ECEN4533 Exam
#2 17 April 2006
1) A router is receiving an average of 601 packets/second that must be
forwarded onto a 3.0 Mbps WAN link. The packets have an
exponentially distributed IAT. Their size is also known to be
exponentially distributed with a mean of 560 bytes.
[10] Calculate the average
time a packet spends in the router. [Answer: 14.57 msec]
[15] If, instead of one 3 Mbps WAN link, two inverse multiplexed 1.5
Mbps WAN links are used, calculate
the average time a packet spends in the router. [Answer: 15.35 msec]
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2) You are the LAN manager for a corporate network in NYC. PC's
at this location need to connect to a server farm in San
Francisco. 100 Mbps switched Ethernet LAN's are at both
locations, and are connected by a cross-country 48 Mbps leased
line. Test have shown that the one-way propagation delay over the
leased line is 34 msec. The fastest PING time you've ever got
from the server farm during an idle period in the wee hours of the
morning is 71 msec. All devices are using TCP/IP.
[25] Estimate the minimum TCP
window size required to insure a single NYC PC downloading a large file
from the server can fully load the connection. You're going to
need to make some assumptions to arrive at your estimate. Clearly
indicate what those assumptions are. [Leased line NPD = 135.4
packets. 3 msec in LAN has an equivalent NPD = 11.94, long term
(100 Mbps LAN speed can only be used for short bursts. 48 Mbps
WAN speed is a choke point that will limit long term transmission
speed.). Roughly 282.7 packets could be in the system at a
time. TCP, tracking the message byes, will need a window size of
about 282.7*1460 = 431.3 KB.]
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3) An Internet router is moving packets over a 7.9 Mbps trunk running
at a 67% load. The average size of the packets is known to be 390
bytes. The H parameter for the traffic has been measured to be
0.76.
[10] Compute the average
number of packets in the router. [14.54]
[15] Compute the average time
a packet spends in the router. [8.571 msec]
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4) A layer 7 application generates messages that are uniformly
distributed between 0 and 4380 bytes. These messages are then
mapped to Ethernet packets such that a message from 0 - 1459 bytes gets
mapped to one small packet, a 1460 byte message gets mapped to a full
sized packet, a 1461 - 2919 byte message gets mapped to one full sized
and one small packet, etc.
[25] Sketch the PDF of the
resulting Ethernet packet size. I.E. if the above application
randomly generated a bunch of Ethernet packets, what would a histogram
of the packet sizes look like? [You should sketch a rectangle of
height 342.5*10-6 and x axis domain of 66-1526
inclusive, and a delta function at 1526 with weight 1/2.]
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