6.8 HIGH-DENSITY BIPOLAR 3 (HDB3)

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6.8 HIGH-DENSITY BIPOLAR 3 (HDB3)

The HDB3 encoding scheme is almost the same as AMI, except for a small change: Two pulses, called violation pulse and the balancing pulse, are used when consecutively four or more zeros occur in the bit stream. When there are four consecutive binary 0 bits, the pulses will be 000V, where V will be same as the previous non-zero voltage. However, this V pulse creates a DC component. To overcome this problem, the B bit is introduced. If there are four consecutive binary 0 bits, the pulse train will be B00V. B is positive or negative to make alternate Vs of opposite polarity.

HDB3 encoding is similar to AMI, except that two pulses called violation pulse (V) and balancing pulse (B) are used when consecutively four or more zeros occur in the bit stream. These pulses eliminate the DC component.

The HDB3 encoding scheme is a standard adopted in Europe and Japan.

Summary

This chapter has presented the various schemes used for digital encoding. The digital encoding scheme should take into consideration the bandwidth requirement, immunity to noise, and synchronization. Encoding schemes can be broadly divided into unipolar encoding, polar encoding, and bipolar encoding. In unipolar encoding, there will be only one level—binary 1 is represented by positive voltage and binary 0 by keeping the line idle. Because the signal will have a DC component, it cannot be used in radio systems. Polar encoding uses two levels: a positive voltage level and a negative voltage level. Non-return to zero level (NRZ-L) and non-return to zero invertive (NRZ-I) are examples of this type of encoding. In biploar encoding, three voltage levels are used: positive, negative, and zero levels. Alternate mark inversion (AMI) and high-density bipolar 3 (HDB3) are examples of this type of encoding. All these types of encoding schemes are used in communication systems.

References

• W. Stallings. Data and Computer Communication. Fifth Edition. Prentice Hall Inc., 1999.

• J. Campbell. C Programmer's Guide to Serial Communications. Prentice Hall Inc., 1997.

Questions

1. What are the requirements of digital encoding?

2. What are the three categories of digital encoding?

3. Explain polar encoding techniques.

4. Explain bipolar encoding techniques.

Exercises

1.	For the bit pattern 10101010100, draw the electrical signal waveforms for NRZ-I and NRZ-L encoding schemes.
2.	For the bit pattern 10101010110, draw the electrical signal waveforms for the Manchester encoding scheme.
3.	If the bit pattern is 101010101110, how does the waveform appear on an RS232 port?
4.	For the bit pattern 10101010100, draw the waveforms for Bipolar AMI and Manchester encoding schemes.
5.	Study the UART (universal asynchronous receive-transmit) chips used for RS232 communication.
6.	Is it possible to generate 5-bit codes from an RS232 interface of your PC? If not, what is the alternative?

Answers

1.	For the bit pattern 10101010100, the NRZ-I waveform is as follows: The NRZ-L waveform is as follows:
2.	For the bit pattern 10101010110, the waveform using Manchester coding is as follows:
3.	For the bit pattern 101010101110, the waveform appear on an RS232 port will appear as:
4.	For the bit pattern 10101010100, bipolar AMI waveform is as follows: The HDB3 waveform is as follows: Note that the HDB3 encoding will be the same as AMI. When four consecutive zeros are in the bit stream, the waveforms will be different.
5.	The UART (universal asynchronous receive-transmit) chip is used to control the serial communication port (COM port) of the PC. The functions of this chip are converting parallel data into serial data and vice versa, adding start and stop bits, creating a hardware interrupt when a character is received, and flow control. Different UART chips support different speeds: 8250 supports 9.6kbps, 16450 supports 19.2kbps, and 16550 supports 115.2kbps. 8250 and 16450 have one byte buffer, and 16550 has 16 bytes of buffer.
6.	To generate 5 bit codes from the RS232 interface of your PC is not possible. In RS232 communication, there will be start bit and a parity bit. Even if there are five data bits, additional bits are added. To generate five bit codes, you need to have special hardware with UART chips.

Projects

1. Write a C program that takes a bit stream and a type of encoding scheme as inputs and produces the electrical signal waveforms for all the codes explained in this chapter.

2. A variety of codes such as CCITT2, CCITT3, CCITT5, TORFEC, etc. have been developed to represent the alphabet (A to Z). Compile a list of such codes. Write a program that takes English text as input and displays the bit stream corresponding to various codes.

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