Computer Science 825
                         Quiz 4              Name _______________________

1. The 802.5 token byte contains 4 fields: For each pair identify WHICH FIELD
   APPEARS BEFORE THE OTHER in the token. In each case identify the
   SPECIFIC PROBLEM that it would prevent the 802.5 from working as
   advertised if the order were inverted (assuming the use of a 1 bit
   store and forward buffer)

   a. P(riority) and R(eservation)






   b. M(onitor) and T(token)




2. What is the minimum Hamming distance of a code that is capable
   of doing double bit error CORRECTION.




3. In an 802.5 token ring system the responsibility for restoring
   the priority on a token to its original level lies initially with

   a. the station that made         b. the last station to transmit
      the high priority reservation    using the higher priority token


   c. the station that actually     d. the monitor station
      issued the high priority
      token.


4. Both the "bitmap" and "binary countdown" MAC protocols are collision
   free. When a large number of stations exist on the network the size
   of a reservation frame is:

   a. the same for bitmap and         b. larger for bitmap
      binary countdown

   c. larger for binary countdown

5. Show the sequence that would actually be transmitted AFTER the
   following message was subjected to BISYNCH style byte stuffing.
   Bisynch byte stuffing is used (Here the leading and trailing
   DLE STX and DLE ETX are the "real thing" indicating start and
   end of the message

   ------------------------------------------------------------------
   | DLE | STX | 'A' | 'B' | DLE | 'C' | DLE | STX | DLE | DLE | ETX |
   ------------------------------------------------------------------


6. Suppose a 7 bit codeword with ODD parity is being used in a
   single bit hamming ECC.  Suppose the  following codeword is
   received.

       1  1  1  0  1  1  1
      a1 a2 a3 a4 a5 a6 a7

   a. Which bit if any is in error?




7. Suppose a distance 3 Hamming code is being used. We all (hopefully)
   know that this code can correct single bit errors. We can also use
   such a code to do ERROR DETECTION ONLY. In such an application the
   best that the code can detect is:

   a. Single bit errors                b. Double bit errors

   c. Triple bit errors                d. No errors at all


8. Suppose a codeword consists of 2400 bits and a distance 3 Hamming Code is
   used to correct bursts of 60 bits or less. How many bits in the
   codeword must be used as ECC bits. (Schemes involving fractional
   bits are not likely to be warmly received.)







9. 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.

10. In the standard bit stuffing the loss of an end flag (framing
   character) can be detected by:

   a. The appearance of some 0's       b. The appearance of some 1's where
      where there should only be 1's      there should only be 0's.

   c. The appearance of all 1's        d. The appearance of all 0's where
      where there should be a mix of      there should be a mix of 1's
      1's and 0's                         and 0's.

11. Suppose a codeword contains "n" total bits. It is broken down
    as "m" msg  bits and "r" ECC bits.  Which of the following
    correctly characterizes the relationship between n, m, and r
    for single bit error correction to be done:

       r   m      n                           m      r
   a. 2 * 2   <= 2              b. (n + 1) * 2   <  2

                 m     n
   c. (n + 1) * 2  < 2          d. None of the above.