Computer Science 825
                         Quiz 3              Name _______________________

1. As the load G on a pure or slotted aloha channel increases,
   the expected number of attempts per successful transmission:

   a. Approaches zero.                 b. Increases linearly

   b. Approaches a fixed upper bound   d. Increases exponentially
      asymptotically

2. Select the appropriate window of vulnerability for collision
   OCCURRENCE in each of the following MAC protocols: (n.b. the
   question says occurrence and not detection)

   ____ a. Pure aloha            1. One byte time
                                 2. One packet time
   ____ b. Slotted aloha         3. Two packet times
                                 4. The one way propogation delay
   ____ c. Ethernet style        5. The two way propogation delay
           CSMA-CD               6. Two times the bit rate

3. 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 shortest possible amount of time after station 1
      starts transmitting that station 2 can hear the collision?


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



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



4. Measurements of an infinite user slotted aloha channel show that 20%
   of the slots are idle.

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



   d. Is the load on this channel below or above the level at which
      maximum throughput is achieved.

5. The main function of the CD portion of a CSMA-CD protocol is to:

   a. Reduce the probability that     b. Reduce amount of wasted time
      a collision will occur.            when a collision occurs.

   c. Ensure that no collisions       d. Both a and b.
      occur.

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 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


8. Suppose packet time on an aloha channel is 40 msec and an infinite
   population is generating a total load (including retransmissions)
   of 50 transmissions per second. Hint: convert the numbers above to
   packets per packet time... then convert back for part b.

   a. What is the probability of a success on the first try on this
      channel?





   b. What is the throughput in packets / second.