Computer Science 825
                         Quiz 6              Name _______________________


1. Consider the following shift register:






   a. What will be the contents of the shift register after the
      input digits 1 and 1 are processed.









   b. What generator polynomial is represented by this shift register.





   c. Does this shift register represent standard long division  or
      is it the pre-multiplying version? (Yes is not the correct answer).




2. In protocol 2, ACK's were used but no sequence numbers were used
   on frames or ACK's.  Which of the following problems could occur.

   a. Lost frames would go          b. Lost ACK's would result in
      undetected.                      lost frames

   c. Lost acks would result in     d. Lost frames would result in
      duplicate frames.                duplicate frames.

3. In protocol 3, frames's had sequence numbers but ACKS did not.
   Protocol 3 could fail in the presence of:

   a. ACK's that were lost          b. ACK's that were excessively
                                       delayed

   c. Frames that were lost.        d. Either a or b


4. When protocol 3 failed, the failure resulted in:

   a. The loss of 2 frames          b. The loss of 1 frame and
                                       the duplication of 1 frame.
   c. The duplication of 2
      frames.


5. Match the amount of channel time lost each time an isolated
   error occurs in each of the following environments:

   ____ a. Go Back N            1. The two way propogation delay

   ____ b. Selective repeat     2. One way propogation delay.

   ____ c. Selective reject     3. One packet time

                                4. The timeout interval

6. For a sliding window protocol to function correctly it is
   necessary that the timeout interval be greater than:

   a. 0                         b. the one-way propagation
                                   delay

   c. the two-way propagation   d. two times the two-way
      delay                        propagation delay.

7. For each of the following combinations of propogation delay
   and error rate identify the most suitable variation of sliding
   window protocol. (The most suitable choice is the SIMPLEST
   one that doesn't incur a big performance penalty.)

   ___ a. No errors but very long     1. Stop and Wait
          propogation delay
   ___ b. No errors and negligable    2. Go back N
          propogation delay.
   ___ c. High errors and very long   3. Selective repeat
          propogation delay
                                      4. Selective reject.

8. Suppose a 6 bit sequence number is used and the sender
   window size is set to 40 packets. What is the largest
   size that the receiver window can be if the protocol is
   to function correctly?



9. Consider the following simplex connection.. Since the connection
   is simplex the sender side needs only nxtack and nxtsend state
   variables and the receiver side needs only nxtrecv. Assume that
   the "ACK next frame expected" convention is used and that the
   protocol is uses sender and receiver window size = N and that
   sequence numbers never wrap.


    nxtack  --------------------> nxtrecv
    nxtsend

   Answer the following T or F. At any instant in time:

   ____ a. nxtack must always be <= nxtsend

   ____ b. nxtrecv may be < nxtack

   ____ c. nxtrecv may be > nxtack

   ____ d. nxtrecv may be > nxtsend

   ____ e. nxtrecv may be < nxtsend