Islamic University of Gaza

Faculty of Engineering

Computer Engineering Department

ECOM4314: Data Communications (Fall 2010)

Date: Jan 13, 2010

Time : 11:00- 13:30

Final Exam

Student Name:
Student Id: / -
Question / Points
Question 1
Question 2
Question 3
Question 4
Question 5
Question 6
Total / 60

Question 1 : (10 Point)

A. State whether each of the following is True (T) or False (F)

1. A digital signal is a composite signal with an infinite bandwidth
2. Segmentation/reassembly is the process of adding/removing headers to the data packets
3. Attenuation is an increasing function of signal frequency
4. We need to send extra bits with data to detect or correct error
5. The hamming distance between two words is the number of difference between
corresponding bits
6. A simple parity-check code can detect an odd number of errors
7. CSMA can reduce the possibility of collision, but it cannot eliminate it.
8. In Frequency-Division Multiple Access (FDMA),each station is allocated a time slot
during which it can send data
9. The chance of collision can't be reduced if a station senses the medium before trying
to use it.
10. In CDMA, one channel carries all transmission simultaneously

B. Please answer the following briefly (generally one to three sentences)

  1. Is packet switching always more efficient than circuit switching?
    ______
    ______
    ______
  2. What is/are the main advantage(s) of using a layered network architecture?
    ______
    ______
    ______
  3. Suppose a computer is moved from the Computer Science Department another department
    in the university campus. Does its MAC address need to be changed? Does the IP address
    need to be changed?

______
______

  1. Why is it generally a good idea to implement reliability (e.g., packet loss recovery) at end
    systems instead of in the middle of a network (e.g., by a router)?

______
______
______

Question 2 : ( 10 Point)

A. Give a definition for each statement . Write your answer clearly

1. The loss of signal’s energy due to the resistance of the medium
2. The division of data into smaller units to accommodate a protocol’s PDU.
3. A digital-to-digital polar encoding method in which a transition occurs at the
middle of each bit interval
4. A code in which the exclusive-OR of two valid codeword create another
valid code word.
5. A code has the property if a codeword is cyclically shifted, the result is
another valid code word.
6. Refers to a set of procedures used to restrict the amount of data that the
sender can send before waiting for acknowledgment.
7. Refers to a set of procedures used to detect and correct errors.
8. A special byte added to the data section of the frame where there is a
character with the same patterns as the flag.
9. The number of signaling events per second
10. The time needed for each intermediate or end device to hold the message
before it can be processed

B. Encode the following bit stream

1 / 1 / 0 / 1 / 0 / 1 / 0 / 1
NRZ-L
`
NRZ-I
RZ
Manchester
Differential Manchester

Question 3 : ( 10 Point)

Consider the network shown in , where the numbers beside the links connecting hosts and routers represents the port numbers of the routers. The table beside the routers represent the VCI translation table at the routers. The VCI translation tables for routers 1-4 are given in the figure , while the VCI translation table for router 5 isnot given. In this particular scenario, based on the VCI translation tables at the other four routers.

  1. Can you infer the VCI translation table at router 5? If your answer is yes, please write down the VCI translation table at router 5. If not, briefly explain why.
  2. How many virtual circuits are there in the network? Please write down thepath each virtual circuit traverses.

Incoming / Outgoing
Port / VCI / Port / VCI
0 / 4 / 1 / 2
0 / 3 / 3 / 2
Incoming / Outgoing
Port / VCI / Port / VCI
0 / 2 / 1 / 1
3 / 2 / 1 / 2
Incoming / Outgoing
Port / VCI / Port / VCI
0 / 2 / 2 / 1
3 / 2 / 2 / 2
3 / 1 / 1 / 3
Incoming / Outgoing
Port / VCI / Port / VCI
3 / 2 / 2 / 2
3 / 1 / 1 / 3

Question 4 : ( 10 Point)

A.Find the Minimum Hamming Distance for the Following cases

  1. Detection of two errors
  2. Correction of two errors
  3. Detection of 3 errors or correction of 2 errors
  4. Detection of 6 errors or correction of 2 errors

B.Show that the Hamming code C(7,4) if it can detect two-bit error but notnecessarily three-bit error by testing the code in the following cases. The character "V" in the burst error means no error; the character "E" means an error.

a. Dataword: 0100 Burst error : VEEVVVV

b. Dataword :0111 Burst error: EVVEVVE

c. Dataword: 1111 Burst error: EVEVVVE

d. Dataword: 0000 Burst error : EEVEVVV

Question 5 : ( 10 Point)

A sender need to send the four data items , 0x3456, 0xABCC, 0x02BC and 0xEEEE

Answer the following

  1. Find the checksum at the sender site
  2. Find the checksum at the receiver site if there is no error
  3. Find the checksum at the receiver site if the second data item is changed to 0xABCE.
  4. Find the checksum at the receiver site if the second data item is changed to 0xABCEand the third is changed to 0x02BA.

Question 6(10 points)

According to paper discussed in the class ("Interference in data communication over narrow-band-PLC"), answer the following questions:

  1. Mention types of interference on power line.
  2. Mention the services provided by PLC

3. Talk about the experiment environment used in the research and the main conclusion point .

Our best Wishes …

Eng. Wazen M. Shbair, Eng.Mahmoud El-Aff , Eng. Daila. A. Awad

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