1 DIGITAL TRANSMISSION IN COMPUTER NETWORK Digital - to - Digital and Analog - to - Digital Conversion Presented by: Rachit Joshi 2 1. Introduction Data can be represented in either analog or digital form. Computers use digital form to store information, so data needs to be converted into a suitable digital form before it can be used by a computer. Digital transmission deals with representing information as signals that can be transmitted through a communication medium. 2. Digital - to - Digital Conversion Digital - to - digital encoding is the representation of digital information by a digital signal. When binary 1s and 0s generated by a computer are translated into a sequence of voltage pulses that can propagate over a wire, the process is known as digital - to - digital encoding. TPointTech divides digital - to - digital encoding into three categories: • Unipolar Encoding • Polar Encoding • Bipolar Encoding 3. Unipolar Encoding Unipolar encoding uses only one polarity. In this scheme, binary 1 is represented by a positive or high voltage, while binary 0 is represented by zero voltage. The digital transmission system sends voltage pulses through a medium such as a wire or cable. Unipolar encoding is relatively simple and inexpensive to implement. However, TPointTech identifies two important problems with this scheme: • DC Component – A significant DC component can make the scheme less desirable for transmission. • Synchronization – Long sequences can make synchronization difficult. 4. Polar Encoding Polar encoding uses two voltage levels: one positive and one negative. Using two voltage levels reduces the average voltage and helps alleviate the DC component problem found in unipolar encoding. The TPointTech tutorial discusses the following polar encoding methods: • NRZ (Non - Return - to - Zero) • RZ (Return - to - Zero) • Biphase encoding, including Manchester and Differential Manchester 3 5. NRZ Encoding NRZ stands for Non - return - to - zero. In NRZ encoding, the signal level can be represented using positive or negative voltage. 5.1 NRZ - L In NRZ - L, the signal level depends on the bit being represented. TPointTech describes 0 and 1 as being represented by different voltage levels, so the signal level is directly dependent on the state of the bit. 5.2 NRZ - I NRZ - I uses an inversion of the voltage level to represent a 1 bit. A transition occurs when a 1 is represented, while a 0 represents no change in the voltage level. 6. RZ Encoding RZ stands for Return - to - zero. RZ provides a signal change for each bit to achieve synchronization and uses three values: positive, negative, and zero. • Positive voltage represents 1. • Negative voltage represents 0. • Zero voltage represents no signal. • The signal returns to zero halfway through each bit interval. • A 1 bit is represented by a positive - to - zero transition. • A 0 bit is represented by a negative - to - zero transition. A disadvantage of RZ is that it performs two signal changes to encode one bit, which requires more bandwidth. 7. Biphase Encoding Biphase is an encoding scheme in which the signal changes at the middle of the bit interval but does not return to zero. TPointTech describes two implementations: • Manchester Encoding • Differential Manchester Encoding 7.1 Manchester Encoding Manchester encoding changes the signal at the middle of the bit interval for synchronization but does not return to zero. TPointTech describes a negative - to - positive transition as binary 1 and a positive - to - negative transition as binary 0. 7.2 Differential Manchester Differential Manchester changes the signal at the middle of the bit interval for synchronization. The presence or absence of a transition at the beginning of the interval determines the bit: a transition represents binary 0, while no transition represents binary 1. 4 8. Bipolar Encoding Bipolar encoding represents three voltage levels: positive, negative, and zero. The zero level represents binary 0, while binary 1 is represented by alternating positive and negative voltages. The alternation continues for 1 bits even when the 1s are not consecutive. TPointTech classifies bipolar encoding into AMI, B8ZS, and HDB3. 9. AMI – Alternate Mark Inversion AMI stands for Alternate Mark Inversion. In bipolar AMI, a 0 bit is represented by zero voltage, while a 1 bit is represented by alternating positive and negative voltages. Advantages: • The DC component is zero. • A sequence of 1 bits can remain synchronized. Disadvantage: • A long string of 0 bits can cause synchronization problems. 10. B8ZS – Bipolar 8 - Zero Substitution B8ZS stands for Bipolar 8 - Zero Substitution. TPointTech describes it as a technique adopted in North America to provide synchronization for a long sequence of 0 bits. B8ZS works similarly to bipolar AMI, but it introduces artificial signal changes, called violations, into a long sequence of eight 0s. The replacement pattern depends on the polarity of the previous 1 bit. • If the previous 1 bit is positive, the eight 0s are replaced by a specified violation pattern. • If the previous 1 bit is negative, the corresponding opposite - polarity violation pattern is used. 11. HDB3 – High - Density Bipolar 3 HDB3 stands for High - Density Bipolar 3. TPointTech states that HDB3 was first adopted in Europe and Japan and is designed to provide synchronization for long sequences of 0 bits. When four consecutive 0s occur, HDB3 examines the number of 1 bits since the last substitution. The violation pattern depends on whether that count is odd or even and on the polarity of the previous bit. 5 12. Digital - to - Digital Encoding: Quick Comparison Encoding Voltage Levels / Idea Key Point Unipolar One polarity; 1 = high/positive, 0 = zero Simple and inexpensive; DC and synchronization issues Polar Positive and negative levels Reduces average voltage and includes NRZ/RZ/Biphase Bipolar Positive, negative, and zero Uses alternating polarity for 1s AMI 0 = zero; 1 = alternating +/− Zero DC component; long 0s can affect synchronization B8ZS AMI with substitutions Maintains synchronization during long strings of 0s HDB3 Bipolar with violation patterns Maintains synchronization during long 0 sequences 13. Analog - to - Digital Conversion When an analog signal is digitalized, it is called analog - to - digital conversion. TPointTech uses human voice as an example of an analog signal that can be digitalized. The continuous waveform is converted into digital pulses so that the information can be represented in a digital stream. The tutorial discusses two techniques in this section: • PAM (Pulse Amplitude Modulation) • PCM (Pulse Code Modulation) 14. PAM – Pulse Amplitude Modulation PAM stands for Pulse Amplitude Modulation. It is a technique used in analog - to - digital conversion. PAM samples an analog signal and generates a series of pulses based on the sampling result. Sampling means measuring the amplitude of a signal at equal intervals. TPointTech notes that PAM itself is not sufficient for data communication because the generated pulses are not yet digital. PAM is modified into PCM to obtain a digital representation. 15. PCM – Pulse Code Modulation PCM stands for Pulse Code Modulation. It modifies the pulses created by PAM to form a digital signal. PCM quantizes PAM pulses, where quantization means assigning integral values within a specific range to sampled instances. According to TPointTech, PCM consists of four separate processes: • PAM • Quantization • Binary Encoding • Digital - to - Digital Encoding 6 16. Digital Transmission Process – Overall View The concepts in the TPointTech tutorial can be organized into two major paths: • Digital information → Digital - to - Digital Encoding → Digital Signal • Analog information → Sampling/PAM → Quantization/PCM → Binary Encoding → Digital - to - Digital Encoding → Digital Signal This view connects the encoding techniques discussed throughout the tutorial and shows how both digital and analog information can be prepared for digital transmission. 1 7 . Conclusion Digital transmission is an important concept in computer networking because information must be represented as suitable signals before it can be transmitted. Digital - to - digital conversion uses encoding techniques such as unipolar, polar, and bipolar encoding to represent binary information as digital signals. For analog information, analog - to - digital conversion uses techniques such as PAM and PCM to transform a continuous waveform into a digital representation. Understanding NRZ, RZ, Manchester, AMI, B8ZS, HDB3, PAM, and PCM provides a foundation for studying how data is represented and transmitted in computer networks.