Answers of Exercise 3

1. Answers of Exercise 3 1. Describe what are the packet, the packet communication, and motivations using the packet in computer communications. Answer: A packet is a data block including information data and control data. The packet communication is based on the transfer of individual packets. Each of them is sent to destination independently. Because of special features of computer data, such as discontinuous and varied data amount as well as high reliability requirement, Motivations using the packet communications are efficiency of sharing network resources, fairness for multiple computers and effective ways to control errors. 2. (1) An ASCII coded data block to be sent as the following: Draw its BSC frame with byte stuffing. Answer: (2) Bit stuff the following data: 00111111111111001011111011 Answer: 00111110111110110010111110011 3. (1) Find the even and odd parity check for a 7-bit data: 1001011. Answer: 10010110 (even check) and 10010111 (odd check) (2) Find the even parity check for each ASCII code of the word “packet”. Anwser: p a c k e t Original data 1110000 1100001 1100011 1101011 1100101 1110100 Data with check 11100001 11000011 11000110 11010111 1100101011101000 esc F r a m e eot B y t e soh soh esc z F r a m e esc y B y t e esc x eot CRC

2. 4. Find the 8 bit (v=8) LRC of the data: 1001 0011 1001 0011 1001 1000 0100 1101 Answer: 5. Find the 16 bit (v-16) checksum of the data: 1001 0011 1001 0011 1001 1000 0100 1101 Answer: 1001 0011 1001 0011 1001 1000 0100 1101 Data 10010011 10010011 10011000 01001101 LRC 11010101 1001 0011 1001 0011 1001 1000 0100 1101 Data 1 1 1111 carrier 1001001110010011 1001100001001101 + 1 sum 0010101111100001 checksum 1101010000011110