Digital Data, Digital Signal | Scrambling Techniques
- 2. Introduction To Digital Data, Digital Signal Digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data are transmitted by encoding the bit stream into signal elements. In the simplest case, one bit is represented by one signal element. - E.g., 1 is represented by a lower voltage level, and 0 is represented by a higher voltage level
- 3. Terminologies Unipolar If all signal elements have the same algebraic sign (all positive or all negative), then the signal is unipolar. Polar One logic state represented by positive voltage, the other by negative voltage Data rate Rate of data transmission measured in bps: bits per second Duration or length of a bit Time taken for transmitter to emit the bit Modulation rate - Rate at which the signal level changes - Measured in baud: signal elements per second
- 4. Interpreting Signals at the Receiver The receiver needs to know • The timing of each signal element, i.e., when a signal element begins and ends • signal levels • These tasks are performed by sampling each element position in the middle of the interval and comparing the value to a threshold. Factors affecting successful interpreting of signals • Signal-to-noise ratio (SNR) • Data rate • Bandwidth Another factor that can improve performance: • Encoding scheme: the mapping from data bits to signal elements
- 5. Line Coding The process for converting digital data into digital signal is said to be Line Coding. Line Coding Helps Error detection - errors occur during transmission due to line impairments. Some codes are constructed such that when an error occurs it can be detected. Noise and interference - there are line encoding techniques that make the transmitted signal “immune” to noise and interference. This means that the signal cannot be corrupted, it is stronger than error detection. Complexity - the more robust and resilient the code, the more complex it is to implement and the price is often paid in baud rate or required bandwidth.
- 6. Uni-polar Encoding Unipolar encoding schemes use single voltage level to represent data. In this case, to represent binary 1, high voltage is transmitted and to represent 0, no voltage is transmitted. It is also called Unipolar-Non-return-to-zero, because there is no rest condition i.e. it either represents 1 or 0.
- 7. Polar Encoding Polar encoding scheme uses multiple voltage levels to represent binary values. Polar encodings is available in four types: Polar Non-Return to Zero (Polar NRZ) • It uses two different voltage levels to represent binary values. Generally, positive voltage represents 1 and negative value represents 0. It is also NRZ because there is no rest condition. • NRZ scheme has two variants: NRZ-L and NRZ-I. NRZ-L changes voltage level at when a different bit is encountered whereas NRZ-I changes voltage when a 1 is encountered.
- 8. Polar Encoding PolarReturn to Zero (RZ) • Problem with NRZ is that the receiver cannot conclude when a bit ended and when the next bit is started, in case when sender and receiver’s clock are not synchronized. • RZ uses three voltage levels, positive voltage to represent 1, negative voltage to represent 0 and zero voltage for none. Signals change during bits not between bits. Manchester • This encoding scheme is a combination of RZ and NRZ-L. Bit time is divided into two halves. It transits in the middle of the bit and changes phase when a different bit is encountered. Differential Manchester • This encoding scheme is a combination of RZ and NRZ-I. It also transit at the middle of the bit but changes phase only when 1 is encountered.
- 9. Bipolar Encoding Bipolar encoding uses three voltage levels, positive, negative and zero. Zero voltage represents binary 0 and bit 1 is represented by altering positive and negative voltages.
- 10. Problems on Uni- Polar Encoding If you encode a long series of ones using unipolar/on-off signaling, then the DC component could fully charge a capacitor, resulting in errors as the signal is brought to zero. With bi-polar signaling, long series of ones are encoded as alternating negative and positive voltages, so the average voltage is zero, which eliminates the DC component issue. The constant transitions on ones with bipolar signaling also allows the circuit to be self clocking, which eliminates the need for a separate clock source.
- 11. So Why Not Bi- Polar Encoding ? Although, the constant transitions on ones with bipolar signaling also allows the circuit to be self clocking, which eliminates the need for a separate clock source. But then you still have issues with runs of zeroes causing a lack of transitions to clock on. This issue has been dealt with using several methods. And that is Dealt with Scrambling Technique.
- 12. Scrambling Scrambling is a technique used to create a sequence of bits that has the required self clocking, no low frequencies, no wide bandwidth. It is implemented at the same time as encoding, the bit stream is created on the fly. The best code is one that does not increase the bandwidth for synchronization and has no DC components. It replaces ‘unfriendly’ runs of bits with a violation code that is easy to recognize.
- 13. Scrambling Cont.. Use scrambling to replace sequences that would produce constant voltage these filling sequences must: • Sequences that would result in a constant voltage are replaced by filling sequences that will provide sufficient transitions for the receiver’s clock to maintain synchronization. • Filling sequences must be recognized by receiver and replaced with original data sequence. • Filling sequence is the same length as original sequence. Design goals • have no dc component • have no long sequences of zero level line signal • have no reduction in data rate • give error detection capability
- 14. Alternate Mark Inversion (AMI) used with scrambling
- 15. Scrambling Techniques Bipolar With 8-Zeros Substitution Based on bipolar-AMI, whose drawback is a long string of zeros may result in loss of synchronization. If octet of all zeros occurs and the last voltage pulse preceding this octet was positive, encode as 000+-0-+ or 000VB0VB. If octet of all zeros occurs and the last voltage pulse preceding this octet was negative, encode as 000-+0+- Causes two violations of AMI code: • Unlikely to occur as a result of noise • Receiver recognizes the pattern and interprets the octet as consisting of all zeros. A. B8ZS
- 16. Two cases of B8ZS scrambling technique The main formula for B8ZS is 000VB0VB Where V stands for “violation bit” Follows the polarity of previous non-zero bit B stands for “bipolar bit” or “balance bit” Inverses the polarity of previous non-zero bit
- 17. High-Density Bipolar-3 Zeros Based on bipolar-AMI String of four zeros is replaced with sequences containing one or two pulses. B. HDB3 Number of Bipolar Pulses since last substitution Polarity of Preceding Pulse Odd Even - 000- +00+ + 000+ -00-
- 18. HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution. If no of non zero pulses is even the substitution is B00V to make total non zero pulse even. If no of non zero pulses is odd the substitution is 000V to make total non zero pulses even. Example 1 of HDB3 encoding The pattern of bits " 1 0 0 0 0 1 1 0 “ encoded in HDB3 is " + 0 0 0 V - + 0 " (the corresponding encoding using AMI is " + 0 0 0 0 - + "). Different situations in HDB3 scrambling technique
- 19. Example 2 of HDB3 encoding The pattern of bits " 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 " encoded in HDB3 is " + 0 - 0 0 0 V 0 + - B 0 0 V - + B 0 0 V 0 0 " which is: " + 0 - 0 0 0 - 0 + - + 0 0 + - + - 0 0 - 0 0 "
- 20. B8ZS and HDB3
- 21. Special Thanks Mr. Shree Krishna Khadka KIST College Of Science And Technology