Computer Science 825
                         Quiz 4              Name _______________________

1. 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. p * k * (1 - k) ^ (p - 1)

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


2. 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.

3. 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. Iecreasing 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
          increase the efficiency of a CSMA LAN

4. For the IEEE token bus match the control packets with the conditions
   underwhich they are used. (Some answers may or may not be used
   multiple times and some may or may not be used at all.)

   ____ a. A station is powered up, listens     1. SOLICIT_SUCCESSOR_2
           for the token and doesn't hear one.
                                                2. WHO_LEADS
   ____ b. A station wishes to leave the net.
                                                3. WHO_FOLLOWS

   ____ c. A station transmits the token twice  4. RESOLVE_CONTENTION
           without hearing its successor
           transmit a token or a packet.        5. CLAIM_TOKEN

   ____ d. A station wishes to invite a new     6. SOLICIT_SUCCESSOR
           station to enter the network.
                                                7. SET_SUCCESSOR
   ____ e. A station recognizes a WHO_FOLLOWS
           packet specifying its predecessors   8. WHO_KNOWS
           address.

   ____ f. A station transmits WHO_FOLLOWS
           twice ad receives no response.


5. A token bus system (not IEEE 802.4) works like this. When the token
   arrives at a station a timer is reset to 0.  The station then transmits
   priority 6 frames unitl the timer reaches T6. It then switches to
   priority 4 until the timer reaches T4.  This algorithm is repeated for
   with priority 2 and 0. If all stations use the values of 60, 100, 130,
   150 for T6 through T0 what percent of the bandwidth will be dedicated
   to class 2 (assume all stations have an infinite backlog of each class.)








6. Starvation of all priority classes except class 6 can occur on
   an IEEE 802.4 token bus:

   a. Never                          b. Any time all stations have a
                                        large backlog of class 6 traffic

   c. Only if THT >= TRT 4           d. Only if N * THT > TRT4



7. Two standard measures of network performance are sustained throughput
   and mean packet delay. High sustained throughput is good and low mean
   packet delay is good.  For each of the following select the answer that
   bests compares CSMA/CD with token passing. (Assume equal bit rates).

   a. Sustainable throughput at very high loads
      1. CSMA/CD is better          2. Token passing is better
      3. No measureable difference

   b. Sustainable throughput at very low loads
      1. CSMA/CD is better          2. Token passing is better
      3. No measureable difference

   c. Packet delay at very low loads
      1. CSMA/CD is better          2. Token passing is better
      3. No measureable difference