EEC4113 Data Communication & Multimedia System Chapter 2: Baseband Encoding by Muhazam Mustapha, July 2010

Learning Outcome By the end of this chapter, students are expected to be able to explain link level baseband encoding for transmission

Chapter Content Polarity in baseband encoding Encoding techniques –NRZ-L, NRZI –Bipolar –Biphase Modulation rate Scrambling techniques

Polarity in Baseband Encoding

Baseband Encoding Definition: encoding of the signal in the spectrum range from 0 Hz to the data rate frequency Use: encoding of data for short distances, LAN, Ethernet

Polarity in Encoding Unipolar –All signals follow the values of the binary Amplitude Time

Polarity in Encoding Polar –One signal sign follows one data binary Amplitude Time

Polarity in Encoding Bipolar –3 levels of signal: +ve, −ve, 0 –Binary 0 is level 0; binary 1 alternates sign Amplitude Time

Encoding Techniques

Nonreturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits –Negative voltage for 1 –Positive voltage for Amplitude Time

Nonreturn to Zero-Inverted (NRZI) Bit 1: Transition at the beginning of bit time Bit 0: No transition A kind of differential encoding – data is represented by transition rather than level Amplitude Time transitions

Advantages of NRZ Coding Easiest to engineer Make efficient use of bandwidth –Most of the energy in NRZ-L & NRZ-I signals (80%) is between DC and half of the bit rate –e.g. If NRZ code is used to generate a signal with data rate of 9600 bps, then most of the energy in the signal is concentrated between DC & 4800 Hz

Spectral Density of Various Schemes Normalized frequency (f/R) NRZ Manchester, differential Manchester B8ZS, HDB3 AM, pseudoternary

Disadvantages of NRZ Coding Presence of DC component (zero frequency) –Presents problems for a system that cannot pass low frequencies e.g. Telephone line can’t pass frequencies below 300 Hz

Disadvantages of NRZ Coding –Also presents problems for a system that uses electrical coupling via transformer There must be direct physical attachment of transmission component Electrical (AC) coupling via transformer, which provides excellent electrical isolation that reduces interference, is not possible e.g. A long distance link may use one or more transformers to isolate different parts of the line electrically

Disadvantages of NRZ Coding Lack of synchronization capability –Consider long string of 1-s and 0-s for NRZ-L or long string of 0-s for NRZI –The output is a constant voltage over a long period of time –A drift between the timing of transmitter & receiver will result in loss of synchronization between both devices

Disadvantages of NRZ Coding Due to these lacking, it is unattractive for signal transmission applications Due to these shortcomings, it is only used in direct devices connection like in digital magnetic recording

Bipolar-AMI Alternate Mark Inversion A kind of multilevel binary encoding Binary 0: No line signal Binary 1: +ve or –ve pulses alternately Time

Advantages of Bipolar-AMI No loss of synchronization if a long string of 1-s occur –Each 1 introduces a transition –Receiver can resynchronize on that transition –Long string of 0-s would still be a problem No net DC component –1-s signals alternate in voltage –0-s is at zero volt –Hence 0 DC component

Pseudoternary Inversion of AMI A kind of multilevel binary encoding Binary 1: No line signal Binary 0: +ve or –ve pulses alternately Time

Disadvantages of Multilevel Binary Long string of 0-s (AMI) and 1-s (pseudoternary) still present a problem –Common technique: insert additional bits that force transition – called scrambling Less efficient than NRZ –The receiver has to distinguish 3 levels –Requires ~3dB of power for the same BER as NRZ –BER is higher for the same SNR as NRZ

Disadvantages of Multilevel Binary BER SNR (dB) AMI, pseudoternary, ASK, FSK NRZ, biphase, PSK, QPSK 3 dB

Manchester A kind of biphase encoding Transition in the middle of each bit period Binary 1: Low to High transition Binary 0: High to Low transition Time

Differential Manchester A kind of biphase & differential encoding Binary 0: Transition at start of bit period Binary 1: No transition at start of bit period Time

Advantages of Biphase Encoding Synchronization –Biphase codes are self-clocking codes –Predictable transition during each bit period –Receiver can synchronize on that transition No DC component Error detection –Absence of expected transition can be used to detect errors Due to these advantages it is popular for LAN connection

Disadvantages of Biphase Encoding Requires double the bandwidth of non- biphase encoding Requires more signaling power Due to these disadvantages it is not popular in long distance connection

Modulation Rate

Data Rate Also known as BIT Rate Definition: The rate at which data (or bits) are communicated per second Unit: bit per second (bps) Example: 1000 bps means 1000 bit is transmitted and received in 1 second

Modulation Rate Also known as BAUD rate or SYMBOL rate Definition: The MAXIMUM no. symbol at which the signal in communication channel can have per second Unit: baud per second Example: Consider an NRZ optical signaling between red & green. If the system has to produce the colors at max 2400 times per second then it is 2400 baud per second

Baud vs Bit Rate 1 signal symbol may represent more that 1 bits Hence this gives room for more than 1 bps in each baud rate –bps = baud × no. bit per baud Example: Consider an NRZ optical signaling between green (00), red (01), yellow (10) and blue (11). If the system has to produce the colors at max 2400 times per second then it is 2400 baud per second. Since there are 2 bits per symbol, then it is 4800 bps.

Baud vs Bit Rate D= R / L = R / log 2 M D=Modulation rate, baud R=Data rate, bps L=Number of bits per symbol or signal element M=Number of different symbols used = 2 L In general:

Baud vs Bit Rate NRZL Time Manchester Time 1 bit/μs 1 sym/μs 1 bit/μs 2 sym/μs Data rate = 1 Mbps Modulation rate = 1 Mbaud Data rate = 1 Mbps Modulation rate = 2 Mbaud

Scrambling Techniques

Multilevel binary with scrambling techniques –Commonly used in long-distance transmission Sequences that would result in a constant voltage level would be replaced with a new filling sequence The filling sequence would provide enough transitions for the receiver’s clock to maintain synchronization

Scrambling Techniques The filling sequence must be recognized by the receiver & to be replaced with the original data sequence The filling sequence is the same length as the original sequence, hence there is no data rate penalty

Scrambling Techniques Design goals: –No DC component –No long sequence of zero-level line signal –No reduction in data rate –Error detection capability Two scrambling techniques commonly used in long-distance transmission –B8ZS –HDB3

B8ZS Bipolar with 8-Zeros Substitution Based on Bipolar-AMI encoding –Long string of 0-s may result in loss synchronization Replaces strings of eight 0-s with: If the last voltage pulse preceding this 8 0-s was +ve, then they are encoded with − 0 − + If the last voltage pulse preceding this 8 0-s was -ve, then they are encoded with − −

B8ZS Bipolar-AMI B8ZS This technique forces 2 code violations, which is unlikely to occur due to noise, and the parity is also maintained

HDB3 High-Density Bipolar 3-Zeros Based on Bipolar-AMI encoding Replaces strings of four 0-s with: No. bipolar pulses since last substitution Polarity of Preceding PulseOddEven −0 0 0 − − 0 0 −

HDB3 Bipolar-AMI HDB3 Consider pulses count at this point is odd