Computer Science 825
                         Quiz 2              Name _______________________

1. Suppose a network has links that carry data at a rate of 10,000
   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 200,000 bit message
      from A to B if message switching is used.







   b. Suppose packet switching is used and no additional bits are
      required for headers. How long will it take to send the entire
      message if 2000 bit packets are used.






2. Suppose headers of length H bits are used in the packet switching
   scheme described above. Suppose packet size is P (where
   P now includes header+data bits).  Under this new scheme the
   time to send a complete packet is:

   a. 10000 * P                     b. P / 10000

   c. 10000 / (P + H)               d. 10000 / (P - H)

3. Under the new scheme the total number of packets that must
   now be transmitted is:

   a. 200,000 * P                   b. 200,000 / P

   c. 200,000 / (P + H)             d. 200,000 / (P - H)


4. Suppose a PCM transmission system has a bit rate of 2^20 bps
   and can carry frequencies between 0 and 2^15 Hz.  Into how
   many discrete levels can the input signal be quantized by the
   A-D converter.










5. The use of very small slot sizes in TDM is

   a. Inefficient and shouldn't       b. Generally shouldn't be used for
      be used in either STDM or ATDM      SDTM but is OK for ATDM

   c. Generally shouldn't be used     d. Perfectly OK for both
      for ATDM but is OK for STDM

6. Measurements of an infinite user slotted aloha channel show that 40%
   of the slots are idle.  What is the throughput,  S.








7. 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 - p) ^ (k - 1)

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


8. If the probability that one station successfully transmits in a
   slot is "A". The expected value for the number of slots between
   successful transmissions is:

   a. A ^ j * (1 - A) ^ (j - 1)   b. Summation  j * (A ^ j) (1 - A)^ (j - 1)
                                     j=0,infinity

   c. Summation  j * A * (1 - A)^ (j - 1)  d. None of the above
      j=0,infinity

9. 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 2
      starts transmitting that station 2 can hear the collision?



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



10. 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