Computer Science 825
                         Quiz 3              Name _______________________

1. Measurements of an infinite user slotted aloha channel show that 60%
   of the slots are idle.

   a. What is the total load (mean number of transmission attempts per
      packet time) = G.






   b. What is the expected number of transmission attempts required
      for each successful transmission.





   c. Is the current load below or above the load required to
      obtain maximum throughput.




2. Suppose the value "t" represents the ONE WAY signal propogation
   delay between two stations on an ethernet and that the two stations
   transmit and cause a collision.  Suppose station 1 began transmitting
   first and station 2 began transmitting second.  Hint: the correct
   answers can be drawn (w/ replacement} from the following "urn":

               {0  t/2  2t 8t}
                \  t/4  4t  /
                 \____t____/

   a. What is the shortest possible amount of time after station 1
      starts transmitting that station 1 can hear the collision?


   b. What is the longest possible amount of time after station 1
      starts transmitting that station 1 can hear the collision?


   c. What is the longest possible amount of time after station 1
      starts transmitting that station 2 can hear the collision?


   d. What is the shortest possible amount of time after station 2
      starts transmitting that station 2 can hear the collision.


3. The objective of the CS part of the CSMA-CD MAC protocol is to:

   a. Reduce the amount of time        b. Reduce the number of
      caused by collisions that           collisions that occur
      do occur

   c. Reduce the amount of idle        d. All of the above.
      time in which all stations
      are silent

4. The objective of the CS part of the CSMA-CD MAC protocol is to:

   a. Reduce the amount of time        b. Reduce the number of
      caused by collisions that           collisions that occur
      do occur

   c. Reduce the amount of idle        d. All of the above.
      time in which all stations
      are silent

5. When compared to 1 persistent protocols, p persistence tends to have

   a. more time wasted on              b. less loss of time when
      collisions but less idle time       collisions do occur

   c. smaller number of collisions     d. better performance at very
      but larger amounts of idle          low loads but worse performance
      time.                               under high loads.

6. For the IEEE token bus match the control packets with the conditions
   underwhich they are used. (Some answers may or may not be used
   multiple times and some may or may not be used at all.)

   ____ a. A station recognizes a WHO_FOLLOWS   1. SOLICIT_SUCCESSOR_2
           packet specifying its predecessors
           address.                             2. SET_SUCCESSOR_2

   ____ b. A station wishes to leave the net.
                                                3. WHO_FOLLOWS

   ____ c. A station transmits the token twice  4. RESOLVE_CONTENTION
           without hearing its successor
           transmit a token or a packet.        5. CLAIM_TOKEN

   ____ d. A station wishes to invite a new     6. SOLICIT_SUCCESSOR
           station to enter the network.
                                                7. SET_SUCCESSOR

                                                8. WHO_CARES

7. Suppose there are 20 stations on a token bus and that the
   THT = 10 msec and TRT4 = 150 msec.  If all stations send
   priorty 6 traffic for 6 msec, how much time does each
   station have for class 4 traffic (Ignore token passing
   latencies).