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COMP 421 /CMPET 401

COMP 421 /CMPET 401. COMMUNICATIONS and NETWORKING CLASS 6. Encoding Techniques. Digital data, digital signal Easy encoding / Less Complex Less Expensive Analog data, digital signal Can transmit data over Digital Network Digital data, analog signal Modems / Fiber / Unguided Media

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COMP 421 /CMPET 401

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  1. COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING CLASS 6

  2. Encoding Techniques • Digital data, digital signal • Easy encoding / Less Complex Less Expensive • Analog data, digital signal • Can transmit data over Digital Network • Digital data, analog signal • Modems / Fiber / Unguided Media • Analog data, analog signal • Cheap & Easy Baseband Transmission / FDM

  3. Analog Data Choices

  4. Digital Data Choices

  5. Transmission Choices • Analog transmission • Only transmits analog signals, without regard for data content • Attenuation overcome with amplifiers • Digital transmission • Transmits analog or digital signals • Uses repeaters rather than amplifiers

  6. Advantages of Digital Transmission • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression

  7. Encoding schemes Analog data, Digital signal Analog data, Analog signal digital analog analog voice Telephone CODEC Digitaldata, Digital signal Digitaldata, Analogsignal analog digital digital digital Modem Digital transmitter

  8. Encoding and Modulation x(t) x(t) g(t) g(t) Encoder Decoder digital or analog digital t s(f) s(t) m(t) Modulator Demodulator m(t) digital or analog analog f fc fc

  9. Why encoding? • Three factors determine successfulness of receiving a signal: • S/N (Signal to Noise Ratio) • Data rate • Bandwidth

  10. Encoding Schemes' Evaluation Factors • Signal spectrum • Clocking • Error detection • Signal interference & noise immunity • Cost and complexity

  11. Digital Data, Digital Signal / Characteristics • Digital signal • Uses discrete, discontinuous, voltage pulses • Each pulse is a signal element • Binary data is encoded into signal elements

  12. Terms (1) • Unipolar • All signal elements have same sign • Polar • One logic state represented by positive voltage the other by negative voltage • Data rate • Rate of data transmission in bits per second • Duration or length of a bit • Time taken for transmitter to emit the bit

  13. Terms (2) • Modulation rate • Rate at which the signal level changes • Measured in baud = signal elements per second • Mark and Space • Binary 1 and Binary 0 respectively

  14. Interpreting Signals • Need to know • Timing of bits - when they start and end • Signal levels • Factors affecting successful interpretation of signals: • Signal to noise ratio • Data rate • Bandwidth

  15. Comparison of Encoding Schemes (1) • Signal Spectrum • Lack of high frequencies reduces required bandwidth • Lack of dc component allows ac coupling via transformer, providing isolation • It is important to concentrate power in the middle of the bandwidth

  16. Comparison of Encoding Schemes (2) • Clocking issues • Synchronizing transmitter and receiver is essential • External clock is one way used for synchronization • Synchronizing mechanism based on signal is also used & preferred (over using an external clock)

  17. Spectral density 1.5 B8ZS,HDB3 NRZ-L, NRZI 1 AMI, Pseudoternary 0.5 Mean square voltage per unit bandwidth Manchester, Differential Manchester 0 0 1 0.5 1.5 -0.5 Normalizedfrequency (f/r)

  18. Comparison of Encoding Schemes (3) • Error detection • Can be built into signal encoding • Signal interference and noise immunity • Some codes are better than others • Cost and complexity • Higher signal rate (& thus data rate) lead to higher costs • Some codes require signal rate greater than data rate

  19. Encoding Schemes • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI (Alternate Mark Inversion) • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3

  20. Digital Data, Digital Signal 0 1 0 0 1 1 0 0 0 1 1 NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester

  21. Nonreturn to Zero-Level (NRZ-L) • Two different voltages: • 0 - Low Level • 1 - High Level • Voltage constant during bit interval • Most often, negative voltage for one value and positive for the other

  22. Nonreturn to Zero Inverted • 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 a binary 1 • No transition denotes binary 0 • An example of differential encoding (Data represented by changes rather than levels)

  23. NRZ

  24. NRZ pros and cons • Pros • Easy to engineer • Makes good use of bandwidth • Cons • dc component • Lack of synchronization capability • Used for magnetic recording • Not often used for signal transmission

  25. 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 happens (zeros still a problem) • No net dc component  Can use a transformer for isolating transmission line • Lower bandwidth • Easy error detection

  26. Pseudoternary • One represented by absence of line signal • Zero represented by alternating positive and negative • No advantage or disadvantage over bipolar-AMI

  27. Bipolar-AMI and Pseudoternary

  28. Trade Off for Multilevel Binary • Not as efficient as NRZ • With multi-level binary coding, the line signal may take on one of 3 levels, but each signal element, which could represent log23 = 1.58 bits of information, bears only one bit of information • Receiver must distinguish between three levels : (+A, -A, 0) • Requires approx. 3dB more signal power for same probability of bit error

  29. Biphase • Manchester • Transition in middle of each bit period • Transition serves as clock and data • One is represented by a transition from low to high • Zero is represented by a transition from high to low • Used by IEEE 802.3 (Ethernet)

  30. Differential Manchester • Always a transition in the middle of the interval for clocking • Transition at start of a bit period represents zero • No transition at start of a bit period represents one • Note: this is a differential encoding scheme used by • IEEE 802.5 (Token Ring)

  31. Biphase Pros and Cons • Con • At least one transition per bit time and possibly two • Maximum modulation rate is twice NRZ • Requires more bandwidth • Pros • Synchronization on mid bit transition (self clocking) • No dc component • Error detection • Absence of expected transition points to error in transmission

  32. Modulation Rate The modulation Rate is at which signal elements are generated In General the Modulation Rate D = R/b where R=Data Rate=bits/sec b=number of bits per signal element Data Rate (bit Rate 1/Tb) where Tb is bit duration For Manchester Encoding maximum Rate is: 2/Tb

  33. Scrambling Techniques • Used to reduce signaling rate relative to the data rate by replacing sequences that would produce constant voltage for a priod of time with a filling sequence that accomplishes the following goals: • Must produce enough transitions to maintain synchronization • Must be recognized by receiver and replaced with original data sequence • is same length as original sequence

  34. Scrambling Techniques • No dc component • No long sequences of zero level line signal • No reduction in data rate • Error detection capability • As an example, fax machines use the modified Huffman code to accomplish this.

  35. B8ZS • B8ZS:Abbreviation forbipolar with eight-zero substitution • Same as Bipolar AMI with 8 Zeros Substitution • Based on Bipolar-AMI • If octet of all zeros and last voltage pulse preceding was positive, encode as 000+-0-+ • If octet of all zeros and last voltage pulse preceding was negative, encode as 000-+0+- • Causes two violations of AMI code • This is unlikely to occur as a result of noise • Receiver detects and interprets the sequence as octet of all zeros

  36. B8ZS • A one is sent on a T1 by sending a pulse, as opposed to not sending a pulse. • The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going. • If a T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred. • In B8ZS a specific combination of valid pulses and bipolar violations is used to represent a string of eight zeroes, whenever the user data contains eight zeroes in a row

  37. B8ZS Since a T1 uses a single pair of wires in each direction and the only signals on those wires are the pulses which represent data; the only way to recover clock and retain synchronization on a T1 is by detecting the rate at which pulses are being received. All of the equipment in a T1 circuit must operate at the same rate because all of the equipment must sense the T1 at the correct time in order to determine if a pulse (1) or no pulse (0) has been received at each bit time. Since only ones are sent as pulses and zeroes are represented by doing nothing, if too many zeroes are sent at a time there will be no pulses on the T1 at all and the clock circuitry in all of the hardware will rapidly fall out of synchronization. Thus the design of AMI requires that a certain ONES DENSITY be maintained, that a certain minimum of the bits over a certain period of time be guaranteed to be a ONE (pulse). This is why AMI circuits require DENSITY enforcement

  38. B8ZS Briefly stated; on average one bit in eight must be a one and no more than (varies according to specific standard) so many zeroes may be sent in a row. In order to be able to satisfy the ones density requirement on an AMI T1 one bit out of every eight is taken away from the user, not available for voice or data traffic, and that 1 bit in 8 is always sent as a one. Once this has been done the requirement for ones density is satisfied and the user is free to send any data pattern in the remaining bandwidth.

  39. B8ZS The rate of a T1 is 1.544 megabits per second. 8K is used for framing leaving 1.536MBPS. The 1.536 is usually divided into 24 timeslots (DS0s) or "channels" each being inherently 64KBPS. By taking the 1 bit in 8 that is reserved to satisfy ones density the user is left with 56K per timeslot.

  40. AMI • AMI = Alternate Mark Inversion. This is the original method of formatting T1 data streams. In AMI a zero is always sent by doing nothing, at the time when a pulse might otherwise be sent, a pulse is not sent to represent a zero. • A one is sent on an AMI T1 by sending a pulse, as opposed to not sending a pulse. • The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going. • If an AMI T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred. • Thus AMI has a rudimentary error checking capability with a 50% probability of detecting altered, inserted or lost bits end to end.

  41. ESF Extended Super Frame A DS level and framing specification for synchronous digital streams over circuits in the North America. A DS1 "frame" is composed of 24 eight-bit bytes plus one framing bit (193 bits). 8000 bytes per second come from each source, and thus 8000 frames per second are transported by the DS1 signal. The result is 193*8000 = 1,544,000 bits per second. In the original standard, the framing bits continuously repeated the sequence 110111001000, and such a 12-frame unit is called a super-frame. In voice telephony, errors are acceptable (early standards allowed as much as one frame in six to be missing entirely), so the least significant bit in two of the 24 streams was used for signaling between network equipments. This is called robbed bit signaling

  42. ESF To promote error-free transmission, an alternative called the extended super-frame (ESF) of 24 frames was developed. In this standard, six of the 24 framing bits provide a six bit cyclic redundancy check(CRC-6), and six provide the actual framing. The other 12 form a virtual circuit of 4000 bits per second for use by the transmission equipment, for call progress signals such as busy, idle and ringing. DS1 signals using ESF equipment are nearly error-free, because the CRC detects errors and allows automatic re-routing of connections.

  43. HDB3 • High Density Bipolar 3 Zeros • Based on bipolar-AMI • String of four zeros replaced with one or two pulses Note: The following is the explanation for the HDB3 code example on the next slide (see rules in Table 5.4, page 142): Assuming that an odd number of 1's have occurred since the last substitution, since the polarity of the preceding pulse is "-", then the first 4 zeros are replaced by "000-". For the next 4 zeros, since there have been no Bipolar pulses since the 1st substitution, then they are replaced by"+00+" since the preceding pulse is a "-". For the 3rd case where 4 zeros happen, 2 (even) Bipolar pulses have happened since the last substitution and the polarity of the preceding pulse is "+", so "-00-" is substituted for the zeros.

  44. B8ZS and HDB3 (Assume odd number of 1s since last substitution) See Table 5.4 for HDB3 Substitution Rules

  45. Digital Data, Analog Signal • Transmitting digital data through PSTN (Public telephone system) • 300Hz to 3400Hz bandwidth • modem (modulator-demodulator) is used to convert digital data to analog signal and vice versa • Three basic modulation techniques are used: • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PSK)

  46. Modulation Techniques

  47. Amplitude Shift Keying • Values represented by different amplitudes of carrier • Usually, one amplitude is zero • i.e. presence and absence of carrier is used • Susceptible to sudden gain changes • Inefficient • Up to 1200bps on voice grade lines • Used over optical fiber

  48. ASK Vd(t) Vc(t) VASK(t) Signal power frequencyspectrum Frequency fc fc+f0 fc+3f0 fc-3f0 fc-f0

  49. Frequency Shift Keying • Values represented by different frequencies (near carrier) • Less susceptible to error than ASK • Up to 1200bps on voice grade lines • High frequency radio (3-30 MHz) • Higher frequency on LANs using co-ax

  50. FSK Data signal vd(t) v1(t) Carrier 1 v2(t) Carrier 2 FSK(t) Signal power frequencyspectrum Frequency f1 f2

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