Quiz 2
COSC650 Computer Networks
Fall 2009
Name ______________________
1. (5 points) Show the NRZ, Manchester, and NRZI encodings for the bit pattern.
Assume that the NRZI signal starts out low.
Bit
1
1
1
0
1
0
1
0
0
1
1
NRZ
Clock
Manchester
NRZI
2. (5 points) Suppose the following sequence of bits arrive over a link:
011010111110101001111111011001111110
Show the resulting frame after any stuffed bits have been removed. Indicare any error
that might have been introduced into the frame.
01101011111^101001111111011001111110
Error
eof
3. (5 points) Suppose we want to transmit the message 1011 0010 0100 1011 and protect
it from error using the CRC8 polynomial G(x) = X8 + X2 + X1 + 1.
1) Use polynomial long division to determine the message that should be transmitted.
1011 0010 0100 1011 1001 0011
2) Suppose the leftmost bit of the message is inverted due to noise on the transmission
link. What is the result of the receiver’s CRC calculation? How does the receiver know
that an error has occurred?
Remainder : 1011 0110 is not zero.
4. (8 points) Consider the following timeline diagram given for the sliding algorithm with
SWS = RWS = 4 frames in the following situation.
Assume the receiver sends a duplicate acknowledgement if it does not receive the
expected frame. For example, it sends DA[2] when it expects to see Frame[2] but
receives Frm[3] instead. Also, the receiver sends a cumulative acknowledgement after
it receives all the outstanding frames. For example, it sends ACK[_(a)__] when it
receives the lost frame Frm[2] after it already received Frm[3], Frm[4], and Frm[5]. Use
a timeout interval of about 2 X RTT.
Frm[1]
Frm[2]
(a) ACK[5]
ACK[1]
1 RTT
(b) Frm[2]
Frm[3]
Frm[4]
Frm[5]
DA[2]
DA[2]
DA[2]
2 RTT
__(b)_______
__(c)_______
3 RTT
(d)
(e)
4 RTT
(c) ACK[5]
Draw the arrow lines, (d) and (e), on
the timeline
(d) Frm[6] and (e) ACK[6]
5. (3 points) Consider the sliding window algorithm with SWS=RWS=3, with no out-oforder arrivals, and with infinite-precision sequence numbers. What is the MaxSeqNum
for sufficient condition?
Answer ____6___________
6. (4 points) List three different methods for Acknowledgement algorithms for a reliable
transmission of sliding window algorithm with their brief explanations.
 Negative acknowledgment
 Duplicate acknowledgment
 Selective acknowledgment