Homework 3 - Chapter 3

Answer the following questions. Each question is worth 2 points.

1. A packet is split into 20 frames. Each frame has an 85% percent chance of being sucessfully transmitted. How many frames on average must be sent for a data link protocol that does no error correction (i.e. the whole packet must be resent when any frame is corrupted)?
2. For the situation in Question 1, how many frames would be transmited if the data link protocol instead used selective repeat to resend just the damaged frames?
3. The message 0110 0010 1111 1110 0111 0011 1110 0000 1100 is transmitted using bit flags with bit stuffing. Show the message after it is encoded with bit stuffing.
4. For the same message as Question 3, now show the message after it is encoded with byte stuffing. For byte stuffing, the FLAG sequence is 0111 1110 and the ESC sequence is 1110 0000
5. Show the message 1100 0001 after it is encoded with even-parity Hamming code. Show all work in computing each check bit.
6. Say bit 9 of your computed message from Question 5 were flipped in transit. Show how the receiver would detect this bad bit and correct it.
7. Compute the CRC checksum for 0110 0010 1111 1110 divided by the generator polynomial 1001. Show all work (similar to Figure 3.8 in the book).
8. What features of the physical line should be considered when choosing a sending window size for a sliding window protocol? Remember that one purpose of sliding window is to keep the line in use as much as possible.
9. Why do most data link protocols put the CRC checksum in the footer instead of in the header?
10. HDLC uses bit stuffing. PPP uses byte stuffing. Give one reason why PPP uses byte stuffing instead of bit stuffing.