ECEN4533 Exam
#2 16 April 2007
1) An application passes messages of three sizes (70, 302, and 1557
bytes) to the TCP stack in a PC. 18% of the messages are 70
bytes, 41% are 302 bytes, and 41% are 1557 bytes. TCP is set with
a Maximum Transmission Unit of 1460 bytes. TCP will segment the
message if necessary, add a 20B header to each segment, and pass the
result to the IP layer. IP adds a 20B header to each segment, and
passes the result to the Ethernet layer. Ethernet adds 26 bytes
and transmits each resulting frame via the 802.3 protocol.
[10] Compute the average
Ethernet frame size on the 802.3 network. (Answer = 615.5 bytes)
[15] Find and sketch the
Probability Density Function of the Ethernet Frame size you expect to
see on this network. (You
should sketch delta functions at 136, 163, 368, and 1526 bytes with
weights 0.1277, 0.2908, 0.2908, and 0.2908 respectively.)
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2) A transmitter is directly connected to a receiver via a 1.5 Mbps
leased line with an end-to-end propagation delay of 2.4 msec.
Assume a logical link has been completely opened and the transmitter
has received a 1,000 byte credit from the receiver. The
transmitter will be sending small fixed-size 410 byte packets, and the
receiver will issue a window credit for an additional 410 bytes with
each ACK. The receiver issues 41 byte ACKnowledgements
immediately after receiving a complete packet. The transmitter
processing time = 0.
[25] Estimate the throughput
in packets/second for an errorless system using TCP window flow
control. (277.6
packets/second)
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3) An ATM switch is moving statistically multiplexed traffic that is
known to be self-similar with an H parameter of .95. The switch
has a trunk speed of 12 Mbps, with a trunk loading of 64%.
[10] Compute the average
number of cells in the switch. (3.105 million)
[5] Compute the average delay an ATM cell experiences in the
switch. (171.4 seconds)
[10] Assume that exactly 100 ATM cells need to be stored in the switch
queue. Let the random variable X be defined as the number of
bytes that need to be stored in the queue. If the queue size is
6000 bytes, compute the probability the queue overflows, i.e. compute
P( X > 6000 / 100 ATM cells need to be stored). (0)
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4) You are looking at a system for possible use on a Voice over IP
(VoIP) network for a corporate branch office. You are analyzing
the outbound link which consists of the VoIP phones feeding a 100 Mbps
switched Ethernet, which in turn feeds a branch office router, which in
turn is attached to corporate HQ by an 400 Kbps leased line.
Tests have shown that to get good quality voice, the time lapse between
your voice hitting a microphone and clearing the branch office router
needs to be no greater than 75 msec. Assuming...
*The VoIP phone codes voice at a fixed rate of 36
Kbps and generates one packet every 60 msec, leaving 15 msec of
allowable delay at the router.
*Each packet contains 47 bytes of overhead (7B PPP,
20B IP, 8B UDP, and 12B RTP).
*Propagation delay between the VoIP phones and
branch office router can be ignored.
[10] Compute the size of a
VoIP packet, including the overhead. (317 bytes)
[15] Suppose there are N active VoIP phones on the branch office
network, and no other traffic. Compute the largest value N can be such
that all VoIP packets completely clear the router by <15 msec.
Assume a worst case situation, where all N active VoIP phones are
somehow synchronized such that the N VoIP packets hit the router
stacked back-to-back. How large can N be such that this group of
N packets is completely transmitted by 15 msec? Time starts when
the leading edge of packet #1 hits the router, and stops when the
trailing edge of the Nth packet is injected onto the 400 Kbps leased
line. (N = a maximum of 2)
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