CS/ECE 338: Communication Networks for Computers Spring 2007
Problem Set 3 Due: start of class, Wednesday, February 28
Direct Link Networks (Part II)
Assigned reading: Peterson and Davie: Chapter 2.5 – 2.9. All problems carry equal weight. For full credit, show all
work.
1. Multiple Access
a. Consider a 10 Mbps Ethernet segment with two nodes, A and B. The propagation delay between two
nodes is 250 bit times. Suppose node A begins transmitting a frame, and before it finishes, node B
begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why
or why not? If the answer is yes, then A incorrectly believes that its frame was successfully
transmitted without a collision.
Now consider a 100 Mbps 100BT Ethernet.
b. In order to have an efficiency of 0.50, what should be the maximum distance between a node and the
hub? Assume a frame length of 64 bytes.
c. Does this maximum distance also ensure that a transmitting node A will be able to detect whether any
other node transmitted while A was transmitting? Why or why not?
d. How does your maximum distance compare to the actual 100 Mbps standard of a maximum of 200 m
between any two nodes?
2. Ethernet Timing
This problem is about the Ethernet/IEEE 802.11 access protocol. To be definite, suppose that if a host detects a
transmission while it is transmitting a frame, then: (i) if the host has already transmitted the 64 bit preamble, the host
stops transmitting the frame and sends a 32 bit jamming sequence; (ii) Else the host finishes transmitting the 64 bit
preamble and then sends a 32 bit jamming sequence. For simplicity, assume a collision is detected as soon as an
interfering signal first begins to reach a host. Suppose the packets are 512 bits long, which is the minimum length
allowed. Hosts A and B are the only active hosts on a 10 Mbps Ethernet and the propagation time between them is
12 μS, or 120 bit durations. Suppose A begins transmitting a frame at time t = 0, and just before the beginning of the
frame reaches B, B begins sending a frame, and then almost immediately B detects a collision.
a. Does A finish transmitting the frame before it detects that there was a collision? Explain.
b. What time does A finish sending a jamming signal? What time does B finish sending a jamming
signal?
c. What time does A first hear an idle channel again? What time does B first hear an idle channel again?
d. Suppose each host next decides to retransmit immediately after hearing the channel idle. After the
resulting (second) collision: When does A next hear the channel idle? When does B next hear the
channel idle?
e. Suppose after the second collision, A decides to wait 512 bit durations to retransmit (if it hears silence
after that long) and B decides to retransmit immediately after hearing a silent channel. Is the
transmission of host B successful?
f. At the time A was planning to send its second retransmission, it senses a carrier present. Suppose at
that particular time A decides to wait 3 x 51.2μs more until its next retransmission. What time does
host A finish sending its packet?
3. Window Size
Consider an error-free 1024-kbps channel used to send 256B data frames in one direction, with very short
acknowledgements coming back the other way. Assume a propagation delay of 50 msec.
a. What is the maximum throughput for window size of 1, 7, 15, 127 and 255?
b. At what minimum window size can the protocol run at the full rate of the channel?
