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IST 220, Test 2 (answer), Fall 2009
Name: _______________________
1. For each question (3 points), there are multiple choices, but only one is correct
1.1) The meaning of '10' in the term '10Base2' is __________
(a) 100 meters
(b) 100 Mbps
(c) 10 meters
Answer: (d)
(d) 10 Mbps
1.2) An even parity checking will fail to detect transmission errors if ____ bits of the
corresponding char are changed.
(a) 1 (b) 2 (c) 3 (d) 5
Answer: (b)
1.3) Every LAN switch uses a specific forwarding table to make decisions about where to
forward a packet. Such decisions are independent of:
(a) the computer who sends out the packet
(b) the computer who is going to receive the packet
(c) the destination address field of the packet
(d) the way the sending computer and the receiving computer are connected
Answer: (a)
1.4) The connectors used in a 10Base2 Ethernet are called a ______
(a) BNC connector (b) RJ-45 connector (c) Fiber connector (d) TP connector
Answer: (a)
1.5) When one bridge and three repeaters are used to build a 10Base2 Ethernet, the maximum
distance between two computers is _______
(a) 185 (b) 185*2 (c) 185*3 (d) 185*4 (e) 185*5 (f) 185*6
Answer: (e)
1.6 _______ When two computers and 1,000 miles away from each other and use phone lines to
do long distance communications, if each phone switch uses an amplifier in the middle, the
circuit that connects the two computers can be called one of the following except _______
(a) A leased line (b) A long distance circuit (c) An analog long distance circuit; (d) A digital
long distance circuit
Answer: D
1.7 _______ Assume 5 signals want to go through a cable with bandwidth 8000HZ at the same
time using FDM. Assume signal 1 needs 2500HZ; signal 2 needs 800HZ; signal 3 needs 3000HZ;
signal 4 needs 2800HZ; signal 5 needs 2200HZ. Maximally how many signals can go through the
cable at the same time without suffering inference? (No signal can be split.)
(a) 1 (b) 2 (c) 3 (d) 4 (e) 5
Answer: C
2. Please say TRUE or FALSE to each of the following statements (2.5 points each)
2.1) __True_____Twisted pair Ethernet can be quicker than coaxial cable Ethernet.
2.2) __False_In an Ethernet, repeaters do not do address filtering, but NIC cards on a bridge do.
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2.3) __False_____When two Ethernet LAN segments are connected by a device, we say the
device is “transparent” to a computer if the NIC card of the computer could not experience any
difference between receiving a packet sent to him from a computer on the same segment and
receiving a packet sent to him from a computer on the other segment. Based on the above
definition, repeaters are transparent to computers in a LAN, but bridges are not.
2.4) _False______Parity checking sometimes could fail to detect transmission errors, but CRC
will never fail.
2.5) __False_____In order to build an Ethernet, we have to use an Ethernet Hub.
2.6) __False_____Similar to CRC and checksums, parity checking is designed to detect
transmission errors of packets.
2.7) __False_____Although 10Base2 and 100BaseT have different transmission speed, they use
the same type of cables.
3. Consider the following LAN where 8 computers are connected. Here B1 and B2 are two
bridges and R is a repeater. Assume that the physical address of NIC-1 is 78DF4C84F2D3,
the physical address of NIC-8 is 7E4C8D4C544C. We assume that computer 8 wants to send
the message "E" to computer 1, and the hexadecimal value of 'E' is 45 (binary 1000101).
3.0) How many LAN segments are there? (3 points)
Answer: 4
3.1) In order to send out the message, P-8 needs to first compose the packet, please draw the
packet before it is forwarded to NIC-8. (3 points)
Answer: |78DF4C84F2D3|7E4C8D4C544C|45|
Note: using “1000101” is also correct.
3.2) Assume NIC-8 uses the following CRC generator.
3.2a) Please calculate the CRC step-by-step. (4.5 points)
Grading note: (a) Since the payload has 7 bits, there should be 7 rounds; in each round, there
should be four steps: shift  feedback  XOR  move in; (b) partial points should be given.
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The result should be: 0001
3.2.b) Please draw the packet before NIC-8 sends out the packet (3 points).
Answer: |preamble|78DF4C84F2D3|7E4C8D4C544C|’E’|0001|
3.3) After a while, NIC-R of B2 will sense the packet, what will NIC-R do before the packet is
discarded or forwarded to P-B2? (5 points)
Answer:
(a) demodulate  put into cache
(b) length checking; if failing, discard the packet
(c) CRC checking; if failing, discard the packet;
(d) Strip off preamble and CRC
(e) Forward the packet to P-B2
Grading note: if address filtering is included, deduct 1 point.
3.4) Say True or False to the following statement: “If P-B2 gets the packet, then the payload of
the packet must not have been corrupted.” (3 points)
Answer: False
3.5) Assume P-B2 gets the packet and then forwards the packet to NIC-L of B2. Then NIC-L will
use CSMA/CD to send out the packet to the bus (where computer 5 is attached), that is, NIC-L
will keep on sensing the bus after the packet is on the bus (where computer 5 is attached). Please
tell when NIC-L will stop sensing the bus. (4 points)
Answer: NIC-L will keep on sensing the bus until the packet arrives at the NIC-R of B1.
Grading note: give 2 points when the answer is “until the packet arrives at NIC-1”
3.6) Assume when NIC-L of B2 sends out the packet, there is a collision on the bus where
computer 2 is attached. Will NIC-L of B2 be able to detect this collision? Why? (4 points)
Answer: NO, because B1 will prevent this collision from propagating to the bus where NIC-L is
attached.
3.7) When NIC-L of B2 sends out the packet, after a while, repeater R will gets the packet, what
will R do? Here we assume there are no collisions on this LAN. (3 points)
Answer: R will simply first amplify the m-carrier (or signal) then forward the amplified signal
onto the bus where B1 is attached.
3.8) After a while, NIC-R of B1 will sense and capture the packet. What is the format of the
packet inside the cache of NIC-R (of B1)? (3 points)
Answer: |preamble|78DF4C84F2D3|7E4C8D4C544C|’E’|0001|
3.9) The packet will be forwarded to P-B1 if there are no errors. When P-B1 gets the packet, what
is the format of the packet? (3 points)
Answer: |78DF4C84F2D3|7E4C8D4C544C|’E’|
3.10) If there is nothing wrong, after a while the packet will be sensed and captured by NIC-2.
What will NIC-2 do then? (4 points)
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Answer:
(a) demodulate  cache
(b) do address filtering; since the packet is not for computer 2, it will be discarded.
3.11) If there are no transmission errors, during the whole process from computer 8 to computer
1, how many NICs have seen the packet? Please consider every computer on the LAN. (3 points)
Answer:
If the NICs inside the bridges are not counted, the answer is: 8
If the NICs inside the bridges are counted, the answer is: 12
Grading note: (a) if the answer is 7 or 11, give 3 points
3.12) If there are no transmission errors, during the whole process from computer 8 to computer
1, how many processors have seen the packet? Please include the processor of the sender. (3
points)
Answer: 4
3.13) If there are no transmission errors, during the whole process from computer 8 to computer
1, how many times is length checking done? Please consider every computer on the LAN. (3
points)
Answer:
If the NICs inside the bridges are not counted, the answer is: 1
If the NICs inside the bridges are counted, the answer is: 3
3.14) If there are no transmission errors, during the whole process from computer 8 to computer
1, how many times is a CRC calculated? (3 points)
Answer: 6
4. Consider the following star topology LAN. Assume the MAC address for NIC-A is
78DF4C84F2D3; for NIC-B is 88DF4C84F2D3; for NIC-C is 98DF4C84F2D3; for NIC-D is
68DF4C84F2D3. (8 points)
Comp A
Comp C
Switch 1
NIC-A
NIC-A
1,1
1,3
P-C
Comp D
Comp B
P-s1
NIC-D
NIC-B
1,2
1,4
P-D
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4.1) What are missing inside the switch? (2 points)
Answer: (a) the queue; (b) the forwarding table
4.2) Say True or False to the following statement: “Assume computers A and B are sending two
packets to computers C and D (respectively) simultaneously, when the two packets arrives at
Switch 1, they will collide with each other.” (2 points)
Answer: False
4.3) What is the forwarding table used by the switch? (3 points)
Answer:
Dest Address
NIC-A’s MAC
NIC-B’s MAC
NIC-C’s MAC
NIC-D’s MAC
NIC
NIC1,1
NIC1,2
NIC1,3
NIC1,4