming code is commonly used. This method is fairly complex from the standpoint of creating and interpreting the error bits. It is implemented in software algorithms and relies on a lot of preliminary conditions agreed upon by the sender and receiver. Error bits, called Hamming bits, are inserted into the message at random locations. It is believed that the randomness of their locations reduces the statistical odds that these Hamming bits themselves would be in error. This is based on a mathematical assumption that because there are so many more messages bits compared to Hamming bits, that there is a greater chance for a message bit to be in error than for a Hamming bit to be wrong. Another school of thought disputes this, claiming that each and every bit in the message, including the Hamming bits, has the same chance of being corrupted as any other bit. Be that as it may, Hamming bits are inserted into the data stream randomly. The only crucial point in the selection of their locations is that both the sender and receiver are aware of where they actually are the first step in the process is to determine how many Hamming bits (H) are to be inserted between the message (M) bits. Then their actual placement is selected. The number of bits in the message (M) are counted and used to solve the following equation to determine the number of Hamming (H) bits:

2H ³ M + H - 1 (3-1)

Hamming code error-correction method based on the number of logic 1 states in a message.Once the number of Hamming bits is determined, the actual placement of the bits into the message is performed. It is important to note that despite the random nature of the Hamming bit placements, the exact same placements must be known and used by both the transmitter and the receiver. This is necessary so that the receiver can remove the Hamming bits from the message sent by the transmitter and compare them with a similar set of bits generated at the receiver

CONCLUSION:

The specific attention has been directed towards the effects of noise on digital communication and it has been shown that the probability of error can be related to SNR at the receiver. This probability can be reduced either by employing SNR enhancement techniques such a matched filtering or by error coding techniques. The major source of signal impairment on data network is due to inter symbol interference and the noise encountered is tend to impulsive rather that Gaussian. This leads to error which occur in bursts and the correction of such errors requires the use of specialised codes and techniques of interleaving

This is also the situation in the digital cellular radio,where burst errors result from multipath propagation which produces signal fading .Error detection and correction methods are necessary to assure the integrity of the datasent from one location to another. The types of methods used to support both asynchronous-and synchronous-type data streams. Asynchronous error detection is facilitated by the use of a parity bit with each character of data sent. Error correction for asynchronous data utilizes the LRC/VRC method, which duplicates the parity process (VRC) and examines each character by bit position (LRC). Synchronous data streams apply CRC or checksum for error detection and the Hamming code for error correction.

It has been concluded from this experiment that how we can detect our original message signal over noisy condition, also learnt that how a message signal when encountered with error how an error technique is used to make the message signal error free. If there is a single error parity bit is used to make it free but if there is more error then hamming code is used to make the data error free.