Communication Systems Revision Set 2

2.

A communication channel modulates waves using 256 QAM and transmits 8 million symbols each second. Approximately how long will it take to transfer 10MB?

64 seconds
8 seconds
0.125 seconds
1.25 seconds

Correct Answer

A. 1.25 seconds

Explanation

The communication channel is using 256 QAM modulation, which means it can transmit 8 bits per symbol. Since there are 8 million symbols transmitted each second, the channel has a data rate of 8 million symbols/second * 8 bits/symbol = 64 million bits/second. To transfer 10MB, we need to convert it to bits, which is 10MB * 8 bits/byte * 1024 kilobytes/MB * 1024 bytes/kilobyte = 83,886,080 bits. Dividing this by the data rate gives us 83,886,080 bits / 64 million bits/second = 1.31 seconds. Rounded to the nearest tenth, the answer is 1.3 seconds. Therefore, the closest option is 1.25 seconds.

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3.

Which of the following includes only baseband communication links?

Ethernet, ISDN
ADSL, ISDN
Ethernet, ADSL
Cable, ADSL

Correct Answer

A. Ethernet, ISDN

Explanation

Ethernet and ISDN are both examples of baseband communication links. Baseband communication refers to the transmission of digital signals without modulation, where the entire bandwidth of the communication medium is used for a single channel. Ethernet is a standard for local area networks (LANs) that uses baseband communication over twisted pair or fiber optic cables. ISDN (Integrated Services Digital Network) is a telecommunications standard that uses baseband communication over digital lines to transmit voice, video, and data. Therefore, the correct answer is Ethernet, ISDN.

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4.

Which of the following is TRUE in terms of 8-bit checksums?

Approximately 99.6% of errors are detected
Approximately 99.6% of data packets will be received correctly
Approximately 99.6% of packets will not be corrupted during transmission
Approximately 99.6% of detected error can be corrected

Correct Answer

A. Approximately 99.6% of errors are detected

Explanation

An 8-bit checksum is a simple error detection technique in which a sum of all the data bits is calculated and appended to the data packet. When the packet is received, the sum is recalculated and compared to the appended checksum. If they match, it is assumed that no errors occurred during transmission. However, if they don't match, it indicates that an error has occurred. Therefore, the statement "Approximately 99.6% of errors are detected" is true because the checksum has a high probability of detecting errors, but it cannot correct them.

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5.

Protocols that include checksums include:

Ethernet and SONET
TCP and IP
ATM and IP
TCP and Ethernet

Correct Answer

A. TCP and IP

Explanation

TCP (Transmission Control Protocol) and IP (Internet Protocol) are both network protocols that are widely used in computer networks. TCP is responsible for establishing and maintaining a reliable connection between two devices, ensuring that data is transmitted accurately and in the correct order. IP, on the other hand, is responsible for addressing and routing packets of data across the network. Both TCP and IP include checksums as a way to detect errors in the data transmission. The checksum is a value calculated from the data being transmitted, and it is compared at the receiving end to ensure that the data arrived intact.

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6.

A parity bit is added to each byte of data sent. If all data bits are reversed what will occur?

The error will always be detected
No error will ever be detected
Some errors will be detected
Most errors will be detected

Correct Answer

A. No error will ever be detected

Explanation

When a parity bit is added to each byte of data sent, it helps in detecting errors in the transmission. However, if all data bits are reversed, the parity bit will also be reversed along with the data bits. As a result, the transmitted data will still have the correct number of 1s and 0s, and the parity bit will still match the data. Therefore, no error will ever be detected in this scenario.

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7.

7-bit ASCII data is sent one character at a time using odd parity. The received data contains errors. Which of the following is most likely?

An odd number of bits in some bytes were corrupted
The parity bit in some bytes was corrupted
An even number of bits in some bytes were corrupted
The reciever has different port setting to the sender

Correct Answer

A. An even number of bits in some bytes were corrupted

Explanation

In odd parity, the number of 1s in a byte should be odd. If the received data contains errors, it is most likely that an even number of bits in some bytes were corrupted. This would result in an incorrect number of 1s in those bytes, causing the parity check to fail.

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8.

The range of frequencies a transmission channel occupies is known as its:

Symbol rate
Baud
Speed
Bandwidth

Correct Answer

A. Bandwidth

Explanation

The range of frequencies a transmission channel occupies is known as its bandwidth. Bandwidth refers to the amount of data that can be transmitted through a channel in a given amount of time. It represents the capacity of the channel to carry information and is typically measured in hertz (Hz). Symbol rate refers to the number of symbol changes per second, baud refers to the number of signal elements per second, and speed is a general term that can refer to various aspects of a transmission channel.

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9.

The most significant advantage of CRCs compared to checksums is:

CRCs are used by lower OSI layer protocols than checksums
CRCs are better at detecting commonly occurring types of transmission errors
Division is a more reliable operation than addition
CRCs are usually implemented within the hardware while checksums are implemented within software

Correct Answer

A. CRCs are better at detecting commonly occurring types of transmission errors

Explanation

CRCs are better at detecting commonly occurring types of transmission errors compared to checksums. CRCs use polynomial division to generate a unique check value for the data being transmitted. This check value is appended to the data and sent along with it. At the receiving end, the same polynomial division is performed on the received data, and if the check value obtained does not match the one sent, it indicates that errors have occurred during transmission. CRCs have a higher probability of detecting errors such as single bit flips, burst errors, and most common types of transmission errors.

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10.

When using parity bits, checksums and CRCs, what must occur for an error to go undetected?

The message must be corrupted such that the parity bit, checksum or CRC is unaltered
An even number of bits within the message must be corrupted
The error must be the result of hardware errors rather than software or interference errors
The message must be corrupted in such a way that it becomes some other legitimate message

Correct Answer

A. An even number of bits within the message must be corrupted

Explanation

When using parity bits, checksums, and CRCs, these error detection techniques rely on the assumption that an odd number of bits will be corrupted. This is because these techniques are designed to detect errors, and an odd number of bit errors will result in a change in the parity bit, checksum, or CRC value. However, if an even number of bits within the message are corrupted, the error detection mechanism will not be able to detect the error, as the parity bit, checksum, or CRC value will remain unaltered.

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Communication Systems Revision Set 2

The number of signal events occurring each second is known as the:

Quiz Preview

A communication channel modulates waves using 256 QAM and transmits 8 million symbols each second. Approximately how long will it take to transfer 10MB?

Which of the following includes only baseband communication links?

Which of the following is TRUE in terms of 8-bit checksums?

Protocols that include checksums include:

A parity bit is added to each byte of data sent. If all data bits are reversed what will occur?

7-bit ASCII data is sent one character at a time using odd parity. The received data contains errors. Which of the following is most likely?

The range of frequencies a transmission channel occupies is known as its:

The most significant advantage of CRCs compared to checksums is:

When using parity bits, checksums and CRCs, what must occur for an error to go undetected?