CS536 Homework 4
due Tue, Apr 28th, 1:30pm
May 2, 2009
Submit your homework electronically in a text or PDF file.
Email your
solution to the professor by 1:30pm on Tuesday, Apr 14th. Late submission will
not be accepted, nor will collaboration.
To receive credit, all submissions should contain this statement at the be
ginning: “By turning in this homework submission, I certify that this work was
done solely by me.” Questions about this homework should be directed to the
TAs or the Professor, not to fellow classmates.
Problem 1 (10 pts)
Suppose two nodes start to transmit at the same time a packet of length
L
over
a broadcast channel of rate
R
. Denote the propagation delay between the two
nodes as
d
prop
. Will there be a collision if
d
prop
< L/R
? Why or why not?
Answer.
Yes, because the remote sender’s packet will arrive before the packet
is done being transmitted.
Problem 2 (10 pts)
Suppose a 10 Mbps adapter sends into a channel an infinite stream of 1s using
a Manchester encoding.
The signal merging from the adaptor has how many
transitions per second?
Answer.
20 million transitions per second.
Problem 3 (10 pts)
In CSMA/CD, after the fifth collision, what is the probability that a node
chooses
K
= 4? The result
K
= 4 corresponds to a delay of how many seconds
on a 10 Mbps Ethernet?
Answer.
After the 5th collision, the adapter chooses from
{
0, 1, 2,, 31
}
. The
probability that it chooses 4 is 1
/
32. It waits 204.8 microseconds.
1
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Problem 4 (10 pts)
Consider the 4bit generator
G
= 1001, and suppose that the data
D
has the
values given below. For each, give the value of the remainder
R
which serve as
the CRC. Show your work.
a. 01010101
Answer.
110 (Note that the subtraction is done without carry, see ex
ample in book)
b. 10010001
Answer.
001 (Note that the leading 0’s will be important for a fixed
width check.)
c. 11011011
Answer.
011
d. 01101011
Answer.
111 (Note that even though 1000
<
1001, you still apply the
subtraction/xor at that point).
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 Spring '08
 Sonia,F
 IP address, slot 2, slot, bit times, Ethernet destination address

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