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 8 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 4000 bit packets are used.





   c. Suppose that packet size remains 4000 bits but that 400 bits
      are now used for headers. How many packets must now be sent.



2. Suppose the bit transmission rate of an infinite population
   slotted aloha channel is 100000 bits per second and that packet
   size is fixed at 1000 bits.  Suppose that 80 transmissions
   per second take place.

   a. Suppose a station decides to transmit in a given slot.
      What is the probability the transmission will succeed?





   b. What is the probability that a randomly selected slot will
      contain a collision?  (N.B. This is NOT the same as asking:
      Suppose I decide to transmit in a slot, what is the probability
      that my transmission will suffer a collision!!!)





3. 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 1
      starts transmitting that station 1 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?


   d. What is the longest possible amount of time after station 2
      starts transmitting that station 2 can hear the collision?


4. 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


5. If K stations share a CSMA 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                           b. k * p * (1 - k) ^  (p - 1)

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

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

   a. A ^ j  * (1 - A) ^ (j - 1)  c. A / j

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

7. Parameters that effect the efficiency of a CSMA channel include
   the bit rate B, the packet size P, and the length of the wire L.
   For each parameter indicate whether increasing the value of the
   parameter will increase (I) or decrease (D) the efficiency of
   the channel.

   ___ a. Bit rate

   ___ b. Packet size

   ___ c. Length of the wire


8. Suppose M$ decides to get in the networking business and introduces
   the M$ ethernet adapter. The M$ adapter randomly either waits 0 or 1 slots
   after a collision.

   a. Describe the effect of using a single M$ adapter on a 64 station
      LAN in which the other 63 stations are IEEE compliant.





   b. Describe the effect of using 64 M$ adapters on the 64 station LAN.