Computer Science 825
                           Quiz 6          Name _______________________

1. Suppose that a satellite channel is in use and that the ROUND TRIP
   propogation delay is 540 ms.  Suppose bit transmission rate is 400,000
   bits per second and packets are 8000 bits long.

   a. What will the efficiency of this channel be if stop and wait
      protocal is used.



   b. How large (in packets) must the SWS be in order to obtain 100%
      efficiency if the channel is guaranteed error free. (i.e. give
      me the MINIMUM SWS for 100% efficient operation).




   c. How large (in packets) must the RWS be in order to obtain 100%
      efficiency if the channel is guaranteed error free.



   d. How many bits should the sequence number be if selective
      reject is used.


2. Show how a sliding window protocol that uses a 2 bit sequence
   number, a sender window size of 3, and a receiver window size of 2
   can be made to fail.















4. The address byte in an S/HDLC packet always carries the address of

   a. The sender                 b. The receiver

   c. The primary station.       d. The secondary station.

5. Select the description which most accurately matches the use of

   the P/F bit in SDLC.

   a. 1 -> Poll and 0 -> Final    b. The bit means poll or not poll
                                     when a secondary sends final or
                                     not final when a primary sends.

   c. The bit means poll or not   d. The bit means poll or not poll
      poll to the sender final       when the primary sends final or
      or not final to the receiver   not final when a secondary sends.

6. The "failure" that the unnumbered protocol is susceptible to
   is:

   a. Undetected loss              b. Undetected duplication of
      of a frame.                     a frame.

   c. Reordering of frames         d. Both a. and b.

7. Suppose unnumbered frames were given sequence numbers and ACK numbers
   just like data frames and that the receiver of an out of sequence
   unnumbered frame would discard it just as it would an out of sequence
   data frame. Would this make the functions performed with unnum frames
   work:

   a. Much better and more         b. Work worse and less reliably.
      reliably.

   c. Make no appreciable difference.

=== The remaining questions address the sample router protocol
=== whose code we read in class

8. The sender window logic used three indexes: nxtsend; nxtqueue; and
   nxtack. We said these values always remained in a specific relative
   order. The order was

   a. ack <= queue <= send              b. ack <= send <= queue

   c. send <= queue <= ack              d. queue <= ack <= send

9. The protocol used 4 state variables: nxtsend, nxtack, nxtqueue,
   and nxtrecv.. For each variable identify the process that was
   responsible for INCREMENTING it.
   (Some answers may be used more than once or not at all).

      ____ 1. nxtsend                      a.  Writelink process

                                           b.  Timeout process
      ____ 2. nxtack
                                           c.  Network layer process

      ____ 3. nxtqueue                     d.  Queuemsg process

      ____ 4. nxtrecv                      e.  Readlink process


10. When a process becomes blocked on a counting semaphore it
    necessarily depends upon some OTHER process to signal the
    semaphore and unblock it.  For each of the following identify
    the process that is responsible for unblocking it AFTER IT
    BLOCKS ON A COUNTING SEMAPHORE.

   ___ a. network layer           1. queuemsg

   ___ b. readlink                2. readlink

   ___ c. writelink               3. writelink

                                  4. network layer