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## Lecture # 15

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**Lecture # 15**Computer Communication & Networks**Today’s Menu**• Encoding/Decoding • Unipolar, Polar and Bipolar encoding**Encoding/Decoding**• Digital-to-Digital conversion or encoding/decoding is the representation of digital information by digital signal • For example when we transmit data from computer to the printer, both original and transmitted data have to be digital • Encoding a digital signal is where 1’s and 0’s generated by the computer are translated into voltage pulses that can be propagated over the wire**Encoding/Decoding**• A digital signal is a sequence of discrete, 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 • An example would be in which binary 0 is represented by a lower voltage level and binary 1 by a higher voltage level • A variety of other encoding schemes are also used**Encoding/Decoding**• Types of Encoding • Unipolar • Polar • Bipolar • 1-Unipolar: • Encoding is simple , with only one technique in use • Simple and primitive • Almost obsolete today • Study provides introduction to concepts and problems involved with more complex encoding systems**Unipolar Encoding**• It works by sending voltage pulses on the transmission medium • The signal elements all have the same algebraic sign, that is, all positive or negative • One voltage level stands for binary 0 while the other stands for binary 1 • It is called Unipolar because it uses only one polarity • This polarity is assigned to one of the two binary states usually a ‘1’ • The other state usually a ‘0’ is represented by zero voltage**Unipolar Encoding**• Figure shows the idea: 1’s are encoded as +ve values, and 0’s are encoded as –ve values**Unipolar Encoding**• Pros and Cons of Unipolar Encoding • Pros • Straight forward and simple • Inexpensive to implement • Cons • DC component • Synchronization**Polar Encoding**• 2-Polar: • Polar encoding uses two voltage levels, positive and negative • One logic state is represented by a positive voltage level, and the other by a negative voltage level • It has 3 subcategories: • Non Return to Zero (NRZ) • NRZL • NRZI • Return to Zero (RZ) • Biphase • Manchester • Differential Manchester**Non Return to Zero (NRZ)**• In NRZ, the level of signal is either positive or negative • NRZ-L (Non-Return-to-Zero-Level) • Level of the signal depends on the type of bit it represents • A +ve voltage usually means the bit is a 1 and a –ve voltage means the bit is a 0 (vice versa)**Non Return to Zero (NRZ)**Problem with NRZ-L: When long streams of 0’s or 1’s are there in data, receiver receives a continuous voltage and should determine how many bits are sent by relying on its clock, which may or may not be synchronized with the sender clock**Non Return to Zero (NRZ)**• NRZ-I (Non-Return-to-Zero-Invert On One) • The inversion of the level represents a 1 bit • A bit 0 is represented by no change • 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**Non Return to Zero (NRZ)**• Problem with NRZ-I • NRZ-I is superior to NRZ-L due to synchronization provided by signal change each time a 1 bit is encountered • The string of 0’s can still cause problem but since 0’s are not as likely, they are less of a problem**Non Return to Zero (NRZ)**• The NRZ codes are the easiest to engineer and, in addition, make efficient use of bandwidth • The main limitations of NRZ signals are the presence of a dc component and the lack of synchronization capability • 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**Return to Zero (RZ)**• Any time, data contains long strings of 1’s or 0’s, receiver can loose its timing • In unipolar, we have seen a good solution is to send a separate timing signal but this solution is expensive • A better solution is to somehow include sync in encoded signal somewhat similar to what we did in NRZ-I but it should work for both strings of 0 & 1 • One solution is RZ encoding which uses 3 values; Positive, Negative and Zero • Signal changes not between bits but during each bit**Return to Zero (RZ)**• Like NRZ-L, +ve voltage means 1 and a –ve voltage means 0, but unlike NRZ-L, half way through each bit interval, the signal returns to zero • A 1 bit is represented by positive to zero and a 0 is represented by negative to zero transition**Return to Zero (RZ)**• Problem with RZ • The only problem with RZ encoding is that it requires two signal changes to encode one bit and therefore occupies more bandwidth • But of the 3 alternatives we have discussed, it is most effective**Biphase**• Best existing solution to the problem of synchronization • Signal changes at the middle of bit interval but does not stop at zero • Instead it continues to the opposite pole • There are two types of biphase encoding • Manchester • Differential Manchester**Manchester**• Uses inversion at the middle of each bit interval for both synchronization and bit representation • Negative-to-Positive Transition = 1 • Positive-to-Negative Transition = 0 • By using a single transition for a dual purpose, Manchester achieves the same level of synchronization as RZ but with only two levels of amplitude**Differential Manchester**• Inversion at the middle of the bit interval is used for synchronization but presence or absence of an additional transition at the beginning of bit interval is used to identify a bit • A transition means binary 0 & no transition means binary 1 • Requires 2 signal changes to represent binary 0 but only one to represent binary 1**3-Bipolar Encoding**• Although the biphase techniques have achieved widespread use in local-area-network applications at relatively high data rates, 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**Bipolar Encoding**• An 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**Bipolar Encoding**• Like RZ, it uses three voltage levels • Unlike RZ, zero level is used to represent binary 0 • Binary 1’s are represented by alternate positive and negative voltages • AMI • Pseudoternary • B8Zs • HDB3**Alternate Mark Inversion(AMI)**• Simplest type of bipolar encoding • 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 • Alternate Mark Inversion means alternate ‘1’ inversion**Alternate Mark Inversion(AMI)**• Pros and Cons: • There will be no loss of synchronization if a long string of is occurs • Each 1 introduces a transition, and the receiver can resynchronize on that transition • A long string of 0s would still be a problem • Because the 1 signals alternate in voltage from positive to negative, there is no net dc component**Pseudoternary**• Inverse of AMI • 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**Pseudoternary**• Two variations are developed to solve the problem of synchronization of sequential 0’s • B8Zs (used in North America) • HDB3 (used in Europe & Japan) • Both modify original pattern of AMI only on case of long stream of zeroes**B8Zs**• Bipolar with 8-zeros substitution • Difference between AMI and B8Zs occurs only when 8 or more consecutive zeros are encountered • Forces artificial signal changes called violations • Each time eight 0’s occur, B8Zs introduces changes in pattern based on polarity of previous 1 (the ‘1’ occurring just before zeros) • Same as bipolar AMI, except that any string of eight zeros is replaced by a string with two code violations**HDB3**• High-Density Bipolar-3 Zeros • Alteration of AMI adopted in Europe and Japan • Introduces changes into AMI, every time four consecutive zeros are encountered instead of waiting for eight zeros as in the case of B8Zs • As in B8Zs, the pattern of violations is based on the polarity of the previous 1 bit • HDB3 also looks at the number of 1’s that have occurred since the last substitution • Same as bipolar AMI, except that any string of four zeros is replaced by a string with one code violation**HDB3**• High-Density Bipolar-3 Zeros**HDB3**• High-Density Bipolar-3 Zeros • If the last violation was positive, this violation must be negative, and vice versa • The table shows that this condition is tested for by knowing whether the number of pulses since the last violation is even or odd and the polarity of the last pulse before the occurrence of the four zeros**HDB3**• High-Density Bipolar-3 Zeros