Question

3. The bit stream 100100 is transmitted using the CRC method, using the generator 1001. Identify the actual bit string transmitted. (6 pts)

Answer 1

First of all let's understand how to calculate the actual transmitted bits ?

Steps to follow:

1.First of all calculate n which will be number of bits of generator -1.

n = number of bits of generator - 1

number of bits of generator = 4 (1001)

so n = 4 - 1 = 3.

2.Now add n number of zeroes at the end of transmitted bits stream using CRC method

which is 100100

Now after adding three 0s message will become 100100000 (dividend)

3. Now divide this message by generator (1001) (divisor)

4.Calculate remainder by dividing.

5.Now actual transmitted message will be formed by adding remainder at the end of transmitted message.

To divide follow these steps.

1.If first bit of dividend is 1 then divisor will run 1 times

2.else if first bit of dividend is 0 then divisor will run 0 times.

3.to calculate remainder use xor operation

0 xor 0 -> 0

1 xor 1 -> 0

1 xor 0 -> 1

0 xor 1 -> 1

Now solving the question

100000 1001 100100000 1001 0000 ОООО 0000 0000 0000 0000 0000 0000 0000 0000

  

000

So remainder is 000  (take n bits)

So actual transmitted message is 100100000 (message + remainder)

Now to check if transmitted message has no error then divide actual transmitted message with generator.

if remainder is 0 then it has no error

otherwise it has errors.

But the actual transmitted message is same as the transmitted message so no need to check again.

Now see its remainder was 0 so the actual transmitted message is correct.