Avionics Multiple Choice Questions on “Digital Transmission of Data”.
1. Which of the following is false with respect to digital data transmission?
a) LAN is a digital data transmission
b) Can transmit binary data
c) Only restricted to communication between computers
d) Can transmit analog data
Answer: c
Clarification: Digital data communication was only restricted to communication between computers. Since analog signals can now be easily converted into digital signals using analog to digital converter, digital transmission can now transmit voice, video and other analog signals in digital form.
2. Why is digital data not easily affected by noise?
a) High power transmission
b) Cannot easily change binary 1 to 0
c) High frequency transmission
d) Low frequency transmission
Answer: b
Clarification: Noise is inevitable in a signal transmission. In a digital transmission the amplitude of the noise must be higher than the amplitude of the signal at binary ’1’, which is generally not the case, and vice versa. The receiver can differentiate between binary ‘1’ and ‘0’ with a significant amount of noise.
3. What is the process of digital communication where a threshold value is set at the receiver to reduce noise?
a) Signal threshold
b) Data regeneration
c) Signal regeneration
d) Signal cut off
Answer: c
Clarification: In the process of signal regeneration a cut off value or threshold value is set in the receiver so that the noise can be clipped off. When the values are set properly only the binary data would pass through the circuit. This produces a clean output pulse.
4. What is the number of error bits that occur for a given number of bits transmitted called?
a) Bit error
b) Error rate
c) Bit error rate
d) Error bit rate
Answer: c
Clarification: Bit rate error(BER) is the number of error bits that are present for a given number of bits transmitted. It depends on several factors like the environment, equipment and other considerations. BER is calculated for a particular set of conditions.
5. Data is transmitted in 512 bytes. What is the average number of errors expected if the system BER is 2:10,000?
a) 1
b) 0
c) 0.542
d) 0.819
Answer: d
Clarification: Total number of bits = 512 x 8 = 4096 bits
Average number of errors = (2/10,000) x 4096 = 0.819.
6. Why can Digital data use time division multiplexing?
a) Less power
b) High power
c) Discrete data
d) Continuous data
Answer: c
Clarification: Digital data is discrete and hence can be transmitted via time division multiplexing, whereas analog data is continuous and can only be transmitted as a whole. However, analog data can be converted into digital by analog to digital converters and then be transmitted via time division multiplexing. Time division multiplexing is one of the biggest advantages of digital communication.
7. Speed of transmission is increased by channel encoding.
a) True
b) False
Answer: b
Clarification: Speed of transmission is reduced when data is channel encoded. For example, to transmit 1 byte of data, the encoding process may add 3 extra bits for a total of 11 bits. If the bit time is 100 ns, then it takes 800 ns to send the data bits but 1100 ns to send the encoded data.
8. Which of the following bit has an error if odd parity is used?
| data | Parity bit | |
| A | 1000 | 0 |
| B | 1001 | 0 |
| C | 1100 | 1 |
| D | 1111 | 1 |
a) A
b) B
c) C
d) D
Answer: b
Clarification: When odd parity is used, the parity bit is 1 when the number of binary ‘1’ is even and the parity bit is ‘0’ when the number of binary 1 is odd. Here, option ‘b’ has even number of 1’s but the parity bit is 0. Thus option b has error.