Computer Science 851
                         Quiz 4              Name _______________________

1. 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

2. Suppose that the average length of a contention interval on
   a CSMA bus is C and that the average time to transmit a packet is P.
   The efficiency of the channel is:

   a.  P / C                        b.  (P + C) / P

   c.  P / (P + C)                  d. None of the above

3. 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 - k) ^ (p - 1)

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

5. In the analytical model of the CSMA-CD bus described in class, suppose
   each station tries to transmit with a probability of 1/2 on each slot.
   As k becomes large,  the probability of one station successfully
   acquiring the channel during a given contention slot:

   a. approaches 1                 b. approaches 0

   c. converges to 1/2             d. converges to 1/e

6. Each time an IEEE 802.3 Ethernet station suffers a collision
   (without having a successful transmission) the mean delay until
   its next attempt

   a. Increase by tau (the one       b. Increases by 2 * tau
      way prop delay).

   c. Doubles                        d. Is multiplied by  2 ^ n
                                        where n is the number of
                                        collisions

7. In a token ring, received bits are drained into the "bit bucket" when the
    interface is operating in:

   a. recieve mode                    b. transmit mode

   c. store and forward mode          d. all of the above



8. For each of the following Token Bus control frame, answer U if the
   frame can be unicast and B if it must be sent as a link level broadcast

   ___ a. SOLICIT_SUCCESSOR


   ___ b. SET_SUCCESSOR


   ___ c. WHO_FOLLOWS

9. 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 recognizes a WHO_FOLLOWS   1. SOLICIT_SUCCESSOR_2
           packet specifying its predecessors
           address.                             2. SET_SUCCESSOR_2

   ____ 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. The station having the lowest MAC    6. SOLICIT_SUCCESSOR
           address on the bus wishes to invite
           a new station to join the net.       7. SET_SUCCESSOR


10. Answer the following T or F. (Assume only the parameter indicated
   is being varied and all of the others remain fixed in each case.
   This question refers to a "traditional" shared bus ethernet.)

   ___ a. Increasing the average packet size will tend to make the efficiency
          of a CSMA CD LAN higher.


   ___ b. Decreasing the bit transmission rate from 10 Mbps to 1 Mbps
          increase the efficiency of a CSMA LAN

11. When a token ring interface is operating in receive mode bits
     flow:

    a. into one of the onboard        b. through the store and forward
       receive buffers                    buffer and on to the next station

    c. into the "bit-bucket"          d. Both a. and b.


11. In a token bus suppose that THT = 20 msec, there are 30 stations,
    and TRT4 = 750ms.  Suppose ALL stations have an infinite backlog of
    class 6 traffic, ONE station also has an infinite backlog of
    class 4 traffic, and no stations have any other traffic. For
    how long can the station with class 4 traffic send each time it
    gets the token.