Computer Science 825
                         Quiz 4              Name _______________________

1. 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.

2. 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.

3. The maximum sustainable througput of a slotted  aloha channel is:

   a. .17 Packets / packet time.      b. .37 Packets / Second

   c. .5 Packet / packet time.        d. None of the above.

4. The window of collision vulnerability for a CSMA/CD Lan is:

   a. 1 Packet transmission time.     c. 1 Signal Propogation time.

   b. 2 Packet transmission times.    d. 2 Signal Propogation times.


5. 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 throughput??



6. Suppose slotted aloha is being used on a satellite based network.
   If the bit rate is 120,000 bits per second and packets are 4000
   bits long. (Speed of light = 180,000 miles/sec).

   a. How long (physically) is a packet traveling through space.




   b. Suppose the satellite flies a height of 25,000 miles.. How many packets
      could a station have in the air at any time.