Computer Science 851
                         Quiz 3              Name _______________________

1. Suppose a WiMAX system uses a bandwidth of 10.0 Mhz and OFDM with QAM.
   There are 100 subchannels of which 80 are used to carry data.

   a. What is the bandwidth of each subchannel





   b. What is the basic FFT symbol time (doesn't include guard intervals)
      in a WiMAX standard system.






   c. Assuming 0 guard intervals and are used what will be the maximum
      sustained throughput in bits/second (assuming 50/50 upstream downstream
      split and QAM 16 1/2 modulation in each channel).





2. Suppose a network has links that carry data at a rate of 50,000
   bits per second.  Suppose Node A and Node B are connected by a
   path that requires 10 hops (transmissions).

   a. What is the minimum time required to send a 1,000,000 bit message
      from A to B if message switching is used.






  b. Suppose packet switching is used with headers of length 200 bits.
     Total packet length (including header) is 2200 bits. How long
     will it take to send the entire message from A to B.







3. For a slotted aloha system  there are two different values of
   G which will result in the throughput being "S". For a given "S"
   the average delay before a packet is successfully transmitted  is:

   a. The same for both "G"s          b. Larger for the small "G"
                                         than for the big "G".

   c. Larger for the big "G"          d. Totally independent of
      than for the small "G".            the "G" value.


4. 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 (successful transmissions per unit time).






   c. What is the probability that a slot contains a collision?







5. In a dual persistent CSMA scheme INCREASING the value of q will
   tend to:

   a. increase both the number of      b. increase idle but reduce
      collisions and the amount           collisions
      of idle time.

   c. increase collisions and reduce   d. have no effect on collisions
      idle                                or idle


6. The objective of the CS part of the CSMA-CD  protocol is to:

   a. Reduce the amount of time        b. Reduce the number of
      caused by collisions that           collisions that occur
      do occur

   c. Reduce the amount of idle        d. All of the above.
      time in which all stations
      are silent

7. If packets are generated at random (exponentially distributed) by
    each station on a shared channel, then statically assigning 1/Nth of
    the bandwidth to each station (rather than having a single waiting
    queue)

    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.