Computer Science 825
                         Quiz 4              Name _______________________


1. 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.








   b. What is the throughput in packets / packet time.






2. The main reason slotted aloha has a higher sustainable throughput
   than pure aloha is:

   a. Slotted aloha uses fixed        b. Forcing all transmissions to
      size packets but pure aloha        start on a slot boundary
      uses variable size packets.        reduces the window of collision
                                         vulnerability.

   c. Contention intervals in slotted  d. Slotted aloha uses smaller
      aloha are packet times but          packet sizes to reduce the
      contention intervals in pure are    window of collision vulnerability.
      signal propogation times.


3. 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


4. If K stations share a CSMA-CD bus and the probability that any station
   will transmit on a given contention slot is "p", then the probability
   that a successful transmission will take place in a given slot is:

   a. p                            c. p * k * (1 - k) ^ (p - 1)

   b. (k ^ p) * (1 - p) ^ (k - 1)  d. None of the above.

5. If the probability computed above is denoted A, then the probability that
   a contention interval will last exactly j slots is:

   a. A ^ j                       c. A / j

   b. ((1 - A) ^ (j - 1)) * A     d. None of the above.


6. Answer the following T or F.

   ___ a. Increasing the pysical length of a CSMA CD LAN will increase the length
          of the contention interval and thus make the efficiency of the
          LAN higher.

   ___ b. Increasing the average packet size will tend to make the efficiency
          of a CSMA CD LAN higher.

   ___ c. Increasing the bit rate of a CSMA CD LAN will make its efficiency
          higher.

7. In the performance models we derived for slotted aloha and for
   CSMA-CD the maximum effective utilization = efficiency =
   throughput in packets per packet time was:

   a. Independent of packet size   b. Independent of packet size in CSMA
      in both aloha and CSMA          but dependent on packet size in
                                      aloha.

   c. Dependent on packet size     b. Independent of packet size in aloha
      in both aloha and CSMA          but dependent on packet size in
                                      CSMA.

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. CSMA's window of collision
      on LANS than in satellites.       vulnerabilty is typically much
                                        smaller than a packet time.

   c. CSMA stations can detect       d. Both b and c.
      collisions and stop
      transmitting.