Data Transmission

Q: What are the two main transmission modes in data communication, and how do they differ?

A: Asynchronous (slower, cheap, used for low-speed like keyboards) and Synchronous (faster, for high-speed applications).

Q: Give an example of asynchronous transmission.

A: Keyboard input, where characters are sent one at a time with unpredictable gaps.

Q: What is line coding?

A: The process of converting binary data into a digital signal for transmission.

Q: Name the three main categories of line coding schemes.

A: Unipolar, Polar, and Bipolar.

Q: What is the key disadvantage of unipolar line coding?

A: It has a DC component and lacks synchronization due to using only one voltage level.

Q: How does NRZ-L differ from NRZ-I?

A: NRZ-L uses fixed voltages for 0/1 (+ve/-ve), while NRZ-I inverts voltage for a 1 bit (transition).

Q: Why does RZ encoding require more bandwidth?

A: It uses three values (+ve, -ve, zero) and two signal changes per bit.

Q: What is the synchronization advantage of Manchester encoding?

A: It inverts at the middle of each bit interval, embedding clocking information.

Q: How does Differential Manchester identify a 0 bit?

A: A transition at the start of the bit interval indicates 0; no transition means 1.

Q: What is the key feature of Bipolar AMI encoding?

A: Alternates +ve/-ve voltages for 1s (0s remain at zero).

Q: What problem does BnZS solve in bipolar encoding?

A: Synchronization for long sequences of 0s by substituting some bits with +ve/-ve pulses.

Q: What is the purpose of block coding?

A: To improve line coding by adding redundancy for synchronization and error detection.

Q: Describe the steps in block coding transformation.

A: 1) Divide bits into groups (m bits), 2) Substitute with n-bit code (e.g., 4B/5B), 3) Apply line coding.

Q: How does 4B/5B encoding work?

A: It replaces 4-bit data groups with 5-bit codes to ensure sufficient transitions for synchronization.

Q: What is the advantage of 8B/10B over 4B/5B?

A: Higher error detection capability due to more redundant bits.

Q: What is the Nyquist theorem for sampling rate?

A: Sampling rate must be at least twice the highest frequency of the signal.

Q: How is bit rate calculated in PCM?

A: Bit rate = Sampling rate × Number of bits per sample.

Q: What are the two steps in Pulse Code Modulation (PCM)?

A: 1) PAM (sampling analog signal), 2) Quantization (assigning digital values to samples).

Q: What is the main drawback of ASK?

A: Susceptibility to noise interference due to amplitude changes.

Q: Why is FSK less affected by noise than ASK?

A: It detects frequency changes rather than amplitude variations.

Q: How does PSK represent binary data?

A: By shifting the phase of the carrier signal (e.g., 0° for 0, 180° for 1).

Q: What is a "dibit" in QPSK?

A: Two bits represented by one phase shift (e.g., 00 = 0°, 01 = 90°).

Q: How does QAM combine ASK and PSK?

A: It modulates two carriers (90° phase-shifted) with different amplitudes/phases for higher efficiency.

Q: Why does QAM use fewer amplitude shifts than phase shifts?

A: Amplitude changes are more noise-sensitive and require larger differences.

Q: What is the relationship between bit rate and baud rate?

A: Baud rate = Bit rate ÷ Number of bits per signal unit.

Q: If a signal has a bit rate of 3000 bps and 6 bits per unit, what is the baud rate?

A: 500 baud/s (3000 ÷ 6).