ECEN4533 Final
Exam 4 May 2005
1) [15] IPv4 addresses consist of 32 bits, in theory yielding 232 =
4.295 billion addresses. However, the address space has been
broken up into class A, B, and C networks. How many possible
class A, B, and C host addresses are there? This question is
asking for a grand total, and not the number of hosts possible per
network. [Answer:
3.759*109]
2) [20] You are trying to estimate the average packet size on a
backbone trunk. The packet size distribution is known to be
Gaussian with a standard deviation of 170 bytes. You start
randomly sampling the packets, adding the sizes together, and dividing
by the # of packets in order to calculate a sample mean. You wish
to be 95% sure your sample mean lies within + 5 bytes of the true (but
unknown) mean. If the sampled packet sizes are statistically
independent of one another, how many packet sizes should be included in
your sample mean? [> 4,441 packets should be included in
your sample mean calculations]
<<<<<>>>>>
3) A single 1200 byte Instant Message (IM) packet is being sent over a
noisy 140 Kbps wireless link with a propagation delay of 2 msec.
The P(packet is corrupted and dropped) = 0.56. The P(ACK is
corrupted and dropped) = 0.0. TCP is being used with a Time Out
value of 3 seconds (time starts when the last bit is transmitted and
stops when an ACK is received). TCP has been set to attempt 4
transmissions of a specific packet, and if all fail, to cease
transmission attempts and request the IM application to post a
"Connection Lost" message on the user's screen.
3a) [10] Compute the probability the packet does not get through and a
"Connection Lost" message is posted. [0.09834]
3b) [10] Compute the probability exactly two transmissions are required
for successful reception. [0.2464]
3c) [10] Given that the packet was received on the 2nd attempt, compute
the time it took to deliver this packet. [3.139 seconds]
3d) [10] Given that a packet was successfully received (meaning it was
successfully transmitted on either the 1st, 2nd, 3rd, or 4th attempt),
compute the average time it takes to deliver such a packet. [9.276 seconds]
<<<<<>>>>>
4) A 64 Kbps statistically multiplexed switched Ethernet WAN
communications link is 85% loaded. Assume exponentially
distributed inter-arrival times, and exponentially distributed Ethernet
packet lengths with a mean of 360 bytes.
4a) [5] Compute the average time a packet spends in the switch. [300 msec]
4b) [10] Compute the carrying capacity of this link. [0.6942]
4c) [10] Now assume the 360 byte cells are being chopped into N ATM
cells at the information sink and then statistically multiplexed onto a
64 Kbps UBR ATM WAN communications link. Assume exponentially
distributed cell inter-arrival times at this ATM switch. Assume a
cell is allowed to spend an average amount of time in the ATM switch
equal to the time calculated in part 4a divided by N, compute the load
that can be placed on the 64 Kbps ATM trunk. [0.9031]
4d) [10] Compute the carrying capacity of this ATM link. [0.7114]
<<<<<>>>>>
5) Two PC's and a server are connected in a star configuration to a
half duplex 10 Mbps store-and-forward Ethernet switch on a small home
TCP/IP network. Each end computer is connected to the switch by a
5 meter cable. Assume the switch & computer processing time
is insignificant, that the system experiences no bit errors, and all
end devices are idle unless stated otherwise.
5a) [15] If a 10 M byte file is downloaded from the server to PC #1
through this switch, compute the time it will take to fully download
the file assuming TCP uses a window size of 1 packet. [17.45 seconds]
5b) [10] Repeat problem 5a assuming TCP uses a window size of 32
packets. [8.364 seconds]
5c) [15] Suppose PC #2 begins uploading a 10 M byte to the server at
the same instant PC #1 begins downloading its 10 M byte file from the
server. Estimate the time it will take PC#1 to fully download its
file. Assume TCP on all information sources uses a window size of
32 packets. [16.73
seconds]
<<<<<end>>>>>