Computer Science 825 Quiz 5 Name _______________________ 1. Suppose the following bit sequence is subject to SDLC style bit stuffing.. How many bits will actually be transmitted? 1 876543210987654321 -------------------------- Last bit --> 011111101111101111 <- First bit transmitted 2. In the standard bit stuffing the loss of an end flag (framing character) can be detected by: a. The appearance of some 0's b. The appearance of some 1's where where there should only be 1's there should only be 0's. c. The appearance of all 1's d. The appearance of all 0's where where there should be a mix of there should be a mix of 1's 1's and 0's and 0's. 3. Suppose a 15 bit codeword with ODD parity is being used in a single bit hamming ECC. Suppose the following codeword is received. X 0 0 1 1 X 0 0 0 1 0 1 1 -------------------------------------------- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a. Fill in the missing ECC bits above .. DON"T bother to compute the X bits. b. Suppose a single bit Hamming ECC is to be performed on a 511 bit codeword. How many bits are in each of the parity checked groups (include the parity bit itself). c. How many bits does any pair of parity checked groups have in common with each other? d. Suppose a two bit error occurs. Circle the correct description of the result. 1. The fact that there is a two error can be detected, but the error can't be corrected. 2. The two bit error will be undetected and the value that results from the correction process will be a legal code word but it will be the wrong one. 3. The two bit error will be undetected and the corrected value will be either the wrong codeword or it might be the correct codeword. 4. The two bit error will be undetected and the corrected value MAY be an illegal code word. 4. Suppose a distance 3 Hamming code is being used. We all (hopefully) know that this code can correct single bit errors. We can also use such a code to do ERROR DETECTION ONLY. In such an application the best that the code can detect is: a. Single bit errors b. Double bit errors c. Triple bit errors d. No errors at all 5. The objective of the reservation based Aloha Schemes was to a. Act like TDM at both b. Act like Aloha at low loads high and low loads. and TDM at high. c. Act like TDM at high d. Act like Aloha at low loads loads and Aloha at lows. and Aloha at high. 6. Suppose a 7 bit codeword with ODD parity is being used in a single bit hamming ECC. Suppose the following codeword is received. 0 1 1 0 1 0 1 b1 b2 b3 b4 b5 b6 b7 Which bit if any is in error: 7. Suppose 2400 bit code words are to be transmitted and it is desired to use a Hamming code to correct error bursts of up to 80 bits. How many of the 2400 bits must be used for ECC. (Hint: Fractional bit based strategies are not encouraged!) 8. Suppose A generator polynomial of x + 1 is used. Answer the following T or F ___ a. Some but not all double bit errors in which the 2 error bits are adjacent in the message will be detected. ___ b. All seventeen (not necessarily adjacent) bit errors can be detected. ___ c. All single bit errors can be detected. 9. Suppose a generator polynomial of G(x) = x^4 is used. G(x) can detect a. Some, but not all, of the errors b. All errors involving the involving the last 4 bits of the last 4 bits of the message message some but not all of the but no errors that do not errors that don't involve the last involve the last 4 bits. 4 bits. c. All errors involving any of d. All single bit errors but no more the last 4 bits of the message than that. and some errors not involving any of the last 4 bits.