Computer Science 825
                         Quiz 3              Name _______________________

1. Measurements of an infinite user slotted aloha channel show that 30%
   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 a pure aloha system is working at optimal throughput and
   one of the stations transmits a frame. What is the probability
   that the frame will collide with another?






These questions pertain to queueing delays experienced by N stations
using a shared channel in a FIFO manner with no collisions.

3. If packets are generated at random (exponentially distributed) by each
   station, then statically assigning 1/Nth of the bandwidth to each station
   (rather than having a central queueing mechanism)

   a. Will have no appreciable effect      b. Will cause the delay to increase
      on the queueing delay.                  by a factor of N

   c. Will cause the delay to be reduced   d. Will cause the delay to increase
      by a factor of N                         by N seconds.

4. Queuing delays are related to both distributional and correlational
   factors in the arrival and service processes..  Increasing the
   ratio of variance to mean interarrival times..

   a. Increases queuing delays.            b. Decreases queuing delays

   c. Has no effect on queuing
      delays.

5. A positive autocorrelation in the arrival process tends to:

   a. Increase queuing delays.            b. Decrease queuing delays

   c. Have no effect on queuing
      delays.


6. 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.


6. In a dual persistent protocol a station transmits with prob p
   when it initially sense a channel idle and with prob q when
   it has to wait for a busy channel to become idle.  The proper
   relationship between p and q is:

   a. p < q                            b. p > q

   c. p = q                            d. q = 1 and p < q


7. The motivation for dual persistence is that the probability that a
   collision will occur is:

   a. higher when  a station has        b. lower when  a station has
      sensed the carrier busy and          sensed the carrier busy and
      waited for it to become idle         waited for it to become idle
      than when it initially senses        than when it initially senses
      the carrier idle.                    the carrier idle.

   c. very low immediately after a
      collision.

8. The main reason(s) that higher efficiencies are possible in CSMA-CD
   LAN's than in aloha is/are:

   a. Higher bit rates are used      b. Prop delay >> packet time in
      on LANS than in satellites.       CSMA-LAN's but the reverse
                                        is true for aloha

   c. Prop delay << packet time in
      CSMA-LAN's but the reverse
      is true for aloha