4.2Components of DataCommunication Data Analog: Continuous value data (sound, light,temperature) Digital: Discrete value (text, integers,symbols) Signal Analog: Continuously varyingelectromagnetic wave Digital: Series of voltage pulses (squarewave)
4.3Analog Data-->Signal Options Analog data to analog signal Inexpensive, easy conversion (eg telephone) Used in traditional analog telephony Analog data to digital signal Requires a codec (encoder/decoder) Allows use of digital telephony, voice mail
4.4Digital Data-->Signal Options Digital data to analog signal Requires modem (modulator/demodulator) Necessary when analog transmission is used Digital data to digital signal Less expensive when large amounts of dataare involved More reliable because no conversion isinvolved
4.54-1 DIGITAL-TO-DIGITAL CONVERSION4-1 DIGITAL-TO-DIGITAL CONVERSIONIn this section, we see how we can represent digitalIn this section, we see how we can represent digitaldata by using digital signals. The conversion involvesdata by using digital signals. The conversion involvesthree techniques:three techniques: line codingline coding,, block codingblock coding, and, andscramblingscrambling. Line coding is always needed; block. Line coding is always needed; blockcoding and scrambling may or may not be needed.coding and scrambling may or may not be needed.Line CodingLine Coding SchemesBlock CodingScramblingTopics discussed in this section:Topics discussed in this section:
4.7Figure 4.2 Signal element versus data elementr = number of data elements / number of signal elements
4.8Data Rate Vs. Signal Rate•Data rate: the number of data elements (bits) sentin 1s (bps). It’s also called the bit rate•Signal rate: the number of signal elements sent in 1s(baud). It’s also called the pulse rate, the modulationrate, or the baud rate.We wish to:1. increase the data rate (increase the speed oftransmission)2. decrease the signal rate (decrease the bandwidthrequirement)3. Worst case, best case, and average case of r4. S = c * N / r baud
4.9Baseline wanderingBaseline: running average of thereceived signal powerDC ComponentsConstant digital signal creates lowfrequenciesSelf-synchronizationReceiver Setting the clock matching thesender’s
4.10Figure 4.3 Effect of lack of synchronization
4.13Digital Encodingof Digital Data Most common, easiest method is differentvoltage levels for the two binary digits Typically, negative=1 and positive=0 Known as NRZ-L, or nonreturn-to-zerolevel, because signal never returns to zero,and the voltage during a bit transmission islevel
4.14Differential NRZ Differential version is NRZI (NRZ, inverton ones) Change=1, no change=0 Advantage of differential encoding is that itis more reliable to detect a change inpolarity than it is to accurately detect aspecific level
4.15Problems With NRZ Difficult to determine where one bit endsand the next begins In NRZ-L, long strings of ones and zeroeswould appear as constant voltage pulses Timing is critical, because any drift resultsin lack of synchronization and incorrect bitvalues being transmitted
4.18Manchester Code Transition in the middle of each bit period Transition provides clocking and data Low-to-high=1 , high-to-low=0 Used in Ethernet
4.19Differential Manchester Midbit transition is only for clocking Transition at beginning of bit period=0 Transition absent at beginning=1 Has added advantage of differentialencoding Used in token-ring
4.20Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
4.21• High=0, Low=1• No change at begin=0, Change atbegin=1• H-to-L=0, L-to-H=1• Change at begin=0, No change atbegin=1
4.22Bipolar schemes: AMI (Alternate Mark Inversion) and pseudoternary
4.23Multilevel Schemes• In mBnL schemes, a pattern of mdata elements is encoded as apattern of n signal elements inwhich 2m≤ Ln• m: the length of the binary pattern• B: binary data• n: the length of the signal pattern• L: number of levels in the signaling• B for l=2 binary• T for l=3 ternary• Q for l=4 quaternary
4.24Figure 4.10 Multilevel: 2B1Q schemeUsed in DSL
4.27Table 4.1 Summary of line coding schemesPolar
4.28Block Coding• Redundancy is needed to ensuresynchronization and to provideerror detecting• Block coding is normally referred toas mB/nB coding• it replaces each m-bit group with ann-bit group• m < n
4.34Scrambling• It modifies the bipolar AMI encoding(no DC component, but having theproblem of synchronization)• It does not increase the number ofbits• It provides synchronization• It uses some specific form of bits toreplace a sequence of 0s
4.35Figure 4.19 Two cases of B8ZS scrambling techniqueB8ZS substitutes eight consecutive zeros with 000VB0VB
4.36Figure 4.20 Different situations in HDB3 scrambling techniqueHDB3 substitutes four consecutive zeros with 000V or B00V dependingon the number of nonzero pulses after the last substitution.
4.374-2 ANALOG-TO-DIGITAL CONVERSION4-2 ANALOG-TO-DIGITAL CONVERSIONThe tendency today is to change an analog signal toThe tendency today is to change an analog signal todigital data.digital data.In this section we describe two techniques,In this section we describe two techniques,pulse code modulationpulse code modulation andand delta modulationdelta modulation..
4.39According to the Nyquist theorem, thesampling rate must be at least 2 timesthe highest frequency contained in thesignal.What can we get from this:1. we can sample a signal only if the signal isband-limited2. the sampling rate must be at least 2 times thehighest frequency, not the bandwidth
4.40Figure 4.23 Nyquist sampling rate for low-pass and bandpass signals
4.41Figure 4.24 Recovery of a sampled sine wave for different sampling rates
4.42Figure 4.25 Sampling of a clock with only one hand
4.43An example related is the seemingly backward rotation ofthe wheels of a forward-moving car in a movie.This can be explained by under-sampling. A movie isfilmed at 24 frames per second. If a wheel is rotatingmore than 12 times per second, the under-samplingcreates the impression of a backward rotation.Example
4.44A complex low-pass signal has a bandwidth of 200 kHz.What is the minimum sampling rate for this signal?SolutionThe bandwidth of a low-pass signal is between 0 and f,where f is the maximum frequency in the signal.Therefore, we can sample this signal at 2 times thehighest frequency (200 kHz). The sampling rate istherefore 400,000 samples per second.Example
4.45A complex bandpass signal has a bandwidth of 200 kHz.What is the minimum sampling rate for this signal?SolutionWe cannot find the minimum sampling rate in this casebecause we do not know where the bandwidth starts orends. We do not know the maximum frequency in thesignal.Example
4.46Figure 4.26 Quantization and encoding of a sampled signal
4.47What is the SNRdB in the example of Figure 4.26?SolutionWe have eight levels and 3 bits per sample, soSNRdB = 6.02 x 3 + 1.76 = 19.82 dBIncreasing the number of levels increases the SNR.Contribution of the quantization error to SNRdbSNRdb= 6.02nb + 1.76 dBnb: bits per sample (related to the number of level L)
4.48A telephone subscriber line must have an SNRdB above40. What is the minimum number of bits per sample?SolutionWe can calculate the number of bits asExampleTelephone companies usually assign 7 or 8 bits persample.
4.50We have a low-pass analog signal of 4 kHz. If we send theanalog signal, we need a channel with a minimumbandwidth of 4 kHz. If we digitize the signal and send 8bits per sample, we need a channel with a minimumbandwidth of 8 × 4 kHz = 32 kHz.The minimum bandwidth of the digital signal is nbtimes greater than the bandwidth of the analogsignal.Bmin= nb x Banalog
4.51DM (delta modulation) finds the change from theprevious sampleNext bit is 1, if amplitude of the analog signal is largerNext bit is 0, if amplitude of the analog signal is smaller
4.544-3 TRANSMISSION MODES4-3 TRANSMISSION MODES1. The transmission of binary data across a link can1. The transmission of binary data across a link canbe accomplished in either parallel or serial mode.be accomplished in either parallel or serial mode.2. In parallel mode, multiple bits are sent with each2. In parallel mode, multiple bits are sent with eachclock tick.clock tick.3. In serial mode, 1 bit is sent with each clock tick.3. In serial mode, 1 bit is sent with each clock tick.4. there are three subclasses of serial transmission:4. there are three subclasses of serial transmission:asynchronous, synchronous, and isochronous.asynchronous, synchronous, and isochronous.
4.58Asynchronous transmission1. We send 1 start bit (0) at the beginning and 1 or more stop bits(1s) at the end of each byte.2. There may be a gap between each byte.3. Extra bits and gaps are used to alert the receiver, and allow it tosynchronize with the data stream.4. Asynchronous here means “asynchronous at the byte level,”but the bits are still synchronized, their durations are the same.
4.59Synchronous transmissionIn synchronous transmission, we send bits one afteranother without start or stop bits or gaps. It is theresponsibility of the receiver to group the bits.