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GATE CSE 2021 Set 2 Q44: CRC check bits for message 11000 with generator x³+x+1

Question

Consider the cyclic redundancy check (CRC) based error detecting scheme having the generator polynomial X3+X+1X^3 + X + 1. Suppose the message m4m3m2m1m0=11000m_4m_3m_2m_1m_0 = 11000 is to be transmitted. Check bits c2c1c0c_2c_1c_0 are appended at the end of the message by the transmitter using the above CRC scheme. The transmitted bit string is denoted by m4m3m2m1m0c2c1c0m_4m_3m_2m_1m_0c_2c_1c_0. The value of the checkbit sequence c2c1c0c_2c_1c_0 is

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Answer: C (official GATE 2021 final answer key)

Concept used: CRC: append degree-many zeros and take the remainder of modulo-2 division by the generator.

First step: The generator is 1011; divide 11000000 by 1011 using XOR.

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Source: official GATE 2021 Set 2 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key