Computer Science 825
                         Quiz 4              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 throughput??





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



2. 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 slot is:

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

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


3. The efficiency of a CSMA CD LAN is a function of the speed of
   light, the packet size, the bit transmission rate, and
   the length of the cable.

   Answer the following T or F. (Assume only the parameter indicated
   is being varied and all of the others remain fixed in each case).

   ___ a. Increasing the 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. Decreasing the bit transmission rate from 10 Mbps to 1 Mbps
          will make the contention interval longer and thus decrease
          the efficiency.

4. The main reason that an ethernet channel can support more packets /
   packet time throughput than a satellite based aloha system is that:

   a. The signal travels faster thru  b. All stations hear every ethernet
      an ethernet cable than thru        transmission but only the dest
      space.                             station hears an aloha transmission

   c. The signal propogation delay    d. Only one station at a time is
      is far shorter in an ethernet      allowed to transmit in an aloha
      than in a satellite net.           network.

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 at all.


6. Consider a p-persistent csma-cd lan. p persistent means that
   after a station senses the carrier idle it transmits with
   probability p and defers with probability (1 - p).  Increasing
   the value of p will tend to:

   a. Reduce latency at low util.      b. Increase latency at low util.
      and increase throughput at          and increase throughput at
      high utilization.                   high utilization.

   c. Increase latency at low util.    d. Reduce latency at low util.
      and increase throughput at          and reduce throughput at
      high utilization.                   high utilization.

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.