Data transmission methods Transmission Transmission is the act of transporting information from one location to another via a signal. The signal may be analog or digital, and may travel in different media. Transmission: Communication of data by propagation and processing of signals. Signal processing is the representation, transformation and manipulation of signals plus the information they contain. Signal Types Signals: An electric or electromagnetic representations of data by which data is propagated (transmitted).
All signals are either analog or digital.An analog signal: is one in which information appears as a continuous variation of some property. Human speech is an example: it produces a continuous variation of air pressure. Examples of media: Copper wire media, (twisted pair and coaxial cable), fiber optic cable and atmosphere or space propagation.
Analogue signals represent some physical quantity and they are a ‘MODEL’ of the real quantity and can propagate analogue and digital data. A digital signal: is one in which information appears as a sequence of binary values 0 and 1. Digital signals have two amplitude levels called nodes.The value of which are specified as one of two possibilities such as 1 or 0, HIGH or LOW, TRUE or FALSE and so on. Digital signals can propagate analogue and digital data To represent these two values, a signal is used in which only two wave shapes are allowed, one representing the binary value 0 and the other representing the binary value 1. By definition, therefore, a digital signal is a restricted form of an analog signal. A human speaker who only utters the two words zero and one is a crude example of a digital signal.
The difference between digital signals and analog signals: . Analog signal is a continuously varying signal while digital signal has discrete values 2. Analog signal has many issues of intensity over a period of time while a digital signal has only a limited number of defined values. 3.
Since digital computers play a central role in data communication, in nearly all cases, digital signals are used. Analog signals are used in cases of equipment which date back to before the advent of digital technology. Existing analog telephone networks are a good example of the latter. AnalogDigital Datacontinuous (e. g. voice)discrete (e. g. , text) Signalcontinuous electromagnetic waves Used mainly for transmitting data across a network.
sequence of voltage pulses Used mainly internally within computers. Transmission1. Transmission of analog signals without regards to their content (the data may be analog or binary).
2. The signals become weaker (attenuated) with the distance. 3.
Amplifiers may be used to strengthen the signals, but as side effect they also boost the noise. 4. This might not be a problem for analog data, such as voice, but is a problem for digital data. 1.
Transmission that is concerned with the content of the signal. 2. Repeaters are used to overcome attenuation. 3. A repeater recovers the digital pattern from the signal it gets, and resubmits a new signal. Analog and Digital Data Transmission Digital and Analog transmission have more to do with the way in which data is conveyed between point A and point B than with whether the data is digital or analog.
Analog data can be transmitted using analog transmission or digital transmission techniques. Likewise, digital data can be conveyed using either digital or analog transmission.Before data is transmitted over a network, it must be encoded depending on what type it is.
For example: Audio, text, video or graphical giving it its computer representation. This encoding depends basically on the physical medium used to transfer the data, the guaranteed data integrity and transmission speed. Data transmissions depend on the characteristics of the signal and of the medium.
•For guided media, the medium is the dominant factor •For unguided media, the bandwidth of the signal is the dominant factor. Transmission Media Digital data can be transmitted over many different types of media.Selecting a transmission medium is guided by comparing transmission requirements against the medium’s characteristics. These important criteria influence the choice: Design factors: Bandwidth: Bandwidth is the maximum frequency range that can be practically supported by a medium. This is usually expressed in kilo Hz (kHz) or mega Hz (MHz).
Greater bandwidth implies higher data rates Transmission impairments/Coverage: The physical characteristics of a medium dictate; how long a signal can travel in it before it is distorted beyond recognition.To cover larger areas, repeaters are needed to restore the signal, and this increases the costs. (Limit distances). Interference: Competing signals in overlapping frequency bands can distort or wipe out a signal.
Number of receivers: A guided link may use a shared link with multiple attachments, with the attachments introducing some attenuation and distortion. Cost: Two types of cost are relevant: i. The cost of installing the medium, including the medium-specific equipment that may be needed, and ii. The cost of running and maintaining the medium and its equipment.
There is usually a need for tradeoff between cost, bandwidth, and distance. Reliability: Some media, by their physical nature, transmit data more reliably than others. Low reliability translates into a higher number of errors, which needs to be balanced against the potential cost of recovering from the errors (e. g. , retransmission, more complex hardware and software). Analogue transmission: Analogue data transmission consists of sending information over a transmission medium in the form of a wave.
Data is transmitted via a carrier wave.The signal is said to be continuous Forms of Analogue transmission 1. Amplitude: is the strength of the signal, expressed as volts or decibels.
The higher the amplitude, the stronger (louder) the signal. 2. Frequency: is the number of oscillations or cycles per second. Measured in Hertz. 3. Phase: is the rate at which a signal changes its relationship to time.
Measured in degrees. Advantages of Analogue Transmission 1. Uses less bandwidth 2. More accurate 3. Digital communications require greater bandwidth than analogue to transmit the same information.
Disadvantages of Analogue 1.The effects of random noise can make signal loss and distortion impossible to recover. Digital Transmission Involves breaking the signal into a binary format where the data is represented by a series of “1”s and “0”s. The signal is said to be discrete. Examples include text and integers. Advantages of digital transmission 1. Technology Sees a drop in cost due to LSI and VLSI 2. Data integrity: Repeaters allow longer distances over lines of lesser quality.
3. Capacity utilization: Digital techniques can be more easily and cheaply utilized, through multiplexing, availability of transmission links of high bandwidth. . Security and privacy: Encryption techniques are more readily applied to digital data 5. Integration: Simplified if digitized data is used everywhere. 6. Compatibility with other digital systems. 7.
Data is easy to manipulate. 8. Data integrity. Data can be carried over longer distances over lower quality lines. The digital signal suffers the same distortion, attenuation and degradation as do the analog signals. 9.
Relatively cheap. Digital equipment is not very expensive. 10. Capacity utilization: High degree of multiplexing easier with digital techniques. 1. Integration: Can treat analog and digital data similarly A shift towards digital transmission despite large analog base; Why? Digital transmission has several advantages over analog transmission: 1. Improving digital technology 2. Data integrity.
Repeaters take out cumulative problems in transmission. Can thus transmit longer distances. 3. Easier to multiplex large channel capacities with digital 4. Easy to apply encryption to digital data 5. Better integration if all signals are in one form. Can integrate voice, video and digital data. 6.
Analog circuits require amplifiers, and each amplifier adds distortion and noise to the signal. 7. In contrast, digital amplifiers regenerate an exact signal, eliminating cumulative errors. An incoming (analog) signal is sampled, its value is determined, and the node then generates a new signal from the bit value; the incoming signal is discarded. With analog circuits, intermediate nodes amplify the incoming signal, noise and all.
8. Voice, data, video, etc. can all by carried by digital circuits. What about carrying digital signals over analog circuit?The modem example shows the difficulties in carrying digital over analog. Disadvantages The Digital Divide Disadvantage Financial ?The second is availability. Like the internet, DVDs, etc ? The third is capacity to understand. People who are not technocrats cannot unlock the advantages of digital technologies. The Obsolescence Digital technology is moving quickly and everybody knows that a new computer is out of date as soon as you’ve bought it.
Fragile Digital systems can be fragile, in that if a single piece of digital data is lost or misinterpreted, the whole data changes meaning (Compare with sampling error).Data transmission is called simple if there are only two machines communicating, or if only a single piece of data is sent. In cases where several transmission lines are installed or where a transmission line is shared among several different communication actors the sharing is called multiplexing. Data and Signal Conversion Usually use digital signals for digital data and analog signals for analog data; however we can use analogue signal to carry digital data using ADC (Analogue to Digital Convertor) and digital signal to carry analogue data using DAC (Digital to Analogue Convertor).Analog-to-digital converter (ADC) An ADC is an electronic device that converts an input analogue voltage (or current or electronic impulse) to a digital number (converts continuous signals to discrete digital numbers. ) A digital-to-analog converter (DAC) Is a device for converting a digital (usually binary) code to an analogue signal (current, voltage or electric impulse). A Modem is an electronic device that converts a computer’s digital signals into specific frequencies to travel over telephone or cable television lines.
At the destination, the receiving modem demodulates the frequencies back into digital data.Computers use modems to communicate with one another over a network. The role of a modem is: When transmitting: to convert digital data (a sequence of 0s and 1s) into analogue signals.
This process is called modulation. When receiving: convert the analogue signal into digital data. This process is called demodulation. In fact, the word modem is an acronym for Modulator/DEModulator Modulation is the process of combining an input signal m(t) and a carrier at frequency fc to produce a signal s(t) whose bandwidth is centered at fc .Motivation for conversion •The data frequencies may not allow for effective transmission •Frequency-division multiplexing Transmission of digital data over an analog line is achieved using a technique called modulation, where the digital bit stream is modulated over an analog carrier signal. A modem (modulator and demodulator) is a commonly used device which employs this technique. As illustrated in Figure 2.
11, a modem converts the outgoing digital bit stream from a device into an analog signal and converts the incoming analog signal into a digital bit stream. Figure shows the Role of modems.Three basic types of modulation are possible 1.
Amplitude Modulation (AM) also called double-sideband transmitted carrier (DSBTC). In AM, the carrier signal’s amplitude is changed according to the modulating digital signal’s bit value. For example, two amplitude sizes (a small and a large one) may be used to, respectively, represent bit values 0 and 1. AM’s main weakness is its susceptibility to distortion.
data carrying signal (modulating wave) envelope with dc components carrier Amplitude modulated wave 1. The outcome is a multiplication of the carrier amplitude by the amplitude of the envelope 2.The dc (direct current) component prevents loss of information that would be cause if the envelope boundaries cross one another. 2. Frequency Modulation (FM).
In FM, the carrier signal’s frequency is changed according to the modulating digital signal’s bit value. For example, two frequency values (a low and a high one) may be used to, respectively, represent bit values 0 and 1. FM is more resistant to distortion than AM. 3. Phase Modulation (PM). In PM, the carrier signal’s phase is changed according to the modulating digital signal’s bit value.A change in the carrier signal’s phase indicates a change in the modulating digital signal’s bit value from 0 to 1 or from 1 to 0. Digitization ;Modulation Digitization is essentially the opposite of modulation.
Whereas in modulation a digital signal is modulated over an analog signal for transmission, in digitization an analog signal is converted into digital format through a process of sampling. For example, the analog signal resulting from human speech can be sampled and converted into digital data, transmitted over digital lines, and converted back to analog signal at the other end.These two functions are performed by a device called codec (coder/decoder) as shown in the figure below Reasons for Choosing Data and Signal Combinations 1. Digital data, digital signal: Equipment for encoding is less expensive than digital-to-analog equipment. 2.
Analog data, digital signal: Conversion permits use of modern digital transmission and switching equipment. 3. Digital data, analogue signal: Some transmission media will only propagate analogue signals Examples include optical fiber and satellite. 4.
Analogue data, analogue signal: Analogue data easily converted to analogue signal.Analogue Transmission Characteristics ?May be analog or digital data. ?Attenuated over distance.
?Use amplifiers to boost signal’s energy for long distances, leads to distortion. Digital Transmission Characteristics ?Attenuation endangers integrity of data ?Digital Signal carrying analogue data i. Repeaters achieve greater distance ii.
Repeaters recover the signal and retransmit ?Analogue signal carrying digital data i. Retransmission device recovers the digital data from analogue signal. ii. Generates new, clean analogue signal. Converting Analogue to DigitalIn digital technology, the analogue wave is sampled at some interval, and then turned into binaries that are stored in the digital device. Sampling measures the analog signal at different moments in time, recording the physical property of the signal (such as voltage) as a number. The coding process generates the sample data from the analog signal. The decoding process regenerates an approximation of the original signal by fitting a smooth curve to the sampled points.
The quality of the regenerated signal can be improved by increasing the sampling rate (i. e. reducing the sampling interval), but up to a limit dictated by the Nyquist’s theorem. This limit is exercised by a popular digitization technique called Pulse Code Modulation (PCM) which uses a sampling rate twice that of the original signal frequency. For example, a 4 kHz speech signal is sampled at a rate of 8000 samples per second.
The main advantage of digitization is that, due to its resistance to distortion, it is much easier to reliably transmit a digital signal over a long distance than an analog signal. Reading off the vertical scale on the left, the following numbers 0, 5, 3, 3, -4, … are transmitted.The number of bits needed to represent them is known as the bit resolution.
Digital recording converts the analog wave into a stream of numbers and records the numbers instead of the wave using an ADC. To play back the music, the stream of numbers is converted back to an analog wave by a DAC. The analog wave produced by the DAC is amplified and fed to the speakers to produce the sound. The analog wave produced by the DAC will be the same every time, as long as the numbers are not corrupted and will also be very similar to the original analog wave if the ADC sampled at a high rate and produced accurate numbers.Converting Digital to Analogue Each dot in the figure above represents one audio sample. In any digital recording technology), the goal is to create a recording with very high fidelity (very high similarity between the original signal and the reproduced signal) and perfect reproduction (the recording sounds the same every single time you play it no matter how many times you play it).
Two factors determine the quality of a digital recording. Sample Rate: The rate at which the samples are captured or played back, measured in Hertz (Hz), or samples per second. An audio CD has a sample rate of 44,100 Hz, often written as 44 KHz for short.Sample Format or Size or Sample Precision: This is the number of digits in the digital representation of each sample. An audio CD has a precision of 16 bits, which corresponds to about 5 decimal digits. The higher the sampling rate and the sample precision, the lower the sampling error, hence the closer the similarity between the original wave and the DAC’s output.
An improved Analogue signal In general, finer resolution (bits on the vertical axis) and faster sampling, gets you better quality (reproduction of the original signal) but the size of the file increases accordingly. Data EncodingDigital transmission is the sending of information over a physical communications media in the form of digital signals. Analogue signals must therefore be digitized first before being transmitted. However, digital information cannot be sent directly in the form of 0s and 1s, it must be encoded in the form of a signal with two states, for example Encoding: This is the process of putting a sequence of characters (like letters, numbers, and certain symbols) into a specialized format for efficient transmission or storage.
Decoding (opposite process): It is the conversion of an encoded format back into the original sequence of characters.Encoding and decoding are used in data communications, networking, and storage Coding Terminology ?Data element: a single binary 1 or 0 ?Signal element: a voltage pulse of constant amplitude ?Unipolar: All signal elements have the same sign ?Polar: One logic state represented by positive voltage the other by negative voltage ? Data rate: Rate of data (R) transmission in bits per second ? Duration or length of a bit: Time taken for transmitter to emit the bit (Tb=1/R) ? Modulation rate: Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digital encoding used.
Mark and Space – Binary 1 and Binary 0 respectively Types of encoding Systems There are various encoding systems which are divided into two categories: 1. Two-level encoding: the signal can only take on a strictly negative or strictly positive value (-X or +X, where X represents a value of the physical quantity being used to transport the signal) 2. Three-level encoding: the signal can take on a strictly negative, null or strictly positive value (-X, 0 or +X) Have already noted in earlier that both analog and digital information can be encoded as either analog or digital signals: Signal Encoding ChoiceReasonDigital data encoded into digital signalssimplest form of digital encoding of digital data Digital data, analog signalA modem converts digital data to an analog signal so that it can be transmitted over an analog Analog data, digital signalsAnalog data, such as voice and video, are often digitized to be able to use digital transmission facilities Analog data, analog signalsAnalog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission systemThe figure below emphasizes the process involved in this. For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carrier signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding source data onto the carrier signal.
All modulation techniques involve operation on one or more of the three fundamental frequency domain parameters: amplitude, frequency, and phase.The input signal m(t) may be analog or digital and is called the modulating signal, and the result of modulating the carrier signal is called the modulated signal s(t). Encoding and Modulation Techniques Digital Signaling Versus Analog Signaling ?Digital signaling ? Digital or analog data is encoded into a digital signal ? Encoding may be chosen to conserve bandwidth or to minimize error ? Analog Signaling ? Digital or analog data modulates analog carrier signal ? The frequency of the carrier frequency currency (fc) is chosen to be compatible with the transmission medium used ?Modulation: the amplitude, frequency or phase of the carrier signal is varied in accordance with the modulating data signal ? by using different carrier frequencies, multiple data signals (users) can share the same transmission medium ? Digital data, digital signal ?Simplest encoding scheme: assign one voltage level to binary one and another voltage level to binary zero ? More complex encoding schemes: are used to improve performance (reduce transmission bandwidth and minimize errors). ?Examples are NRZ-L, NRZI, Manchester, etc. ?Analog data, Digital signal Analog data, such as voice and video ?Often digitized to be able to use digital transmission facility ? Example: Pulse Code Modulation (PCM), which involves sampling the analog data periodically and quantizing the samples ?Digital data, Analog Signal ?A modem converts digital data to an analog signal so that it can be transmitted over an analog line ? The digital data modulates the amplitude, frequency, or phase of a carrier analog signal ? Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) ? Analog data, Analog Signal Analog data, such as voice and video modulate the amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band ? Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM) ?Digital Data, Digital Signal Encoding – Digital data to digital signals: A digital signal is a sequence of discrete, it has discontinuous voltage pulses. Each pulse is a signal element.
Binary data are transmitted by encoding each data bit into signal elements. In the simplest case, there is a one-to-one correspondence between bits and signal elements.More complex encoding schemes are used to improve performance, by altering the spectrum of the signal and providing synchronization capability. In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment Interpreting Signals Need to know The tasks involved in interpreting digital signals at the receiver can be summarized as follows. 1. Timing of bits – when they start and end: The receiver must know the timing of each bit, knowing with some accuracy when a bit begins and ends. 2.
Signal levels: The receiver must determine whether the signal level for each bit position is high (0) or low (1). These tasks can be performed by sampling each bit position in the middle of the interval and comparing the value to a threshold. Because of noise and other impairments, there will be errors. Three factors are important: I. The signal-to-noise ratio, II. The data rate, and III.
The bandwidth. With other factors held constant, the following statements are true: •An increase in data rate increases bit error rate (BER). •An increase in synchronization (SNR) decreases bit error rate. An increase in bandwidth allows an increase in data rate. There is another factor that can be used to improve performance, and that is the encoding scheme. The encoding scheme is simply the mapping from data bits to signal elements. Comparison of Encoding Schemes/Techniques Before describing the various encoding techniques, consider the following ways of evaluating or comparing them: I.
Signal Spectrum: Lack of high frequencies reduces required bandwidth, lack of direct current (dc) component allows ac coupling via transformer, providing isolation, should concentrate power in the middle of the bandwidth II.Clocking: need for synchronizing transmitter and receiver either with an external clock or with a sync mechanism based on signal III. Error detection: useful if can be built into signal encoding IV.
Signal interference and noise immunity: some codes are better than others V. Cost and complexity: Higher signal rate (; thus data rate) lead to higher costs, some codes require signal rate greater than data rate Encoding Schemes NonReturn to Zero-Level (NRZ-L) ?Two different voltages for 0 and 1 bits ?Voltage constant during bit interval ?no transition, i. e. no return to zero voltage more often, negative voltage for binary one and positive voltage for binary zero The most common, and easiest, way to transmit digital signals is to use two different voltage levels for the two binary digits.
Codes that follow this strategy share the property that the voltage level is constant during a bit interval; there is no transition (no return to a zero voltage level). Can have absence of voltage used to represent binary 0, with a constant positive voltage used to represent binary 1. More commonly a negative voltage represents one binary value and a positive voltage represents the other.This is known as Nonreturn to Zero-Level (NRZ-L).
NRZ-L is typically the code used to generate or interpret digital data by terminals and other devices. NonReturn to Zero INVERTED (NRZI) A variation of NRZ is known as NRZI (Nonreturn to Zero, invert on ones). As with NRZ-L, NRZI maintains a constant voltage pulse for the duration of a bit time. The data bits are encoded as the presence or absence of a signal transition at the beginning of the bit time. A transition (low to high or high to low) at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0. NRZI is an example of differential encoding.
In differential encoding, the information to be transmitted is represented in terms of the changes between successive signal elements rather than the signal elements themselves. The encoding of the current bit is determined as follows: 1. If the current bit is a binary 0, then the current bit is encoded with the same signal as the preceding bit; 2. If the current bit is a binary 1, then the current bit is encoded with a different signal than the preceding bit. One benefit of differential encoding is that it may be more reliable to detect a transition in the presence of noise than to compare a value to a threshold (entrance).
Another benefit is that with a complex transmission layout, it is easy to lose the sense of the polarity of the signal. In summary ?Nonreturn to zero inverted on ones ?Constant voltage pulse for duration of bit ?Data encoded as presence or absence of signal transition at beginning of bit time ? transition (low to high or high to low) denotes binary 1 ? no transition denotes binary 0 ?Example of differential encoding since have ?data represented by changes rather than levels ?more reliable detection of transition rather than level Advantages and disadvantages of NRZ-L, NRZIThe NRZ codes are the easiest to engineer and, in addition, make efficient use of bandwidth. Most of the energy in NRZ and NRZI signals is between dc and half the bit rate. The main limitations of NRZ signals are the presence of a dc component and the lack of synchronization capability. Consider that with a long string of 1s or 0s for NRZ-L or a long string of 0s for NRZI, the output is a constant voltage over a long period of time.
Under these circumstances, any drift between the clocks of transmitter and receiver will result in loss of synchronization between the two.Because of their simplicity and relatively low frequency response characteristics, NRZ codes are commonly used for digital magnetic recording. However, their limitations make these codes unattractive for signal transmission applications. In summary ?Easy to engineer ?Make efficient use of bandwidth However ?Suffers from the presence of dc component ?Lack of synchronization capabilities due to potential of long runs of unchanged voltage levels. ?Attractive for digital magnetic recording, but not for signal transmissions. ?The spectral density graph shows that most of the energy spent between dc and half the bit rateMultilevel Binary Bipolar Alternate Mark Inversion (AMI) It is a category of encoding techniques known as multilevel binary addresses some of the deficiencies of the NRZ codes.
These codes use more than two signal levels. In the bipolar-AMI scheme, a binary 0 is represented by no line signal, and a binary 1 is represented by a positive or negative pulse. The binary 1 pulses must alternate in polarity. There are several advantages to this approach.
1. There will be no loss of synchronization if a long string of 1s occurs. Each 1 introduces a transition, and the receiver can resynchronize on that transition.A long string of 0s would still be a problem. 2. Because the 1 signals alternate in voltage from positive to negative, there is no net dc component. Also, the bandwidth of the resulting signal is considerably less than the bandwidth for NRZ. 3.
The pulse alternation property provides a simple means of error detection. Any isolated error, whether it deletes a pulse or adds a pulse, causes a violation of this property. In summary ?Use more than two levels (three levels, positive, negative and no line signal) ? Bipolar-AMI ?zero represented by no line signal one represented by positive or negative pulse ?one pulses alternate in polarity ?no loss of sync if a long string of ones ?long runs of zeros still a problem ?no net dc component ?lower bandwidth ?easy error detection Multilevel Binary Pseudoternary The comments on bipolar-AMI also apply to pseudoternary. In this case, it is the binary 1 that is represented by the absence of a line signal, and the binary 0 by alternating positive and negative pulses. There is no particular advantage of one technique versus the other, and each is the basis of some applications. In summary Binary one represented by absence of line signal ?Binary zero represented by alternating positive and negative pulses ? No advantage or disadvantage over bipolar-AMI ?Each used in some applications Advantages and disadvantages Although a degree of synchronization is provided with these codes, a long string of 0s in the case of AMI or 1s in the case of pseudoternary still presents a problem. Several techniques have been used to address this deficiency.
One approach is to insert additional bits that force transitions. This technique is used in ISDN (integrated services digital etwork) for relatively low data rate transmission. Of course, at a high data rate, this scheme is expensive, because it results in an increase in an already high signal transmission rate. To deal with this problem at high data rates, a technique that involves scrambling the data is used.
Thus, with suitable modification, multilevel binary schemes overcome the problems of NRZ codes. Of course, as with any engineering design decision, there is a tradeoff. With multilevel binary coding, the line signal may take on one of three levels, but each signal element, which could represent log2 3 = 1.
8 bits of information, bears only one bit of information, since the receiver of multilevel binary signals has to distinguish between three levels (+A, –A, 0) instead of just two levels in the signaling formats previously discussed. Because of this, the multilevel binary signal requires approximately 3 dB more signal power than a two-valued signal for the same probability of bit error. Put another way, the bit error rate for NRZ codes, at a given signal-to-noise ratio, is significantly less than that for multilevel binary. In summary Advantages: ?No loss of synchronization if a long string of 1’s occurs, each introduce a transition, and the receiver can resynchronize on that transition ? No net dc component, as the 1 signal alternate in voltage from negative to positive ? Less bandwidth than NRZ ?Pulse alternating provides a simple mean for error detection ? Disadvantages ?receiver distinguishes between three levels: +A, -A, 0 ?a 3 level system could represent log23 = 1.
58 bits ?requires approx. 3dB more signal power for same probability of bit errorTheoretical Bit Error Rate (BER) For Various Encoding Schemes Although a degree of synchronization is provided with these codes, a long string of 0s in the case of AMI or 1s in the case of pseudoternary still presents a problem. Several techniques have been used to address this deficiency.
One approach is to insert additional bits that force transitions. This technique is used in ISDN (integrated services digital network) for relatively low data rate transmission. Of course, at a high data rate, this scheme is expensive, because it results in an increase in an already high signal transmission rate.
To deal with this problem at high data rates, a technique that involves scrambling the data is used. Thus, with suitable modification, multilevel binary schemes overcome the problems of NRZ codes. Of course, as with any engineering design decision, there is a tradeoff. With multilevel binary coding, the line signal may take on one of three levels, but each signal element, which could represent log2 3 = 1. 58 bits of information, bears only one bit of information, since the receiver of multilevel binary signals has to distinguish between three levels (+A, –A, 0) instead of just two levels in the signaling formats previously discussed.Because of this, the multilevel binary signal requires approximately 3 dB more signal power than a two-valued signal for the same probability of bit error. Put another way, the bit error rate for NRZ codes, at a given signal-to-noise ratio, is significantly less than that for multilevel binary. Manchester Encoding There is another set of coding techniques, grouped under the term biphase, that overcomes the limitations of NRZ codes.
Two of these techniques, Manchester and differential Manchester, are in common use. In the Manchester code, there is a transition at the middle of each bit period.The midbit transition serves as a clocking mechanism and also as data: a low-to-high transition represents a 1, and a high-to-low transition represents a 0.
Biphase codes are popular techniques for data transmission. The more common Manchester code has been specified for the IEEE 802. 3 (Ethernet) standard for baseband coaxial cable and twisted-pair bus LANs. In summary ?has transition in middle of each bit period ?low to high represents binary one ?transition serves as clock and data ?high to low represents binary zero ?used by IEEE 802. 3 (Ethernet) LAN standardDifferential Manchester Encoding In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 is represented by the presence of a transition at the beginning of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period. Differential Manchester has the added advantage of employing differential encoding.
Differential Manchester has been specified for the IEEE 802. 5 token ring LAN, using shielded twisted pair. In summary ?midbit transition is clocking only ?transition at start of bit period representing binary 0 no transition at start of bit period representing binary 1 ? used by IEEE 802. 5 token ring LAN Advantages and disadvantages of Manchester Encoding All of the biphase techniques require at least one transition per bit time and may have as many as two transitions. Thus, the maximum modulation rate is twice that for NRZ; this means that the bandwidth required is correspondingly greater.
The bandwidth for biphase codes is reasonably narrow and contains no dc component. However, it is wider than the bandwidth for the multilevel binary codes.On the other hand, the biphase schemes have several advantages: •Synchronization: Because there is a predictable transition during each bit time, the receiver can synchronize on that transition, known as self-clocking codes. •No dc component: Biphase codes have no dc component •Error detection: The absence of an expected transition can be used to detect errors. Noise on the line would have to invert both the signal before and after the expected transition to cause an undetected error. In summary ?Disadvantages ?at least one transition per bit time and possibly two ?maximum modulation rate is twice NRZ requires more bandwidth ?Advantages ?synchronization on mid bit transition (self clocking codes) ? has no dc component ?has error detection capability (the absence of an expected transition can be used to detect errors) Modulation Rate versus Data Rate When signal-encoding techniques are used, a distinction needs to be made between data rate (expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is 1/Tb, where Tb = bit duration. The modulation rate is the rate at which signal elements are generated.
Consider, for example, Manchester encoding.The minimum size signal element is a pulse of one-half the duration of a bit interval. For a string of all binary zeroes or all binary ones, a continuous stream of such pulses is generated. Hence the maximum modulation rate for Manchester is 2/Tb. This situation is illustrated in Stallings DCC8e Figure 5.
5, which shows the transmission of a stream of binary 1s at a data rate of 1 Mbps using NRZI and Manchester. One way of characterizing the modulation rate is to determine the average number of transitions that occur per bit time. In general, this will depend on the exact sequence of bits being transmitted.Stallings DCC8e Table 5. 3 compares transition rates for various techniques. In summary ?Data rate (expressed in bps) ?Data rate or bit rate R=1/Tb=1/1? s=1Mbps ?Modulation Rate (expressed in baud) is the rate at which signal elements are generated ? Maximum modulation rate for Manchester is D=1/(0. 5Tb)=2/1? s=2Mbaud Scrambling Although the biphase techniques have achieved widespread use in local area network applications at relatively high data rates (up to 10 Mbps), they have not been widely used in long-distance applications.
The principal reason for this is that they require a high signaling rate relative to the data rate.This sort of inefficiency is more costly in a long-distance application. Another approach is to make use of some sort of scrambling scheme. The idea behind this approach is simple: sequences that would result in a constant voltage level on the line are replaced by filling sequences that will provide sufficient transitions for the receiver’s clock to maintain synchronization.
The filling sequence must be recognized by the receiver and replaced with the original data sequence. The filling sequence is the same length as the original sequence, so there is no data rate penalty.The design goals for this approach can be summarized as follows: •No dc component •No long sequences of zero-level line signals •No reduction in data rate •Error-detection capability B8ZS and HDB3 Two techniques are commonly used in long-distance transmission services; these are illustrated in Stallings DCC8e Figure 5. 6. A coding scheme that is commonly used in North America, based on a bipolar-AMI, is known as bipolar with 8-zeros substitution (B8ZS).
To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: a.If an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+–0–+. b. If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000–+0+–. This technique forces two code violations (signal patterns not allowed in AMI) of the AMI code, an event unlikely to be caused by noise or other transmission impairment.
The receiver recognizes the pattern and interprets the octet as consisting of all zeros.A coding scheme that is commonly used in Europe and Japan is known as the high-density bipolar-3 zeros (HDB3) code. It is also based on the use of AMI encoding. In this case, the scheme replaces strings of four zeros with sequences containing one or two pulses. In each case, the fourth zero is replaced with a code violation. In addition, a rule is needed to ensure that successive violations are of alternate polarity so that no dc component is introduced.
Thus, if the last violation was positive, this violation must be negative and vice versa. Neither of these codes has a dc component.Most of the energy is concentrated in a relatively sharp spectrum around a frequency equal to one-half the data rate. Thus, these codes are well suited to high data rate transmission. Bipolar with 8-Zero Substitution (B8ZS) Two techniques are commonly used in long-distance transmission services; these are illustrated in Stallings DCC8e Figure 5. 6. A coding scheme that is commonly used in North America, based on a bipolar-AMI, is known as bipolar with 8-zeros substitution (B8ZS). To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: a.
If an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+–0–+. b. If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000–+0+–. This technique forces two code violations (signal patterns not allowed in AMI) of the AMI code, an event unlikely to be caused by noise or other transmission impairment. The receiver recognizes the pattern and interprets the octet as consisting of all zeros.A coding scheme that is commonly used in Europe and Japan is known as the high-density bipolar-3 zeros (HDB3) code. It is also based on the use of AMI encoding. In this case, the scheme replaces strings of four zeros with sequences containing one or two pulses.
In each case, the fourth zero is replaced with a code violation. In addition, a rule is needed to ensure that successive violations are of alternate polarity so that no dc component is introduced. Thus, if the last violation was positive, this violation must be negative and vice versa.
Neither of these codes has a dc component.Most of the energy is concentrated in a relatively sharp spectrum around a frequency equal to one-half the data rate. Thus, these codes are well suited to high data rate transmission. In summary ?To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: ? If 8 zeros occurs and the last voltage pulse was positive, then the 8 zeros are encoded as 000+–0–+ ? If zeros occurs and the last voltage pulse was negative, then the 8 zeros are encoded as 000–+0+– High Density Bipolar-3 zeros (HDB3) The scheme replaces strings with 4 zeros by sequences containing one or two pulses ? In each case, the fourth zero is replaced with a code violation (V) ? successive violations are of alternate polarity Digital Data, Analog Signal We turn now to the case of transmitting digital data using analog signals. The most familiar use of this transformation is for transmitting digital data through the public telephone network. The telephone network was designed to receive, switch, and transmit analog signals in the voice-frequency range of about 300 to 3400 Hz.
It is not at present suitable for handling digital signals from the subscriber locations (although this is beginning to change). Thus digital devices are attached to the network via a modem (modulator-demodulator), which converts digital data to analog signals, and vice versa. Modulation involves operation on one or more of the three characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly, there are three basic encoding or modulation techniques for transforming digital data into analog signals, as illustrated in Stallings DCC8e Figure 5. (next slide): amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK).
In all these cases, the resulting signal occupies a bandwidth centered on the carrier frequency. In summary ?Main use is public telephone system ?has freq range of 300Hz to 3400Hz ?use modem (modulator-demodulator) ?The digital data modulates the amplitude A, frequency fc , or phase ? of a carrier signal ? Modulation techniques ?Amplitude Shift Keying (ASK) ?Frequency Shift Keying (FSK) ?Phase Shift Keying (PSK) Modulation TechniquesAs stated earlier, modulation involves operation on one or more of the three characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly, there are three basic encoding or modulation techniques for transforming digital data into analog signals, as illustrated in Stallings DCC8e Figure 5. 7 (above): amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). In all these cases, the resulting signal occupies a bandwidth centered on the carrier frequency Amplitude Shift Keying (ASK) In ASK, the two binary values are represented by two different amplitudes of the carrier frequency.Commonly, one of the amplitudes is zero; that is, one binary digit is represented by the presence, at constant amplitude, of the carrier, the other by the absence of the carrier, as shown in Stallings DCC8e Figure 5. 7a. ASK is susceptible to sudden gain changes and is a rather inefficient modulation technique.
On voice-grade lines, it is typically used only up to 1200 bps. The ASK technique is used to transmit digital data over optical fiber, where one signal element is represented by a light pulse while the other signal element is represented by the absence of light.In summary ?In ASK, the two binary values are represented by to different amplitudes of the carrier frequency ?The resulting modulated signal for one bit time is ?Susceptible to noise ?Inefficient modulation technique ?used for ?up to 1200bps on voice grade lines ?very high speeds over optical fiber Binary Frequency Shift Keying (BFSK) The most common form of FSK is binary FSK (BFSK), in which the two binary values are represented by two different frequencies near the carrier frequency, as shown in Stallings DCC8e Figure 5. 7b. BFSK is less susceptible to error than ASK.On voice-grade lines, it is typically used up to 1200 bps. It is also commonly used for high-frequency (3 to 30 MHz) radio transmission. It can also be used at even higher frequencies on local area networks that use coaxial cable.
Full-Duplex BFSK Transmission on a Voice-Grade line ?Voice grade lines will pass voice frequencies in the range 300 to 3400Hz ? Full duplex means that signals are transmitted in both directions at the same time Multiple FSK (MFSK) A signal that is more bandwidth efficient, but also more susceptible to error, is multiple FSK (MFSK), in which more than two frequencies are used.In this case each signaling element represents more than one bit. To match the data rate of the input bit stream, each output signal element is held for a period of Ts = LT seconds, where T is the bit period (data rate = 1/T). Thus, one signal element, which is a constant-frequency tone, encodes L bits. The total bandwidth required is 2Mfd. It can be shown that the minimum frequency separation required is 2fd = 1/Ts.
Therefore, the modulator requires a bandwidth of Wd = 2Mfd = M/Ts. In summary ?More than two frequencies (M frequencies) are used ?More bandwidth efficient compared to BFSK More susceptible to noise compared to BFSK ?MFSK signal: Multiple FSK (MFSK) A signal that is more bandwidth efficient, but also more susceptible to error, is multiple FSK (MFSK), in which more than two frequencies are used. In this case each signaling element represents more than one bit. To match the data rate of the input bit stream, each output signal element is held for a period of Ts = LT seconds, where T is the bit period (data rate = 1/T).
Thus, one signal element, which is a constant-frequency tone, encodes L bits. The total bandwidth required is 2Mfd.It can be shown that the minimum frequency separation required is 2fd = 1/Ts. Therefore, the modulator requires a bandwidth of Wd = 2Mfd = M/Ts. Example ?With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit data combinations: This scheme can support a data rate ofPhase Shift Keying (PSK) In PSK, the phase of the carrier signal is shifted to represent data.
The simplest scheme uses two phases to represent the two binary digits (Figure 5. 7c) and is known as binary phase shift keying.An alternative form of two-level PSK is differential PSK (DPSK). In this scheme, a binary 0 is represented by sending a signal burst of the same phase as the previous signal burst sent. A binary 1 is represented by sending a signal burst of opposite phase to the preceding one.
This term differential refers to the fact that the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal. In differential encoding, the information to be transmitted is represented in terms of the changes between successive data symbols rather than the signal elements themselves.DPSK avoids the requirement for an accurate local oscillator phase at the receiver that is matched with the transmitter. As long as the preceding phase is received correctly, the phase reference is accurate. Differential PSK (DPSK) ?In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal ? Binary 0:signal burst with the same phase as the previous one ? Binary 1:signal burst of opposite phase to the preceding one Four-level PSK: Quadrature PSK (QPSK)More efficient use of bandwidth can be achieved if each signaling element represents more than one bit. For example, instead of a phase shift of 180? , as allowed in BPSK, a common encoding technique, known as quadrature phase shift keying (QPSK), uses phase shifts separated by multiples of ? /2 (90? ).
Thus each signal element represents two bits rather than one. The input is a stream of binary digits with a data rate of R = 1/Tb, where Tb is the width of each bit. This stream is converted into two separate bit streams of R/2 bps each, by taking alternate bits for the two streams.The two data streams are referred to as the I (in-phase) and Q (quadrature phase) streams.
The streams are modulated on a carrier of frequency fc by multiplying the bit stream by the carrier, and the carrier shifted by 90?. The two modulated signals are then added together and transmitted. Thus, the combined signals have a symbol rate that is half the input bit rate. The use of multiple levels can be extended beyond taking bits two at a time.
It is possible to transmit bits three at a time using eight different phase angles. Further, each angle can have more than one amplitude.For example, a standard 9600 bps modem uses 12 phase angles, four of which have two amplitude values, for a total of 16 different signal elements. In summary ?More efficient use of bandwidth if each signal element represents more than one bit ? eg.
shifts of p/2 (90o) ?each signal element represents two bits ?split input data stream in two & modulate onto the phase of the carrier ? can use 8 phase angles & more than one amplitude ?9600bps modem uses 12 phase angles, four of which have two amplitudes: this gives a total of 16 different signal elements